final report compost stability: impact and assessment stability... · final report compost...

Final report

Compost stability:

Impact and assessment

Project code: OMK009-001 Date: July 2015 Research date: January-May 2014

WRAP‟s vision is a world where resources are used sustainably. Our mission is to accelerate the move to a sustainable resource-efficient economy through re-inventing how we design, produce and sell products; re-thinking how we use and consume products; and re-defining what is possible through re-use and recycling. Find out more at www.wrap.org.uk

Document reference: WRAP, 2014, Banbury, Compost stability: Impact and assessment, Prepared by Dimambro, M. E., Frederickson, J., Aspray, T. J, Wallace, P.

Document reference: [e.g. WRAP, 2006, Report Name (WRAP Project TYR009-19. Report prepared by…..Banbury, WRAP]

Written by: Mary Dimambro (Cambridge Eco Ltd), Jim Frederickson (Open University), Thomas Aspray (Heriot-Watt University) and Phil Wallace (Phil Wallace Ltd)

Front cover photography: Compost stability tests. Clockwise from top left: DR4, Dewar self-heating, static respiration, oxygen uptake rate (photos courtesy of Jim Frederickson and Thomas Aspray).

While we have tried to make sure this report is accurate, WRAP does not accept liability for any loss, damage, cost or expense incurred or arising from reliance on this

report. Readers are responsible for assessing the accuracy and conclusions of the content of this report. Quotations and case studies have been drawn from the public

domain, with permissions sought where practicable. This report does not represent endorsement of the examples used and has not been endorsed by the organisations

and individuals featured within it. This material is subject to copyright. You can copy it free of charge and may use excerpts from it provided they are not used in a

misleading context and you must identify the source of the material and acknowledge WRAP‟s copyright. You must not use this report or material from it to endorse or

suggest WRAP has endorsed a commercial product or service. For more details please see WRAP‟s terms and conditions on our website at www.wrap.org.uk

http://www.wrap.org.uk/

Compost stability: Impact and assessment 1

Executive summary

Introduction The British Standards Institution Publically Available Specification 100 (BSI PAS 100) for composted materials was first introduced in 2002, when the majority of composting sites were processing green waste in open windrow systems. Since then a shift in the type of compost feedstocks as well as processing conditions has been observed. The use of in-vessel composting technologies (IVC) has increased significantly in the UK as processing food waste has become more common. Factors such as feedstock type, specific operating (or process) conditions of the in-vessel stage, as well as the subsequent stabilisation and maturation stages, may influence the effectiveness of the composting process, and hence the stability of the end product. This industry shift raises the question as to whether the current UK compost stability test method (ORG0020), as well as the compost stability limit prescribed by PAS 100, are still meaningful. Since the introduction of ORG0020 in 2005, the upper test limit for compost stability has been set at 16 mg CO2 / g organic matter (OM) / day to cover all market areas as a baseline quality specification. A more stringent stability limit applies to certified composts used in growing media (10 mg CO2 / g OM). Around 50 % of compost is currently certified to PAS 100 and the remainder is for the most part applied to land under the various UK waste regulatory regimes. A critical re-evaluation of the compost stability limit in the UK was proposed by the industry, particularly for composts used in agriculture. For non-PAS 100 compliant composts, the question whether stability limits should be imposed on very unstable materials when stored and used in the field was of significant regulatory concern. The overarching objectives of this project were to design and implement a programme of desk, field and laboratory work to provide information and data regarding:

whether the PAS 100 stability test was fit for purpose, particularly for determining the stability of composts produced by IVC processes;

whether the PAS 100 stability baseline could be changed, particularly for PAS 100 composts used in agriculture;

whether stability limits should be set for non-PAS composts, and if so, what test could be used, and how might the limit be selected; and

the financial and commercial (operational) implications for PAS and non-PAS compost producers of the implementation of any of the recommendations made in this work.

This project was aimed at collating currently available information and relevant data to enable WRAP and the Environment Agency to derive their own, evidence-based recommendations. This project should be considered a „scoping study‟ that informs possible future more detailed investigations into specific aspects of compost stability.


Outline of methods and materials A range of methods have been established for measuring compost stability, with carbon dioxide, heat evolution and oxygen uptake forming the basis of European and international standards. For the purposes of comparison with the ORG0020 (PAS 100) stability test, and on the basis of common European practice and proposals made for common Europe-wide compost stability testing, four test methods were selected; modified DR4, Dewar self-heating, oxygen uptake rate (OUR) and static respiration. Two REAL (Renewable Energy Assurance Ltd) appointed laboratories (Lab 1 and Lab 2) carried out the PAS 100 test, with the Open University conducting the DR4 and OUR tests and Heriot Watt University carrying out the Dewar self-heating and static respiration tests. Compost samples were collected from ten IVC sites: Six producing PAS 100 certified material and four producing non-PAS material. Three sites were based in Scotland and seven based in England. Materials collected were fresh waste (feedstock), „in process‟ materials and final screened compost samples. At two sites single batches were followed through the whole composting process. At the remaining eight sites a range of material samples were collected on a single day. The composting process was monitored using CompostManager equipment to assess the temperature, oxygen (O2), carbon dioxide (CO2) and moisture of post-IVC stages of stabilisation and maturation. Screened compost samples were tested for basic physicochemical and biological characteristics; seven samples were tested for culturable microbial numbers. Following claims by the industry that sample „passes‟ could be affected using prior sample treatment, one sample was microwaved and subsequently assessed for any effects on its apparent stability level. Summary of findings PAS 100 test Based on the results of 14 compost samples, a good correlation was found between Lab 1, Lab 2, DR4 and OUR results at the lower range of compost stabilities. However, near the threshold for PAS 100 of 16 mg CO2 / g OM / day, Lab 1 was found to produce much lower values (ie, indicating greater stability) than both Lab 2 and DR4, the results for these two being comparable up to almost double the threshold i.e. 30 mg CO2 / g OM / day. Differences between certified laboratories have been observed in previous PAS 100 stability ring tests, although some variability is to be expected when testing small samples for biological activity. Microwaving one immature compost sample did not greatly affect the stability result compared to the equivalent non microwaved sample using the PAS 100, DR4 and OUR tests. However, the Dewar self-heating and static respiration tests were greatly affected in their accuracy, with the microwaved sample result being reported as very stable. Given that there was no compelling evidence to the contrary, it is suggested that the PAS 100 stability test method be retained for all processing technologies and waste feedstocks. However, due to the PAS 100 stability test results being less consistent between laboratories for less stable non-PAS composts (e.g. >30 mg CO2 / g OM / day), this test may not be appropriate if an accurate assessment of such unstable composts is required.


To address inter-laboratory variability for ORG0020 test results, it is suggested that all of the REAL appointed laboratories be audited to aim to establish the reason for the reported variability, particularly for the less stable materials. PAS 100 baseline stability threshold The only European countries which make a link between compost end use and stability are the BGK quality assurance scheme in Germany and Luxembourg, with fresh compost used in agriculture, mature compost in horticulture, and very mature compost used in growing media. Thus the majority of evidence found on the use of fresh (or immature) composts (as discussed in the project literature review) is from Germany, where agricultural and field horticultural trials have generally shown agronomic benefits on crop yield and soil properties when less mature composts have been used. Yield effects may be linked to the available nitrogen released within a season (associated with the C:N ratio of the compost), for example, or the particular soil it is applied to. In a few cases, short term nitrogen „lock-up‟ has been experienced when using fresh or mature compost. To avoid detrimental effects on crop yield, those studies have recommended applying the compost well in advance, such as in the autumn before a spring sown crop, or to apply additional inorganic nitrogen fertiliser to compensate for any locked up nitrogen. The PAS 100 stability baseline is currently 16 mg CO2/g organic matter/day. Recently proposed Europe-wide criteria for compost stability criteria are:

Rottegrad III, IV or V (self-heating test temperature rise of max. 30oC above ambient

temperature).

Respirometric index (OUR) result of maximum 25 mmol O2/kg organic matter/h.

The limited dataset generated during this project suggests that alignment of the UK approach with the rest of Europe would require amendment of the PAS 100 stability threshold from 16 to 20 mg CO2/g organic matter/day. However further work is required to thoroughly compare the different stability methods/levels proposed. In the growing media sector, the largest potential market share for the use of stable green waste compost to replace peat is in bagged growing media which is stored for long periods. No EU wide stability limit for the use of compost in growing media was identified; however a more stable compost is typically deemed more appropriate in a number of quality assurance schemes across Europe. No evidence was found in the literature to amend the baseline stability threshold for growing media (10 mg CO2 / g OM / day). Stability of non-PAS materials Approximately 1.2 million tonnes of non-PAS composts were utilised in agriculture and field horticulture in 2012, through the regulatory regime of deployments and permits. Under „Standard Rules‟, waste materials should be sufficiently processed so as to become „nominally stable‟ and have reached a „degree of processing and biodegradation at which the rate of biological activity has slowed to an acceptably low and consistent level and will not significantly increase under favourable, altered conditions‟. These rules also state that „Each composting batch shall undergo an identifiable sanitisation and stabilisation stage‟. Unlike PAS 100, these terms do not specify a quantitative threshold that has to be met. However, the literature review, composting theory and data gathered in this project indicate that certain biological and chemical phases should be completed before a material can be classed as „nominally stable‟. A low cost and simple voluntary test may be of use to the industry to help determine whether waste is „nominally stable‟. Examples of possible indicators include pH, temperature


and volatile fatty acid (VFA) content. For example, the literature review indicated that a poorly composted material which is very unstable may have a low pH (unless masked by additives such as lime), and a high concentration of VFAs. However, these indicators would require further investigation as to their general suitability. Laboratory data obtained from this project showed that even the non-PAS, very unstable composts that did not pass the stability test, did pass the PAS 100 bioassay („plant response‟) test. Nevertheless, a number of researchers in Germany recommended delaying the planting of sensitive crops after the application and incorporation of „fresh‟ composts. Although concern has been expressed that unstable material may result in leachate breakout and odour, no evidence of harm was found in the UK from use of composts with differing stabilities. Based on this, there is insufficient evidence to support a stability test requirement for non-PAS composts. Moreover, the NVZ regulations, Defra‟s Code of Good Agricultural Practice, and the current deployment requirements should, in theory, be sufficient to protect water bodies from any leachate released from temporary compost storage heaps and from compost applied to land. Financial and commercial impacts of any changes that might be introduced No changes are currently suggested to the method of stability testing for PAS 100, subject to an audit of laboratories. Moreover, there was no evidence to support the introduction of a stability test for non-PAS composts. Therefore no financial or operational impacts are foreseen for the industry. Suggestions for future work Further work is suggested covering the following aspects:

Auditing the performance and processing of the PAS 100 stability method at each of the REAL appointed laboratories.

Establishing the correlation between stability levels for ORG0020 and proposed Europe-wide stability methods (Dewar and OUR).

Comparing microwaved and non-microwaved samples with a range of stability levels, including levels close to the pass/fail threshold specified in PAS 100 to understand whether such treatment could result in unsuitable material being deemed to have passed the stability test.

Investigating low cost monitoring tests of the composting process and materials in storage such as pH and VFA concentrations, relating to the phases and degree of composting and „nominal stabilisation‟ in parallel to stability.

Field monitoring of the potential for in-field storage heaps or the spreading of very unstable materials to cause environmental harm or offensive odour.

Assessing the effects of immature composts on crops and the environment.


Contents

1.0 Introduction ................................................................................................. 8 1.1 Compost stability considerations for IVC composts ........................................ 8

1.1.1 The PAS 100 stability method – previous ring testing .......................... 9 1.1.2 Alternative compost stability tests ..................................................... 9

2.0 Materials and methods ............................................................................... 10 2.1 On site sampling procedure and sample preparation ................................... 13

2.1.1 Microwave method ......................................................................... 13 2.2 On site assessment of oxygen, carbon dioxide and temperature ................... 13 2.3 Standard laboratory analyses .................................................................... 14 2.4 Compost stability tests .............................................................................. 14

2.4.1 Dry matter and loss on ignition ....................................................... 14 2.4.2 PAS 100 stability test ...................................................................... 14 2.4.3 Oxygen uptake rate ........................................................................ 15 2.4.4 DR4 test ........................................................................................ 15 2.4.5 Dewar self-heating test .................................................................. 16 2.4.6 Static respiration test ..................................................................... 16

2.5 Viable mesophilic bacterial counts.............................................................. 16 3.0 Results and Discussion: Stability testing .................................................... 17

3.1 Assessment of the PAS 100 stability test .................................................... 19 3.2 Comparison of stability tests ..................................................................... 22

3.2.1 DR4 and PAS 100 stability test comparison ....................................... 23 3.2.2 OUR and PAS 100 stability test comparison ...................................... 23 3.2.3 Dewar and PAS 100 stability test comparison ................................... 24 3.2.4 Static respiration 24 h and PAS 100 stability test comparison ............. 25

4.0 Results and discussion: Understanding how operational conditions relate to stability ................................................................................................................ 26

4.1 Summary of CompostManager findings with particular emphasis on relating pile oxygen status with the stability of composts ......................................................... 26 4.2 Compost processing results: DR4 stability testing ........................................ 28 4.3 Processing factors .................................................................................... 30 4.4 Nutrients, physiochemical properties and pathogens ................................... 31

4.4.1 Pathogens ..................................................................................... 31 4.4.2 Chemical parameters ...................................................................... 33

4.5 Results: Plant bioassay ............................................................................. 35 4.6 Results: Viable mesophilic bacterial counts ................................................. 39

5.0 Summary and conclusions .......................................................................... 40 5.1 The PAS 100 stability test method ............................................................. 40 5.2 The PAS 100 stability baseline – agriculture and field horticulture ................. 41 5.3 The recommended stability baseline for growing media ............................... 42 5.4 Stability of non-PAS materials.................................................................... 42 5.5 Financial and commercial (operational) implications of changed in stability methodology ...................................................................................................... 43

6.0 Further work ............................................................................................... 43 6.1 PAS 100 stability test laboratory audit ........................................................ 43 6.2 Correlating PAS 100 stability with EU recommended stability tests: OUR and self-heating ....................................................................................................... 43 6.3 Test using VFAs ....................................................................................... 44 6.4 Further investigating the impact of microwaving on compost stability and microbial abundance........................................................................................... 44 6.5 Assessment of in-field compost storage ...................................................... 44


6.6 Providing evidence of the performance of non-PAS IVC composts in UK agriculture ......................................................................................................... 45

7.0 References .................................................................................................. 46 Appendix 1. Sampling methodology ..................................................................... 48 Appendix 2. CompostManager protocol ............................................................... 50 Appendix 3. CompostManager results .................................................................. 51 Appendix 4. Feedback from the UK laboratories .................................................. 56 Appendix 5. Ranking of the stability tests ............................................................ 58

Glossary

ABP Animal by-products ABPR Animal By-Products Regulations Biowaste Biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers and retail premises, and comparable waste from food processing plants CFW Commercial food waste C:N Carbon:nitrogen ratio CO2 Carbon dioxide CoGAP Code of Good Agricultural Practice DM Dry matter DR4 Dynamic respiration test EC Electrical conductivity EoW End of waste EU European Union GFVC Garden, fruit, vegetable and cardboard GW Green waste JRC Joint Research Centre. The European Commission's science service

which carries out research to provide independent scientific advice and support to EU policy

IVC In-vessel composting MBT Mechanical Biological Treatment MC Moisture content NH4-N Ammoniacal nitrogen NO3-N Nitrate nitrogen NVZ Nitrate Vulnerable Zone OM Organic matter OUR Oxygen uptake rate PAS Publically Available Specification R2 R-squared (r2), which is often called the co-efficient of determination,

is a number between zero and one that determines how closely a graph's trendline corresponds to the actual data points on the graph. A r2 close to one represent a trendline that is almost identical to the data points

REAL Renewable Energy Assurance Limited SSHW Source segregated household waste VFA Volatile fatty acids VS Volatile solid


Acknowledgements

Thanks to

The compost site operators for facilitating compost sample collection and site work.

WRAP and the Environment Agency for funding the project.

ORG for their support and advice.

Eric Crouch for in-depth advice and discussion regarding the CompostManager data

interpretation.

Dr Francis Rayns, Prof Ralph Noble, Dr Joachim Steiner and Graham Howell for

technical advice.

The analytical laboratories for their support and considerations.


1.0 Introduction The overarching objectives of this project were to design and implement a programme of desk, field and laboratory work to provide information and data regarding:

whether the PAS 100 stability test remained fit for purpose, particularly for determining

the stability of composts produced by in-vessel composting processes;

whether the PAS 100 stability baseline could be changed, particularly for PAS 100

composts used in agriculture;

whether stability limits should be set for non-PAS composts, and if so, what test could

be used, and how a limit might be selected; and

to understand the financial and commercial (operational) implications for PAS and non-

PAS compost producers of the implementation of any recommendations for change

made in this work.

Phase 1 of the work programme comprised a literature review (reported separately) (WRAP, 2014), which focussed on three main areas:

The influence of the various processing factors and operational conditions of in-vessel

systems on the stability of the resulting composts;

The impacts of compost stability on compost storage and end use; and

Compost stability tests used in Europe.

Phase 2 was informed by the literature review. This technical report describes Phase 2; the on-site field work and laboratory work – and discusses these within the context of the four main project objectives listed above, as well as the findings of Phase 1. It should be considered as a „scoping study‟ that aims to inform subsequent investigations into specific aspects of compost stability. The aim of this report is to highlight possible benefits and dis-benefits relating to the four project objectives – to enable WRAP and the Environment Agency to derive their own, evidence-based recommendations. 1.1 Compost stability considerations for IVC composts Since PAS 100 was first developed, the shape of the UK composting industry has changed significantly, with an increasing deployment of in-vessel composting (IVC) technologies. In 2012 there were 58 operational IVC sites, treating 40% of organic waste composted (WRAP, 2013) compared to 33 IVC sites in 2009 and 23 in 2007/08 (Anonymous, 2010). As the number of IVC plants in the UK is increasing so is the range of IVC techniques. For example, some in-vessel systems operate with a lower air supply than others and the duration of the in-vessel phase may be one or two weeks (with turning) depending on the requirements of the Animal By-Product Regulations (ABPR) at each site (Anonymous, 2011). The literature review (Phase 1 of this project) highlighted that the specific operating (or process) conditions of both the in-vessel stage and the subsequent stabilisation and maturation stages contribute to compost stability (WRAP, 2014). These process conditions will influence the effectiveness of the composting process, which is correlated with the maintenance of aerobic conditions within well-structured composting piles, optimum moisture conditions and nutrients for growth of aerobic micro-organisms, optimum C:N ratio in feedstocks and use of appropriate composting times. As a result it was recommended that process conditions and process stability for both stages (IVC and subsequent windrow) were individually assessed at a number of IVC sites. While it is normally assumed that the in-vessel stage of composting is highly aerobic, the in-vessel stage for many ABP composting plants may actually be partly anaerobic, since this stage is considered by some operators to represent an ABPR sanitisation stage rather than a traditional composting stage (Frederickson et al., 2013). This confirms that it is important to characterise each of the two composting stages in terms of their potential oxygen profiles, to


better understand and predict changes in stability taking place during the complete composting process. In 2012, the majority of UK composting sites had a composting period of between 5 and 15 weeks, with the average composting period being 11.4 weeks in 2012 compared with 12 weeks in 2010 (WRAP, 2013). There are pressures on compost manufacturers to process waste as rapidly as possible, due to contractual pressures and to meet the agriculture and field horticulture markets. Composts spread in these markets may either be PAS 100 certified (which requires stability testing) or non-PAS (as a waste which does not require stability testing) – at present the PAS:non-PAS is split around 50:50 (WRAP, 2013). Hence it was deemed appropriate to consider both PAS and non-PAS 100 sites in Phase 2 of this project.

1.1.1 The PAS 100 stability method – previous ring testing

In the last ten years, three ring tests have been undertaken on the ORG0020 stability method (for PAS 100). The first two found that, although differences in compost stability could be discerned, there were also differences between laboratories for the same samples (ADAS, 2005; Wood et al., 2009). A further inter-laboratory test was carried out in 2011, and as a result, improvements were made to the method, including temperature and conditioning parameters (WRAP, 2012).

1.1.2 Alternative compost stability tests

There are a wide range of alternative stability tests to the ORG0020 method for compost. Stability tests were reviewed during development of the original PAS 100 method (ADAS, 2005) and again in 2009 (Wood et al., 2009), with the latter providing a good summary of the history of stability testing, including the development of tests in the US and Europe at that time. At present, there is no EU-wide standard for determination of stability in compost or digestate, and Member States use diverse standards and systems, with EN standards available for oxygen uptake rate (OUR) and self-heating tests (EN 16087-1 and EN 16087-2) (Saveyn and Eder, 2014). Further information on the range of compost stability test methods already used in the EU can be found in the literature review (WRAP, 2014). The OUR and self-heating tests have been included within proposals to develop Europe-wide end of waste (EoW) criteria for composts used as soil improvers and growing media ingredients (Saveyn and Eder, 2014). For compost stability, the proposals recommend that compliant material have a Rottegrad index of III, IV or V (together with a self-heating test temperature rise of maximum 30 oC above ambient temperature) or a respirometric index result of maximum 25 mmol O2 / kg organic matter / h. Some stability tests are based on the measurement of respiration including the dynamic respiration test (DR4) (Turrell et al., 2009), which formed part of the UK protocols for evaluating the bio-treatment performance of mechanical biological treatment (MBT).The requirement to evaluate MBT plants in this way was removed in 2013 with the cessation of the Landfill Allowances Trading Scheme, but the DR4 test is still used extensively to assess MBT bio-treatment performance. The DR4 test method is based on international established test methods (e.g. BS EN 14855) and is considered to be „robust‟ because it incorporates a compost inoculum within which the test sample is mixed (which provides an added microbial population and buffering capacity); the compost inoculum should be sieved mature green waste compost with a respiration value between 2 and 20 g O2 /kg VS. Also included in the test mix is nutrient solution to minimise possible nutrient limitations. The method is dynamic in that a controlled air supply is continually fed through the compost matrix/test sample to provide sufficient oxygen for effective decomposition to take place. The method also allows for the use of a positive control (cellulose) to ensure that the test system is functioning correctly.


The current PAS 100 stability test (ORG0020) lacks some of the positive control features of the DR4 test in the sense that no inoculum or nutrient solution is added. Also, the DR4 requires no equilibrium period whereas the ORG0020 method incorporates a three day equilibrium period during which time much carbon dioxide may be evolved and not recorded. Finally, a positive control is an essential component of the DR4 method, while it is not possible to have a positive control for the ORG0020 method – which relies on the inherent microbiological capacity of the sample under test. The DR4 test is robust in other ways too, for example, the sample size used in the test is relatively large (dry matter 100 g) and this is mixed with a similar weight of compost inoculum. This is in contrast to the much smaller sample size typically used for the OUR test which is equivalent to 1-2 g of volatile solids and this corresponds to much less than 5g dry mater for typical composts. Larger sample sizes would be preferred to minimise sub-sampling problems due to the heterogeneous nature of many compost samples. 2.0 Materials and methods Ten IVC sites were selected to represent a range of IVC systems operating in England and Scotland, as detailed in Table 2-1. The feedstocks included source-segregated biodegradable household waste (including food waste) – termed biodegradable municipal waste (BMW), commercial food waste (CFW) and garden waste (GW). Some sites re-incorporated various proportions of screened oversize (o/s) material as a bulking agent and, where reported, the estimated amounts of o/s material in typical batches are given below. In order to maintain site anonymity, the IVC systems are not described.

Table 2-1 Details of the compost sites

Site Feedstock PAS 100 certified

A GW, BMW (including o/s when required) Yes

B BMW, cardboard Yes

C BMW, cardboard, commercial GW Yes

D GW, BMW, woodchip Yes

E BMW, CFW (including o/s) Yes

F GW, BMW, CFW Yes

G BMW (including ~ 10% o/s) No

H BMW (including ~ 33% o/s) No

I BMW (including o/s) No

J BMW No

At each of the ten sites, four samples of material were taken, where possible or where available. These samples were: 1. Freshly shredded IVC input waste: FW 2. Post IVC (sanitised) material: POST IVC 3. Unscreened compost, on the maturation pad1(as mature/stable as possible): MAT 4. Screened final compost: SC

1 The term “maturation” also includes compost stabilisation processes when conducted after the IVC or sanitisation stage.


More information on the samples obtained can be found in Table 2-1 and Table 2-2. Compost sampling was undertaken during January-March 2014. The sampling methodologies and sample details are listed in Appendix 1. Sites were generally visited once during the course of the project, with the exception of site A which was visited four times and site G which was visited twice. For sites A and G the intention was to sample from the same batch as material passed through the various stages of processing – resource constraints meant that this was not possible for the other sites and consequently samples taken at these sites were taken from different batches during one visit. Hence, it should be noted that for sites visited and sampled on one occasion only, results relating to changes to material or processing characteristics should be considered to be indicative only. At site A, FW, POST IVC, MAT and SC samples were taken from one single batch as the material passed through the various stages of processing. In addition to the final SC sample which was taken after 31 days on the maturation pad, 3 intermediate screened compost (SC) samples were also taken from the pad on three separate occasions, as listed below (with the full sampling regime for site A shown in Table 2-2). 1. 0 days on maturation pad (after 23 days in IVC), screened SC1

2. 10 days on the maturation pad, screened SC2

3. 19 days on the maturation pad, screened SC3

4. 31 days on the maturation pad, final compost, screened SC4

At Site G (non-PAS) a single batch was also followed with FW, POST IVC and SC samples relating to one single batch obtained (Section 2.4.4 for the method and Section 4.2 for the results). DR4 tests only were carried out on the unscreened material from three locations at each site (coded FW, POST IVC and MAT samples) to help develop a profile for each site representing changes to the stability of material resulting from site processing operations. Other tests were applied as listed in Table 2-2.


Table 2-2 Sampling details, showing number of days in the IVC and in windrows DR4 = only DR4 test, All = all compost stability tests (ORG0020 (PAS 100) method, Oxygen Uptake Rate (OUR), modified DR4 test, Dewar self-heating test and a static respiration test), n/a not obtainable

Site

Days IVC Days windrow

Total days Stability Tests

A FW 0 DR4

Post IVC 23 0 23 DR4

MAT 23 31 54 DR4

SC 1 23 0 23 All

SC 2 23 10 33 All

SC 3 23 19 42 All

SC 4 23 31 54 All

B FW 0 DR4


MAT 14 42 56 DR4

SC 14 48 62 All

C FW 0 DR4

Post IVC 28 28 DR4

MAT 28 28 56 DR4

SC 28 35 63 All

D FW 0 DR4


MAT 14 98 112 DR4

SC 14 112 126 All

E FW 0 DR4

Post IVC 3 1 4 DR4

MAT 3 56 59 DR4

SC 3 105 108 All

F FW 0 DR4

Post IVC 7 6 13 DR4

MAT 7 33 40 DR4

SC 7 52 59 All

G FW 0 DR4


MAT n/a

SC 28 1 29 All

H FW 0 DR4


MAT n/a

SC 21 4 25 All

I FW 0 DR4


MAT n/a

SC 20 40 60 All

J FW 0 DR4

Post IVC n/a

MAT 5 28 33 DR4

SC 5 28 33 All


2.1 On site sampling procedure and sample preparation The sampling methodology is described in Appendix 1. Samples taken from composting sites were transported to relevant University laboratories on the same day. All samples were stored at 4°C on receipt at University laboratories. Samples of screened compost were subsampled and sent out to subcontracted laboratories the next working day. Sample preparation and chemical analyses were as follows. All samples were stored at 4°C on receipt. Where individual samples were supplied in multiple bags, these were mixed together to homogenise the sample. Samples of screened compost were subsampled, placed in tamper-proof sealed containers and despatched in cool boxes to subcontracted laboratories the next working day. Maximum sample delivery time was estimated to be two days. The remaining sample was re-mixed and subsampled by cone and quartering. Dry matter content was determined on all samples as received by weight loss at 105°C according to BS13040. The fractions of 10 mm, 20 mm and > 20 mm material in 5 litres of the screened compost samples were determined. All screened compost samples were sieved to 20 mm while unscreened samples for DR4 testing had large and non-grindable material removed prior to drying and grinding. 2.1.1 Microwave method Site G was selected to provide screened compost for a trial to determine the effect of micro-waving on compost stability and microbial number. Site G was selected on the basis of its typical short duration IVC stage with no further maturation of sanitised compost on site. 80˚C was selected as the benchmark for the completion of microwaving. This temperature was selected since it represented a sensible balance between the need to subject sample microbial populations, in particular mesophilic micro-organisms, to an appropriately high thermal death point (Golueke, 1982) and the requirement to satisfy laboratory health and safety considerations. It should be noted that the stability tests, to which the screened composts including the micro-waved sample were subjected, operate in the mesophilic range. A preliminary experiment was undertaken to identify the degree of microwaving required to raise the temperature of compost to 80˚C. One litre of sample was microwaved in 30 second increments with a 750W microwave on full power. The temperature was measured with a temperature probe after each 30s interval. Once the sample reached 80˚C the total time necessary was recorded (total time needed was two minutes). This time was verified by microwaving a further one litre sample for two minutes and checking the temperature. The screened compost sample was then subsampled and each one litre subsample microwaved (750W) for two minutes. The subsamples were then placed into a clean receptacle and thoroughly mixed. The micro-waved samples were bagged up and sent out to subcontracted laboratories the next working day. 2.2 On site assessment of oxygen, carbon dioxide and temperature The CompostManager (CM) system was used to obtain data on oxygen, carbon dioxide and moisture levels in the compost piles, in addition to the temperature. The CompostManager system is an instrument that records and analyses temperature, moisture, O2 and CO2 simultaneously within compost piles, using a single probe and custom-designed software. If dissolved O2 concentrations in windrows are 1ppm or more for at least 75% of the time, then oxygen is deemed not to be limiting the composting process, whereas dissolved O2 levels consistently below 0.5ppm are more likely to result in odour problems (Sauer and Crouch, 2013).


Where possible, compost just exiting the IVC, half way through maturation process and in the screened final product was investigated. The sampling protocol can be found in Appendix 2 and the points for CompostManager sampling at each site are presented in Appendix 3. 2.3 Standard laboratory analyses The standard laboratory analyses (see Table 2-3) were all undertaken by an REAL appointed UK laboratory on the screened compost samples (coded SC), the microwaved screened compost sample (SC, Site G) and also the three intermediate SC samples obtained from Site A.

Table 2-3 Standard PAS 100 compost laboratory tests

Analysis Method

NH4-N (ammonium-N) BS EN 13040:2000

NO3-N (nitrate-N) BS EN 13040:2000

Total C AOAC Official Methods of Analysis (1990) Method 949.12

Total N AOAC Official Methods of Analysis (1990) Method 949.12

Bulk Density BS EN 13040:2000

Dry matter BS13040

Organic Matter BS EN 13039:2000

pH BS EN 13037:2000

Electrical conductivity (EC) BS EN 13038:2000

Portion of particles < 20 mm BS EN 13040:1999

E. coli at 44°C BS ISO 16649-2

Salmonella spp. BS EN 6579

Plant bioassay OFW004-004, PAS 100 Annex D

2.4 Compost stability tests Five stability tests were undertaken on the final SC samples from all 10 sites and also the three intermediate SC samples obtained from Site A. The PAS 100 stability test was undertaken by two REAL appointed UK laboratories. The other four tests were carried out at the Open University (OUR and modified DR4) and Heriot-Watt University (static respiration in a sealed vessel and self-heating in Dewar flasks). The Open University also undertook standard DR4 stability testing on samples of unscreened material taken from various operational stages at each site. An outline of the methods used is provided below. 2.4.1 Dry matter and loss on ignition Dry matter content was first determined on all samples by weight loss at 105°C according to BS13040. Loss on ignition at 450°C was determined according to BS13039. These results were used for the calculations of compost stability by the REAL appointed laboratories enabling an additional comparison to be made between the laboratories. 2.4.2 PAS 100 stability test Method: ORG0020 (WRAP, 2012). The main principle of the test is as follows (Llewelyn, 2005b). The fist test was used to assess moisture content, with adjustment as necessary. Moisture-adjusted compost was incubated at 30°C with continuous replacement of carbon dioxide-free air. Carbon dioxide evolved from the compost was captured in a sodium hydroxide solution as sodium carbonate. The carbonate was then precipitated as barium carbonate by the addition of excess barium chloride. The concentration of carbon dioxide evolved by the compost was measured by titration of the residual sodium hydroxide with standard acid. The test was run with triplicate samples of compost in addition to one blank with reagent only. The method can be adapted to use other CO2 measuring devices connected directly to the outlet of the incubation vessels, although the two laboratories


undertaking this test used the titration method. The aerobic biological activity was measured as the evolved carbon dioxide and expressed in units of mg CO2 / g OM / day. 2.4.3 Oxygen uptake rate Oxygen Uptake Rate (OUR) was determined on fresh material screened to 10 mm according to BS16087-1. Dry matter content was determined on all 10 mm samples as received by weight loss at 105°C according to BS13040. Loss on ignition (equivalent to „volatile solids‟) at 450°C was determined BS13039. In brief, fresh sample equivalent to 2g volatile solids was mixed with a standard volume of water, nutrient solution, pH buffer and a nitrification inhibitor in a 1 L vessel. Batches of vessels including blanks were shaken and incubated at 30°C. After 4 hours the vessels were sealed with a suspended portion of soda lime to absorb any CO2 produced. Regular pressure readings were then taken using Oxitop® sensors over the course of 7 days. All samples and blanks were run in duplicate. OUR results were expressed as mmol O2 / kg OM / h. The British Standard BS16087-1 specifies the range of the test as reaching a pressure between -2 and -5 kPa after three days, with a linear pressure decrease. The greater pressure drop defines the maximum value measurable for a given size of reaction vessel and sample size. The Open University system is a standard system, selected to determine the stability of a wide range of composts from stable to very unstable. The system utilizes a 1 L bottle to cover a range of stabilities well in excess of the proposed EU threshold of 25 mmol O2/kg organic matter/h using the minimum sample size. During this study, it was found that results above 50 mmol O2 / kg organic matter / h gave a non-linear response, making the standard calculation invalid. Samples that were out of range after 3 days according to BS16087-1 were re-run with different quantities of sample, to a minimum of 1g volatile solids, less than this being considered impossible to subsample reliably. 2.4.4 DR4 test Two forms of the DR4 test were used in this study. Firstly, the standard form (Turrell et al., 2009) was used to determine the stability of unscreened samples from key stages in the composting process to provide insights into how material stability was related to the effectiveness of different operational conditions. Samples of this unscreened process material (i.e. coded as FW, POST-IVC and MAT) were dried at 70°C and ground so that most particles passed a 4 mm sieve. Any non-grindable objects (such as large rocks or metal fragments) were removed from the sample and the weight recorded. Loss on ignition at 450°C was determined as per BS13039. The test mixture (100g DM of sample plus 100g DM of compost inoculum) was placed in a respirometer chamber, with forced aeration at between 300 and 500 ml/min. Chambers were incubated at 30°C for 7 days. Flow rate and CO2 in the ambient air input and chamber output gas streams were measured using in-line analysers (Sable Systems International, Inc.). Each sample, inoculum-only negative control and cellulose positive control were run in triplicate. Total CO2 produced over the first 4 days was calculated for each replicate and mean production from the inoculum only was subtracted.DR4 results were expressed as g CO2 / kg VS / 96 h throughout. A modified DR4 test was developed for the project based on the standard DR4 test, which is typically used to test highly heterogeneous and recalcitrant residual waste (Turrell et al., 2009). The modified test method was used to determine the stability of the screened compost samples. The only modification to the standard DR4 test was that the test samples comprised fresh screened compost rather than dried, ground material; this was to maintain consistency with the other stability test methods and because any future DR4 test for compost would probably use fresh compost rather than dried, ground compost. Fresh compost material screened to 20 mm was mixed with an inoculum of well-matured,


commercially available green waste compost (PAS 100 certified) and nutrients added as for a DR4 test. Water was added to adjust moisture content to at least 50% by weight, and no mixture had greater than 57% moisture. The test mixture (100g DM of sample plus 100g DM of compost inoculum) was placed in a respirometer chamber, with the remainder of the test performed as described above for the standard DR4 method. Results were expressed as g CO2 / kg VS / 96 h. 2.4.5 Dewar self-heating test The self-heating test was carried out as per EN 16087-2:2011 (Determination of the aerobic biological activity Part 2: Self heating test for compost). Briefly, samples were screened using a 10 mm mesh size sieve before being incubated in a temperature controlled incubator at 22°C overnight. The fist test was used to assess moisture content, with adjustment as necessary. For the determination of self-heating, each sample was tested in duplicate using 100 mm internal diameter, 1.5 L capacity Dewar flasks. The flasks were loosely filled with compost and the base tapped 3-5 times on a hard surface. A digital min/max temperature probe was inserted to 30 mm from the internal bottom of each flask. The flasks were incubated in a temperature controlled incubator at 22°C for 10 days. Current and maximum stored temperatures were recorded daily. Results were reported as average max temperature (Tmax) of the duplicate flasks as per EN 16087-2:2011, for direct comparison with the German Rottegrad classification. In addition, results were reported as rise above ambient temperature (22°C) of the duplicate flasks for comparison with the suggested EU EoW stability grading. 2.4.6 Static respiration test Samples were screened using a 20 mm mesh size sieve. Moisture content was assessed gravimetrically and adjusted as necessary to between 50-60 % (w/w). Samples were then incubated in a water bath at 30°C overnight to equilibrate. Aliquots of 10 g dry weight equivalent were dispensed into Nalgene® 250 ml jars. The jars were connected to a Respicond conductimetric automated respirometer (Respicond; A. Nordgren Innovations AB, Sweden) and set to monitor CO2 efflux every hour for a minimum 72 h. Each sample was analysed in quintuplicate. A covered water bath was used to maintain the samples at 30°C for the duration. Results are reported as mg CO2 accumulation after 24 h and 48 h per gramme organic matter. 2.5 Viable mesophilic bacterial counts Compost microbiology was assessed on final screened compost samples from four sites (G, D, E, F). Samples from the maturation pad (MAT) from sites E and F were also assessed, in addition to the microwaved sample from site G. Samples were screened using a 20 mm mesh size sieve. Moisture content was assessed gravimetrically. Triplicate subsamples of equivalent one gramme dry weight of compost were individually mixed with 9 ml of sterile extraction buffer (0.85 % (w/v) NaCl and 0.1 % (w/v) sodium hexa metaphosphate (NaPO3)6). The suspensions were then serially diluted using 0.85 % (w/v) NaCl under aerobic or anaerobic conditions as required. Aliquots (50 µl of dilutions in the range from 10-4 to 10-8) were spread on tryptone soya agar and Wilkins Chalgren agar for aerobic and anaerobic counts, respectively. Wilkins Chalgren agar plates were pre-incubated overnight under anaerobic conditions before spreading. After spreading, aerobic and anaerobic plates were incubated at 30 and 32°C, respectively. Colonies were counted after 48-72 h and results reported as colony forming units (cfu) per gramme (dry weight) of compost.


3.0 Results and discussion: Stability testing The results from the range of stability tests for the ten IVC sites undertaken is shown in Table 3-1, with results from the batch analysis and microwaved sample in Table 3-2.

Table 3-1 Compost stability test results for the ten IVC sites for the final screened compost (SC) samples

Site

Test Units A B C D E F G H I J

PAS-certified process? Yes Yes Yes Yes Yes Yes No No No No

Lab 1. PAS

100 stability

mg CO2 / g OM

/ day 8.3 7.3 10.8 5.2 14.3 16.5 24.3 13.6 29.7 16.4

Lab 2. PAS

100 stability

mg CO2 / g OM

/ day 10.9 7.0 7.1 5.1 31.7 17.0 37.3 25.7 37.8 29.2

DR4 gCO2 / kgVS /

96 h 52 28 48 24 279 121 315 262 231 266

OUR mmol (O2) / kg / h

16.6 5.7 9.1 5.3 >50 23.1 >50 >50 >50 >50

Dewar

(Rottegrad

stability grading)

T max °C 26.2

(V)

25.1

(V)

24

(V)

25.1

(V)

62.7

(I)

64.7

(I)

49.6

(III)

64.6

(I)

25.3

(V)

64.5

(I)

Dewar

Temperature rise above

ambient (22°C)

4.2 3.1 2.7 3.1 40.7 42.7 27.6 42.6 3.3 42.5

Static respiration

mg CO2 / g OM / 24 h

9.1 11.8 3.5 4.7 15.8 13.9 8.7 14.2 0.4 12.4

Static respiration

mg CO2 / g OM / 48 h

15.0 17.3 7.7 9.3 15.8 16.1 10.4 16.3 2.4 13.9

Notes:

PAS 100 stability limit is 16 mg CO2/g organic matter/day. Samples higher than this are highlighted in green.

OUR proposed EU EoW limit is 25 mmol O2/kg organic matter/h. Samples higher than this are highlighted

in blue.

Dewar: Rottegrad classification (German stability rating) I (immature): >60°C; II - III (fresh compost):

40.1-60°C. Samples within this range are highlighted in orange.

Dewar: proposed JRC EoW limit is < 30°C above ambient. Samples higher than this are highlighted in blue.


Table 3-2 Full set of stability results for the batch analysis at site A, with the number of days on the maturation pad noted. Results for the SC compost from site G and also the microwaved sample, which was a sub-sample taken from the same aged compost and then microwaved (see Section 2.1.1). Note: The two screened final compost samples (SC4 for site A and SC for site G) are the same as in Table 3-1.

Site A Site G

Test Units SC1 Day 0

SC2 Day 10

SC3 Day 19

SC4 Day 31

SC Micro- waved

Lab 1. PAS 100 stability

mg CO2 / g OM / day

16.7 8.87 7.68 8.27 24.3 27.6


mg CO2 / g OM / day

12.1 10.8 8.0 10.9 37.3 30.7

DR4

gCO2 / kgVS /

96 h 103 52 56 52 315 236

OUR mmol (O2) /

kg / h 38.8 11.3 12.1 16.6 >50 >50

Dewar

(Rottegrad

stability grading)

T max °C 50

(III)

30.3

(IV)

29.6

(V)

26.2

(V)

49.6

(III)

24

(V)

Dewar

Temperature

rise above

ambient (22°C)

28 8.3 7.6 4.2 27.6 2.0

Static

respiration

mg CO2 / g

OM / 24 h 14.2 9.02 3.89 9.1 8.7 0.49

Static

respiration

mg CO2 / g

OM / 48 h 16.2 15.2 8.61 15.0 10.4 0.74

Notes:

PAS 100 stability limit is 16 mg CO2/g organic matter/day. Samples higher than this are highlighted in green.

OUR proposed EU EoW limit is 25 mmol O2/kg organic matter/h. Samples higher than this are highlighted

in blue.

Dewar: Rottegrad classification (German stability rating) I (immature): >60°C; II - III (fresh compost):

40.1-60°C. Samples within the I-III range are highlighted in orange.


3.1 Assessment of the PAS 100 stability test Two REAL appointed laboratories were used to test the final SC from the 10 sites plus the site A time series and microwaved sample. The PAS 100 results from the two laboratories are shown in Figure 3-1 indicating that a range of stabilities was measured and that there were differences between the two laboratories. Composts from the four non-PAS 100 sites (G-J) were less stable (i.e. produced more CO2) than composts from most of the PAS 100 sites.

Figure 3-1 PAS 100 stability results from the two laboratories

A comparison of the test results showed that for Lab 2 (in general, and especially for less stable materials – as indicated by a number of stability measures), more CO2 evolution was measured compared with Lab 1 (see Figure 3-2). This difference between the two laboratories for aliquots of the same sample was greater than the tolerance of 2.0 mg /CO2 / g OM / d recommended for intra-lab performance by Llewelyn (2005a) during development of the PAS 100 stability test. The variability of PAS 100 stability test results between laboratories is consistent with previous ring tests (ADAS, 2005; Llewelyn, 2005a; Wood et al., 2009) and may indicate that the recent modifications to the test (WRAP, 2012) have either not been fully implemented or effective at minimising inter-laboratory differences – particularly for less stable materials.


Figure 3-2 PAS 100 stability results from the two laboratories, with 1:1 trendline

The results from each of the laboratories over the full range of samples were then compared with the DR4 results for the same samples (Figure 3-3 and Figure 3-4), as the DR4 test is considered to be a „robust‟ stability test as outlined in Section 1.1.2. The figures show the data from the 10 sites, including the time series at site A, and the microwaved sample.

Figure 3-3 Lab 1 PAS 100 stability results plotted against DR4 results


Figure 3-4 Lab 2 PAS 100 stability test results plotted against DR4 results

The PAS 100 stability values were correlated with the DR4 values, with a close agreement for Lab 2 (r2=0.92), but less so for Lab 1 (r2=0.59)2. Close to, and above, the PAS 100 limit (16 mg CO2 / g OM / day) Lab 1 results, especially, were more variable. Up to DR4 values of 250 g CO2 / kg VS / 96 h, the PAS 100 and DR4 results were well aligned. However, for the very unstable materials above this, it would appear that the PAS 100 method was not generating and/or collecting as much evolved CO2 as might be expected in comparison with the DR4 method. To illustrate this, the DR4 test generated and/or collected 5 times the average amount of CO2 / kg VS from the 5 least stable composts compared with the 5 most stable composts; for Lab 2 this performance figure was 3.4 times. This strongly suggests that the DR4 method, compared with Lab 2, was generating and/or collecting significantly more CO2 per kg VS from the least stable composts. This effect is especially notable for Lab 1, but it should be noted that the values for the least stable composts are well above the PAS 100 threshold. For the microwaved sample, the PAS 100 Lab 2 and DR4 test showed a slight increase in stability compared to the non-microwaved counterpart, although Lab 1 showed a reduction in stability (Table 3-2). However, for both labs, the microwaved sample still failed the PAS 100 stability test. Additional data were available from the two laboratories including the OM contents (these were additional tests as part of the method) but there were no major differences between the data from the two laboratories. The findings from this limited analysis of in-vessel materials derived from food wastes suggests that, even for IVC material with short retention times, if the current PAS 100 test ORG0020 is carried out by Lab 2, there is no evidence that the test is not fit for purpose within the range approximately double that of the threshold value (up to 30 mg CO2 / g OM / day).

2 An r2 value close to 1 indicates that the two data sets show good correlation.


However, the degree of inter-laboratory variability is concerning and should be fully investigated in the near future. 3.2 Comparison of stability tests Results for the SC samples were ranked according to the relative stability data for each test, with data rounded up to the nearest whole number (Table 3-3, with the full ranking table in Appendix 5). The Lab 2 PAS 100 and DR4 results showed the closest relationship of all of the stability tests with only one compost sample, from site I, differing in ranking. In the Lab

2 ranking, Site I is placed in 10th position as the least stable compost whereas, in the DR4 ranking, Site I is placed in 6th position. The reason for this is not clear but in the previous section it was demonstrated that the Lab 2 stability results may have been significantly depressed for the least stable composts. From an analysis of Figure 3-4 it could be argued that this may have been the case for samples from Sites E, G, H and J, which resulted in them being ranked as more stable than Site I. The DR4 and Lab 2 PAS 100 test results ranked the stabilities of the test samples very similarly, while the ranking based on Lab 1 PAS 100 results was very different to both. On the basis of the close relationship between the DR4 and Lab 2 rankings, the Lab 2 stability results were selected to represent the current PAS 100 stability test method and were used for further data analysis.

Table 3-3 Ranking of compost stability test vs site. For the ranking 1=most stable – 10=least stable

Site Lab 2

PAS 100 DR4 OUR Dewar

Lab 1 PAS 100

Sealed vessel (24 h)

D 1 1 1 1 1 3

B 2 2 2 1 2 6

C 3 3 3 1 4 2

A 4 4 4 5 3 5

F 5 5 5 8 8 8

H 6 7 6 8 5 9

J 7 8 6 8 7 7

E 8 9 6 7 6 10

G 9 10 6 6 9 4

I 10 6 6 1 10 1 Note: The Dewar temperature values have been rounded up to the nearest whole number and similar values grouped to give equal rankings.


3.2.1 DR4 and PAS 100 stability test comparison There is a good correlation between Lab 1, Lab 2 and the DR4 method towards the lower end of the stability scale (Figure 3-5). However, Lab 1 may fail near the PAS 100 threshold limit as some very unstable materials pass the threshold (circled in Figure 3-5) whereas Lab 2 agrees with DR4 that the same materials are more unstable than Lab 1 suggests. It would appear that the PAS 100 method, as conducted by Lab 2, is effective up to values of 30 mg CO2 / g OM / day and that this is equivalent to DR4 values between 200 and 250 g CO2 / kg VS / 96 h.

Figure 3-5 PAS 100 stability results for Labs 1 and 2 plotted against DR4 results

3.2.2 OUR and PAS 100 stability test comparison With the OUR test results, half of the final SC test samples were found to be over range using the BS method and the standard equipment – these were the most active samples (Table 3-1 and Figure 3-6).

It was decided that it was best to simply record these very high results as >50 mmol O2 / kg / h. The European Commission JRC EoW proposals suggest that composts with an OUR rate of >30 mmol O2 / kg / h should be classed as very unstable (Saveyn and Eder, 2014) and although the standard method is sufficiently robust to encompass materials at this limit, it is not well designed to encompass very unstable materials. Further work would be required to ensure that the OUR method/equipment is fit for purpose for assessing more accurately the stability of very active composts. The range could be extended by:

Using a larger sample bottle, maximum 2.5L to conform to BS16087-1. This would allow samples of up to approximately 100 mmol O2/kg organic matter/h to be tested.

Using sample size of less than 1g volatile solids. This is excluded in BS16087-1, and obtaining a representative analytical sample of this size would be problematic. An increased number of replicates may overcome this to some extent.

Considering whether it may be that the high results could be inferred from the pressure curve obtained in the tests as run, for instance using the maximum slope, however this would need further investigation, and as with any proposed calculation would need to be verified.

More data in the intermediate range between stable and very unstable would facilitate more accurate correlation of the OUR and PAS 100 methods. This could be deemed important should the JRC EoW proposals be converted into legislation, to demonstrate that the current


PAS 100 stability test could be used to assess compost stability of EoW composts in the UK, rather than introducing a new compost stability assessment method in the UK.

Figure 3-6 OUR plotted against Lab 2 PAS 100 stability

3.2.3 Dewar and PAS 100 stability test comparison Figure 3-7 shows the Dewar self-heating versus the PAS 100 respiration (ORG0020) values generated by Lab 2. The Figure shows for the more stable composts that as the ORG0020 results increase, so do Dewar self-heating maximum temperatures (Tmax). However, with less stable material, whereas the PAS 100 test shows increasing microbiological activity, the Dewar self-heating test appears to plateau at Tmax of ~64°C. A clear limitation of the EN 16087-2:2011 self-heating method is that is does not take into consideration packing density of the material, which is known to affect the ability of samples to heat. Besides this, three samples which were deemed unstable by the PAS 100, DR4 and OUR methods appeared to be more or less stable with the self-heating test:

The first of these was the finished compost sample from site I – one potential reason

for this is due to the low pH (5.1) of the sample as it has been previously reported that

low pH can give false positive results i.e. indicating stable material (Brinton, 1995).

The finished compost from site G, which was the other sample with a very low pH

(4.8), also suggested comparably more stable material with self-heating than the PAS

100, DR4 and OUR methods.

The third false positive was the microwaved sample from site G which the self-heating

test reported as stable (Tmax = 24°C). In addition to low pH, heat damage is known as

a factor which can give false positive results for the self-heating test (Brinton, 1995).


Figure 3-7 Dewar self-heating plotted against Lab 2 PAS 100 stability

3.2.4 Static respiration 24 h and PAS 100 stability test comparison Figure 3-8 shows the static respiration test (24 h) against the Lab 2 PAS 100 values. Compared to other stability methods there was limited correlation. In particular, for less stable material the efflux of CO2 was limited in the static respiration method, most likely due to O2 limitation in the sealed vessels i.e. unlike the ORG0020 method there is no continuous air/oxygen supply. As with the Dewar self-heating method, the screened composts from sites I and G suggested comparatively more stable material than the DR4, OUR and PAS 100 tests. Given the high ammonia concentrations, ambient temperature and availability of oxygen it is likely that ammonia oxidising bacteria and Archaea were active, further contributing to the depletion of oxygen in this static sealed system – obviously this is not an issue in DR4 and PAS 100 tests where air is constantly replenished. Otherwise, ammonia oxidising bacteria (whose metabolism utilises oxygen without release of CO2) are inhibited in the OUR test due to addition of a chemical nitrification inhibitor. With the static respiration test the microwaved sample from site G appeared much more stable than its non-microwave treated counterpart.

Figure 3-8 Static respiration test plotted against Lab 2 PAS 100 stability


4.0 Results and discussion: Understanding how operational conditions relate to stability

This section of the report relates to operational conditions during compost processing and with reference to monitoring of O2, CO2, moisture and temperature during compost maturation. The ten sites were visited during the winter months (January to March 2014) and the CompostManager system employed to assess a number of post-IVC stabilisation/maturation piles on each site. The CompostManager system was used to help characterise the operational conditions prevailing within the compost maturation windrows for the selected sites which provided samples of compost for stability testing (Sauer and Crouch, 2013) The original CompostManager data set contained results from a sample size of 42 compost batches which were monitored for temperature, moisture, O2 and CO2 on-site during this project. However, this data set was reduced to 25 batches to reflect data collected under comparable process conditions. There are two main reasons for this:

Firstly, as the gas data measurements represent the difference in supply and demand

of O2 and CO2, it is not meaningful to compare data from screened material (with a

very dense structure and slow gas transfer) with that of unscreened material. For the

purposes of this analysis, data collected from screened batches were excluded.

Secondly, as the action of turning a batch (be it formation after shredding or unloading

of a composting vessel) generally brings about a marked increase in the rate of

respiration, it is quite normal to observe a drop in O2 level immediately after the turn.

This is a consequence of the fact that the rate of O2 supply (be it passive or forced

aeration) cannot initially keep up with the very high O2 demand. Provided the aeration

mechanism is effective, O2 levels should begin to recover to a steady state within a few

days of turning. For this reason, data collected from any batch which had been turned

or moved within five days of the measurement being taken were excluded.

A discussion of the reduced data set, followed by the complete set of CompostManager results of the on-site measurements of O2, CO, temperature and moisture of the composts from all sites are given in Appendix 3, with a summary below. 4.1 Summary of CompostManager findings with particular emphasis on relating pile

oxygen status with the stability of composts All measurements and findings relate to compost maturation windrows. Batch ages relate to total age of material including the IVC/sanitisation stage and maturation stage. Maximum dissolved O2 levels in the liquid phase were calculated and presented for each datapoint from O2 readings corrected for moisture condensation according to Environment Agency technical guides 2 and 3 (Environment Agency, 2012a; Environment Agency, 2012b). If dissolved O2 concentrations in windrows are 1ppm or more for at least 75% of the time, then oxygen is deemed not to be limiting the composting process, whereas dissolved O2 levels consistently below 0.5ppm are more likely to have odour problems (Sauer and Crouch, 2013).


Compost processing results: Site A Site A represents the only instance where the same batch was visited on a number of separate days and the CompostManager unit was used to monitor conditions within the same maturation windrow; the test windrow was passively aerated only and was not turned during this period or disturbed except for material being taken on specific days using a loading shovel for screening to produce compost from which samples were taken for laboratory analysis. The final compost was well stabilized and passed the PAS 100 stability test with a value of 10.9 mg CO2/g OM/day (Lab 2). Readings from CompostManager indicated that the repeated O2 levels in the maturation windrow that was sampled were extremely high (i.e. average of dissolved O2 was 6.2 ppm) and the CO2 levels low. For this site, the maturation pile that was sampled was well aerated and the corresponding stability of the final compost was high. For the batch studied the feedstock was predominantly woody green waste with relatively low initial biodegradability (as shown by low DR4 value) so this combined with long IVC duration (~3 weeks) and maturation period (~4 weeks) means that well aerated maturation piles and well stabilized compost might be expected. Compost processing results: Site B Compost from site B was found to be well stabilized and passed the PAS 100 stability test with a value of 7.0 mg CO2/g OM/day (Lab 2). While some material stabilization seemed to take place during the IVC period (2 weeks) as shown by reduction in DR4 values, most stabilization occurred during the relatively long subsequent stabilization and maturation phase (8 weeks). The maturation pile at 42 days showed a significantly higher level of oxygenation (i.e. average of dissolved O2 1.13 ppm) compared with 28 days. Weather conditions during this time were very wet and the higher moisture content for the younger compost may have reduced porosity, as indicated by the respective lower dissolved O2 reading. Compost processing results: Site C There were no oxygen data meeting the relevant criteria for this site. The complete set of CompostManager results are shown in Appendix 3. Compost processing results: Site D Compost from site D was found to be well stabilized and passed the PAS 100 stability test with a value of 5.1 mg CO2/g OM/day (Lab 2). With reference to the CompostManager findings, conditions on site D appeared to vary greatly between different batches. At one extreme are the 42, 49, 63 and 77 day old batches which seem to be operating under anaerobic conditions below the 1ppm dissolved O2 level, which contrast with the 14, 56 and 126 day old batches, which were recorded as composting efficiently under aerobic conditions. This site was characterised by very long compost maturation times (typically 16 weeks). Compost processing results: Site E Compost from site E was found to be poorly stabilized and failed the PAS 100 stability test with a value of 31.7 mg CO2/gOM/day (Lab 2). However, processing conditions on site E appeared to be good when monitoring was undertaken, as dissolved oxygen levels in maturation piles (range 1.41 – 4.22 ppm) indicated that there was sufficient oxygen available and the CO2 levels suggest that the aerobic composting process was proceeding efficiently. This site was characterised by relatively long compost maturation times (typically 8 weeks) so it is not clear why the final compost was poorly stabilized. Factors to consider may have been the typically short IVC duration (3 days) and the very high biodegradability of the feedstock when monitoring took place (DR4 value 620 gCO2/kgVS/96h).


Compost processing results: Site F Compost from site F was found to be reasonably well stabilized and only just failed the PAS 100 stability test with a value of 17.0 mg CO2/g OM/day (Lab 2). Dissolved O2 levels in maturation windrows were generally high, with the exception of one batch. However, the CO2 levels imply that the rate of composting could be fairly low for some of the batches. One factor to consider would be the typically short IVC duration (7 days); while the biodegradability of the input waste appeared to be anomalously low, the post-IVC material was recorded as having a relatively high biodegradability when monitoring took place (DR4 value 369 g CO2/kg VS/96h). It is possible that decomposition during maturation and the maturation period of ~7 weeks were both insufficient to enable the compost to meet the PAS 100 stability threshold. Compost processing results: Sites G - H There were no oxygen data meeting the relevant criteria for these sites. Compost processing results: Site I Compost from site I was found to be poorly stabilized and failed the PAS 100 stability test with a value of 37.8 mg CO2/g OM/day (Lab 2). At site I, the batch that was valid in terms of meeting CompostManager criteria was approximately two weeks into maturation. The results suggest that anaerobic conditions prevailed, as there was virtually no oxygen present in the material (dissolved O2 0.09 ppm). These results may suggest that the structure of the material and/or the aeration mechanism during maturation were not suitable for supplying sufficient oxygen to the process, particularly given that the material also had a high moisture content. Compost processing results: Site J Compost from site J was found to be poorly stabilized and failed the PAS 100 stability test with a value of 29.2 mg CO2/g OM/day (Lab 2). O2 levels in the maturation batch aged 23 days suggest that active composting was taking place under aerobic conditions. However, for an older batch (age 37 days), high O2 and low CO2 levels were recorded, suggesting that the rate of activity was lower. This may have been due to a low moisture content and the absence of turning for nearly one month prior to sampling. The results suggest that the relatively short duration (typically 4 weeks) actively-aerated maturation system was working ineffectively. 4.2 Compost processing results: DR4 stability testing Table 4-1 presents DR4 biodegradability values for unscreened materials going to the sanitisation (IVC) stage; emerging from the sanitisation (IVC) stage and after a period of further stabilisation/maturation. From Table 4-1, it can be seen that the biodegradability of input wastes being composted appears to vary widely from site to site. DR4 values in this table are intended to reflect the actual composition of input material going to the sanitisation stage (IVC stage) which includes any oversize material being added to input wastes as bulking agents. DR4 values for sites H and F are very low and would appear to be anomalous (96 and 116 g CO2 / kg VS / 96 h); these values may reflect values for bulking agents (such as screened oversize material) rather than biodegradable fresh inputs (see Table 2-1 for feedstock information). Excluding these values, the range in the biodegradability of input material (177 to 620 g CO2 / kg VS / 96 h) is still very large. It may be assumed that, input materials for composting characterised as highly biodegradable will require more effective processing techniques and longer durations during the sanitisation and stabilisation/maturation stages, compared with low biodegradability inputs, if highly stable compost outputs are required.


It should be noted that these DR4 values should be treated as indicative only since the materials sampled at different stages, in general, did not relate to the same batch and for some sites post-IVC samples had been further matured prior to sampling. Two sites (A and G) were selected as representing contrasting approaches to producing compost. Site A produces PAS 100 compost whereas Site G produces non-PAS compost. For these sites, one batch was monitored from start to finish of the process. The Site A process that was monitored involved IVC sanitisation in an aerated tunnel (~ 3 weeks) followed by outdoor maturation (~ 4 weeks). The Site G process that was monitored involved IVC sanitisation in an aerated tunnel (~ 4 weeks) followed by screening and use with no further maturation.

Table 4-1 DR4 stability results for selected operational stages compared with stability of screened compost, with Lab 2 stability included

Site

Input

waste

(FW)

DR4 results

(gCO2/kgVS/96hr)

Post

Sanitisation

(POST IVC)

DR4 results

(gCO2/kgVS/96hr)

Unscreened

compost

(MAT)

DR4 results

(gCO2/kgVS/96hr)

Screened

compost

stability (SC)

Modified DR4

results

(gCO2/kgVS/96hr)

Screened compost

stability

(SC)

LAB 2 (ORG0020)

results

(CO2/g OM/day)

A 177 141 83 52 10.9

B 373 327 74 28 7.0

C 244 195 145 48 7.1

D 280 129 61 24 5.1

E 620 392 561 279 31.7

F 116 369 222 121 17.0

G 414 325 n/a 315 37.3

H 96 243 n/a 262 25.7

I 404 232 n/a 231 37.8

J 232 n/a 381 266 29.2

n/a sample not available

Reduction in material biodegradability as determined by reduction in DR4 value during IVC sanitisation for Site A was 20% and for site G this reduction was 22%. However, although reductions in biodegradability were equivalent, the input material being composted by Site G was clearly much more biodegradable (BMW plus some oversize) compared with the material being composted by Site A (mainly GW) and this was reflected in the post-IVC DR4 values with Sites A and G of 141 and 325 g CO2 / kg VS / 96 h, respectively. Although Table 4-1 gives mainly indicative values it would appear that in general reductions in biodegradability occur principally during the stabilisation/maturation phase. For Site A, further reduction in material biodegradability, as determined by reduction in DR4 value, for the sanitised material during outdoor maturation was 41%. Site G material was not matured but was screened and immediately deployed whilst still exhibiting a high degree of biodegradability as compared to the PAS 100 stability threshold. The CompostManager results suggest that the 4 week duration maturation phase for Site A was very aerobic and this will have enhanced the stability of the final compost. This may have been due to a


number of factors including the woody nature of the input material and its low initial biodegradability. The PAS 100 stability test results for the Site A samples (1-4) showed that after one month in the IVC the material was already close to the PAS 100 stability threshold (Lab 2 value 12.67 mg CO2 / g OM / day). After a subsequent 10 days on the maturation pad, this decreased to 10.8 mg CO2 / g OM / day with little change thereafter. The feedstock material at Site A was based on green waste plus BMW and, as shown by the DR4 values, was less active than many other sites. Samples of the screened compost which had undergone IVC and maturation/stabilisation from sites B, C and D also passed the PAS 100 stability threshold and these sites were characterised by employing sanitisation stages in the range of two to four weeks combined with compost maturation periods in the range of 4 to 16 weeks. Input feedstocks were characterised as having low to medium biodegradability based on DR4 values. The CompostManager results suggest that the maturation windrows for sites B and D were acceptably aerobic (no results are available for site C). 4.3 Processing factors The stability of the compost produced will be affected by a range of factors such as the initial feedstock, pile structure, moisture, temperature management, but the degree of aeration received (and oxygen supplied) will play a very important role in achieving the optimum conditions for microbial degradation of the wastes. The chronological age of each of the composts is shown in Table 2-2. This is a crude measure of processing as it does not specifically illustrate any of the individual factors mentioned above. This study included a range of IVC technologies with different aeration systems treating different feedstocks. Overall, this shows that the time in process is not a useful indicator of the degree of stabilisation (Figure 4-1).

Figure 4-1 Lab 2 PAS 100 stability test results against compost processing time (including IVC) in days

During development of the PAS 100 stability method, three IVC composts containing catering wastes, all 8 weeks old, were analysed, with stabilities being 12.2-16.3 mg CO2/g VS/day (Llewelyn 2005a). A stability index was derived (reproduced as Table 4-2) at that time,


mainly based on windrow composting data. An efficient in-vessel system may reduce the time to achieve a given stability point, while a high food waste input may require a longer processing time. Such site/process-specifics mean that it is appropriate to challenge former assumptions around processing times and compost stability.

Table 4-2 Index for compost stability and time (based predominantly on green waste open windrow composting)(Llewelyn 2005a)

Microbial respiration rate (mg CO

2/g

VS/day)

Stability index description

Typical age (days of composting / storage)

Typical age (weeks of composting / storage)

> 20.0 Very high < 42 days < 6 weeks

16.0 – 19.9 High 42 – 56 days 6 – 8 weeks

12.0 – 15.9 Medium 56 – 70 days 8 – 10 weeks

8.0 – 11.9 Low 70 – 78 days 10 – 14 weeks

6.0 – 7.9 Very low 78 – 140 days 14 – 20 weeks

< 6.0 Negligible > 120 days > 20 weeks

4.4 Nutrients, physiochemical properties and pathogens This section focusses on the nutrients, physiochemical and pathogenic properties of the composts, with data presented in Table 4-3 (for the final screened composts) and Table 4-4 (for the site A batch composts and the site G microwaved sample). Selected data were compared to the PAS 100 stability test results for Lab 2, to assess whether there were any correlations with stability and other parameters. 4.4.1 Pathogens It was found that two PAS 100 composts did not pass the E. coli test and that Site A also failed on Salmonella (see Table 4-3). Site A (see CompostManager results in Appendix 3) had well aerated maturation piles with temperatures that were mesophilic, and possibly ideal for the incubation of E. coli. (Table 4-4 shows that E. coli persisted through the composting process although a potential explanation might be cross contamination from loading and screening equipment).Thus further stabilisation and maturation may improve stability, but may also result in E.coli regrowth. The non-PAS site J had a high E. coli count and the compost pH was slightly alkaline compared with the other non-PAS site composts, which were acidic. Both the original SC and microwaved sample (Site G) had low levels of E. coli (with Salmonella absent from both un-microwaved and microwaved samples).


Table 4-3 Nutrients, physiochemical properties and pathogen content of the compost from the ten IVC sites. Note: Lab 2 PAS 100 stability has been added for ease of reference.

Site

Test Units A B C D E F G H I J

PAS-certified process? Yes Yes Yes Yes Yes Yes No No No No


mg

CO2/g OM/day

10.9 7.0 7.1 5.1 31.7 17.0 37.3 25.7 37.8 29.2

NH4-N mg/kg 146 1117 246 378 2827 2971 2339 3051 2888 1022

NO3-N mg/kg <1 <1 271 258 <1 <1 <1 <1 <1 <1

Nitrogen As N mg/kg 20100 19400 21000 17300 24100 17100 21400 16200 16800 18300

Total Carbon % w/w 29.06 29.27 22.75 25.39 40.15 29.14 37.93 31.73 34.75 35.2

C:N Ratio 14.5 15.1 10.8 14.7 16.7 17.0 17.7 19.6 20.7 19.2

Bulk Density g/l 403 524 473 613 672 550 495 316 482 420

Dry Matter % m/m 52.8 39.2 72.3 44 36.7 62 41.9 44.4 44.2 55.5

Organic Matter

(Loss On Ignition) % m/m 57.7 45.5 43.8 43.7 68.4 52.3 72.6 58.1 62.3 64.9

pH 8.6 8.9 8.8 8.4 7.3 7.6 4.8 5.6 5.1 7.5

Electrical

Conductivity

uS/cm

@ 20°C 1024 1273 1550 1206 2710 2850 2950 1670 2550 1970

E. coli CFU/g 11000 <10 <10 130 10 250 <10 <10 <10 6100

Salmonella spp Abs/ Pres

Pres Abs Abs Abs Abs Abs Abs Abs Abs Abs

Portion of

particles <20mm % g/g 90 90 100 95 100 90 97 85 95 97

Results highlighted in green exceeded the PAS 100 thresholds of 1000 CFU/g fresh mass for E. coli and/or showed the presence of Salmonella


Table 4-4 Nutrients, physiochemical properties and pathogen content of the compost for the batch analysis at site A, and also the microwaved sample. Note: The screened final compost samples (SC4 for site A) are the same as in Table 4-3. The site A key refers to the number of days of composting on the maturation pad, which follows 23 days in the IVC.

Site A Site G

Test Units SC1

Day 0

SC2

Day 10

SC3

Day 19

SC4

Day 31 SC

Micro-

waved

Lab 2. PAS 100

stability

mg CO2/g

OM/day 12.1 10.8 8.0 10.9 37.3 30.7

NH4-N mg/kg 431 465 310 146 2339 2347

NO3-N mg/kg <1 <1 <1 <1 <1 <1

Nitrogen As N mg/kg 22900 20600 20200 20100 21400 20700

Total Carbon % w/w 33.28 30.56 31.03 146 37.93 36.1

C:N Ratio 14.5 14.8 15.4 14.5 17.7 17.4

Bulk Density g/l 433 396 383 403 495 464

Dry Matter % m/m 42.4 48.3 59.3 52.8 41.9 42.3

Organic Matter (Loss On Ignition)

% m/m 57.3 51.3 60.2 57.7 72.6 67.4

pH 8.6 8.6 8.8 8.6 4.8 5.9

Electrical Conductivity

uS/cm @ 20°C 970 940 832 1024 2950 1400

E. coli CFU/g 97000 400000 4400 11000 <10 <10

Salmonella spp Abs/

Pres Abs Abs Abs Pres Abs Abs

Portion of particles <20mm

% g/g 85 90 92 90 97 95

Results highlighted in green exceeded the PAS 100 thresholds of 1000 CFU/g fresh mass for E. coli and/or showed the presence of Salmonella

4.4.2 Chemical parameters The composts from the six PAS 100 sites all had neutral or slightly alkaline pH levels ranging from 7.3-8.9, and three of the non-PAS 100 composts had acidic pH levels of 4.1-5.6 (Table 4-3). The fourth non-PAS Site J material was slightly alkaline but was still very unstable. The unstable material from the PAS 100 site E had a pH of 7.3. These results indicate that acidic pH (less than pH 6) could be used as a preliminary screen to identify unstable materials but that an alkaline pH is not necessarily an indicator of stability. Ammonia is volatile and is more so at elevated temperatures above 45oC and pH above 7.5. Therefore, ammonia concentrations tend to diminish over time. The more stable the material, the less ammonium-N is present in the extract, as shown in Figure 4-3. Site E was considered unusual in that food waste intake was high and lime was added to the mix to raise pH and improve decomposition. The ammonium levels had not diminished (Table 4-3) possibly due to the pH being insufficiently alkaline (pH 7.3) or due to processing factors (aeration may have been limiting as the material had a high moisture content and bulk density).


Site J had a relatively low ammonium-N content for an unstable compost (33 days in process, Lab 2 stability 29 mg CO2/g OM/day); the pH was 7.5 and the compost was being well aerated (according to the CompostManager data).

Figure 4-2 Compost pH and Lab 2 PAS 100 stability test results

Figure 4-3 Ammonium concentrations and Lab 2 PAS 100 stability test results

Figure 4-3 shows that ammonium-N was greater where pH was low and that pH tended to increase with the degree of processing as reflected in the Lab 2 stability data (Figure 4-2 above). However, neither pH nor ammonium-N on their own or in combination can be used as a reliable predictor of stability but could be useful on-site indicators of potential instability. Ammonium-N tended to be at greater concentrations in less stable materials (Figure 4-3). This was reflected in the EC levels, which were generally higher in less stable composts (Figure 4-5).


Figure 4-4 Ammonium-N and pH

Figure 4-5 Electrical conductivity and Lab 2 PAS 100 stability test results

4.5 Results: Plant bioassay The PAS 100 bioassay (also known as the „plant response test‟) is designed to be responsive to any phytotoxic factors present in the test sample. Bioassay results are shown in Table 4-5 and Table 4-6. All of the composts passed the PAS 100 bioassay test, although the two least stable materials (G and I) were only just above the 80% „test-viability‟ threshold for growth per plant (Figure 4-6). Plant growth was poorest (less than 90% of peat controls) in all of the non-PAS materials. For all compost samples, no abnormal tomato plants were observed.


Figure 4-6 Plant bioassay % germination and growth of tomato compared to the peat control

Plant growth (average weight per tray as % of control) was plotted against the Lab 2 results and showed that plant growth improved very slightly with increasing stability (Figure 4-7). (There were insufficient data to determine whether this improvement were statistically significant). However, as all the test materials passed the bioassay test, this suggests that even such immature materials are unlikely to be injurious to field crops, especially when the mixing factor of soil to compost in a field situation may be 50:1 compared with up to 4:1 in the bioassay.


Table 4-5 Plant bioassay results for the ten sites, showing the ratio of peat added to the sample and the quantity of lime added to the mixture

Site

A B C D E F G H I J

PAS-certified

process? Yes Yes Yes Yes Yes Yes No No No No

PAS 100 stability

(Lab 2. mg

CO2/g OM/day)

10.9 7 7.1 5.1 31.7 17 37.3 25.7 37.8 29.2

NH4-N (mg/kg) 431 1117 246 378 2827 2971 2339 3051 2888 1022

pH 8.6 8.9 8.8 8.4 7.3 7.6 4.8 5.6 5.1 7.5

EC (uS/cm @ 20°C)

970 1273 1550 1206 2710 2850 2950 1670 2550 1970

Bioassay results

Ratio:1* 2 3.64 4 3.5 4 4 4 4 4 4

Lime added (g) 13.2 14.1 14.4 14 14.4 14.4 14.4 14.4 14.4 14.4

Germination (% compared to control) 10 days after

sowing 97 90 90 82 93 97 79 90 72 96

14 days after

sowing 100 100 100 104 100 100 96 100 100 100

28 days after sowing

100 100 100 104 100 100 96 100 100 100

Growth (% compared to control) Total g/tray 111 100 127 139 142 98 79 85 86 81

Average g/plant 111 100 127 82 82 84 81 134 142 99 Results highlighted in green exceeded the PAS 100 thresholds *Ratio of peat to test sample

Table 4-6 Plant bioassay results for the for the batch analysis at site 1, and also the microwaved sample, showing the ratio of peat added to the sample and the quantity of lime added to the mixture. Note: The screened final compost samples (SC) are the same as in Table 4-5.

Site A Site G

Test Units SC1 Day 0

SC2 Day 10

SC3 Day 19

SC4 Day 31

SC Micro- waved

PAS 100 stability (Lab 2)

mg CO2/g OM /day

12.1 10.8 8.0 10.9 37.3 30.7

NH4-N mg/kg 431 465 310 146 2339 2347

pH 8.6 8.6 8.8 8.6 4.8 5.9

EC uS/cm @ 20°C 970 940 832 1024 2950 1400

Bioassay results

Ratio:1* 2 2.4 2 2.7 4 4

Lime added g 12 12.7 12.0 13.2 14.4 14.4

Germination (% compared to control)

10 days after sowing 72 103.4 100 100 79 90



Growth (% compared to control)

Total g/tray 85.5 134.2 138.4 141.9 79 165

Average g/plant 85.4 129.5 138.6 141.9 82 171

Results highlighted in orange exceeded the PAS 100 thresholds *Ratio of peat to test sample


The germination at 10 days after sowing was lowest with the most unstable materials (G and I) and also the relatively stable material from site A (post IVC Day 0). There may be more than one factor causing this effect, such as VOCs, EC, pH, etc, which could be examined with further trials, and it would be interesting to explore whether species other than tomato would be more affected by the less stable composts. It has been suggested, based on field trial results, as highlighted in the Phase 1 literature review (WRAP, 2014), that planting of sensitive crop seeds be delayed after the addition of immature composts to land so that adverse effects on germination are not seen. This is also recommended for inorganic fertilisers (RB209) due to salt effects especially in dry conditions and sandy soils. Less mature composts are likely to have higher EC levels than more mature composts due to the presence of greater concentrations of ammonium-N (see Figure 4-3 and Figure 4-5 above). However, the less stable composts did not adversely affect tomato germination at 10 days any more than some of the more stable materials as shown in Figure 4-8.

Figure 4-7 Plant growth and Lab 2 PAS 100 stability test results

Figure 4-8 Germination after ten days and Lab 2 PAS 100 stability test results


4.6 Results: Viable mesophilic bacterial counts Viable aerobic and anaerobic bacterial plate counts were performed to assess relative abundance of these populations during maturation and in screened composts. In addition, the method was used to assess the impact of microwave treatment on aerobic and anaerobic bacterial populations in compost.

Table 4-7 Viable aerobic and anaerobic bacterial plate counts from selected site samples

Colony forming units (cfu) g-1 Site/Sample ID

Aerobic Anaerobic

D SC 1.70 X 1010 3.37 x 109

E SC 3.13 x 1011 1.37 x 1010

E MAT <107* 5.37 x 1010

F SC 4.25 x 109 5.33 x 107

F MAT 2.10 x 108 2.28 x 108

G SC 7.77 x 108 2.91 x 108

G SC Microwave 1.29 x 109 1.29 x 108

*Note that the higher serial dilutions for this sample were not plated-out

The results show that in all screened compost samples the aerobic bacterial population was consistently higher than the anaerobic bacterial population (Table 4-7 and Figure 4-9). It is not possible to explain the smaller difference in aerobic and anaerobic bacterial populations in site G SC without further testing. However, given the low pH (4.8) of this material it suggests that the compost microbial community at this specific time point is dominated by other microorganisms unable to grow on the selective media used. Certainly it is widely known that different microbial populations grow and decline during the composting process (Tiquia, 2002). Most likely in the sample from site G SC, fungi (known to be more acid tolerant than bacteria), thermophilic bacteria and/or other specific groups (which were unable to grow on specific media or incubation conditions used) were dominating.

Figure 4-9 Aerobic and anaerobic microorganisms SC= screened final compost, MAT = compost on the maturation pad, Micro = microwaved sample


For the microwaved sample (G SC Micro), the aerobic bacterial population was similar to the non-microwaved sample. Given that the material was heated to 80°C it is not surprising to find this situation as although the process will have killed some bacteria this would have released cellular content providing nutrients to stimulate subsequent growth and multiplication of the remaining population. In addition, heating to 80°C is known to stimulate the germination of dormant spore forming bacteria (Hassen, 2001) and as such this „specific‟ group may have contributed to the increased aerobic bacterial count. There was no increase in the anaerobic bacterial count following the microwave treatment suggesting that anaerobic bacteria were not stimulated or at least did not multiply during subsequent storage of this sample. Similar bacterial counts for G SC and G SC Micro are supported by stability results for methods with longer pre-incubation times (e.g. ORG0020 and DR4) where mesophilic populations are assumed to have recovered from the microwave treatment. Further research looking at bacteria population dynamics after autoclaving would provide further evidence.

For the two mature samples tested, the aerobic bacterial count for site E was < 107 cfu g-1. As this material had a pH of 5.5 it is likely that as with site G the compost microbial community was dominated by other groups. The site F mature compost (F MAT) sample had similar aerobic and anaerobic bacterial population counts. Unfortunately, due to an electrical failure of the CompostManager probe during the collection of sample E MAT we do not have an oxygen reading from the sampled pile. In summary, from the small number of samples surveyed for viable mesophilic bacterial counts, two conclusions can be drawn:

Viable mesophilic plate counts are able to differentiate between mature and screened

composts in terms of the relative abundance of aerobic and anaerobic bacteria. A wider

suite of media and incubation conditions (i.e. selective for fungi and thermophilic

bacteria) would aid interpretation further.

Comparable mesophilic plate counts for SC G and SC G Micro correspond with stability

results of the ORG0020, DR4 and OUR methods. The limitations of Dewar self-heating

and static respiration tests with sample G have already been discussed.

5.0 Summary and conclusions This section focusses on the outcomes of the research in relation to the four main objectives, which were to provide information and data regarding:

whether the PAS 100 stability test was fit for purpose, particularly for determining the

stability of composts produced by in-vessel composting processes;

whether the PAS 100 stability baseline could be changed, particularly for PAS 100

composts used in agriculture;

whether stability limits should be set for non-PAS composts, and if so, what test could

be used, and how the limit might be selected; and

to understand the financial and commercial (operational) implications for PAS and non-

PAS compost producers of the implementation of any of the recommendations made in

this work.

5.1 The PAS 100 stability test method The data from this study of ten IVC sites indicates that there is no compelling evidence to suggest that the PAS 100 stability test is not suitable for testing compost stability up to 30 mg CO2/g organic matter/day for all composts, including food wastes treated through an IVC. This is supported by the very strong correlation of Lab 2 stability data with the DR4


data (section 1.1.2 explains why the DR4 test is considered to be a robust test capable of testing composts over a large range of stabilities). There is, therefore, no evidence from this empirical study that a change in method is required. However, it is recommended that the range within which the test is valid should be investigated in more detail, especially if the ORG0020 test will be increasingly applied to less stable composts, for example those composts associated with very short maturation periods intended for agricultural applications. Microwaving one immature compost sample did not greatly affect its stability when tested using the PAS 100, DR4 or OUR tests. By comparison the Dewar and static respiration tests showed the microwaved sample to be very stable, although both of these tests had short pre-incubation periods (24 h) and in the case of the Dewar test are known not to be reliable for low pH samples. However, differences between the two laboratories were evident and a thorough audit of all laboratories carrying out the PAS 100 stability test is recommended. If such an audit suggested that the causes for any discrepancy could not be readily identified, then the regulator and PAS 100 Steering Group would need to consider whether an alternative test, which is more easily reproducible, would be more appropriate. Moreover, as a pragmatic response to the observed inter-laboratory variability, the current stability limit could have a narrow margin of error built in (e.g. 2 mg CO2 / g OM / day). Thus if there was a single batch PAS 100 failure between 16 and 18 mg CO2 / g OM / day, the batch could still be dispatched as conforming for agriculture. 5.2 The PAS 100 stability baseline – agriculture and field horticulture In order to consider whether the current PAS 100 limit is appropriate for agricultural and field horticulture, evidence on the use of immature composts in these applications was considered during the Phase 1 literature review (WRAP, 2014). To date research on green waste, BMW and industrial food waste composts has been undertaken predominantly in Germany, with several studies from Luxemburg and Austria. Their focus was on the use of immature composts, generally of stability Rottegrad II – III (termed „fresh‟ composts, with Rottegrad III equating approximately to 20 mg CO2/g organic matter/day). These European studies found that application of fresh composts tended to result in agronomic benefits on crop yield and soil properties for a range of crop types. Yield effects were in some studies linked to the available nitrogen released within a season (associated with the C:N ratio of the compost) as, in the field situation, composts can supply both carbon (as a source of energy for soil microorganisms) and nutrients, generally leading to increased soil microbial activity, improved soil conditions and plant growth. There therefore needs to be a balance between stabilising (thereby rendering less readily available) carbon forms during the composting process against providing the soil with energy for microorganisms that can help improve soil structure through the utilisation of the added carbon and nutrients. The Phase 1 literature review (WRAP, 2014) also found that, in some instances, fresh compost (Rottegrad I-III) was found to increase available N, with some temporary N lock-up experienced in other studies. Hence some authors recommended application of fresh composts well in advance of sowing, such as in the autumn before a spring sown crop, to avoid any potential detrimental crop yield effects. Moreover, in Germany the application of fresh compost in the autumn was considered unlikely to cause significant leaching during the cold winter months, with N becoming available to crops the following spring as the temperature increases. To date UK field trials have focussed on using green waste composts and in most trials compost stability testing was not included as part of the suite of compost analyses


undertaken (WRAP, 2014). Thus, there is insufficient evidence to be able to directly translate the German and Luxemburg data on „fresh‟ composts into a UK context for agricultural growing conditions and compost types. In a UK agricultural and field horticultural setting, PAS 100 compost is applied in relation to requirements for nutrient loading. For non-PAS composts, heavy metal loading also governs the rate of application. In this project, all of the composts passed the PAS 100 bioassay although the two least stable materials were only just above the 80% threshold for growth. Thus, for the ten composts tested, the bioassay results suggest that if applied to soil any effects would be minimal – especially if planting was carried out after a period of equilibration in the soil. For non-PAS materials, the suitable end point for the treatment by composting to define fitness for purpose without pollution risk is not well defined in the UK regulations or guidance – and additional testing (coupled with field or laboratory growth trials) would be necessary to explore this further. The PAS 100 stability baseline is currently 16 mg CO2/g organic matter/day. The European Commission JRC proposed EoW stability criteria (Saveyn and Eder, 2014) are:

Rottegrad III, IV or V (self-heating test temperature rise of max. 30oC above ambient

temperature).

Respirometric index (OUR) result of maximum 25 mmol O2/kg organic matter/h.

The limited dataset generated during this project suggests that alignment of the UK approach with the rest of Europe would require relaxation of the PAS 100 stability threshold from 16 to 20 mg CO2/g organic matter/day. However further work is required to thoroughly compare the different stability methods/levels proposed. 5.3 The recommended stability baseline for growing media The Phase 1 literature review (WRAP, 2014) revealed that the only EU countries which have a distinction between compost end use and stability are for a specific quality assurance scheme (BGK) in Germany and Luxembourg, with „fresh‟ compost used in agriculture, „mature‟ compost in field horticulture, and substrate („very mature‟) compost used in growing media. This is not a legal requirement. Compost used in growing media does need to be very stable due to the long storage periods typically seen for bagged retail products, and the risk of nitrogen lock-up. There is a significant market for such products. For example, in 2009 in the UK, 4.18 million tonnes of growing media was supplied to the amateur gardening market (predominantly bagged and sold for retail), compared to 1.17 million m3 for professional growers (mostly un-bagged) (Defra, 2010). The results of the literature review did not suggest that the PAS 100 recommended threshold for growing media of 10 and a target of 8 CO2 / g OM / day (WRAP, 2011) should be changed. 5.4 Stability of non-PAS materials Non-PAS composted materials are utilised in agriculture and field horticulture, in addition to landscaping, brownfield sites and as landfill cover. A requirement for non-PAS composts is that they must be beneficial to soils and crops to be used in agriculture and field horticulture. Under „Standard Rules‟ for composting in open and closed systems (Environment Agency, 2010b), waste materials should be sufficiently processed so as to become „nominally stable‟ and have reached a „degree of processing and biodegradation at which the rate of biological activity has slowed to an acceptably low and consistent level and will not significantly increase under favourable, altered conditions‟. The standard rules also require that „Each composting batch shall undergo an identifiable sanitisation and stabilisation stage‟.


As shown in this study, stability levels of the ten composts tested covered a wide range, with the four non-PAS composts (samples G-J) generally being much less stable than the PAS 100 composts, with the exception of the PAS 100 site D. No evidence was found during the Phase 1 literature review regarding ground water pollution and greenhouse gas emissions during the storage of PAS or non-PAS composts. Moreover, no evidence of harm was unearthed in the UK from use of PAS 100 composts or waste composts applied under an Environmental Permitting Regulations exemption or permit, although the regulator has expressed concerns. The NVZ regulations (Defra, 2013), Defra‟s Code of Good Agricultural Practice (Defra, 2009), and the current deployment requirements (Environment Agency, 2010a) should – in theory – be sufficient to protect water bodies from any leachate released from temporary compost storage heaps and during compost application. As discussed above, the evidence from the Phase 1 literature review has shown that immature composts containing food waste (broadly comparable to UK IVC composts in some cases), can offer benefits to crops and the soil under some circumstances (WRAP, 2014). 5.5 Financial and commercial (operational) implications of changed in stability

methodology Lack of evidence to support change means that it is not suggested that the PAS 100 test be changed for the time being, but that the reasons for the differences between laboratories be explored and corrected. Should a change in method be deemed necessary in future, then the summary of initial discussions with the UK PAS 100 laboratories (Appendix 4) is intended to be informative. The differences between the PAS 100 stability results between the two labs could result in a pass or fail, with the latter having financial implications for the site operator. No recommendations are made within this report for changes to the PAS 100 threshold for agriculture nor the introduction of a non-PAS stability test, therefore no detailed costings have been carried out. However, if the stability threshold for PAS 100 composts was to be relaxed (such as from 16 to 20 mg CO2/g organic matter/day), savings could be made by the processors as the number of batch failures would be likely to decrease. In addition, it is possible that processing time could be reduced at some sites, reducing operational costs. 6.0 Further work This scoping study has highlighted a number of areas where further research would enhance the knowledge to facilitate the UK regulators in considering the current questions regarding compost stability in the UK. The main topics are discussed below. 6.1 PAS 100 stability test laboratory audit As discussed above, a thorough audit of the PAS 100 stability test laboratories should be implemented to understand the causes of the current inter-laboratory differences in stability results. 6.2 Correlating PAS 100 stability with EU recommended stability tests: OUR and self-

heating At the time of writing, national stability tests (such as ORG0020) are thought to be acceptable in the JRC‟s technical proposal for end of waste. However, a specific more in-depth comparison of OUR and self-heating (Dewar) stabilities (the two EU wide preferred stability tests) with the PAS 100 stability test would ensure the UK composting industry is


prepared should the JRC technical proposal be adopted, especially should the situation change regarding applicability of national stability tests. Given that samples in this study were either highly stable or unstable, a more in-depth comparison should specifically include a range of composts with intermediate stability i.e.10-30 mg CO2/g organic matter/day range for ORG0020. 6.3 Test using VFAs As discussed in the literature review (WRAP, 2014) a well composted, stable product is likely to have an alkaline pH and very low concentrations of VFAs due to a good aeration regime and a succession of heating phases over a suitable time period. A poorly composted material that is very unstable may have a low pH, unless masked by lime, and a high concentration of VFAs due to poor aeration, a lack of time, and phase changes relating to temperature and activity of different microbial communities. Batches of PAS 100 and non-PAS materials could be tested at intermediate stability levels through the phases of decomposition over time for pH, VFAs and stability to assess if a VFA threshold could be established that indicates whether a material has been „adequately treated and nominally stabilised‟. A recommended method for VFA determination would be to firstly prepare samples by standard extraction (i.e. same as pH) and then stabilise using phosphoric acid. VFAs would then be measured by GC-FID as for the PAS110 residual biogas potential test for digestates (WRAP, 2010). However, further research would be required into VFA testing in composts to develop this idea. 6.4 Further investigating the impact of microwaving on compost stability and microbial

abundance Although some work has been carried out here on the impact of microwave treatment on compost stability and microbial abundance of a single immature compost sample, this was not a main focus and as such was not exhaustive. Further work should look at the impact of microwave treatment at temperatures above 80°C which may be easily achieved with readily available equipment. In terms of measurements following microwave treatment, these should be carried out at multiple time points to assess potential recovery of the microbial community (in terms of numbers and activity). Moreover, compost samples with a range of stabilities should be assessed to ascertain whether the apparent increase in stability after microwaving of an immature sample (Lab 2 and DR4 data) would be the same for compost on the borderline between pass/fail stability levels for PAS 100. As well as looking at generic microbial numbers, this work could be extended to monitor specific bacterial pathogens numbers (e.g. Escherichia coli) which are likely to be irreversibly affected by such treatments, and currently assessed as part of the PAS 100 suite. 6.5 Assessment of in-field compost storage No direct evidence was found in the literature to assess any potential run-off during in-field storage of immature composts, or pollution incidences resulting from this scenario. Thus a survey is recommended, to gain more evidence in this area. This would involve the long-term assessment of a range of in-field compost piles from a range of IVC sites, ideally from creation of the storage pile through to final deployment. Ideally this would include both PAS 100 and non-PAS composts as a comparison. Monthly measurements could include stability, odour, temperature and oxygen concentration, VFAs and leaching of nitrate as well as basic compost analysis of pH, EC, moisture, C:N ratio and water extractable ammonium and nitrate-N.


6.6 Providing evidence of the performance of non-PAS IVC composts in UK agriculture No direct evidence was found in the literature of any impacts of using immature composts in UK agriculture and field horticulture on the environment (water, air and soil) and the crop. This suggests that no change is currently necessary. However, if further consideration of change does prove necessary in the future (whether through regulatory or market interests) then robust field evidence would be required from a UK context to inform the discussion and any decision-making process.


7.0 References ADAS 2005. Assessment of options and requirements for stability and maturity testing of

composts (Issue 2). WRAP report. Banbury. ANONYMOUS 2010. Survey of the UK organics recycling industry 2008/09. Wellingborough:

The Association for Organics Recycling. ANONYMOUS 2011. The Animal By-Products (Enforcement) (England) Regulations 2011. In:

ANIMAL HEALTH (ed.). BRINTON, W. F., EVANS, E., DROFFNER, M.L., BRINTON, R.B. 1995. A standardized Dewar

test for evaluation of compost self-heating. BioCycle, 36, 1-16. DEFRA 2009. Protecting our Water, Soil and Air: A Code of Good Agricultural Practice for

farmers, growers and land managers. DEFRA 2010. Monitoring the horticultural use of peat and progress towards the UK

Biodiversity Action Plan target (SP08020). DEFRA 2013. Guidance on complying with the rules for Nitrate Vulnerable Zones in England

for 2013 to 2016. ENVIRONMENT AGENCY 2010a. Chapter 4, The Environmental Permitting (England and

Wales) Regulations 2010. Standard rules SR2010No9_50Kte. Use of waste for reclamation, restoration or improvement of land.

ENVIRONMENT AGENCY 2010b. Chapter 4, The Environmental Permitting (England and Wales) Regulations 2010. Standard rules SR2011No1_500t. Composting in open and closed systems.

ENVIRONMENT AGENCY 2012a. Correcting Oxygen Measurements in Compost. Odour Technical Guide 2. Version 1.1.

ENVIRONMENT AGENCY 2012b. Oxygen solubility in compost. Odour Technical Guide 3. Version 1.0.

FREDERICKSON, J., BOARDMAN, C. P., GLADDING, T. L., SIMPSON, A. E., HOWELL, G. & SGOURIDIS, F. 2013. Biofilter Performance and Operation: as related to Commercial Composting. Environment Agency.

GOLUEKE, C. G. 1982. When is compost safe? Biocycle, 28, 38. HASSEN, A., BELGUITH, K., JEDIDI, N., CHERIF, A., CHERIF, M., BOUDABOUS, A. 2001.

Microbial characterisation during composting of municipal solid waste. Bioresource Technology, 80, 217-225.

LLEWELYN, R., H. 2005a. Development of standard laboratory based test to measure compost stability Project code ORG0020. Banbury: WRAP.

LLEWELYN, R., H. 2005b. Development of standard laboratory based test to measure compost stability - Annex A. Project code ORG0020. Banbury: WRAP.

SAUER, N. & CROUCH, E. 2013. Measuring oxygen in compost. Biocycle, 12, 23-26. SAVEYN, H. & EDER, P. 2014. End-of-waste criteria for biodegradable waste subjected to

biological treatment (compost & digestate): Technical proposals. European Commission Joint Research Centre.

TIQUIA, S. M., WAN, H.C., TYAM, N.F.Y. 2002. Microbial population dynamics and enzyme activities during composting. Compost Science & Utilization, 10, 150-161.

TURRELL, J., GODLEY, A., AGBASIERE, N. & LEWIN, K. 2009. Guidance on monitoring of MBT and other treatment processes for the landfill allowances schemes (LATS and LAS) for England and Wales. Bristol: Environment Agency.

WOOD, M., WALLACE, P., BECVAR, A. & WALLER, P. 2009. BSI PAS 100 Update – Review of Stability Testing. WRAP.

WRAP 2010. Development and evaluation of a method for testing the residual biogas potential of digestates. OFW004-005. Banbury: Written by: Walker, M., Banks, C., Heaven, S., and Frederickson, J.

WRAP 2011. Guidelines for Specification of Quality Compost for Use in Growing Media. Banbury.


WRAP 2012. Development of standard laboratory based test to measure compost stability – Annex A. Standardised method for the determination of compost stability by measurement of evolved carbon dioxide. Originally published in 2005. Banbury.

WRAP 2013. A survey of the UK organics recycling industry in 2012. WRAP Project RAK-005-002. Written by Horne J, Scholes P, Areikin E, Brown B.

WRAP 2014. Literature review: Compost stability impact and assessment. Written by Dimambro M, Steiner J, Rayns F, Schmutz U, Wallace P: Banbury.


Appendix 1 Sampling methodology

Sampling methodology for obtaining samples 1, 2 & 3 for DR4 testing only

1. There are three sampling locations: Freshly shredded IVC input waste for Sample 1;

Post - IVC sanitised material in maturation windrow for Sample 2; the maturation pile

containing unscreened final compost for Sample 3. Note, these samples will be taken

from different batches.

2. For each location estimate the approximate volume of the material to be sampled and

determine the number of sample increments to be taken with reference to the REAL

“Guidelines on Sampling Compost” document. It is recommended that only one batch

is sampled on each occasion. The minimum number of incremental samples to be

taken at each location will be 12.

3. These samples will only be used for DR4 testing. It is permitted to use clean but not

disinfected equipment for taking samples.

4. Visually identify a minimum of 12 incremental sample points in the material to be

sampled to ensure an even distribution of sampling points. Ensure that no surface

samples are taken and that samples are taken equally from near surface and deeper

points within the pile. For safety reasons do not engage the use of a mechanical

loading shovel to dig into piles and it will be sufficient to take deeper samples at a

depth of approximately 0.5m below the surface. One incremental sample is one

manual shovelful.

5. Spread out the clean tarpaulin on the ground adjacent to the pile to be sampled. For

each sample point in turn, use the shovel to extract a suitable incremental sample

and place this into the clean bucket. The use of the bucket should speed up sampling

and indicate sample size i.e. two incremental samples (two large shovels) would be

approximately 15 litres.

6. Empty each bucket to form a conical pile on the tarpaulin. Assuming 12 incremental

samples (6 buckets), the volume of the completed pile should be approximately 90

litres.

7. Mix the pile thoroughly.

8. Cone and quarter the pile (discarding opposite quarters) until an approximate 20 litre

sample remains.

9. Place the approximate 20 litre sample in a heavy duty sack, secure using a cable tie

and use the marker pen to code and date the sample.

10. The codes to be used are:

Freshly shredded IVC input waste Code is: Site number FW Post – IVC sanitised material Code is: Site number POST IVC Unscreened final compost Code is: Site number MAT


Sampling methodology for obtaining sample 4 (screened compost for all stability tests)

1. There is one sampling location: the pile containing screened final compost.

2. For each location estimate the approximate volume of the material to be sampled and

determine the number of sample increments to be taken with reference to the REAL

“Guidelines on Sampling Compost” document. It is recommended that only one batch

is sampled on each occasion. The minimum number of incremental samples to be

taken at each location will be 12.

3. These samples will be used for all stability tests and will be screened for E. coli and

Salmonella. Disinfected equipment must be used at all times for taking screened

compost samples.

4. Visually identify a minimum of 12 incremental sample points in the material to be

sampled to ensure an even distribution of sampling points. Ensure that no surface

samples are taken and that samples are taken equally from near surface and deeper

points within the pile. For safety reasons do not engage the use of a mechanical

loading shovel to dig into piles and it will be sufficient to take deeper samples at a

depth of approximately 0.5m below the surface. One incremental sample is one

shovelful.

5. Spread out a new tarpaulin on the ground adjacent to the pile to be sampled. For

each sample point in turn, use the disinfected shovel to extract a suitable incremental

sample and place this into the disinfected bucket.

6. Empty each bucket to form a conical pile on the tarpaulin. Take as many incremental

samples as is needed to form a pile approximately 100 litres in volume.

7. Mix the sample pile thoroughly.

8. Quarter the sample pile.

9. Place each of the quartered samples into a heavy duty sack, secure using cable ties

and use the marker pen to code and date each sample.

The code to be used is: Screened final compost Code is: Site number SC


Appendix 2 CompostManager protocol

The main purpose of the sampling exercise The main purpose is to measure oxygen levels in the maturation windrows as a means of characterising the degree of aerobicity of the windrows which is known to be related to compost stability. It is anticipated that the process of taking oxygen measurements should take approximately 1 hour. The Compost Manager probe should be used to take oxygen measurements from a maximum of 6 locations including all or a selection of the maturation windrows and, if possible, also from material in an IVC. It is permissible to select fewer than six locations depending on site conditions, site operations and circumstances. Oxygen measurements should be taken from the maturation windrows, representing increasing time of maturation from deposition of post-IVC sanitised material through to full maturation of compost (i.e. the oldest windrow from which the final screened and unscreened compost samples are taken). If possible and with the agreement of the site manager, also take oxygen measurements from the material in the IVC vessel: for example from the face of the waste at the front of the vessel with the doors opened or from the top of the vessel with the top cover pulled back. Sampling protocol Pre-select the locations where oxygen measurements are to be taken. At each of the selected locations, identify a sampling point at each windrow/pile. At each sampling point, 6 oxygen measurements should be taken within an area of approximately 1 m2. The procedure is for the Compost Manager probe to be inserted into the waste pile at waist height for a period of one minute and a measurement taken with a further five measurements being taken within the 1 m2 area. If possible, follow the same procedure for taking measurements at a selected sampling point within an IVC. Record precisely the locations of the oxygen measurements (e.g. batch number) as well as full details of material being assessed (e.g. windrow age) and windrow characteristics (e.g. windrow height) – importantly record when the windrows/piles were last turned or aerated. If measurements are taken from material in the IVC vessel, full IVC operating characteristics at the time should be noted (e.g. operation of the fans).


Appendix 3 CompostManager results

Table 7-1 Compost process results for site A

Table 7-2 Compost processing results for site B Average of Batch Age (days) 28 42

Average of Time Since Turn (days) 6 6

Average of Oxygen (%) 3.8 9.5

Average of CO2 (%) 20.1 9.2

Average of Dissolved O2 (ppm) 0.58 1.13

Average of Pile Temperature (°C) 66.1 74.8

Average of Moisture (%) 54.3 45.8

Compost processing results: Site C There were no data meeting the relevant criteria for this site. The complete set of CompostManager results are shown below.

Table 7-3 Compost processing results for site D

Average of Batch Age (days) 14 42 49 56 63 77 91 126

Average of Time Since Turn (days) 14 28 14 7 7 7 14 14

Average of Oxygen (%) 15.6 0.7 0 16.4 2.5 5.2 8.4 14.5

Average of CO2 (%) 3.5 21.3 18 3.6 18 13 8.5 6.8

Average of Dissolved O2 (ppm) 2.7 0.16 0 2.74 0.41 0.74 1.05 4.83

Average of Pile Temperature (°C) 62.8 51.3 41.8 64.1 65.9 69.2 73 34.4

Average of Moisture (%) 56 57.9 57 57.9 57.9 57.9 52.7 57.9

Table 7-4 Compost processing results for site E

Average of Batch Age (days) 21 39 53 74 91 105

Average of Time Since Turn (days) 8 26 40 15 29 43

Average of Oxygen (%) 12 9 10 8.4 15 15.3

Average of CO2 (%) 7.2 15.1 12 12.3 5.3 4.4

Average of Dissolved O2 (ppm) 4.22 2.32 2.62 1.41 2.96 2.51

Average of Pile Temperature (°C) 32.5 46.6 45.9 63.6 57.9 64.9

Average of Moisture (%) 47.7 49.8 39.8 55.3 54.6 50.1

Average of Batch Age (days) 29

Average of Time Since Turn (days) 16

Average of Oxygen (%) 20.3

Average of CO2 (%) 0.5

Average of Dissolved O2 (ppm) 6.20

Average of Pile Temperature (°C) 39.6

Average of Moisture (%) 22.7


Table 7-5 Compost processing results for site F

Average of Batch Age (days) 21 24 31 38 47

Average of Time Since Turn (days) 7 10 17 24 33

Average of Oxygen (%) 18 16.5 17.4 12.4 2.3

Average of CO2 (%) 1.8 3 2.3 10.4 20.5

Average of Dissolved O2 (ppm) 4.92 2.31 2.88 3.52 0.81

Average of Pile Temperature (°C) 44.1 69.9 64.4 42.9 32.6

Average of Moisture (%) 60.2 55.2 53 57 52

Compost processing results: Sites G - H There were no data meeting the relevant criteria for these sites. The complete set of CompostManager results are shown below.

Table 7-6 Compost processing results for site I Average of Batch Age (days) 32

Average of Time Since Turn (days) 14

Average of Oxygen (%) 0.4

Average of CO2 (%) 38.8

Average of Dissolved O2 (ppm) 0.09

Average of Pile Temperature (°C) 55.4

Average of Moisture (%) 56.9

Table 7-7 Compost processing results for site J Average of Batch Age (days) 23 37

Average of Time Since Turn (days) 14 28

Average of Oxygen (%) 10.1 18.9

Average of CO2 (%) 8.6 1.7

Average of Dissolved O2 (ppm) 1.24 4.48

Average of Pile Temperature (°C) 73.8 51.3

Average of Moisture (%) 52.2 28.3


Table 7-8 Oxygen and carbon dioxide concentrations, moisture content and temperature of the composts at a number of stages during composting measured using the CompostManager

Site Compost

Manager profile taken

from:

Description Days

since turned

Oxygen

(%)

Carbon

dioxide (%)

Temp-

erature (oC)

Moisture

A Outdoor pad Days on pad = 0 1 20.23 0.45 46 27

Outdoor pad Days on pad = 10 11 20.06 0.64 38 26

Outdoor pad Days on pad = 19 20 No data No data No data No data


B Outdoor pad Days on pad = 0 0 15.73 3.34 61 45




C Freshly shredded indoor input pile

Days since shredded = 0

0 4.07 29.23 55 45

Indoor composting windrow

Unscreened material 4 weeks old (end of sanitisation/ABPR phase)

3 -1.76 15.78 61 36

Indoor screened compost pile

Screened compost 6 weeks old in “stabilisation” phase

0 0.12 36.34 38 43

D Unscreened material (indoor)

~5 weeks post IVC 14 8.42 8.48 73 53

Unscreened material (indoor)

~11 weeks post IVC 14 3.46 13.48 39 52

Unscreened material = Post IVC

Two weeks in IVC n/a 15.61 3.5 63 56


4 weeks post IVC Not turned.

n/a 14.05 6.32 62 53


6 weeks post IVC 7 5.01 15.44 65 42


7 weeks post IVC 7 4.60 13.96 69 58


9 weeks post IVC 7 12.17 8.87 40 54

Screened material (indoor)

16 weeks post IVC. Screened ~ 2 weeks ago. Probe to half depth

14 18.99 1.09 36 66


Site Compost


from:

Description Days

since turned

Oxygen

(%)

Carbon

dioxide (%)

Temp-

erature (oC)

Moisture

E Unscreened material (indoor) =Post IVC

1 day post IVC Aerated floor

1 0.11 55.05 43 58


~1 week post IVC Aerated floor

n/a 12.00 7.18 33 48


~3-4 weeks post IVC. Aerated floor

n/a 8.98 15.09 47 50



n/a 10.00 12.01 46 45



~14 8.45 12.31 64 55



~28 14.95 5.31 58 55



~42 15.28 4.38 65 50

Screened material (outdoor)

~16 weeks post IVC 14 8.76 17.20 50 60

F Unscreened material (indoor) =Post IVC

7 days post IVC 7 18.00 1.80 44 60


10 days post IVC 10 16.55 2.97 70 55


17 days post IVC 17 17.40 2.33 64 53


24 days post IVC 24 12.41 10.38 43 57

Unscreened material (indoor) = MAT

33 days post IVC Probe to half depth

33 2.28 20.54 33 52

Screened material (indoor)

38 days post IVC 0 0.03 20.56 43 60

Screened material (indoor) =SC

52 days post IVC ~14 0.36 21.66 46 55


Site Compost


from:

Description Days

since turned

Oxygen

(%)

Carbon

dioxide (%)

Temp-

erature (oC)

Moisture

G Screened compost pile (outside)

Batch 110 (typically takes 3 days to empty and screen IVC and to form pile – this batch was ~50% complete)

0 9.25 11.47 29 57

Screened compost pile (outside)

Misc. batch (in IVC for 2.5 weeks) and screening was completed –pile 3 days old

3 2.52 40.83 37 57

H Screened compost pile (indoor)

Pile 4 days old (~400m3) and facility does not mature compost prior to deployment

4 3.95 16.97 71 51

I Screened compost pile (indoor)

Mature screened compost (5-6 weeks in maturation) – awaiting removal from site

0 9.37 6.30 43 54


Unscreened material (~4 weeks in maturation)

5 13.33 5.82 49 55


Unscreened material (~2 weeks in maturation)

~14 -1.74 38.77 55 56


Unscreened material (~1 day in maturation)

1 11.36 7.24 39 53

J Unscreened material (outdoor)

Aerated maturation windrows (~4 weeks in maturation).

28 18.52 1.72 51 28

Unscreened material (outdoor)

Aerated maturation windrows (~2 weeks in maturation).

14 8.93 8.55 74 52

Unscreened material (outdoor)

Aerated maturation windrows (~4 days in maturation).

4 7.46 9.30 80 41


Appendix 4 Feedback from the UK

laboratories

The four main UK laboratories undertaking PAS 100 testing and a laboratory which currently undertakes EoW testing, were asked if they were able to provide some indicative costings, in the scenario that the stability test would be changed to one of the four test methods trialled during this project. The laboratories were provided with the exact methodologies used during this project. Not all laboratories were able to provide costs, as these are not generally tests that are currently offered, and equipment would need to be purchased. Thus costs have not been included herein, as they would not be representative for all laboratories. A range of aspects would need to be considered by each laboratory to arrive at a final costing, to include: 1. The capital cost of equipment. 2. Costs for chemicals, software, etc. 3. The amount of space that the equipment would require. 4. Manpower in terms of sample preparation time, the number of samples that have to be

prepared for testing, checks whilst samples are running, logging, interpreting and reporting data, material disposal and clean down.

However, the majority of the laboratories did provide some initial considerations on the logistics and feasibility of each method, as summarised below. DR4 test The DR4 test is relatively similar to the PAS 100 ORG0020 test but with the addition of an inoculum (plus a few other minor differences). The DR4 test used in this project included providing a continuous record of CO2 production using an in line analyser. The laboratories highlighted that to purchase a Sable Systems analyser or equivalent is very expensive, and if this was necessary, some would be unable to offer the DR4 test in-house, whereas others considered that this initial outlay would be manageable. However, an alternative option would be to measure CO2 evolved by alkalinity titration at regular intervals, which would not require a high capital expenditure. Titration is already used for the current ORG0020 stability test. Oxygen uptake rate The responses for oxygen uptake rate tests were varied. The capital cost for an Oxitop system for 12 samples is approximately £6,000. Two of the laboratories already undertake a comparative test, another stated that the OUR test would be relatively easy to set up using current space and facilities. Two labs highlighted that the requirement to repeat the analysis if the compost sample is out of range could be an issue, for example with the need to ensure staff availability at the weekend, before knowing whether a repeat will be necessary or not. The need to repeat was also questioned, and whether it would be sufficient to simply have a pass or fail result. From the Open University‟s experience in running the OUR test, they observed that some additional checking is required, as it does go out of range for very active composts using standard equipment. A simple fail for very active composts over a threshold value of, for example, 50 mmol O2/kg/h would seem to be reasonable to keep costs down.


Dewar self heating The Dewar test was considered to be relatively cheap and easy to set up using current space and facilities. A laboratory would need to purchase at least 2 x Dewar flasks (approximately £100 each) as specified in the standard, in addition to 2 x max/min or datalogger probe thermometers (approximately £50 each). The probes record the temperature automatically, and so the test can be run over the weekend. Static respiration The system used in this project, which measures CO2 evolution by change in CO2 trap conductivity retails at £60,000. Two laboratories felt that the equipment needed (i.e. Static respiration incorporating Respicond respirometer/ continuous oxygen analyser) would be sufficiently costly that they would have to consider very carefully whether the initial set up costs would make this test viable. However, any lab offering the PAS 100 stability test will have chemicals/glassware to analyse CO2 evolution using the manual titration test. It should be noted that if a manual titration approach is used, sample vessels could be keep in a constant temperature room or incubator, rather than water bath. Laboratories already offering the ORG0020 test should be using one or other of these already.


Appendix 5 Ranking of the stability tests

Lab 1 Stability Lab 2 Stability DR4 test OUR

Site mg CO2/g OM /day

Rank mg CO2/g OM /day

Rank

g CO2/

kg VS/

96hr

Rank

mmol

(O2) kg-1

h-1

Rank

D 5 1 5 1 24 1 5.3 1

B 7 2 7 2 28 2 5.7 2

C 11 6 7 2 48 3 9.1 3

A SC3 8 3 8 4 56 6 12.1 5

A SC2 9 5 11 5 52 5 11.3 4

A SC4 8 3 11 5 52 4 16.6 6

A SC1 17 11 12 7 103 7 38.8 8

F 16 10 17 8 121 8 23.1 7

H 14 7 26 9 262 11 >50 9

J 16 9 29 10 266 12 >50 9

G micro 28 13 31 11 236 10 >50 9

E 14 8 32 12 279 13 >50 9

G 24 12 37 13 315 14 >50 9

I 30 14 38 14 231 9 >50 9

Lab 2 Stability Dewar test Static

respiration

Site mg CO2

/g OM /day

Rank Tmax °C

Rank Rotte-grad

mg CO2/g

organic matter

/24 h

Rank

D 5 1 25 2 V 4.7 5

B 7 2 25 2 V 11.8 9

C 7 2 25 2 V 3.5 3

A SC3 8 4 30 7 V 3.9 4

A SC2 11 5 30 7 IV 9.0 7

A SC4 11 5 26 6 V 9.1 8

A SC1 12 7 50 9 III 14.2 12

F 17 8 65 11 I 13.9 11

H 26 9 65 11 I 14.2 13

J 29 10 65 11 I 12.4 10

G micro 31 11 24 1 V 0.5 2

E 32 12 63 11 I 15.8 14

G 37 13 50 9 III 8.7 6

I 38 14 25 2 V 0.4 1

www.wrap.org.uk

final report compost stability: impact and assessment stability... · final report compost...

Documents