three vermicomposting trials

8/6/2019 Three Vermicomposting Trials

1/30

Organic Food Waste

Treatment DevelopmentProject

Organisation Partners

Open University

Urban Mines

Worm Research Centre

Biffaward Project Number: B/1859

ENTRUST Project Number: 204032.030

January 2004


2/30

S:\biffa reports worms\Final Report.doc

S:\biffa reports worms\Final Report.doc - 1 -

Name of Organisation Partners: Open University, Urban Mines, Worm Research Centre

Project Name: Organic Food Waste Treatment Development Project

Biffaward Project Number: B/1859 ENTRUST Project Number: 204032.030

Summary of Project

The main aim of the research programme was to devise and undertake appropriate trials inorder to contribute to knowledge and understanding of composting systems which utiliseearthworms as the main processing agent (vermicomposting). The programme aimed to buildon previous research carried out at the Worm Research Centre (WRC) into vermicomposting.

The research programme summarised in this report was based on large-scalevermicomposting trials, which were undertaken at WRC and devised and supervised by theOpen University. This programme took place over the period of almost 18 months, up untilNovember 2003. During the first few months, equipment was put in place and the worm bedswere stocked up so that the first technical work started in November 2002. A report detailingthe technical trials carried out for this project can be found in Appendix One.

Objectives and Achievements

! The effect of bed temperature on vermicomposting rates and greenhouse gas emissions,using potato waste as a feedstock material

Five experimental earthworm composting beds were built for this trial. Individual bedsmeasured 1.5m x 6.6m, with a depth of 0.75m. Each bed was filled with composted horsemanure/wood shavings bedding material, to a depth of 0.25m when settled. The waste usedas feed material was locally produced potato waste, which is a highly putrescible and wet by-product of the food processing industry.

The block of beds were protected from rain by impermeable but well ventilated covers. Liquidby products of the process were collected in a leachate drainage and collection system. The

earthworm species employed during the trial was Dendrobaena veneta and populationssampled in November 2002 were found to 1.2 to 1.4 kgm-2

of bed. The first waste applicationfor this trial and the commencement of bed monitoring programme began in December 2002.

The aim was to produce a range of bed temperatures from 5 C to 25 C. To achieve the lowtemperatures within the range, the trial was commenced during the winter months so thatambient air temperatures would naturally lower bed temperatures. The heated cables withinbeds were individually thermostatically controlled and this enabled the higher temperatures tobe achieved during the winter months. During the first three months it was possible tomaintain the bed temperatures broadly within the required range. For the following threemonths, the bed temperatures tended to reflect ambient temperatures, as might be expected.Mean bed temperatures achieved during the six-month study are presented in Appendix Two.

From Tables 1 and 2 in Appendix Two, for the first three-month period it can be seen that the

lowest temperature bed (Bed B at 7 C) processed the least material while the highesttemperature bed (Bed C at 24 C) processed approximately 60% more. More importantly,

maintaining a moderate and achievable bed temperature of around 13-16 C during coldambient conditions resulted in a 50% increase in waste processing rate. Earthwormpopulations were broadly comparable in all beds during the first three-month period.

For the second three month period (months 4-6), the bed temperatures tended to reflect thehigher ambient temperatures of spring and early summer, except for Bed C, whose

thermostat was set at the highest temperature (25 C). Hence in general the bedtemperatures were higher and more uniform during this period and this resulted in greater andmore uniform waste processing rates throughout.

For the beds operating under higher ambient temperatures during spring, around 560 kg of


3/30



sludge was processed in 3 months per bed. This is equivalent to 0.62 kg of waste processedper square metre of bed per day and is similar to previous findings with this type of waste.

The trial showed that unheated vermicomposting beds are likely to suffer from reducedprocessing rates during periods of cold weather. The best processing rates were obtained

from the bed heated to the optimum temperature for worm composting (20-25 C). However,heating beds to these relatively high temperatures is not likely to be cost-effective. Heating

beds to moderate temperatures (approximately 15 C) during periods of low ambienttemperatures is achievable in practice. The trial confirmed that processing rates at moderatetemperatures are acceptable.

The static chamber method was used to monitor greenhouse gas emissions from thevermicomposting beds. This method is commonly used to measure gas fluxes from surfaceemissions and has been validated in comparison to micrometeorological methods.

The trial clearly confirmed that nitrous oxide emissions could be a potential problem for large-scale vermicomposting systems especially when operating at higher temperatures (seeAppendix Two, Figures 1 and 2). It recommended that further research is urgently undertakento determine the full extent of potential problems and to identify mitigation measures in orderto minimise harmful emissions.

! Comparison between vermicomposting and windrow composting of plastic wrappedvegetable waste

The feedstock waste used for the trial was predominantly whole cucumbers in plasticwrappers. It is not normally possible or cost-effective for the suppliers of the waste to removethe individual wrappers, so the waste is usually landfilled.

Although the mixed, whole cucumber waste had a relatively soft texture and would havebroken down readily during biological processing, the plastic wrappings surrounding much ofthe waste were intact. Therefore two methods of pre-treating the waste prior to biologicaltreatment were utilised in order to partially disrupt the plastic wrappers. These were:

i. shredding the waste to create a slurry

ii. lightly crushing the material to maintain the structure of the vegetables

The windrow was mechanically turned every seven days. Very little of the original cucumberfeedstock remained in the windrow at the end of the eight week composting process. Theplastic wrapping material, which formed 2% of the original waste also remained intimatelymixed within the bulking agent.

A particular feature of the vermicomposting operation was that both types of cucumber wastewere applied directly to the surface of the processing bed. This enabled the earthworms toenter the plastic wrappers; consuming and processing the cucumber. The consequence ofthis was that the plastic wrappers remained on the surface of the bed and was readilyremoved once processing has been completed. This makes removal of the plastic from theprocessing system a very simple and cost-effective operation compared with the composting

process, which required the use of a bulking agent and which trapped the plastic.

! Feasibility of combining vermicomposting with an in-vessel composting system

With the introduction of the Animal By-Products Regulations (ABPRs) in 2003, the treatmentof catering wastes using vermicomposting, without the wastes having first undergone asanitisation process in a closed composting reactor is no longer permitted.

All catering wastes, including source segregated household wastes, must be composted in a

closed reactor at a temperature of 60 C for 2 days (40 cm particle size) or 70 C for 1 hour (6cm particle size). Further processing requirements for those wastes containing meat includesubjecting the waste to a further composting stage to be carried out at the temperatures given


4/30



above. For non-meat wastes it is sufficient to store the partially composted material for aminimum of 18 days before use.

It is important to note that while the composting times given in the regulations relate totemperature-time relationships for disease suppression, they do not take into account themuch longer periods of composting which are required to produce stable, compostedproducts, so that the second stage will need effective composting management rather than"storage" in order to produce compost.

It is clear from the regulations that all catering wastes must first be composted at hightemperatures in a closed reactor and meat-containing wastes must undergo furtherthermophilic composting to comply with the regulations. Hence, vermicomposting would notbe considered to be a suitable technology for the treatment of meat-containing wastes, sinceit operates in the low temperature or mesophilic range. However, for non-meat containingwastes, low temperature processes such as vermicomposting can be used to satisfy the "18-day storage" requirement. Indeed, the use of vermicomposting to accelerate the compostmaturation process and to enhance the partially-composted material from the closed reactorstage would appear to be a very good option for some composting operations. For non-meatcontaining wastes, other composting systems such as open air mechanically turned windrowsystems would also be suitable for the second stage and for this application the composting

temperatures and turning regimes need not comply with the ABPRs.

Combining the closed reactor stage with vermicomposting for the treatment of sourcesegregated household waste may offer many benefits but very little research has been carriedout into this type of combined system. In particular, many practical aspects of combiningsystems are unclear. For example, it is not known if hot, partially composted material from in-vessel systems can be applied directly to earthworm beds without killing the earthwormpopulations. Equally, although vermicomposting is known to accelerate the maturationprocess for some wastes, it is not known if maturation can be achieved more rapidly thanother cost-effective processes, such as windrow composting systems.

Also, in terms of the environmental impact of vermicomposting and windrow compostingsystems when operated in combination with in-vessel systems, it is important to assess thegreenhouse gas emissions from both approaches (see Appendix Three).

This trial was devised in order to address some of these fundamental questions, aiming toexplore the practical aspects of combining vermicomposting with in-vessel systems. Partiallycomposted material from an in-vessel system was applied to vermicomposting beds, andmaturation rates, as well as greenhouse gas emissions were measured. The same materialwas also windrowed and measurements for the two trials were compared.

The trial showed that in practice vermicomposting can be combined with in-vessel compostingsystems. Vermicomposting was shown to be an effective method for fully stabilising andmaturing the partially composted material from in-vessel composting systems. Windrowcomposting was equally effective but requires considerably more resources. Turning theheaps involves regular use of people, machinery and time. Therefore, these extra resourcesmust be taken into account when deciding which system to use.

Pre-composting material using an in-vessel system prior to vermicomposting appeared toreduce nitrous oxide emissions from vermicomposting. It is recommended that greenhousegas emissions from the overall in-vessel/vermicomposting system is investigated.

! Optimising key operating parameters for vermicomposting systems and developing a bedsystem

i. Bed system

The main progress in this area was the development of the plastic bed system. WRC havedesigned and developed a modular plastic bed system using rotationally moulded recycledplastic. These are lightweight and clip together to the required size, and can be placed on anyflat ground. They have drainage channels, so that leachate can be safely collected and used


5/30



as a liquid fertiliser in controlled conditions. This collection is a requirement of current UKlegislation, which forbids the direct drainage of such a material directly onto land, which wormbeds in the past have neglected to take into consideration.

ii. Cover system

One area still requiring further work is the development of a suitable covering system. The

cover is very important in maintaining constant conditions within the system. It also has to beeasy and efficient for an operator to use. Due to the length of the beds, the covers are heavyand therefore a system that easily maneuvers these items is taking time to develop asdifferent solutions are tested.

iii. Feeding

The most efficient way to feed the worms with large quantities of waste was also developed.The use of a tractor with a tank is optimal, since this requires minimal investment and tankconversion is reasonably simple. In terms of waste type applied to vermicomposting beds, ithas been found that liquid sludge waste is easier to administer onto the beds by mixing it withshredded cardboard.

Others bodies involved in the project

Since the project is research based, there are few beneficiaries as of yet. The WRC has helda number of open days where representatives from local composting organisations have beenshown the work which is taking place at the WRC. These groups will ultimately benefit fromthe overall research which will be shared at the end of the project. The WRC also giveongoing assistance to individuals and groups involved in composting by phone and throughthese people visiting the centre. Thirty five such visits were made in the last year.

Urban Mines (UM) and WRC are currently working with Lattice Foundation, which providedsome third party funding for this project. The aim is to set up a link between the WRC and oneof the young offenders institutions which Lattice work with. It is hoped that the youngoffenders will learn about worm farming and about environmental issues associated withwaste. The end result from this will enable these individuals to set up worm beds at the prison

in order to convert vegetable-derived canteen waste into compost to be used on theirgardens. Discussions are ongoing regarding this.

Funding was also obtained from TXU energy for the installation of a solar panel and windturbine. These power sources were used to supply heat to one bed to evaluate how effectiveand reliable this source of energy could be. Enough electricity is also generated from the windturbine to power the electric fences in the worm beds.

Classes from two local primary schools have visited WRC, allowing pupils to see the practicalaspects of waste management through vermiculture. This fits in with the Science NationalCurriculum at Key Stages One and Two, both in the areas of waste and biology (usingvermicompost as a growth media). It is hoped that this will roll out to other local schools.

Media CoverageIn April 2002, Radio Humberside, visited the Worm Research Centre as part of a visit by theNorthern Recycling Group. Information about the Biffa funded work at WRC was broadcaston the news every half-hour. An interview with the Centres Director, Steve Ross-Smith, wasbroadcast on the 6pm slot.

In September 2002, UM published a newsletter, which included a piece about the project (seeAppendix Four). UM has made updates to their website regarding the Biffa project at WRC,see http://www.urbanmines.org.uk/wrc.htm. This also shows the installation of a windmill toprovide energy to the electric fences within the beds, that keep the worms inside.

The 2003 summer issue of Composting News contained an article on the Biffa funded work atWRC, see Appendix Four.
http://www.urbanmines.org.uk/wrc.htmhttp://www.urbanmines.org.uk/wrc.htmhttp://www.urbanmines.org.uk/wrc.htm


6/30

S:\biffa reports worms\Final Report app1.doc

APPENDIX ONE

SUMMARY REPORT:

THREE VERMICOMPOSTING TRIALS

FUNDED BY BIFFAWARD

2002 - 2003

Prepared by Jim Frederickson

Integrated Waste Systems

Open University

December 2003


7/30

Aims of the Research Programme

The main aim of the research programme was to devise and undertake appropriate trials in orderto contribute to knowledge and understanding of composting systems which utilise earthworms asthe main processing agent (vermicomposting). The programme aimed to build on previousresearch carried out at the Worm Research Centre (WRC) into vermicomposting. This work hasbeen completed and the findings published. A detailed report on the findings of previous work can

be found at http://www.wormresearchcentre.co.uk/.

The research programme summarised in this report was based on three separate large-scalevermicomposting trials, which were undertaken at the Worm Research Centre (WRC) anddevised and supervised by the Open University.

The trials were:

1. The effect of bed temperature on vermicomposting rate and greenhouse gas emissions

2. Feasibility of combining vermicomposting with an in-vessel composting system

3. Using vermicomposting to process plastic wrapped vegetable waste

In particular the project aimed to investigate new approaches to stabilising waste such as usingselected species of earthworms to compost difficult and contaminated waste types and looked atcombined systems for possible enhanced performance. It investigated new methods andprotocols for monitoring composting performance such as respirometry. Very importantly, itaddressed the environmental impact of composting systems. A key objective was to build onprevious research into the emission of greenhouse gases from vermicomposting which had beenidentified as potentially problematic.

A particular feature of previous research at WRC was an attempt to apply rigorous scientificmethod to the evaluation of both the process of vermicomposting and the products ofvermicomposting. This approach was adopted throughout this programme and a considerableemphasis was placed on analysing physico-chemical characteristics of waste materials as theyunderwent vermicomposting and composting. The environmental impact of both vermicompostingand composting was also investigated in great detail, in particular with regard to the emission of

greenhouse gases. Trials 1 and 2 addressed greenhouse gas emissions and in the UK, thisimportant research is unique to the work carried out at WRC in collaboration with the OpenUniversity.

Another unique feature of the work carried out in this programme was the use of respirometrytechniques to measure the "stability" of waste (i.e. the degree of biological activity) duringcontrolled vermicomposting and composting and this is a key parameter when assessing systemperformance. The respirometric method and its application to evaluating composting systems isdescribed in the report.

While previous research at the Worm Research Centre focused mainly on vermicomposting, theresearch programme described in this summary report placed considerable emphasis oncombining and comparing vermicomposting with other composting systems.

This brief summary report is not meant to be a "guide to vermicomposting" neither is it meant to

be a manual on good practice when operating vermicomposting systems or composting systems.The research programme attempted to contribute to the debate about choosing appropriatecomposting systems to achieve specific waste management objectives and particular emphasiswas placed on assessing the relative merits of different approaches to composting. It also aimedto provide some much needed dada on the environmental impact of large-scale vermicomposting,in particular greenhouse gas emissions.
http://www.wormresearchcentre.co.uk/http://www.wormresearchcentre.co.uk/http://www.wormresearchcentre.co.uk/


8/30

TRIAL 1

The effect of bed temperature on vermicomposting rate and on greenhouse gas emissions

Aims and backgroundPrevious research at WRC suggested that maintaining moderate bed temperatures was good forincreasing worm populations and this was also associated with increased waste processing rates.Unfortunately maintaining higher temperatures during vermicomposting seemed to be alsoassociated with increased greenhouse gas emissions (N2O).

This trial aims to investigate the effect of different temperature regimes on waste processing ratesand greenhouse gas emissions. In particular, the trial was set up to determine the reason for theincreased rates associated with higher temperatures; increased worm numbers or the highertemperatures per sewhich would have stimulated production of microbial biomass and increaseddecomposition rates. It also aims to look in more detail at the release of nitrous oxide (N2O)during vermicomposting at different temperatures to see if increasing temperatures (andincreased waste application rates) would be likely to increase the environmental impact ofvermicomposting systems.

Methane (CH4) and nitrous oxide (N2O) are included in the 6 greenhouse gases listed in theKyoto protocol that require emission reduction. To meet reduced emission targets governmentsneed first to quantify their contribution to global warming. Composting has been identified as animportant source of CH4 and N2O. With increasing divergence of biodegradable waste from landfillinto the composting sector, it is important to quantify emissions of CH4 andN2O from all forms ofcomposting. The contribution to the greenhouse effect of methane (CH4) and nitrous oxide (N2O)has been well documented. CH4 is second to CO2 in contributing to global warming (around 18%of the enhanced global greenhouse effect), CH4 absorbs and re-radiates 21 times the energy ofCO2. Its atmospheric concentration has doubled in the past several hundred years to the present1.7ppm which is rising by around 4ppbyr

-1. N2O contributes to global warming (around 6% of the

enhanced global greenhouse effect) and stratospheric ozone depletion. It absorbs and re-radiates310 times the energy of CO2, with an atmospheric concentration of approx. 311ppb, rising by

around 0.75ppbyr

-1

.

Objectives

1. To investigate the effect of different temperature regimes on waste processing rates.

2. To investigate the effect of different temperature regimes on greenhouse gas emissions

Trial details

Five experimental earthworm composting beds built as one block were used for the trial andthese are shown in Figure 1. More details about this type of bed construction can be found athttp://www.wormresearchcentre.co.uk/.

Each individual bed was 1.5 metres wide by 6.6 metres long and beds were approximately 0.75metres deep. The block was 1.5m wide by 25m long and total block area was approximately 50m

2. Each bed was filled with composted horse manure/wood shavings bedding material

(approximately 0.25 m deep when settled) to contain the earthworm populations.
http://www.wormresearchcentre.co.uk/http://www.wormresearchcentre.co.uk/http://www.wormresearchcentre.co.uk/


9/30

Figure 1: Bed layout

The block of beds were protected from rain by impermeable but well ventilated covers and theblock was equipped with a leachate drainage and collection system. All beds were heated andaccurately controlled temperatures for each bed were achieved using individual electric heatingcables and thermostats located in the bedding material. Thermocouples and data loggerscontinuously recorded bed and air temperatures to ensure that composting rates and earthworm

populations were linked to prevailing environmental conditions.

The waste used as feed material for the earthworms during the vermicomposting trials was locallyproduced potato waste, which is a highly putrescible and very wet by-product of the foodprocessing industry. Previous research at WRC established that the potato slurry used as feed forearthworms supported good levels of earthworm growth, was not toxic to earthworms andproduced good quality vermicompost. The potato waste when delivered to site by tanker (around20 tonnes per load) was often fresh from the factory as shown by its elevated temperature. Theslurry contained a mixture of steamed potato flesh in homogenised form and fine potato skins. Inbrief, the chemical analysis of the potato waste when applied to the processing beds showed it tobe very wet (around 90% moisture content), relatively acidic (pH 4 - 5), very rich in dissolvednutrients (electrical conductivity approximately 8 mS/cm). Nutritionally and in terms of wasteprocessing the potato slurry would be considered to be highly nutritious and very putrescible, withthe solid material in the slurry having a relatively high protein content of approximately 20% (3.2% total nitrogen content x 6.25), a carbon to nitrogen ratio (C:N) of 15:1 and with most of the solidmaterial being readily amenable to decomposition (high organic matter content 88%). The bulkdensity was approximately 1 kg/litre.

The beds selected for the trial were chosen because they had been previously used forvermicomposting similar potato waste and hence the earthworm populations and micro-organisms in the beds were well acclimatised to the environmental conditions. The earthwormspecies employed in the trial was Dendrobaena veneta. Earthworm populations were sampledinNovember 2002 and the initial biomass of earthworms in the five beds was found to be between


10/30

1.2 to 1.4 kg m-2

of bed. The first waste application for this trial and the commencement of bedmonitoring programme began on 17

thDecember 2002.

Bed temperatures

The aim was to produce a range of bed temperatures from (5 C to 25 C). To achieve the low

temperatures within the range, it was necessary to commence the trial during the cold wintermonths so that ambient air temperatures would naturally lower bed temperatures. The heatedcables within beds were individually thermostatically controlled and this enabled the highertemperatures to be achieved during the winter months. The project began in December 2002 andduring the first three months it was possible to maintain the bed temperatures broadly within therequired range. For the following three months, the bed temperatures tended to reflect ambienttemperatures, as might be expected. Mean bed temperatures achieved during the six-monthstudy are presented in Table 1.

Table 1: Mean bed temperatures for trial periods 1-3 months and 4-6 months

Trial period Bed A

(C)

Bed B

(C)

Bed C

(C)

Bed D

(C)

Bed E

(C)

Ambient

(C)

Months

1-3

13 7 24 16 11 6

Months

4-6

16 16 23 21 15 15

Vermicomposting rates

Table 2: Amounts of potato sludge applied to beds maintained at different temperatures

Trial period Bed A

Litresapplied

Bed B

Litresapplied

Bed C

Litresapplied

Bed D

Litresapplied

Bed E

Litresapplied

Months

1-3

490 330 530 490 370

Months

4-6

560 560 560 560 560


11/30

From Tables 1 and 2 for the first three-month period it can be seen that the lowest temperature

bed (Bed B at 7 C) processed the least material while the highest temperature bed (Bed C at 24

C) processed approximately 60% more. More importantly, maintaining a moderate and

achievable bed temperature of around 13-16 C during cold ambient conditions resulted in a 50%increase in waste processing rate. Earthworm populations would have been broadly comparablein all beds during the first three-month period.

For the second three month period (months 4-6), the bed temperatures tended to reflect thehigher ambient temperatures of spring and early summer, except for Bed C, whose thermostat

was set at the highest temperature (25 C). Hence in general the bed temperatures were higherand more uniform during this period and this resulted in greater and more uniform wasteprocessing rates throughout.

For the beds operating under higher ambient temperatures during spring, around 560 kg ofsludge was processed in 3 months per bed. This is equivalent to 0.62 kg of waste processed persquare metre of bed per day and is similar to previous findings with this type of waste.

Monitoring greenhouse gas emissions

The static chamber method was used to monitor greenhouse gas emissions from the

vermicomposting beds (see Figure 2). The static chamber method is commonly used to measuregas fluxes from surface emissions and has been validated in comparison to micrometeorologicalmethods. Static chambers have been extensively employed to measure methane emission fromrice paddies and composting. For this study cylinders of 0.0707m

2cross sectional area and

height of around 0.3m were pressed into the compost material to a depth of around 0.05m, afterallowing time for gas evolved due to disturbance of the material to disperse, the cylinders weretopped and sealed. The closed chamber (0.25m x 0.0707m

2in volume) now captured any gas

flux from the bed. Samples of around 60ml were taken at t=0 (when the cylinders were topped)t=10 minutes, 20 minutes and 30 minutes. Once a sample was removed (via syringe) it wasimmediately injected into an evacuated glass vial and labelled. The glass vials were thentransported to the Open University and analysed for CH4 and N2O content on a gaschromatograph (Ai Cambridge GC94m) fitted with a flame ionisation detector for CH4 and anelectron capture detector for N2O.


12/30

Figure 2: Static chamber method

Results

From Figure 3 it can be seen that methane (CH4) emissions from the beds were generally verylow and this would be expected since methane is formed in anaerobic conditions (i.e. devoid ofoxygen). Vermicomposting beds are required to be well aerated in order to provide high oxygenlevels for the earthworms, therefore methane emission from beds would not be expected.

However, nitrous oxide (N2O) emissions from beds were significant at all temperatures,confirming findings from previous studies at WRC (see Figure 4). This study has shown that at

high vermicomposting temperatures (20 - 25 C) very high fluxes (in excess of 35 mg hr-1

m-2

)were observed.

In general, fluxes of N2O found in this study exceeded those of previous studies at WRC (e.g.1.24 - 4.75 mg hr

-1m

-2) and in summary, it would appear that the release of nitrous oxide from the

surface of the beds, rather than methane, is much more problematic. For example, the range ofnitrous oxide fluxes found during the project can be compared to other emission sources such asthe range of fluxes reported for garden soil 0.0031 - 0.031 mg hr

-1m

-2.

Clearly the issue of nitrous oxide emissions from vermicomposting is a potentially serious and, asyet, unrecognised problem. There is a pressing need to investigate the extent of the problem assoon as possible and to identify mitigation options, if appropriate. Research undertaken for thisproject, has identified vermicomposting as one of the most significant point sources of nitrousoxide emissions yet discovered. For example, riparian zones in the UK associated with intensiveagriculture have been identified as the largest emitters of nitrous oxide to date, with levels of N2O

N of around 38 kg ha-1

yr-2

. Recent emission figures for N2O N from vermicomposting havebeen found to many times greater than this. Although the total area of land devoted tovermicomposting operations would never be comparable to areas of riverbank in sensitive,agriculturally intensive locations, the first vermicomposting operation larger than 1ha in area hasalready been established.

It is recommended that further research is undertaken into the effect of temperature on N2Oemissions and into ways of reducing emissions form these systems.


13/30

Figure 3: Methane emissions

Figure 4: Nitrous Oxide emissions

Conclusions

1. The trial showed that unheated vermicomposting beds are likely to suffer from reducedprocessing rates during periods of cold weather. The best processing rates were obtained

from the bed heated to the optimum temperature for worm composting (20-25 C). However,heating beds to these relatively high temperatures is not l ikely to be cost-effective. Heating

beds to moderate temperatures (approximately 15 C) during periods of low ambient

temperatures is achievable in practice. The trial confirmed that processing rates at moderatetemperatures are acceptable.

2. The trial clearly confirmed that nitrous oxide emissions could be a potential problem for large-scale vermicomposting systems especially when operating at higher temperatures. Itrecommended that further research is urgently undertaken to determine the full extent ofpotential problems and to identify mitigation measures in order to minimise harmfulemissions.

0

2

4

6

8

10

12

0 5 10 15 20 25

Bed Temperature (oC)

CH4mgm-2hr-1

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25


N2O

mgm-2hr-1


14/30

TRIAL 2

Feasibility of combining vermicomposting with an in-vessel composting system

Aims and backgroundWith the introduction of the Animal By-Products Regulations (ABPRs) in 2003, it no longerpermitted to treat catering wastes using vermicomposting, without the wastes having firstundergone a sanitisation process in a closed composting reactor. All catering wastes, includingsource segregated household wastes, must be composted in a closed reactor at a temperature of

60 C for 2 days (40 cm particle size) or 70 C for 1 hour (6 cm particle size). Further processingrequirements for those wastes containing meat include subjecting the waste to a furthercomposting stage to be carried out at the temperatures given above. For non-meat wastes it issufficient to store the partially composted material for a minimum of 18 days before use. It isimportant to note that while the composting times given in the regulations relate to temperature-time relationships for disease suppression, they do not take into account the much longer periodsof composting which are needed in the second stage to produce stable, composted products.Hence in reality, the duration of the second stage of composting will be much longer than the 2-

day requirement and the second stage will need effective composting management rather than"storage" to produce compost.

It is clear from the regulations that all catering wastes must first be composted at hightemperatures in a closed reactor and meat-containing wastes must undergo further thermophiliccomposting to comply with the regulations. Hence, vermicomposting would not be considered tobe a suitable technology for the treatment of meat-containing wastes, since it operates in the lowtemperature or mesophilic range. However, for non-meat containing wastes, low temperatureprocesses such as vermicomposting can be used to satisfy the "18-day storage" requirement.Indeed, the use of vermicomposting to accelerate the compost maturation process and toenhance the partially-composted material from the closed reactor stage would appear to be avery good option for some composting operations. For non-meat containing wastes, othercomposting systems such as open air mechanically turned windrow systems would also besuitable for the second stage and for this application the composting temperatures and turning

regimes need not comply with the ABPRs.

Combining the closed reactor stage with vermicomposting for the treatment of source segregatedhousehold waste may offer many benefits but very little research has been carried out into thistype of combined system. In particular, many practical aspects of combining systems are unclear.For example, it is not known if hot, partially composted material from in-vessel systems can beapplied directly to earthworm beds without killing the earthworm populations. Equally, althoughvermicomposting is known to accelerate the maturation process for some wastes, it is not knownif maturation can be achieved more rapidly than other cost-effective processes, such as windrowcomposting systems. Also, in terms of the environmental impact of vermicomposting and windrowcomposting systems when operated in combination with in-vessel systems, it is important toassess the greenhouse gas emissions from both approaches. This trial was devised in order toaddress some of these fundamental questions.

Objectives

1. To explore practical aspects of combining vermicomposting with in-vessel systems

2. To investigate the effect of in-vessel composting on source segregated waste and asses itssuitability to undergo further maturation

3. To apply partially composted material from an in-vessel system to vermicomposting beds andmonitor vermicompost maturation rate using respirometry


15/30

4. To windrow compost partially composted material from an in-vessel system and monitorcompost maturation rate using respirometry

5. To monitor greenhouse gas emissions from vermicomposting and windrow compostingsystem when operated in combination with the in-vessel system

Trial details

The study was undertaken from November 2002 to February 2003.

The waste used in this study was double shredded source segregated household waste (85%green waste, 10% non-meat kitchen waste, 5% inert contaminants). Initial 1 week in-vesseltreatment of the material was undertaken at the Cleanaway landfill/waste treatment site inRainham, East London using a Waste Solutions Sirocco invessel system. After pre-treating thematerial for 1 week and subjecting the material to the required temperature conditions to complywith the Animal By-Product Regulations, the unit and its contents were transported to the WRCexperimental site.

Half of the material (approximately 3.5t) was formed into a windrow (2m high, 10m long and 3mwide) which was situated on a concrete surface and covered with a porous membrane, 2 installedtemperature probes provided continuous logging of the pile temperature. The windrow was turnedweekly for the first month the every fortnight thereafter. The remaining material was depositedonto the surface of 4 brick build vermicomposting beds the depth of the layer was 10-15cm (seeFigure 5). The purpose built vermicomposting beds were of concrete and brick construction withbuilt-in drainage/leachate collection, electric cable heating and temperature data logging andsimilar to beds used in Trial 1. The bedding material for the composting worms consisted ofmixed woodchip to a depth of around 20 cm. The beds were maintained at a constant 20

oC

temperature and stocked with 2 kg worms m-2

of bed (Dendrobaena veneta).

Figure 5: Partially composted material from the In-vessel system being applied tovermicomposting beds


16/30

It can be seen from Figure 6 that the partially composted material from the in-vessel systemreadily underwent further composting and maturation using the WRC windrow operation.Temperatures are given for the first 8 weeks of composting only since windrow temperaturesthereafter followed ambient conditions. Regular reductions in temperature coincided with periodicwindrow turning.

Figure 6: Windrow composting temperatures

Using respirometry to monitor vermicompost and compost maturation rates

To assess the maturity (stability) of the material a respirometer facility was developed at the OpenUniversity. The respirometer gauges the microbiological activity of composting material, anindicator of compost maturity and stability. This allows comparison of windrow andvermicompostings effectiveness in degrading organic material. Figure 7 shows the respirometerset-up; the design was adapted from the basic system recommended by the manufacturer (SableSystems, Connecticut, USA). The system employs the flow through dynamic method and allowsfor higher quantities of compost to be analysed. The system employed in this study had 3chambers each holding 4Kg of material. CO2 production rate was determined by subsampling theoutput stream using a multiplexing unit to alternate between subsample lines. The compostmoisture was amended to the optimum 60% prior to being analysed, and the water bathtemperature was set at 35

oC maximising CO2 production by providing conditions favourable to

most of the microbe population.

In the Open University system, consumption of O2, production of CO2 (a measure of microbial

activity) aeration flow rate and temperature are continually logged. Material make-up (C:N ratio,volatile solids, moisture, pH) is recorded and samples are also taken for N2O and CH4 analysis.Nitrogen transformations are followed by ion chromatograph analysis of NH4 and NO3.

0

10

20

30

40

50

60

70

Time (days)

Temperature(oC)

7 14 21 28 35 42 49 560


17/30

Figure 7: Open University Respirometry facility

The respiration studies showed that the in-vessel treatment, which lasted 7 days, was veryeffective in stabilising the source segregated household waste (see Figure 8). Thereafter, bothvermicomposting and windrow composting were equally effective in fully stabilising and maturingthe organic material. The total time for composting for both combined systems was approximately12 weeks. Vermicomposting may be considered to be a suitable process for combining with in-

vessel composting but there was no evidence that vermicomposting showed any advantagesover windrow composting in this mode of operation.

Figure 8: Respiration rates for in-vessel, windrow and vermicomposting systems

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80 90 100

Time (days)

Respirationrate

(mgCO2/hr/Kg)

Vermicomposting

Windrow

In-vessel


18/30

Monitoring greenhouse gas emissions

The material was deposited at the experimental site on 12 November 2002, and after beingallowed to equilibrate, gas and material samples were taken the following day. Gas and materialsamples were taken every week for the first 3 weeks then every fortnight for the following 10weeks. The static chamber method was used for gaseous emission measurements from thesurface of the compost material from both the windrow and vermicomposting beds. For one

sampling run 4 static chambers were used on the windrow and 4 chambers on thevermicomposting bed. Figures 9 and 10 show the chamber method being used for thevermicomposting beds and the windrow system respectively.

Figure 11 demonstrates that methane emissions from vermicomposting are generally very low asfor Trial 1. However, the windrow composting operation produced significant amounts ofmethane, which reduced over time due to reduced availability of substrate as confirmed by therespirometer studies. Nitrous oxide fluxes were again significant (Figure 12) but were not aspronounced as found for previous research at WRC. This may be related to the high nitrogenlosses found for the initial 7 day composting programme using the in-vessel system. Prior tobeing in-vessel composted, the nitrogen content of the fresh waste was found to be 1.5% (dryweight) but when composted for 7 days the nitrogen content was only 1.2% (dry weight). In-vessel composting is known to volatilise large amounts of nitrogen and in this case the reducednitrogen content of the waste being vermicomposted may have been responsible for reducing

further nitrogen losses as nitrous oxide. This would have also reduced the environmental impactof the vermicomposting system. Further research into mitigation of greenhouse gas emissions isurgently required.

Figure 9: Static chamber method applied to vermicomposting beds


19/30

Figure 10: Static chamber method applied to windrow composting

Figure 11: Methane emissions from vermicomposting and windrow composting

0

1

2

3

4

5

6

7

7 14 21 35 50 64 78 92

Time (days)

CH4flux(mgm-2hr-1)

WindrowVermicom ostin


20/30

Figure 12: Nitrous Oxide emissions from vermicomposting and windrow composting

Conclusions

1. The trial showed that in practice vermicomposting can be combined with in-vesselcomposting systems.

2. Vermicomposting was shown to be an effective method for fully stabilising and maturing thepartially composted material from in-vessel composting systems. However, windrow

composting was equally effective.3. Pre-composting material using an in-vessel system prior to vermicomposting appeared to

reduce nitrous oxide emissions from vermicomposting. It is recommended that greenhousegas emissions from the overall in-vessel/vermicomposting system is investigated.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

7 14 21 35 50 64 78 92

Time (days)

N2O

flux(mgm-2hr-

1)

Windrow

Vermicomposting


21/30

TRIAL 3

Using vermicomposting to process plastic wrapped vegetable waste

Aims and backgroundThe feedstock waste used for the trial was predominantly whole cucumbers in plastic wrappersmixed with a small amount of discarded mixed vegetables, such as peppers. The mix ofcucumbers and other vegetables is classified as waste as a result of failure to meet quality controlcriteria. Due to the presence of the wrappers, this waste stream is typically landfilled. It is notnormally possible or cost-effective for the suppliers of the waste to remove the individualwrappers, to enable more sustainable treatment options, such as composting, to be considered.

The aim of this trial was to explore the use of vermicomposting as a possible sustainabletreatment option for the mixed vegetable waste stream. Without the plastic wrapping,vermicomposting and composting would normally be appropriate treatment options for processingmixed vegetable waste. However, the presence of the plastic wrappers would be expected toreduce the effectiveness of both processes and to present serious contamination problemsrelated to the removal of plastic, subsequent to treatment. The trial was undertaken to investigatetwo main issues.

Objectives

1. To determine the extent to which vermicomposting could be used to stabilise the organiccontent of the mixed vegetable stream

2. To determine if such a vermicomposting system could minimise or overcome the plasticcontamination problem by rendering the plastic amenable to collection after processing.

In order to help address these objectives, the performance of a vermicomposting system wascompared with a suitable windrow composting system and similar mixed vegetable waste wasused for each process.

Trial details

The trial commenced on 11 September 2003 and was terminated on 5 November 2003.

The mixed cucumber waste was delivered to site in 7.5 tonne loads. The waste contained (bymass) 95% whole cucumbers, 3% other vegetable matter and 2% plastic wrappings. Initialanalysis of the mixed waste delivered to site confirmed that the mix had a very high moisturecontent (around 97%). In terms of earthworm nutrition the mix had a moderate protein content ofapproximately 9% and a carbon to nitrogen ratio (C:N) of 30:1 and with most of the solid materialbeing readily amenable to decomposition. The mixed cucumber waste potato slurry would beconsidered to be moderately nutritious to earthworms and moderately putrescible making itamenable to both composting and vermicomposting. The most serious barrier to effectivestabilisation using biological methods was identified as the very high moisture content whichcould significantly inhibit microbial decomposition and reduce the effectiveness of bothcomposting and vermicomposting.


22/30

Preparation for composting and vermicomposting

Although the mixed, whole cucumber waste had a relatively soft texture and would have brokendown readily during biological processing, the plastic wrappings surrounding much of the wastewere intact (see Figure 13).

Figure 13: Waste feedstock mainly comprising cucumbers and wrappers

Two methods of pre-treating the waste prior to biological treatment were identified in order topartially disrupt the plastic wrappers. The two methods were shredding the waste to create aslurry and lightly crushing the material to maintain the structure of the vegetables.

Figure 14 shows the shredding process in operation. The waste was first passed through aspecially adapted shredder before being pumped into the storage tank prior to being used in thecomposting and vermicomposting trials. The resulting cucumber slurry in the storage tank wasfound to be moderately acidic (pH 4.5), highly polluting (BOD 8,000 mg/l) and rich in dissolvednutrients (electrical conductivity approximately 5 mS/cm). The bulk density of the slurry wasapproximately 1 kg per litre.

Another batch of cucumber waste was only lightly crushed by driving over the waste using a

tractor. This material was also used in the composting and vermicomposting trials. The bulkdensity of the lightly crushed material was approximately 0.53 kg per litre.


23/30

Figure 14: Shredding the cucumber-based waste

Windrow composting

Wood chips were used as the bulking agent for the cucumber waste composting trial. Anexperiment was carried out to determine the positive and negative aspects of using each type ofpre-treated waste for composting. Figure 15 shows the cucumber slurry being added to the woodchips. Slurry (2,000 litres) was pumped on to the chips (3.5 m

3) until saturation of the chips was

achieved. At the ratio of slurry to chips used in the experiment, a significant amount of slurry run-off occurred. Subsequent laboratory trials showed that at this ratio as much as 50% of slurry maybe lost as run-off. A much higher ratio of chips to slurry would appear to be necessary to avoidthe loss of potentially polluting slurried feedstock.


24/30

Figure 15: Pumping shredded waste onto the wood chip bulking agent

The wood chip/cucumber slurry mix was formed into a windrow approximately 4 m3

in volume.The temperature of the windrow was monitored for 10 days. No rise in temperature was observedindicating that composting was not taking place (see Figure 16).

Figure 16: Windrow formation

After ten days the windrow was dismantled and the lightly crushed cucumber (2.5 m3) was added

to with the wood chip/cucumber slurry mix. It can be seen from Figure 17 that the temperature in


25/30

the windrow increased to above 60 C confirming that composting had commenced. The windrowwas mechanically turned every seven days and composting process was monitored for 56 days intotal. Very little of the original cucumber feedstock remained in the windrow at the end of the eightweek composting process. The only evidence of the original material was a thin bio-film layer ofstabilised matter firmly adhering to the wood chip bulking agent.

Figure 17: Windrow temperatures for the first 8 weeks of composting

The plastic wrapping material, which formed 2% of the original waste also remained intimatelymixed within the bulking agent. In laboratory trials it was not possible to remove the plasticcontaminants from the bulking agent by screening and it is likely that the technique of airclassification would be the only way of removing the plastic from the bulking agent. This would bevery expensive and time consuming.

Vermicomposting

Vermicomposting of the cucumber slurry and the lightly crushed whole cucumber waste wascarried out in one vermicomposting bed (10 m

2) commencing 11 September 2003 as described in

Trial 1. The earthworm density was 4 kg m-1

of bed and the bed was maintained at a temperature

of 21 C. The cucumber slurry was vermicomposted first and this slurry was applied to the bed for

25 days. Application rates are given in Table 3. All new waste batches were applied only afterprevious applications had been processed. The lightly crushed cucumbers were then applied tothe bed for 30 days.

It can be seen from Table 3 that the vermicomposting rate for the cucumber slurry wasapproximately double that for the more solid lightly crushed cucumber. This may be due to anumber of factors such as greater surface area for the earthworms to act on but may also be dueto seepage of some liquid from the slurry directly through the bed.

0

10

20

30

40

50

60

70

Time (days)

Temperature(oC)

7 14 21 28 35 42 49 560


26/30

A particular feature of the vermicomposting operation was that both types of cucumber wastewere applied directly to the surface of the processing bed. This enabled the earthworms to enterthe plastic wrappers; consuming and processing the cucumber. This is shown in Figure 18. Theconsequence of this is that the plastic wrappers remain on the surface of the bed and can bereadily removed once processing has been completed. This makes removal of the plastic fromthe processing system a very simple and cost-effective operation compared with the compostingprocess, which required the use of the bulking agent and which trapped the plastic.

Figure 18: Earthworms processing cucumber waste while leaving plastic wrappers on bed surface

Comparison of vermicomposting with windrow composting

It is not easy to directly compare the two processes since vermicomposting is a continuous flowsystem and composting is a batch process. However, it has been noted that the vermicompostingsystem has the very positive advantage over windrow composting that physical contaminants arerestricted to the surface of the processing beds, making recovery relatively straightforward.

Previous research has established that both processes can be very similar in the times taken tostabilise waste materials (see Trial 2). One useful point of comparison that is increasinglyimportant is the area of land required to process specific amounts of waste. In this example, thevermicomposting process took around eight times more ground area to process the same amountof material compared with windrow composting and this is broadly in line with other studies.


27/30

Table 3: Waste application rates

Waste type Vermicompostingtime

(days)

Volume of wasteapplied

(litres)

Mass of wasteapplied

(kg)

Vermicompostingrate

(Kg m-1

d-1

)

Slurry 25 600 600 2.4

Lightly

Crushed

30 600 320 1.1

Conclusions

1. The vermicomposting process was found to be highly suitable for the treatment of cucumberwaste. It is recommended that the waste is first shredded to accelerate the processing rate.

2. Vermicomposting was found to be particularly appropriate for treating waste types containingphysical contaminants since these can be easily recovered from the surface of beds onceprocessing is complete.

3. Windrow composting appears to be a suitable treatment option for cucumber-based wastestreams. Shredding the waste prior to composting to help disrupt the plastic wrappings wouldnot be recommended in this case. Composting only slightly crushed wrapped cucumberwaste would be recommended using chipped wood as a bulking agent.


28/30


APPENDIX TWO

Table 1 Mean bed temperatures for trial periods 1-3 months and 4-6 months

Trial period Bed A

(C)

Bed B

(C)

Bed C

(C)

Bed D

(C)

Bed E

(C)

Ambient

(C)

Months 1-3 13 7 24 16 11 6

Months 4-6 16 16 23 21 15 15

Table 2 Amounts of potato sludge applied to beds maintained at different temperatures

Trial period Bed A (Lapplied)

Bed B (Lapplied)

Bed C (Lapplied)

Bed D (Lapplied)

Bed E (Lapplied)

Months 1-3 490 330 530 490 370

Months 4-6 560 560 560 560 560

Figure 1 Nitrous Oxide emissions

Figure 2 Methane emissions

0

2

4

6

8

10

12

0 5 10 15 20 25


CH4mgm-2hr-1

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25


N2O

mgm-2

hr-1


29/30


APPENDIX THREE

Figure 3 Nitrous Oxide emissions from vermicomposting and windrow composting

Figure 4 Methane emissions from vermicomposting and windrow composting

0

1

2

3

4

5

6

7

7 14 21 35 50 64 78 92

Time (days)

C

H4flux(mgm-2hr-1)

WindrowVermicom ostin

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

7 14 21 35 50 64 78 92

Time (days)

N2Of

lux(mgm-2hr-1)

Windrow

Vermicomposting


30/30

APPENDIX FOUR

Media Coverage

1. Urban Mines Newsletter

2. Composting News, Volume 7 Issue 3 Summer 2003

three vermicomposting trials

Documents