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Final report

Use of food-based PAS110

digestate to grow energy

grasses on brownfield land as

an AD feedstock

Demonstration trial to evaluate the economics of using green PAS100

compost and food-based PAS110 certified digestate to grow energy

grasses on brownfield land as an AD feedstock

Project code: OIN005-005

Research date: July 2015-March 2017 Date: September 2017

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Written by: Anne Bhogal, Alison Rollett & John Williams

Front cover photography: Grass crop in July 2016 on the former landfill site at Llwyn Isaf, grown using food-based digestate

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WRAP - Use of food-based PAS110 digestate to grow energy grasses on

brownfield land as an AD feedstock 3

Executive summary

In recent years, Wales has seen a rapid expansion in the anaerobic digestion (AD) of food waste for bio-methane production leading to an estimated 87,500 tonnes of whole digestate being produced each year. Whilst agriculture is by far the largest market for digestate, alternative options would help the AD industry be more resilient to fluctuations in demand. Using digestate on brownfield land as a biofertiliser for energy crop production is potentially beneficial, as it provides a feedstock for AD (creating a ‘closed-loop’ system) without taking land out of agricultural production and provides an opportunity for the site owners to generate income. This report details the findings of a demonstration project undertaken on a former landfill site at Llwyn Isaf in Gwynedd, North Wales. The objective of the project was to establish whether it was financially and practically viable to use food-based digestate produced by the adjacent AD plant to grow energy grass as a feedstock for AD. Green compost was used to improve the soil prior to the establishment of a ‘high energy’ grass seed mixture in autumn 2015. The site was divided into two plots to compare the effect of food-based digestate against ammonium nitrate (AN) fertiliser application. The two plots were treated on three occasions during the subsequent growing season with food-based digestate or ammonium nitrate (AN) fertiliser, supplying c.270 kg/ha crop available N ahead of three grass silage cuts. Grass production from the three cut grass silage system was similar at 12 t/ha grass dry matter with a biochemical methane potential (BMP) of c.335 L.CH4/kg volatile solids (VS). There was no difference in grass yield or biochemical methane production between the different nitrogen sources (digestate or AN fertiliser). As a feedstock, the grass production was estimated to be worth c.£240/t VS or c.£34/t fresh grass. Cost-benefit analysis indicated that using digestate to produce grass feedstock on a brownfield site was financially viable, producing a net benefit of c. £2300/ha, approximately £400/ha higher than that achievable with AN fertiliser. The main benefit of using digestate was the saving in digestate haulage incurred by having the AD plant adjacent to the brownfield site. The cost benefit analysis assumed the operator had access to a silage clamp and the AD plant and brownfield site were co-located. If this was not the case, the benefit would be marginal (estimated at £500-£700/ha). A number of legislative, and practical barriers and limitations were identified, most notably, the need to change the Quality Protocol for Anaerobic Digestion to enable the approved use of whole digestate for energy crop production on brownfield sites. Without this, the site owner would have to demonstrate the benefit of this practice to the brownfield site and apply for a U11 exemption. Practical barriers, such as site, soil conditions and the presence of infrastructure, would make operations difficult and potentially more costly, but would not be insurmountable. Overall, it can be concluded that using digestate as a biofertiliser for energy crop production on brownfield sites, may provide a viable alternative use for food-based digestate, and a beneficial use of brownfield sites, particularly where the agricultural land bank is limited.

WRAP –Use of food-based PAS110 digestate to grow energy grasses on brownfield land as an AD feedstock 4

Contents

1.0 Introduction ................................................................................................. 5 2.0 Objectives ..................................................................................................... 5

2.1 Overall aim ................................................................................................ 5 2.2 Specific objectives ...................................................................................... 5

3.0 Methodology ................................................................................................. 5 3.1 Demonstration site ..................................................................................... 5 3.2 Topsoil improvement .................................................................................. 7 3.3 Grass establishment ................................................................................... 8 3.4 Treatment application ................................................................................. 8 3.5 Grass harvest & analysis ........................................................................... 10

4.0 Results ........................................................................................................ 11 4.1.1 Grass yields and nitrogen offtake .................................................... 11 4.1.2 Biogas potential ............................................................................. 12 4.1.3 Cost-benefit analysis ...................................................................... 13

5.0 Discussion................................................................................................... 15 5.1 Biochemical methane potential of the grass ................................................ 15 5.2 Financial and practical viability of growing energy grasses on brownfield sites as a feedstock for AD plants. ............................................................................... 16

6.0 Conclusions ................................................................................................ 17 Appendix 1: References........................................................................................ 17

Acknowledgements

The authors wish to thank Gwynedd Council for access and use of the landfill site at Llywn Isaf for the demonstration trial, Dave Herbert Biogen’s GwyriAD Plant Site Manager for supplying the food-based digestate, Aldwyn Clarke (ADAS) for managing the demonstration site at Llwyn Isaf and Matthew Smyth and Matt Taylor (Aqua Enviro) for the BMP testing and interpretation.


1.0 Introduction Welsh Government’s policy is to increase the quantity of food waste processed via anaerobic digestion (AD) to exceed the 220,000 tonnes of organic waste which was composted and digested in 2012. This has led to a rapid expansion in AD capability in Wales, from two commercial plants processing food waste in 2012, to nine plants in 2017 (www.biogas-info.co.uk). The most recent, became operation in April 2017 and is capable of processing 35,000 tonnes of food waste. It is estimated that current throughput of food waste in Wales is approximately 90,000 tonnes producing around 87,300 tonnes of whole digestate annually. Agriculture is by far the largest market for digestate in the UK, accounting for over 98% of digestate recycled in 2013 (WRAP, 2014). However, alternative options are required as suitable agricultural land is not always available close to AD plants, and hauling digestate long distances is not economically viable. Land restoration has been identified as a significant sector that could benefit from the use of digestate, by providing nutrients and organic matter for soil formation. The use of land reclamation sites to grow energy crops is particularly beneficial, by providing feedstock for energy production, without taking land out of agricultural production, as well as providing site owners an opportunity to generate income. Wales has a long industrial history, which has resulted in numerous brownfield sites that could be utilised for growing energy crops, not only as a feedstock for biomass burners (e.g. short rotation coppice, miscanthus etc.), but also for use in AD plants. The economic and sustainable production of feedstocks is likely to become increasingly important as the competition for land bank and other sources of feedstock increases. 2.0 Objectives 2.1 Overall aim

To evaluate if it is financially viable to grow energy grasses on brownfield land which can

then be used as an additional AD feedstock; and

To demonstrate best practice for energy production on brownfield land through a

demonstration trial.

2.2 Specific objectives

Use PAS100 certified compost to manufacture a soil on a former landfill site;

Establish a suitable grass crop on the manufactured soil;

Use PAS110 certified compliant whole digestate as a biofertiliser for the grass crop;

Harvest the grass during the growing season and determine the annual biogas yield;

Identify the cost of producing energy grasses as a feedstock to the AD plant;

Provide a full financial appraisal of the cost-benefits of growing energy grasses as a

feedstock to the AD plants; and

Produce a case study of the demonstration trial with details of both the financial and

practical viability of growing energy grasses on brownfield sites as a feedstock for AD

plants.

3.0 Methodology 3.1 Demonstration site The demonstration project was undertaken at the former landfill site at Llwyn Isaf in Gwynedd, North Wales. The site had a domed profile with only mineral material (i.e. a heavy

http://www.biogas-info.co.uk/

http://www.biogas-info.co.uk/


clay textured soil deficient in nitrogen, potassium and organic matter; Table 1) covering the landfill cap. Before the project was set up the vegetation was extensive grassland comprising a mixture of grasses, clover and rushes (Plate 1) that was grazed by sheep, as a means of maintaining the site, rather than for food (livestock) production. The demonstration site was comprised of two 12 x 12m plots (separated by a 2m ‘guard’ strip) located on slightly sloping land between four gas wells. Table 1 Baseline soil analysis results (July 2015)

Property Topsoil analysis result

Total Nitrogen (%) 0.11

Extractable Phosphorus (mg/l) 17 (ADAS Index 2)

Extractable Potassium (mg/l) 35 (ADAS Index 0)

Extractable Magnesium (mg/l) 64 (ADAS Index 2)

pH 7.3

Organic matter (%) 1.97

Total Zinc (mg/kg dm) 118

Total Copper (mg/kg dm) 42.9

Total Nickel (mg/kg dm) 32.0

Total Cadmium (mg/kg dm) 0.45

Total Lead (mg/kg dm) 37.0

Total Chromium (mg/kg dm) 42.0

Total Mercury (mg/kg dm) 0.05

Bulk density (g/cm3) 0.62

% = percentage; mg/l = milligrams per litre; mg/kg dm = milligrams per kilogram dry matter

Plate 1. Location of the demonstration trial on the former landfill site at Llywn Isaf (prior to soil improvement)


3.2 Topsoil improvement The vegetation from an area 26 m by 12 m was removed by cutting and spraying (glyphosate) in August 2015. Green compost, certified by the Compost Certification Scheme (CCS) to PAS100 specification (BSI, 2011), was sourced from a local supplier and spread at a rate equivalent to c.100 t/ha dry solids (186 t/ha fresh weight-FW; calculated assuming a dry matter content of 54%, Table 2). The compost was subsequently incorporated to a depth of c.30cm using a mini-digger (Plate 2). Based on the analysis in Table 2 and an application rate of 186 t/ha (FW), the compost supplied c.430 kg/ha of total N (of which c.13 kg/ha was readily available), c.325 kg/ha of phosphate and c.280 kg/ha of potash and c.14 t/ha of organic matter. Heavy rainfall in late summer/early autumn 2015 combined with poor drainage made site access difficult and delayed the compost application and incorporation until late September/early October. Table 2 Compost analysis results (at spreading, September 2015)

Property Green compost analysis result

Dry matter (%) 54

Total Nitrogen (kg/t fw) 2.3

Readily Available N (kg/t fw) 0.07

Total Phosphate (P2O5; kg/t fw) 1.76

Total Potash (K2O; kg/t fw) 1.50

Total Magnesium (MgO; kg/t fw) 10.0

Total Sulphur (SO3; kg/t fw) 1.29

pH 7.7

Electrical Conductivity (us/cm) 694

Loss on Ignition (LOI; % dm) 14.9

Organic carbon (% dm) 8.1

Stability (mg/CO2/gVS/day) 2.2

Total Neutralising Value (TNV; % dm as CaO) 6.2

Total Zinc (mg/kg dm) 133

Total Copper (mg/kg dm) 39.6

Total Nickel (mg/kg dm) 17.1

Total Cadmium (mg/kg dm) 0.21

Total Lead (mg/kg dm) 34.9

Total Chromium (mg/kg dm) 40.4

Total Mercury (mg/kg dm) 0.07

E.coli (cfu/g) 60

% = percent; kg/t fw = kilograms per tonne fresh weight; % dm = percent dry matter; mg CO2/g VS/day = milligrams of carbon dioxide per gram of volatile solids per day; kg dm = milligrams per

kilogram dry matter; cfu/g = colony forming units per gram;


Plate 2. Topsoil improvement using green compost at LLywn Isaf in autumn 2015 (a) compost delivery; (b) compost spread; (c) incorporation

using a mini digger; (d) site after incorporation

3.3 Grass establishment Following incorporation of the compost, the site was rotovated and a grass seed mix comprising equal parts of Aberniche festulolium, Gemini tetraploid Italian ryegrass, Talladega tall fescue & Amba cocksfoot, was hand-sown at a rate equivalent to c.35 kg/ha (Plate 3). This mix of grasses was designed to create a short-term (up to 4 years), high yielding grassland that produced grass with a high energy value that was suitable for anaerobic digestion.

Plate 3. Grass establishment at Llwyn Isaf; a) sowing the grass see mix; b) establishment after 1 month (November 2015).

3.4 Treatment application The improved area was split into two demonstration plots (each 12 x 12m in size), to allow for two treatments to be applied:


Food based digestate was applied on three occasions during the growing season ahead of

three grass cuts.

Ammonium nitrate fertiliser was applied on three occasions ahead of three grass cuts at

rates to match the crop available N supplied by the digestate

The ammonium nitrate fertiliser treatment represented typical farm practice for grass silage production and was included as a comparison to digestate as an N source. As the application of whole food-based digestate to brownfield land is not a permitted ‘designated market’ within the Quality Protocol for the use of PAS110 certified digestate (WRAP/EA, 2014), it was necessary to obtain an environmental permit or trial exemption. An application (trial request) to Natural Resources Wales (NRW) was made in August 2015, and although this was initially refused (due to the lack of innovation), a local decision was made to approve the project in accordance with the U11 waste exemption (spreading waste to benefit non-agricultural land; EA, 2014). RB209 7th Edition (MAFF, 2000a) was used to determine the nitrogen requirement for the grass, assuming the site had a low soil N supply (SNS). This recommended a total of 150 kg N/ha for the first grass cut (split applied, with 40 kg/ha early and 110 kg/ha applied at least six weeks ahead of cutting), 110 kg N/ha for the second cut and 80 kg N/ha for the third cut (340 kg/ha over the whole season). As yield potential was limited by climatic and soil conditions the recommended N rates were reduced to 100 kg/ha for first cut and 85 kg/ha for subsequent cuts (270 kg/ha over the whole season). Food-based digestate was supplied by the Biogen AD plant at Llwyn Isaf, located immediately adjacent to the demonstration site. A sample of the digestate was analysed prior to application (Table 3), and the application rate needed to meet the required crop available N supply was calculated using MANNER-NPK (Nicholson et al., 2013). The rates of digestate application before each cut, the amount of crop available N supplied by the applications and the rate of ammonium nitrate fertiliser applied to the fertiliser demonstration plot are given in Table 4. Table 3 Composition of the food-based digestate

Property Food-based digestate analysis result

Dry matter (%) 2.12

Total Nitrogen (kg/t fw) 4.30

Readily Available N (kg/t fw) 4.18

Total Phosphate (P2O5; kg/t fw) 0.63

Total Potash (K2O; kg/t fw) 2.64

Total Magnesium (MgO; kg/t fw) 0.02

Total Sulphur (SO3; kg/t fw) 0.44

pH 8.5

% = percent; kg/t fw = kilograms per tonnes fresh weight A topsoil sample taken prior to treatment application (February 2016) was analysed for pH (7.3), extractable phosphorus-P (16 mg/l – Index 2), potassium-K (87 mg/l – Index 1) and magnesium-Mg (78 mg/l – Index 2). In order to ensure nutrient supply was not limiting, the following fertiliser applications based on RB209 recommendations were made to both plots


before first cut: 40 kg/ha phosphate (P2O5), 50 kg/ha potash (K2O) and 40 kg/ha sulphur (SO3). A further 40 kg/ha of potash and sulphur (as sulphate of potash) was also applied to both plots following each grass cut. In practice, the digestate would supply sufficient potash and some of the post-cutting sulphur requirement (Table 3), so an operator could potentially reduce/remove these applications altogether and save on costs. Table 4 Application rates and dates of digestate and fertiliser

Activity First cut

(21/6/16) Second cut (26/7/16)

Third cut (6/9/16)

Total

Application date 9/5/16 21/6/16 26/7/16

Food-based digestate:

Application rate (m3/ha) 35 30 30 95

Total N loading (kg N/ha) 150 129 129 408

RAN (kg N/ha)1 146 125 125 396

Crop available N (kg N/ha)2 100 87 87 274

Ammonium nitrate (kg N/ha) 100 85 85 270

1RAN = Readily available N 2Crop available N estimated using MANNER-NPK

3.5 Grass harvest & analysis Before the treatments were applied a 0.5m2 area was cut from each plot to give an estimate of the grass growth over winter and before any fertiliser or digestate was applied in order to understand how the grass responded to the subsequent digestate and ammonium nitrate applications. The grass was subsequently cut using a mower on three occasions during the season (June, July and September), and the total dry matter yield determined. Samples were taken and analysed for total N content and biochemical methane potential (BMP). BMP estimates the amount of methane that could be produced from anaerobically digesting organic waste at mesophilic temperatures, and was undertaken by Aqua Enviro. The method involves taking a known quantity of the grass and adding it to a seed inoculum which is then anaerobically digested for 28 days. The quantity and quality of the biogas produced during the 28 day period is quantified. Results are expressed in terms of litres of methane produced per kilogramme of volatile solids added. Following each cut, the grass was raked off the plots and removed from the site (Plate 4).


Plate 4. Grass growth immediately prior to the second cut (a) and following cutting (b)

4.0 Results 4.1.1 Grass yields and nitrogen offtake Grass growth prior to fertiliser and digestate application was estimated at 1.2 t/ha and 1.7 t/ha (dry matter – dm) on the ammonium nitrate (AN) and digestate (AD) plots respectively. By first cut in June, 42 days following treatment application, yields had increased to 7.0 and 5.6 t/ha dm on the AN and AD plots, respectively (Table 5). Grass yields were lower at the second (35 days following the second treatment application) and third cuts (42 days following the third treatment application). Dry matter yields were similar on the two treatments at these subsequent cuts, with slightly higher yields (0.2-0.8 t/ha difference) measured on the AD treatment. As a result, the total yield for the season was almost identical on each treatment at c.12.5 t/ha dm (Table 5). These yields are comparable to those achievable on agricultural soils i.e. an average of 10-11 t/ha cut grass on deep soils in high rainfall areas (Anon, 2010). Table 5 Grass yield and N offtake at each cut

Treatment Fresh yield

(t/ha) DM (%)

Dry yield

(t/ha)

Total N

(%)

N offtake

(kg/ha)

First cut (21 June 2016)

Ammonium Nitrate 30.8 22.8 7.0 1.16 81.7

Food-based Digestate 30.4 18.3 5.6 1.49 82.8

Second cut (26 July 2016)



Third cut (6 September 2016)



Total or average for all cuts*

Ammonium Nitrate 71.3 16.8 12.7 1.58 183

Food-based Digestate 86.4 14.1 12.3 1.74 209

*Total fresh and dry yield and N offtake; Average %N.

As this was a demonstration project, without any replication of treatments, it was not possible to determine whether the differences in yield between the treatments at each cut were statistically significant, or whether they were just due to underlying site variability.


The total N content of the grass was lowest at the first cut, reflecting the higher yields at this cut which caused a ‘dilution’ of the grass N content. The total amount of N removed by the grass was 183 kg/ha where AN had been applied and marginally higher at 209 kg/ha following the digestate applications. Taking into account N uptake prior to the treatment applications, the amount of N recovered by the grass was similar on both treatments at 62% of the N applied on the AN treatment and 69% of the crop available N supplied with the digestate (Table 6).

Table 6 Nitrogen recovery

Treatment Total N applied

(kg/ha) Crop available

N* (kg/ha) Dry yield

(t/ha) N offake**

(kg/ha) N recovery

(%)**

AN 270 270 11.52 168.6 62.4

AD 409 274 10.63 188.7 68.9

*Actual amount of N applied with AN; MANNER-NPK prediction of crop N availability – note Manner

predicted 87kg/ha for the 2nd and 3rd applications, but only 85 kg/ha AN was applied.

**Assumes N offtake of 14.4 kg/ha and 20.4 kg/ha from AN and AD plots prior to treatment

application (quadrat yields of 1.2 and 1.7 t/ha, respectively and estimated N content of 1.2%)

4.1.2 Biogas potential The biochemical methane potential (BMP) of the grass harvested at each cut was expressed as litres of methane produced per kg of volatile solids (Table 7; equivalent to m3/t). The BMP was lowest at the first cut (@ c.240 m3/t), but similar at the second and third cuts (@ c.380 m3/t). This was related to the total N content of the grass (which was c.1.3% at first cut and 1.8% at the subsequent cuts; Table 5), with higher BMP associated with a higher total N (protein) content of the grass. There was very little difference in the BMP (and volatile solids) of grass grown with AD compared to AN. Table 7 Biochemical methane potential (BMP) and volatile solids (VS) of the grass at each cut

Treatment VS (%) BMP (L.CH4/kg.VS)

First cut

Ammonium Nitrate 92.5 214

Food-based Digestate 93.0 267

Second cut



Third cut



Average of all cuts



Using the average BMP across all cuts, the energy value (£/t VS) was determined assuming the grass was used to power a >500kW CHP unit whose engine was 38% efficient and 95% available (Table 8). On average across both treatments and for the whole season (3 cuts), the potential revenue achievable from using the grass as a feedstock for anaerobic digestion was £2,734/ha or £238/t volatile solids (Table 8). This was equivalent to c. £34/t fresh grass (assuming a dry matter of 15.5% and volatile solids of 92%).

Table 8 Energy value of the grass


Treatment

Dry

yield (t/ha)

VS

(%)

VS

(t/ha)

BMP

(L CH4/kg VS)

Methane Yield

(m3/ha)

Energy value

after CHP (mWh)1

Energy value (£)2

/ha /t VS

AN 12.7 91.7 11.7 330 3,861 13.9 2,744 235

AD 12.3 91.7 11.3 340 3,842 13.8 2,724 241

Average 12.5 91.7 11.5 335 3,852 13.8 2,734 238 1Assumes the energy content of methane is 9.97kWh/Nm3 (Banks, 2009) and a CHP unit that is 38% efficient and 95% available (Aqua Enviro, pers comm.) i.e. the potential energy production will be 3.6

kWh/m3 from this CHP unit. 2Assumes a generation tariff of 6.33p/kWh plus and export tariff of 4.91p/kWh under the Feed in

Tariff (FIT) scheme (i.e. 11.24p/kWh or £112.40/MWh; Ofgem, 2016)

4.1.3 Cost-benefit analysis A simple cost-benefit of the demonstration project is presented in Table 9 in order to assess the economic viability of growing energy grass on brownfield land as a feedstock for anaerobic digestion. The analysis assumed compost was required to improve the soil prior to both the AN and digestate fertiliser options as part of the land fill restoration process. The Good practice guide for use of PAS100 compost in landscape & regeneration indicates a cost of £2,420/ha for cost of sourcing, hauling and spreading 128 t/ha compost (10%) to a depth of 25cm (2007 prices); these costs have not been included within the cost-benefit analysis. The underlying assumption was that the AD plant was immediately adjacent to the brownfield site where the energy grass was being grown, so there would be a saving on digestate haulage to its existing market. As most operators rarely look to move digestate to markets located more than a 10-20 mile radius away from the AD plant (WRAP, 2013), a distance of 10 miles was assumed for the haulage costs. If the bio-methane production and digestate spreading operations were not co-located, the cost of hauling the digestate for application and grass back to the AD plant would need to be considered in the economic assessment. Haulage costs were estimated at £380 for the digestate (three trips x 30m3) and £240 for grass (two journeys per cut for a tractor, trailer & man @ c.£40/hour; NAAC 2016). The costs also assume the AD plant has a silage clamp (with the cost of clamping included in the grass harvesting operational costs). This is not the case at Llwyn Isaf. Although fresh grass could be fed directly into the digester, its value as a feedstock would be lower and more inconsistent, and there would be potential storage problems. Given the size of the landfill at Llwyn Isaf, only a small silage clamp would be required. For example, 200 tonnes fresh grass (achievable from an area c.3-5ha in size) would need a silage clamp with a capacity of c.270m3 (equating to a floor area of c.100m2, walling 3m high and a 5500litre effluent tank), and would cost c. £15k to build or c.£22k if a roof was required (C. Bentley, pers. comm). The capital repayment costs for a clamp of this size amortised over 20 years at an interest rate of 5.25% would add another £1,200/yr to the economic assessment. An alternative would be to make big bale silage after harvesting the grass, although the plastic wrapping would need to be removed prior to digestion, and the cost of baling would also be in the region of £1,200/ha (assuming baling and wrapping cost £7.50/bale, annual grass yield was 80t/ha fresh material and a bale contained c.0.5t grass; NAAC, 2016 & Table 5). Based on these assumptions, there would be no difference between the AN and digestate treatments in the cost of establishing the grass, which given the short-term nature of the grass species used (chosen specifically for their energy value) would be incurred every 4-5 years in order to maintain productivity. Annual costs would also be similar for both the treatments, even though the cost of spreading digestate would be greater than the cost of purchasing and spreading AN, because of the potential saving on potash and sulphur fertiliser costs where digestate was used as the N source.


Harvesting costs were identical for both the AN and digestate treatments, and the yield and energy value of the grass was no different. The economic analysis demonstrates that it is financially viable to grow energy grass on a brownfield site as a feedstock for AD (assuming the site has access to a silage clamp). The over-riding benefit of using digestate rather than ammonium nitrate as the N fertiliser source is the saving in digestate haulage cost by diverting it away from its existing market (assuming this was within a 10 mile radius of the AD plant). The cost of building a silage clamp (c.£1,200/yr for a small silage clamp suitable for a 3-5ha site), or making big bale silage (up to £1,200/ha, depending on the yield and dry matter), would almost halve the return achievable. Moreover, if the biomethane production and digestate application operations were not co-located, additional haulage costs (c.£620/ha) would make the use of digestate to produce grass AD feedstock marginal (net benefit of £500-£700/ha). It should be noted that the AD plant at Llywn Isaf is already at maximum capacity in terms of feedstock, and would not be able to take the grass silage if the surrounding landfill was converted to this land-use. Table 9 Cost benefit analysis1

Scenario AN grass2 AD grass3 Notes

Establishment Costs (£/ha):

Seed costs 123 123 Special Short Term AD mixture @35kg/ha; The productivity of this mixture would potentially drop after 2 years, with re-seeding required after 4 years

Base fertiliser for grass establishment

59 59

Same for both systems; Base cost for grass establishment (40, 50, 40 kg/ha P2O5, K2O, SO3) https://dairy.ahdb.org.uk/resources-library/market-information/farm-expenses/fertiliser-prices; TSP = £272/t (60p/kg P2O5); Potash = £231/t (38p/kg K2O /kg); Sulphur (10p/kg SO3); Spreading = £11.81/ha for 100kg/ha (NAAC contractor costs)

Drilling 23 23 Costs would be the same for all options; NAAC 2016/17 contractor costs for broadcasting grass seed = £22.66/ha

Total establishment costs

205 205 These would be incurred every 4-5 years if productivity is to be maintained (reseeding)

Annual costs (£/ha):

Fertiliser following

each cut 72 20

40 kg/ha of Potash and Sulphur after each cut. The operator could reduce this where digestate was applied as 30m3/ha digestate would supply sufficient potash (@2.64kg/t K2O) and S applications could be reduced to 30 kg/ha/cut (0.44 kg/t SO3). See above for fertiliser prices and spreading costs. P2O5 requirement would need to be assessed by soil analysis going forward

Digestate haulage N/A N/A Assume AD plant on site: digestate could be directly pumped either by umbilical to the surrounding landfill or into the tanker used for spreading.

Digestate application N/A 228 NAAC 2016/17 £2.40/m3 slurry spreading (broadcast)

Cost of AN 147 N/A https://dairy.ahdb.org.uk/resources-library/market-

information/farm-expenses/fertiliser-prices; £188.5/t for AN (or 54.5p/kg N) 2017

Spreading AN 32 N/A NAAC 2016/17 fertiliser distribution by spinner £11.81/ha per 100kg/ha


Grass harvesting &

carting 361 361

NAAC 2016/17 forage harvesting including carting (3 trailers) and clamp = £130.42 for first cut and £115.40 for subsequent cuts. Assume AD plant is on site, so carting is local, no baling required, as will go into a clamp before digestion.

Total Annual Costs 612 609

BENEFIT (£/ha)

Energy value of grass 2,737 2,737 Measured as part of the study; £238/tVS; 11.5 t/ha VS from 3 cuts

Digestate diverted N/A 380

Assume existing market for digestate is within 10 miles; Whole digestate haulage cost estimated at £3-4/m3 for 10 mile delivery (WRAP, 2013); 95m3 total used in a year.

Total Benefit 2,737 3,117

Cost/Benefit (£/ha)

Year 1 1,920 2,303

Includes grass establishment costs (excluding the compost application as assumes no requirement after establishment); Note these would be incurred every 4 years as reseeding would be required to maintain the high yielding sward.

Year 2 2,125 2,508 1Assumes the AD plant and brownfield site are at the same location, with access to a silage clamp; 2AN Grass: Ammonium nitrate - 270 kg/ha N (100, 85, 85 kg/ha before each cut); 3AD Grass: Food-based digestate - 95 m3/ha digestate (35, 30, 30 m3/ha)

5.0 Discussion 5.1 Biochemical methane potential of the grass The BMP values of a range of feedstocks are given in Table 10. Assuming the grass feedstocks in Table 10 had a 92% volatile solids content and a methane concentration (from a full scale plant) of 50%, this would provide methane yields (BMP m3/t VS) of: 296, 231 and 265 for meadow grass green, grass silage and clover hay first cut, respectively. The energy value (BMP) of the grass grown at Llywn Isaaf ranged between 214-423 m3/tVS (mean: 330-340m3/t), and was therefore higher than these similar feedstocks. This is most likely due to the choice of grass species used, with the seed mix specifically designed to produce a grass sward with a high yield and energy potential. Table 10 Biochemical methane potential of difference feedstocks

Feedstock % Dry

solids

Biogas yield (m3 /tonne

dry solids)1

Methane yield (m3 /tonne

volatile solids)

Source data

Sewage sludge 2.0 - 6.2

160-320

Aqua Enviro database

Thermally hydrolysed

sewage sludge 7.5 – 11.8 270-405

Source separated food

waste 10.3 – 19.8 262-614

Sugar beet pulp 30.8 405

Potato waste 34.5 255

Meadow grass green 18 544

Andersons Calculator2

Grass silage 40 425

Maize silage 33 621

Barley straw 86 363

Clover hay first cut 86 487

FYM 40 113

Poultry excrement 15 375

Glycerine 100 846


Bakery wastes 87.7 742 1Raw data provided in terms of yield on a fresh weight basis, converted here to a dry solids basis 2Andersons Calculator (http://www.biogas-info.co.uk/about/feedstocks/)

5.2 Financial and practical viability of growing energy grasses on brownfield sites as a feedstock for AD plants.

The cost benefit analysis clearly demonstrated that co-locating an AD plant adjacent to a brownfield site and using the digestate produced as a biofertiliser for growing energy grasses as a feedstock for the AD process, is financially viable, particularly if the operator has access to a silage clamp. Moreover, it is more cost-effective than using manufactured fertilisers to grow the grass, as the digestate supplies not only nitrogen, but also all of the potash and some of the sulphur required by the grass crop, and there are significant savings to be made from not having to pay for haulage of the digestate to alternative, more distant markets. If the two operations are not co-located, the model is still financially viable up to a radius of up to 20 miles (beyond which the cost of hauling digestate to the brownfield site and carting the grass off site, estimated at £620/ha, is likely to be prohibitive). In this case, using manufactured fertilisers as the N source would become more cost-effective. However, if the AD plant does not have a silage clamp, then the additional cost of either building a clamp or making big bale silage (estimated at £1200/ha), would need to be taken into account. There are also a number of practical and legislative barriers that could not only make this model difficult to achieve in practice, but also prevent it occurring altogether:

Quality protocol AD: Grass silage is a permitted feedstock for AD, but may not necessarily be on the AD operator’s environmental permit that applies to the anaerobic digestion of the waste. This would be relatively simple to rectify. However, more significant is the fact that brownfield sites are not currently a ‘designated market’ for the use of whole food-based digestate (separated fibre digestate may be used in land restoration). The Quality Protocol would need revising to reflect this additional market, or the site operator would be required to apply for a U11 exemption and demonstrate that the spreading of digestate to the brownfield site would confer benefit. However, the U11 exemption currently restricts the application of digestate to a maximum of 50 m3/ha in any one season (the case study applied the equivalent of 95 m3/ha, but as the plots were 0.014ha in size, the total volume of digestate applied during the year was only 1.3m3). A maximum rate of 50m3/ha would only supply about half of the nitrogen and potash required by the grass, so the operator would need to apply additional manufactured fertiliser to achieve optimum grass yields, and only 50m3/ha of digestate could be diverted away from other agricultural markets. The net effect would be to reduce the cost-benefit by about £160/ha, as a result of additional haulage costs associated with spreading the digestate offsite.

Code of good agricultural practice (COGAP): Although brownfield sites are not subject to COGAP guidelines, COGAP recommends a field application rate of not more than 250 kg/ha total N in the form of organic materials. If the agricultural field is within a Nitrate Vulnerable Zone, this is mandatory with additional restrictions over the timing of applications. The demonstration study described here applied c.400 kg/ha total N in the form of digestate, of which only 270 kg/ha was considered to be crop-available (taking into account losses following application). In order to keep within the 250 kg/ha total N recommendation that applies to agricultural situations, only c.60m3/ha digestate could be applied. This would therefore have a similar effect on the cost-benefit as described above.

Practical viability: The logistics of applying organic materials to brownfield sites are more complicated than to agricultural land because of the quality of on-site material (e.g. poor quality stoney material, often containing large boulders, heavy textured soil susceptible to waterlogging), shallow nature of the material, poor drainage and the


presence of other infrastructure (e.g. gas heads on a landfill site). Waterlogging in particular will limit site access and therefore the timing of operations, potentially leading to restricted yields (e.g. if fertiliser applications were delayed or missed entirely, or grass cuts were delayed leading to deterioration of the grass crop). These issues are not insurmountable and their effect will partly be dependent on the weather and site conditions. At a minimum they are likely to make applications more time consuming which will lead to increased application costs. The costs for spreading fertiliser and digestate as well as harvesting the grass given in the analysis above were based on data from the National Association of Agricultural Contractors (NAAC, 2016) and consequently biased towards agricultural situations. However, even if they were doubled, the model would still be financially viable.

Environmental impact: As a result of the high readily available N content of digestate it is important that it is applied to a growing crop, or incorporated into the soil to make best use of the nitrogen supplied, limit ammonia emissions to air and prevent runoff to surface water systems.

6.0 Conclusions This study has clearly demonstrated that co-locating an AD plant adjacent to a brownfield site, such as a landfill, and using the digestate as a biofertiliser for growing energy grasses as a feedstock for the AD process, is financially viable, particularly if the operator has access to a silage clamp. The operation would be encouraged if the Quality Protocol for anaerobic digestion was revised to allow the use of whole digestate as a biofertiliser for energy crop production on such sites. Failure to revise the QP will require operators to demonstrate a benefit to the land under a U11 exemption. This would limit the amount of digestate that could be diverted to brownfield land (to 50m3/ha). There are also a number of practical limitations that would make operations difficult, but not insurmountable, although these could potentially reduce the overall return from the system. Moreover, if the biomethane production and land application of digestate could not be co-located or the operator did not have access to a silage clamp, the financial viability of the operation is reduced. Where the agricultural land bank for digestate is limited (either due to competition from other organic materials, or, as is often the case where food waste is the feedstock, AD plants are not located near agricultural land), use of brownfield sites for energy grass production using digestate as a biofertiliser may become the most appropriate practice.

Appendix 1: References

BSI (2011) PAS100:2011 Specification for composted materials EA (2010) The quality protocol for the production and use of quality compost from source segregated biodegradable waste. The Environment Agency, July 2010. www.environment-

agency.gov.uk/business/topics/waste/114395.aspx

EA (2014) Waste exemption: U11 spreading waste to benefit non-agricultural land https://www.gov.uk/guidance/waste-exemption-u11-spreading-waste-to-benefit-non-agricultural-land

https://www.gov.uk/guidance/waste-exemption-u11-spreading-waste-to-benefit-non-agricultural-land

https://www.gov.uk/guidance/waste-exemption-u11-spreading-waste-to-benefit-non-agricultural-land


NAAC (2016) NAAC contracting charges guide 2016/17. www.naac.co.uk/userfiles/files/Contracting%20charges%202016-17%20V2.pdf WRAP/EA (2014) Quality Protocol Anaerobic digestate. End of waste criteria for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste

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