Organic Food Waste Treatment Development Project

Open University Urban Mines Worm Research Centre

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

January 2004

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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 in order to contribute to knowledge and understanding of composting systems which utilise earthworms as the main processing agent (vermicomposting). The programme aimed to build on previous research carried out at the Worm Research Centre (WRC) into vermicomposting.

The research programme summarised in this report was based on large-scale vermicomposting trials, which were undertaken at WRC and devised and supervised by the Open University. This programme took place over the period of almost 18 months, up until November 2003. During the first few months, equipment was put in place and the worm beds were stocked up so that the first technical work started in November 2002. A report detailing the 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 beds measured 1.5m x 6.6m, with a depth of 0.75m. Each bed was filled with composted horse manure/wood shavings bedding material, to a depth of 0.25m when settled. The waste used as 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. Liquid by products of the process were collected in a leachate drainage and collection system. The earthworm species employed during the trial was Dendrobaena veneta and populations sampled in November 2002 were found to 1.2 to 1.4 kgm^-2 of bed. The first waste application for 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 low temperatures within the range, the trial was commenced during the winter months so that ambient air temperatures would naturally lower bed temperatures. The heated cables within beds were individually thermostatically controlled and this enabled the higher temperatures to be achieved during the winter months. During the first three months it was possible to maintain the bed temperatures broadly within the required range. For the following three months, 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 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 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 the higher 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 higher and more uniform during this period and this resulted in greater and more uniform waste processing rates throughout.

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

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sludge was processed in 3 months per bed. This is equivalent to 0.62 kg of waste processed per 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 reduced processing 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 ambient temperatures is achievable in practice. The trial confirmed that processing rates at moderate temperatures are acceptable.

The static chamber method was used to monitor greenhouse gas emissions from the vermicomposting beds. This method is commonly used to measure gas fluxes from surface emissions 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 (see Appendix Two, Figures 1 and 2). It recommended that further research is urgently undertaken to determine the full extent of potential problems and to identify mitigation measures in order to minimise harmful emissions.

Comparison between vermicomposting and windrow composting of plastic wrapped vegetable waste

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

Although the mixed, whole cucumber waste had a relatively soft texture and would have broken down readily during biological processing, the plastic wrappings surrounding much of the waste were intact. Therefore two methods of pre-treating the waste prior to biological treatment 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 cucumber feedstock remained in the windrow at the end of the eight week composting process. The plastic wrapping material, which formed 2% of the original waste also remained intimately mixed within the bulking agent.

A particular feature of the vermicomposting operation was that both types of cucumber waste were applied directly to the surface of the processing bed. This enabled the earthworms to enter the plastic wrappers; consuming and processing the cucumber. The consequence of this was that the plastic wrappers remained on the surface of the bed and was readily removed once processing has been completed. This makes removal of the plastic from the processing 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 treatment of catering wastes using vermicomposting, without the wastes having first undergone a sanitisation 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 (6 cm particle size). Further processing requirements for those wastes containing meat include subjecting the waste to a further composting stage to be carried out at the temperatures given

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

It is important 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 periods of composting which are required to produce stable, composted products, 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 high temperatures in a closed reactor and meat-containing wastes must undergo further thermophilic composting to comply with the regulations. Hence, vermicomposting would not be considered to be a suitable technology for the treatment of meat-containing wastes, since it operates in the low temperature or mesophilic range. However, for non-meat containing wastes, low temperature processes such as vermicomposting can be used to satisfy the "18day storage" requirement. Indeed, the use of vermicomposting to accelerate the compost maturation process and to enhance the partially-composted material from the closed reactor stage would appear to be a very good option for some composting operations. For non-meat containing wastes, other composting systems such as open air mechanically turned windrow systems 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 source segregated household waste may offer many benefits but very little research has been carried out into this type 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 be applied directly to earthworm beds without killing the earthworm populations. Equally, although vermicomposting is known to accelerate the maturation process for some wastes, it is not known if maturation can be achieved more rapidly than other cost-effective processes, such as windrow composting systems.

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

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

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

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

Optimising key operating parameters for vermicomposting systems and developing a bed system

i. Bed system

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

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as a liquid fertiliser in controlled conditions. This collection is a requirement of current UK legislation, which forbids the direct drainage of such a material directly onto land, which worm beds 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 be easy and efficient for an operator to use. Due to the length of the beds, the covers are heavy and therefore a system that easily maneuvers these items is taking time to develop as different 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 tank conversion is reasonably simple. In terms of waste type applied to vermicomposting beds, it has been found that liquid sludge waste is easier to administer onto the beds by mixing it with shredded cardboard.

Others bodies involved in the project

Since the project is research based, there are few beneficiaries as of yet. The WRC has held a number of open days where representatives from local composting organisations have been shown the work which is taking place at the WRC. These groups will ultimately benefit from the overall research which will be shared at the end of the project. The WRC also give ongoing assistance to individuals and groups involved in composting by phone and through these 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 provided some third party funding for this project. The aim is to set up a link between the WRC and one of the young offenders institutions which Lattice work with. It is hoped that the young offenders will learn about worm farming and about environmental issues associated with waste. 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 their gardens. Discussions are ongoing regarding this.

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

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

Media Coverage

In April 2002, Radio Humberside, visited the Worm Research Centre as part of a visit by the Northern Recycling Group. Information about the Biffa funded work at WRC was broadcast on the news every half-hour. An interview with the Centre’s Director, Steve Ross-Smith, was broadcast on the 6pm slot.

In September 2002, UM published a newsletter, which included a piece about the project (see Appendix 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 to provide 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 at WRC, see Appendix Four.

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Prepared by Jim Frederickson Integrated Waste Systems Open University

December 2003

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Aims of the Research Programme

The main aim of the research programme was to devise and undertake appropriate trials in order to contribute to knowledge and understanding of composting systems which utilise earthworms as the main processing agent (vermicomposting). The programme aimed to build on previous research carried out at the Worm Research Centre (WRC) into vermicomposting. This work has been 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-scale vermicomposting trials, which were undertaken at the Worm Research Centre (WRC) and devised 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 using selected species of earthworms to compost difficult and contaminated waste types and looked at combined systems for possible enhanced performance. It investigated new methods and protocols for monitoring composting performance such as respirometry. Very importantly, it addressed the environmental impact of composting systems. A key objective was to build on previous research into the emission of greenhouse gases from vermicomposting which had been identified as potentially problematic.

A particular feature of previous research at WRC was an attempt to apply rigorous scientific method to the evaluation of both the process of vermicomposting and the products of vermicomposting. This approach was adopted throughout this programme and a considerable emphasis was placed on analysing physico-chemical characteristics of waste materials as they underwent vermicomposting and composting. The environmental impact of both vermicomposting and 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, this important research is unique to the work carried out at WRC in collaboration with the Open University.

Another unique feature of the work carried out in this programme was the use of respirometry techniques to measure the "stability" of waste (i.e. the degree of biological activity) during controlled vermicomposting and composting and this is a key parameter when assessing system performance. The respirometric method and its application to evaluating composting systems is described in the report.

While previous research at the Worm Research Centre focused mainly on vermicomposting, the research programme described in this summary report placed considerable emphasis on combining 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 appropriate composting systems to achieve specific waste management objectives and particular emphasis was placed on assessing the relative merits of different approaches to composting. It also aimed to provide some much needed dada on the environmental impact of large-scale vermicomposting, in particular greenhouse gas emissions.

TRIAL 1

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

Aims and background

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

This trial aims to investigate the effect of different temperature regimes on waste processing rates and greenhouse gas emissions. In particular, the trial was set up to determine the reason for the increased rates associated with higher temperatures; increased worm numbers or the higher temperatures per se which would have stimulated production of microbial biomass and increased decomposition 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 (and increased waste application rates) would be likely to increase the environmental impact of vermicomposting systems.

Methane (CH4) and nitrous oxide (N2O) are included in the 6 greenhouse gases listed in the Kyoto protocol that require emission reduction. To meet reduced emission targets governments need first to quantify their contribution to global warming. Composting has been identified as an important source of CH4 and N2O. With increasing divergence of biodegradable waste from landfill into the composting sector, it is important to quantify emissions of CH4 and N2O from all forms of composting. 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 of CO2. It’s atmospheric concentration has doubled in the past several hundred years to the present 1.7ppm 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-radiates 310 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 and these are shown in Figure 1. More details about this type of bed construction can be found at http://www.wormresearchcentre.co.uk/.

Each individual bed was 1.5 metres wide by 6.6 metres long and beds were approximately 0.75 metres deep. The block was 1.5m wide by 25m long and total block area was approximately 50 m². Each bed was filled with composted horse manure/wood shavings bedding material (approximately 0.25 m deep when settled) to contain the earthworm populations.

Figure 1: Bed layout

The block of beds were protected from rain by impermeable but well ventilated covers and the block was equipped with a leachate drainage and collection system. All beds were heated and accurately controlled temperatures for each bed were achieved using individual electric heating cables and thermostats located in the bedding material. Thermocouples and data loggers continuously 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 locally produced potato waste, which is a highly putrescible and very wet by-product of the food processing industry. Previous research at WRC established that the potato slurry used as feed for earthworms supported good levels of earthworm growth, was not toxic to earthworms and produced good quality vermicompost. The potato waste when delivered to site by tanker (around 20 tonnes per load) was often fresh from the factory as shown by its elevated temperature. The slurry contained a mixture of steamed potato flesh in homogenised form and fine potato skins. In brief, the chemical analysis of the potato waste when applied to the processing beds showed it to be very wet (around 90% moisture content), relatively acidic (pH 4 - 5), very rich in dissolved nutrients (electrical conductivity approximately 8 mS/cm). Nutritionally and in terms of waste processing the potato slurry would be considered to be highly nutritious and very putrescible, with the 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 solid material being readily amenable to decomposition (high organic matter content 88%). The bulk density was approximately 1 kg/litre.

The beds selected for the trial were chosen because they had been previously used for vermicomposting similar potato waste and hence the earthworm populations and microorganisms in the beds were well acclimatised to the environmental conditions. The earthworm species employed in the trial was Dendrobaena veneta. Earthworm populations were sampled in November 2002 and the initial biomass of earthworms in the five beds was found to be between 1.2 to 1.4 kg m^-2 of bed. The first waste application for this trial and the commencement of bed monitoring programme began on 17^th December 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 winter months so that ambient air temperatures would naturally lower bed temperatures. The heated cables within beds were individually thermostatically controlled and this enabled the higher temperatures to be achieved during the winter months. The project began in December 2002 and during the first three months it was possible to maintain the bed temperatures broadly within the required range. For the following three months, the bed temperatures tended to reflect ambient temperatures, as might be expected. Mean bed temperatures achieved during the six-month study are presented in Table 1.

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

Trial period	Bed A	Bed B	Bed C	Bed D	Bed E	Ambient
	(°C)	(°C)	(°C)	(°C)	(°C)	(°C)
Months	13	7	24	16	11	6
1-3
Months	16	16	23	21	15	15
4-6

Vermicomposting rates

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

Trial period	Bed A	Bed B	Bed C	Bed D	Bed E
	Litres	Litres	Litres	Litres	Litres
	applied	applied	applied	applied	applied
Months	490	330	530	490	370
1-3
Months	560	560	560	560	560
4-6

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 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 the higher 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 higher and more uniform during this period and this resulted in greater and more uniform waste processing rates throughout.

For the beds operating under higher ambient temperatures during spring, around 560 kg of sludge was processed in 3 months per bed. This is equivalent to 0.62 kg of waste processed per square 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 measure gas fluxes from surface emissions and has been validated in comparison to micrometeorological methods. Static chambers have been extensively employed to measure methane emission from rice paddies and composting. For this study cylinders of 0.0707m² cross sectional area and height of around 0.3m were pressed into the compost material to a depth of around 0.05m, after allowing time for gas evolved due to disturbance of the material to disperse, the cylinders were topped and sealed. The closed chamber (0.25m x 0.0707m² in 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 was immediately injected into an evacuated glass vial and labelled. The glass vials were then transported to the Open University and analysed for CH4 and N2O content on a gas chromatograph (Ai Cambridge GC94m) fitted with a flame ionisation detector for CH4 and an electron capture detector for N2O.

Figure 2: Static chamber method

Results

From Figure 3 it can be seen that methane (CH4) emissions from the beds were generally very low and this would be expected since methane is formed in anaerobic conditions (i.e. devoid of oxygen). Vermicomposting beds are required to be well aerated in order to provide high oxygen levels 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^-1m^-2 ) were observed.

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

-2

1.24 - 4.75 mg hr^-1m) 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 of nitrous oxide fluxes found during the project can be compared to other emission sources such as

-2

the range of fluxes reported for garden soil 0.0031 - 0.031 mg hr^-1m.

Clearly the issue of nitrous oxide emissions from vermicomposting is a potentially serious and, as yet, unrecognised problem. There is a pressing need to investigate the extent of the problem as soon as possible and to identify mitigation options, if appropriate. Research undertaken for this project, has identified vermicomposting as one of the most significant point sources of nitrous oxide emissions yet discovered. For example, riparian zones in the UK associated with intensive agriculture have been identified as the largest emitters of nitrous oxide to date, with levels of N2O – N of around 38 kg ha^-1yr^-2. Recent emission figures for N2O – N from vermicomposting have been found to many times greater than this. Although the total area of land devoted to vermicomposting operations would never be comparable to areas of riverbank in sensitive, agriculturally intensive locations, the first vermicomposting operation larger than 1ha in area has already been established.

It is recommended that further research is undertaken into the effect of temperature on N2O emissions and into ways of reducing emissions form these systems.

12

10

8

6

CH4 mg m-2 hr -1

4

N2O mg m-2 hr -1

2

0 0 5 10152025