Worm Research Centre 3


Vermicomposting of Pre-composted Mixed Fish/Shellfish and Green Waste

July 2004

SR566

J. FredericksonOpen University

S. Ross-SmithThe Worm Research Centre

PROJECT PART-FINANCED BY THE EUROPEAN UNION THROUGH THE FINANCIAL INSTRUMENTS FOR FISHERIES GUIDANCE

Working with the seafood industry to satisfy consumers, raise standards, improve efficiency and secure a sustainable future.

The Sea Fish Industry Authority (Seafish) was established by the Government in 1981 and is a Non Departmental Public Body (NDPB).

Seafish activities are directed at the entire UK seafood industry including the catching, processing, retailing and catering sectors.

ISBN 0 903941 67 8

Vermicomposting of pre-composted mixed fish/shellfish and greenwaste

Prepared by:

Jim Frederickson Open University (j.frederickson@open.ac.uk) and Steven Ross-Smith The Worm Research Centre Ltd Phoenix Farm Asselby Howden (srsphoenix@lineone.net)

Date submitted:

26/7/04

Executive summary

Partially composted mixed fish/shellfish and green waste was further vermicomposted on a large-scale for seven weeks. The composition of the pre-composted waste was such that it had elevated levels of pH and electrical conductivity. In the large-scale vermicomposting system, the earthworms appeared to be reluctant to enter the waste directly for the first three weeks but then actively processed the waste. However, laboratory experiments confirmed that the earthworms would have been capable of processing the waste during this period and increasing their biomass significantly as a result. It is likely that the earthworms were ingesting and processing the waste during the first three weeks while remaining in the bedding material. Greenhouse gas emissions were found to be low, probably due to the pre-composting phase reducing carbon and nitrogen contents. The vermicompost which was derived from the process showed the high nitrate concentration which is characteristic of typical “vermicomposts”. On the basis of this and other characteristics, it would appear that the vermicompost would be capable of being used as a basis for the formulation of high value composts.

Waste composition and in-vessel composting

The partially composted material used in the vermicomposting trial was derived from in-vessel composting of shellfish and fish with green waste.

Four different types of shellfish were used (crab, whelks, mussels and Nephrops) and two types of fish: oily fish (mackerel) and mixed whitefish (cod, haddock etc). Shellfish waste comprised shell and flesh waste – not just shell on its own. Mackerel waste was derived from every part of the fish but the fillet. Whitefish waste was largely fish frames, with possibly some fish heads as well.

1.106 tonnes of shellfish (comprising the four types in approximate equal parts) were mixed with 1.404 tonnes of mackerel and 1.156 tonnes of whitefish (total of 3.666 tonnes of mixed fish and shellfish). This mix was then combined with about 10 tonnes of green waste and loaded into three composting chambers. The material sent to the Worm Research Centre was a combination of material from all three chambers.

The mixed fish and shellfish composting trial started on 12th December and was completed on 21st January. The waste was composted in the in-vessel system for a total of 40 days.

Details of waste preparation and vermicomposting process

Vermicomposting is the use of selected species of earthworms to help decompose and transform organic wastes into stable and useful compost. In vermicomposting systems, it is the earthworms that fragment, mix and help aerate the waste. This is compared with traditional composting where the compost piles (known as compost windrows) are mixed and aerated mechanically. See Appendix 1 for more details about vermicomposting.

The mixed fish/shellfish waste was stored at the Worm Research Centre for approximately four weeks prior to commencement of the vermicomposting trial. On arrival, the waste emitted a very strong odour which was characteristic of ammonia gas. This odour was found to reduce gradually over 7 days until no detectable pungent odour remained. During the vermicomposting process, monitoring of odour took place every 3-4 days, however, this proved to be unnecessary as odour from the processing beds was negligible.

The vermicomposting bed (area 10 metre2) was prepared for use on 13th February 2004. Bedding material was moist, composted wood shavings and coir with a large mesh wire screen placed on top (area 6 metre2) to keep the waste separated from the bedding. Temperature probes and data loggers were installed and the bed heating thermostat controlling electrically heated cables was set at 15 oC.

The earthworm density in the bed was determined as 3kg per metre2 of bed. Earthworm species was Dendrobaena veneta. The mixed fish/shellfish/greenwaste was prepared for vermicomposting by saturating it with water until its maximum moisture holding capacity was reached and a small amount of leachate was produced. The waste was weighed as shown in Figure 1 and then manually placed on the vermicomposting bed. The weight of the saturated waste placed on the bed was approximately 1 tonne and this was placed directly on the wire mesh to a depth of approximately 0.3 m. Figure 2 is a sample of the earthworm inoculum used for the trial. Figure 3 shows the location of the waste relative to the bed and the bed was then covered by an impervious membrane to exclude rain (shown retracted). Samples of the waste were sent to the Open University for chemical analysis. The vermicomposting trial commenced on 16th February 2004.

Figure 1 Weighing the waste prior to application to the vermicomposting bed Figure 2 Earthworms used for the trial

Figure 3 Vermicomposting bed

Progress of the trial:

16th February to 8th March 2004 The earthworms made no significant movement from the bedding material into the waste during the first three weeks. However, there was clear evidence that they were ingesting the bottom layer of waste while remaining in the bedding. Waste samples were removed for determination of pH.

th March to 15th March 2004 The waste showed signs of drying and water was applied to the bed. The first significant movement of earthworms into the waste was recorded (9th March). Waste samples were removed for determination of pH.

16th March to 6th April 2004 During this period a high density of earthworms were detected throughout the waste. Sampling the surface of the waste for the emission of greenhouse gases was undertaken. Waste samples were removed for determination of pH. The trial was terminated on 7th April and final samples taken for chemical analysis.

Characteristics of the mixed fish/shellfish and greenwaste feedstock as applied to the vermicomposting bed

Table 1

Waste type Dry Loss on Organic Carbon pH Electrical
Matter Ignition Carbon to Conductivity
Nitrogen
ratio
% % DM % DM ÁS/cm
Fish/shellfish/GW 52.1 57.6 32.0 17:1 8.1 2040
Table 2
Waste Nitrogen NO3-N NH4-N
content (nitrate (ammonium
(Kjeldahl) content) content)
(mg/kg (mg/kg
(mg/kg DM) (% DM) DM) DM)

Fish/shellfish/GW 19000 1.90 Negligible 144

A respirometer (see Appendix 3) was used to determine the microbiological activity of partially composted fish/shellfish/greenwaste material, which is an indicator of compost stability. Stability is defined as the degree of decomposition or maturity of the composting material. The system employed in this study had 3 chambers each holding 4kg of material. The compost moisture was amended to the optimum 60% prior to being analysed. The operating temperature was 35oC, maximising carbon dioxide (CO2) production by providing conditions favourable to most of the microbial population.

The respiration rate for the waste which was applied to the vermicomposting bed was found to be 336 mgCO2 /hour/kg waste. It can be seen from Figure 4 for a comparable waste (taken from Hobson A.M., Frederickson J. and Dise N. B. 2004) that the waste supplied for vermicomposting in this case had a respiration rate which was relatively low suggesting that the waste had been stabilised during in-vessel composting. However, while the waste was relatively stable prior to vermicomposting, it can also be seen from Figure 4 that further maturing of the waste was clearly required to lower the respiration rate to levels typical of mature composts.

Time (days)

Figure 4 Respiration rates for source segregated household waste; in-vessel composted (7 days) followed by windrow composting or vermicomposting

Environmental impact of vermicomposting process

Leachate:

The processing bed was covered during vermicomposting to exclude rain and no leachate was detected.

Greenhouse gas emissions (methane and nitrous oxide):

Methane and nitrous oxide emissions were monitored once during vermicomposting using the static chamber method. A full account of the method can be found in Hobson A.M., Frederickson J. and Dise N. B., (2004). Table 3 shows the gas fluxes that were detected from the vermicomposting of the fish/shellfish and green waste. These were relatively low and are comparable to fluxes found for similar waste types such as mixed green waste and source segregated household waste (see Appendix 2 for these data).

Table 3

Waste type Rate CH4 N2O
(methane) (mg m-2 hr -1) (nitrous oxide) (mg m-2 hr -1)
Fish/shellfish
and green waste mean rate 0.04 0.69
peak rate 0.08 1.46

Table 4

Date pH of waste

16/2/04 8.1 24/2/04 8.1 10/3/04 8.0 17/3/04 7.7 26/3/04 7.6 07/4/04 7.4

Characteristics of mixed fish/shellfish and greenwaste vermicompost

Table 5 Characteristics of the Fish/shellfish/GW vermicompost (screened to under 10mm) compared with typical composts

Compost Dry Loss on Organic Carbon pH Electrical
Matter Ignition Carbon to Conductvity
% % DM % DM Nitrogen ÁS/cm
ratio
Fish/shellfish/GW
Vermicompost 46.1 42.5 23.6 12:1 7.2 1442
Typical green
waste compost 76.1 17.7 10.3 15:1 8.0 600
Typical
vermicompost 24.7 72.7 40.4 31:1 4.9 741

Table 6 Characteristics of the Fish/shellfish/GW vermicompost (screened to under 10mm) compared with typical composts

Compost Nitrogen NO3-N NH4-N content (nitrate (ammonium (Kjeldahl) content) content)

(mg/kg DM) % DM (mg/kg DM)(mg/kg DM)

Fish/shellfish/GW
vermicompost 19900 1.99 4820 Negligible
Typical green
waste compost 6824 0.68 9 Negligible
Typical
vermicompost 13300 1.3 5300 Negligible

Laboratory studies

Laboratory studies were conducted on earthworms when fed partially composted fish/shellfish/greenwaste material to determine:

  1. earthworm mortality
  2. earthworm growth rates over time

Five pots (0.5l) each containing coir bedding and five adult earthworms (mean individual biomass 1.1g) were fed approximately 50g of the fish/shellfish/greenwaste material. No unfed control was used since earthworms are known to fail to gain weight on coir alone and quickly die. After 22 days the total earthworm biomass had increased by 30%. One earthworm died during the experiment. The earthworms gained weight at the rate of 8mg per worm per day and this rate is typical for earthworms fed on partially composted material (Frederickson, J., Butt, K.R. Morris,

R. M. & Daniel, C. 1997).

Observations from the trial

The fish/shellfish/greenwaste material that was supplied for vermicomposting had been previously composted in an in-vessel system for 40 days. Respirometry evaluation showed the material to have been well stabilised by the composting process but also confirmed that the material needed to be further matured before it could be considered to be an acceptable compost for high specification use. In this project the material was subjected to further maturation using vermicomposting. In terms of the waste’s suitability for vermicomposting, a number of points are worth noting. Firstly, Frederickson, J., Butt, K.R. Morris, R. M. & Daniel, C. 1997 reported that earthworms grew and reproduced better in fresh waste compared with pre-composted waste and that the degree of pre-composting affected the long term sustainability of the system. Pre-composting waste normally has the effect of greatly reducing the carbon content and nutrient value of the waste for subsequent worm composting. From Tables 1 and 2 it can be seen that the waste carbon and nitrogen contents were relatively low prior to vermicomposting and it is also likely that these compounds would have been in stable and humified forms. Hence, it would be expected that the waste would experience very little further mass losses as a result of worm composting.

Also from Tables 1 and 2, it can be seen that both the pH and electrical conductivity of the pre-composted waste are very high and these characteristics are known to have a negative effect on earthworms. It is likely that the high levels of these parameters would have deterred the earthworms from entering the waste during the early stages of worm composting and observation of the bed confirmed this. However, in the laboratory experiments the worms gained 30% in weight during the first three weeks when placed in the same waste, suggesting that the worms were capable of ingesting and processing the waste despite the apparently hostile environmental conditions to which they were subjected. It is likely that the earthworms were ingesting and processing the waste during the first three weeks while remaining in the bedding material. Table 4 shows how the waste pH dropped over time probably due to conversion of ammonia to nitrate, making the waste less hostile to the earthworms.

Table 3 showed that greenhouse gas emissions were not significant during vermicomposting due to the pre-composting stage reducing both carbon and nitrogen contents of the waste.

The earthworms were observed to be actively processing the waste after 3 weeks and after approximately seven weeks vermicomposting, the project was terminated and the final material was screened to 10mm. Approximately 60% m/m of the screened vermicomposted waste was found to be under 10mm. The main feature of the vermicompost was the high nitrate content as shown in Table 6. This is a key indicator of maturity and on this basis the vermicompost would appear to be suitable for high specification plant growth applications when blended with other materials.

References

Hobson A.M., Frederickson J. and Dise N. B., (2004). Emission of CH4 and N2O from composting: comparing mechanically turned windrow and vermicomposting systems. In proceedings: Treatment of biodegradable and residual waste. Harrogate, UK. ISBN 0-9544708-1-8.

Frederickson, J., Butt, K.R. Morris, R. M. & Daniel, C. (1997) Combining vermiculture with traditional green waste composting systems. Soil Biology and Biochemistry 29, 725-730. 0038-0717.

Appendix 1

Vermicomposting and traditional composting

Vermicomposting is the use of selected species of earthworms to help decompose and transform organic wastes into stable and useful compost. In vermicomposting systems, it is the earthworms that fragment, mix and help aerate the waste. This is compared with traditional composting where the compost piles (known as compost windrows) are mixed and aerated mechanically. There are many different methods of vermicomposting, making it impossible to present a definitive guide to best practice and systems will vary depending on whether the aim is to produce vermicompost or earthworms, or both.

While vermicomposting and composting both involve the aerobic decomposition of organic matter by microorganisms, there are important differences in the way the two processes are carried out. The most notable being that vermicomposting is carried out at relatively low temperatures (under 25C), compared with composting, where pile temperatures can exceed 70C. The intention with traditional composting is to stack waste material in sufficiently large piles so that the heat produced in the intense breakdown of organic matter is retained in the compost pile. This temperature increase stimulates the proliferation of heat loving (thermophilic) microorganisms and it is mainly these that are responsible for the decomposition. With vermicomposting it is vitally important to keep the temperature below 35C, otherwise the earthworms will be killed. It is the joint action between earthworms and the aerobic microorganisms that thrive in these lower temperatures (mesophilic) that breaks down the waste. Hence it is common with vermicomposting systems to apply waste frequently in thin layers, a few centimetres thick, to beds or boxes containing earthworms in order to prevent overheating and to help keep the waste aerobic.

It is difficult to directly compare composting with vermicomposting in terms of the time taken to produce stable and mature compost products. With vermicomposting, particles of waste spend only a few hours inside the earthworm’s gut and most of the decomposition is actually carried out by microorganisms either before or after passing through the earthworm. Hence, earthworms accelerate waste decomposition rather than being the direct agent. With windrow composting it usually takes at least six to twelve weeks to produce a stable compost and research suggests that vermicomposting takes around the same time. However, processing rates will crucially depend on many factors such as the system being used, the processing temperature and other factors, the nature of the wastes and the ratio of earthworms to waste.

One advantage that vermicomposting has over composting is that a net excess of earthworms can be produced and these may be harvested for a variety of purposes. It should be noted that it can take many months to build up a large working population of earthworms capable of vermicomposting significant quantities of waste. Vermicomposting does have one significant disadvantage and this is to do with the destruction of human and plant pathogens that can be present in some wastes. Destruction of most pathogens is more easily achieved in windrow composting due to the high operating temperatures and the intense microbial reactions taking place. Although the destruction of human pathogens has also been shown to be possible with vermicomposting, elimination of pathogens requires very effective management of the vermicomposting process. It is often recommended that wastes, such as sewage sludge, which are known to contain human pathogens, are either pre-composted before vermicomposting or else the resulting casts should be sterilized before use.

Vermicompost is the matured, processed material that is egested from earthworms as casts. As earthworms feed on the rich diet of organic matter and micro-organisms in waste, this ingested material is finely ground by the earthworms gut. This helps micro-organisms decompose the organic matter and stimulates mineralisation of complex compounds into simple nutrients, easily utilised by plants. At the same time the organic matter and microbial cells are glued together by the secretions from the earthworms gut forming casts. The amount of time that the waste spends in the earthworm gut is only a few hours and therefore the egested cast material is very microbially active and continues to decompose for some time. Once matured, the casts are known as vermicompost and this can have excellent physical and chemical characteristics. Compared with windrow composts, vermicomposts are likely to contain higher levels of nitrogen because vermicomposting temperatures and nitrogen losses are typically much lower.

Appendix 2

Greenhouse gas emissions

The data below for mixed green waste and source segregated household waste has been taken from Hobson A.M., Frederickson J. and Dise N. B., (2004). The waste had been subjected to in-vessel composting for the first seven days before being composted and vermicomposted.

Measured static chamber CH4 (methane) flux from windrow, vermicomposting and control.

Day Windrow CH4 flux mgm-2hr-1 Windrow temperature Vermicomposting CH4 flux mgm-2hr-1 Control CH4 flux mgm-2hr-1
7 6.604 36.075 0.016 0.003
14 4.142 40.800 0.057 -0.010
21 1.055 60.862 0.019 -0.006
35 6.120 50.112 0.040 0.001
50 5.020 46.683 0.076 -0.007
64 0.862 18.758 0.027 -0.002
78 0.050 14.150 0.027 -0.014
92 0.215 8.562 0.378 0.016

Measured static chamber N2O (nitrous oxide) flux from windrow, vermicomposting and control.

Day Windrow N2O flux mgm-2hr-1 Vermicomposting N2O flux mgm-2hr-1 Control N2O flux mgm-2hr-1
7 0.370 0.425 0.025
14 0.271 0.807 0.120
21 0.006 0.117 -0.068
35 0.029 0.627 0.030
50 0.627 0.526 0.080
64 0.030 0.541 0.031
78 0.005 1.013 -0.012
92 0.025 1.457 0.071

Appendix 3 Respirometer for measuring waste stability

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