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Vermicomposting of Pre-composted MixedFish/Shellfish and GreenWaste
July 2004
SR566
J. FredericksonOpen University
S. Ross-SmithThe Worm Research Centre
PROJECT PART-FINANCED BYTHE EUROPEAN UNION
THROUGH THE FINANCIALINSTRUMENTS FOR FISHERIES
GUIDANCE
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ISBN 0 903941 67 8
Working with the seafood industry to satisfy consumers, raisestandards, improve efficiency and secure a sustainable future.
The Sea Fish Industry Authority (Seafish) was established by theGovernment in 1981 and is a Non Departmental Public Body (NDPB).
Seafish activities are directed at the entire UK seafood industry includingthe catching, processing, retailing and catering sectors.
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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
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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 wastedirectly 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 andNephrops) andtwo 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 21
st
January. The waste was composted in the in-vessel system for atotal 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.
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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 processingbeds 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 wasDendrobaena veneta. The mixed fish/shellfish/greenwaste
was prepared for vermicomposting by saturating it with water until its maximummoisture 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 applicationto the vermicomposting bed
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Figure 2
Earthworms used for the trial
Figure 3Vermicomposting bed
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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 sampleswere removed for determination of pH.
9th 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 wasterminated 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
Matter
Loss on
Ignition
Organic
Carbon
Carbon
to
Nitrogenratio
pH Electrical
Conductivity
% % DM % DM S/cm
Fish/shellfish/GW 52.1 57.6 32.0 17:1 8.1 2040
Table 2
Waste Nitrogen
content(Kjeldahl)
NO3-N
(nitratecontent)
NH4-N
(ammoniumcontent)
(mg/kg DM) (% DM)
(mg/kg
DM)
(mg/kg
DM)
Fish/shellfish/GW 19000 1.90 Negligible 144
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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 35o
C, maximising carbondioxide (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.
Figure 4
Respiration rates for source segregated household waste;
in-vessel composted (7 days) followed by windrow composting or vermicomposting
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40 50 60 70 80 90 100
Time (days)
RespirationratemgCO2/hr/Kg
Vermicomposting
Windrow
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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(methane)
(mg m-2
hr-1
)
N2O
(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
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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
Matter
%
Loss on
Ignition
% DM
Organic
Carbon
% DM
Carbon
to
Nitrogen
ratio
pH Electrical
Conductvity
S/cm
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
content
(Kjeldahl)
NO3-N
(nitrate
content)
NH4-N
(ammonium
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
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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 forearthworms 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. Interms of the wastes 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 andprocessing 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 screenedvermicomposted waste was found to be under 10mm. The main feature of the
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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.
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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
Biochemistry29, 725-730. 0038-0717.
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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 vermicompostingsystems, 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 containingearthworms 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 earthworms 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 processingtemperature 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
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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 ascasts. 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 nitrogenlosses are typically much lower.
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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 hadbeen 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 CH4flux mgm
-2hr
-1Windrow
temperatureVermicomposting CH4
flux mgm-2
hr-1
Control CH4 fluxmgm
-2hr
-1
7 6.604 36.075 0.016 0.003
14 4.142 40.800 0.057 -0.01021 1.055 60.862 0.019 -0.00635 6.120 50.112 0.040 0.001
50 5.020 46.683 0.076 -0.00764 0.862 18.758 0.027 -0.00278 0.050 14.150 0.027 -0.01492 0.215 8.562 0.378 0.016
Measured static chamber N2O (nitrous oxide) flux from windrow, vermicomposting
and control.
Day Windrow N2Oflux mgm-2hr-1
Vermicomposting N2Oflux mgm-2hr-1
Control N2O fluxmgm-2hr-1
7 0.370 0.425 0.025
14 0.271 0.807 0.12021 0.006 0.117 -0.06835 0.029 0.627 0.03050 0.627 0.526 0.08064 0.030 0.541 0.03178 0.005 1.013 -0.012
92 0.025 1.457 0.071
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Appendix 3
Respirometer for measuring waste stability
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