BioCycle April 2006, Vol. 47, No. 4, p. 70
British trials determine safety and practicality of seafood by-product composting, plus how biological treatment relates to marketability of finished material.
Michaela Archer and David Baldwin

RECENT BIOSECURITY and animal feed issues have contributed to regulations regarding animal by-products that are now formalized in The Animal By-Product Regulations 2003 (ABPR2003). At the same time, the United Kingdom government has resorted to regulation and economic instruments such as The Landfill Regulations and Landfill Tax to promote techniques for waste recovery and recycling. Estimates show that seafood waste disposal or use is a significant issue across the UK.
The ABPR2003 brings significant new controls to the seafood industry. Seafood processing waste is regulated as Category 3 animal by-products and as such, may be utilized in preparation of fertilizer or soil conditioning material subject to being composted or anaerobically digested.
A review of past research identified fishing regions in areas of North America, Newfoundland, and British Columbia as having investigated composting of seafood waste. Being rich in timber production and processing, their methods generally entailed use of timber processing residues (sawdust and woodchips) as a bulking and amendment material for open windrow composting using passively ventilated windrow or small “in-bin” systems.
Other research has used the “Gore composting system” where actively ventilated windrows were individually covered with “Gortex” textile sheeting for rainwater protection and controlled air exhaust. The trial work is well documented and provides much data regarding analyses, temperature regime and material stability. These, plus later trials using a small-scale reactor type composting vessel, successfully processed mussels and salmon mixed with dust from wood-board manufacture.
It was concluded that composting of seafood processing waste can be a viable option and that different methods are available depending upon the size of the business and amount of seafood waste generated. This research project was designed to fill the gap between windrow composting and reactor type processing by enabling high temperature composting of a range of mixes of seafood waste with household green waste (i.e. grass clippings, hedge trimmings and garden vegetable residues) in each case followed by bioassay and plant growing trials on the resultant composted material.
The seafood composting project was devised to determine the safety and practicality of seafood by-product composting, while also identifying the potential marketability of the compost itself. Specific objectives were: To determine the effectiveness of biological treatment for the proper stabilization and pathogen kill for specific types of seafood waste; To evaluate different feedstocks (each mixed with typical green waste) including demersal fish only, pelagic fish only, shellfish, mussels and whelks; and To determine the safety and marketability of all the resultant material.
METHODS FOR COMPOSTING COMPARED
The seafood waste composting trials were designed to enable comparisons between different seafood waste types mixed with shredded green waste materials. Four different types of shellfish were used (crab, whelks, mussels and Nephrops) and two types of fish: pelagic (mackerel) and mixed demersal (cod, haddock, etc).
Facilities were established where six enclosed composting chambers were used concurrently to compost batches of approximately four to five metric tons of mixed seafood and green waste material in each chamber.
Each chamber was 2.4m x 2.4m x 2.4m dimensions, built of timber beams and sheets with integral insulation material to provide walls and roof panels of 150mm thickness.The roof panels and fronts were removable to enable entry for filling and emptying. At the base of each chamber was an air plenum, comprising a suspended floor with a slatted cover. Vertical air ducts together with a ventilation fan arrangement provided either recirculated air or ambient air to the plenum. The plenum facilitated air distribution to the material within the chamber. Fans were controlled by a system of simple time switches and thermostatic controls. Any displaced air from the system was exhausted to a water wash (scrubber) and wood/bark biofilter. Multiple temperature probes connected to a data logger recorded the process temperatures at set intervals.
Following preliminary trials and general procedural assessment, four seafood and green waste mixtures were composted. These included whitefish (demersal) only, oily (pelagic) fish only, mixed shellfish only and mixed fish/shellfish. Each composting trial was operated for approximately six weeks, followed by a growing trial using tomato and barley plants grown in pots in a glasshouse to assess the effect of the finished composts produced on plant growth. The growing trials were run as a multiple trial after the completion of all of the phases of composting.
FEEDSTOCK MIXTURE RATIOS AND PROCESS
The whitefish and mackerel trials comprised three replicates each of whitefish mixed with green waste; and mackerel mixed with green waste. A mixture ratio of the order of 1:3 by fresh weight was planned, i.e. 1.3 metric tons of fish waste to 3.9 metric tons of green waste in each instance. In practice, there was a small variation in the ratios on account of a shortfall in the green waste delivered and a surplus of melted ice-water weight in the whitefish.
In the mixed shellfish and overall mixed seafood based composting trials, 1.1 metric tons of shellfish (comprising the four types in approximate equal parts) were mixed with 1.4 metric tons of mackerel and 1.2 metric tons of whitefish (total of 3.7 metric tons of seafood). This mix was then combined with about 10 metric tons of green waste, thoroughly mixed and loaded into three of the composting chambers. Similarly a shellfish mixture was thoroughly mixed with another 10 metric tons of green waste and loaded to the other three composting chambers. This gave approximately 1.2 metric tons of shellfish mixed with 3.4 metric tons of green waste in each chamber.
The feedstock mixes were prepared and loaded to the various composting chambers. The regime for oxygen control was set to maintain oxygen levels in the range of 14 percent to 18 percent with higher levels being avoided to preclude over cooling. Some minor ventilation was allowed to occur via the air input, induced by the suction effect of the wet-scrubber/biofilter exhaust fan. The trial was continued until the process cooled down and the material became more stable. The temperatures were recorded by using 4 probes per chamber, in a high/low and front/rear array.
The mixes of seafood waste to green waste were characterized by analytical methods of the ingredients and carbon to nitrogen ratio (C:N ratio), carbohydrate composition, nitrogen availability, potentially toxic elements (PTO) and trace elements inclusion. Gross energy and respiration indices were included. The PTO included cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) in metallic form as well as their salts and oxides.
RESULTS OF GROWING TRIALS
The aim of the growing trials was to simulate the effect of using the composted seafood materials as if they were being used in agriculture as a soil conditioner and fertilizer type product. Seafood waste composts produced were added to the plant pots in amounts, based on pot surface area, to give 100, 200, 300 and 400 kg/ha of total compost nitrogen. The compost was incorporated evenly into the full volume of soil in the pot. There were also three control pots with no compost added.
For each compost treatment, nitrogen rate and control, separate pots were planted or sown with three tomato seedlings or 25 barley seeds. Tomatoes were used as they are more sensitive to phytotoxicity and are known to reveal symptoms of nutrient shortage or other adverse conditions more clearly. Barley was selected as being representative of a crop that would be most likely to receive this type of compost in agricultural practice. The pots with barley seed were covered with plastic sheet during seed germination and emergence.
During the growing trial period, plants were kept in a carefully controlled regime of temperature (18°C), lighting and watering. Watering was done via capillary matting. Growth was then observed in a heated glasshouse over 56 days. Measurements were made at various stages of plant growth and “plant vigour” was assessed by using a “scoring system” that included plant bushiness, color and growth as criteria. Records of the actual plant numbers established were compared with the 25 seeds sown as well as being compared with the control. Plant vigor results are restricted to comments in this paper. At the termination of the growing trial (56 days) the barley and the tomatoes were “harvested” by trimming the plants off at ground level. The plants were then weighed to give their fresh weight, inclusive of moisture content.
The pH’s ranged from 4.9 for mussels to 7.9 for Nephrops. The dry matters reflected the influence of the “wetter” (low dry matter) white fish compared to the mackerel but in both treatments the mixes provided ideal starting moisture levels. The heavy metals i.e. potentially toxic element values (PTO) in the seafood were in most cases found to be very low in relation to PAS100 standards, whereas the green waste showed PTO were present at a level of around 20 percent of the threshold.
All the seafood types had high nitrogen contents, with Nephrops, crab, mussels and mackerel being similar and white fish containing at least twice as much as the others (dry matter basis). The C:N ratios were quite low in all cases (lowest being white fish C:N at 3:1) and the green waste was relatively low, being less than the ideal level for composting. The green waste was comparatively high in bulk density (weight per unit volume) and wetter than typical green waste due to the inclusion of shredded leaves.
The C:N ratios directly relate to the higher percentage of nitrogen in the white fish and the higher percentage of carbon in the mackerel. The potassium content is generally linked to the green waste, whereas the phosphorous is linked to the bone and frame material in the fish, predominantly in the whitefish. The ash content reflected the presence of inert material in the green waste. The carbon content reflected the fact that mackerel has a high organic carbon content.
In regard to microbiological analyses, the materials were sub-sampled and the seafood waste was found clear of the main indicator pathogens Escherichia coli, Clostridium perfringens and Salmonella spp. By comparison, the green waste contained relatively high levels of each of these with the exception of Salmonella spp. These levels served as indicators within the trials and enabled some evaluation of pathogen destruction to be made in relation to time and temperature.
Each temperature is the mean of 12 results (i.e. for the three treatment replicate chambers and the four measurement positions for each chamber). The lower initial values on the shellfish and mixed seafood profiles reflect the lower ambient temperatures at the start of each composting trial.
In each case, the temperatures quickly rose to levels in excess of 65°C for all probe locations in the chambers, with the exception of one replicate of the treatment for whitefish based feedstock where the effect of cold air entry suppressed temperature rise. After the third day, all the temperature sensors had achieved similar levels >65°C.
Given that the average temperatures ranged mainly between 65-70°C for several days at the start of these trials, effective pathogen destruction in all treatments and replicates was to be expected. However, at day two, the mean temperature of the whitefish material was slightly suppressed owing to the material at the rear of one chamber being significantly cooler than the rest. The effect of this temperature variation was borne out in the pathogen sampling, which showed pathogens present in the part of the whitefish chamber containing the temperature deficiency.
Regarding analysis of composts, all treatments followed almost identical trends with pH and dry matter values steadily rising during the composting process toward a terminal pH of ~9 (except mackerel) and dry matter value of ~ 65% (except whitefish which was wetter). The electrical conductivity values are consistent with typical composted green waste values and are the primary reason why these materials are not suitable for use as growing media. The PTE levels in the composts were well within the acceptable ranges of the standard.
At the end of composting, samples taken for microbiological analysis from all positions and treatments showed less than 10 cfu/g (colony forming units per gram) for Clostridium perfringens, Enterobacteriaceae and Escherichia coli. Salmonella spp were not detectable in 25 grams.
ANALYSIS OF GROWING TRIALS
In nearly all cases, treated pots performed better than the control pots. In all but one instance, the overall establishment percentage was over 90 percent. Plant vigor was variable. For the mackerel composted materials, it was evident that the oil content produced a slight inhibitive effect, characterized by mould growth at the soil surface. For ease of comparison, the data is presented by referencing the weight of plants for each treatment in a series at termination against the control within that series – i.e. the control termination weight was taken as one, and all other plant weights were indexed to that weight.
For the tomatoes treated with mixed seafood or shellfish compost supplying over 180 kg/ha of total compost nitrogen, plant fresh weight was increased by about 50 percent compared to the control with no compost. The composts containing shellfish materials appeared to perform better than the whitefish or mackerel compost which may have been due to variable total and available (ammoniacal) nitrogen available in the whitefish compost and oil related phytotoxicity in the mackerel compost.
It should be noted that the whitefish actual nitrogen levels were significantly lower than the target levels due to variation between the pre-screened compost which was sampled for analysis and the screened compost actually used in the growing trial. The change in nitrogen levels was due to the combined effect of aging and screening. This also means that the curve produced is extrapolated further forward than ideal. The mackerel compost gives no increase in growth at low compost total nitrogen application rates and only small increases at higher rates. This was linked with the reduced growth rate in the tomatoes during the final two weeks of the trial for the 100 and 200kg/ha treatments and also poorer early (28 day) vigor in the 400kg/ha treatment.
For the barley, with mixed seafood compost supplying over 180 kg/ha of total compost nitrogen, plant fresh weight was increased by about 50 percent compared to the control with no compost. The shellfish compost performed less well than mixed seafood compost for barley. For barley, as with tomatoes, the whitefish compost gave lower yield increases than mixed seafood or shellfish composts but similar provisos about the amount of compost nitrogen apply. The mackerel compost reduced yield at the lower application rates for barley.
REVIEW OF COMPOSTING TRIALS
Generally, the results of the composting trials have served to confirm and provide added confidence with regard to the sanitization of cocomposted seafood waste and green waste within recognized time and temperature levels. The temperature curves followed typical patterns once the system was operational, and the trial served to demonstrate that repeatable results could be obtained without undue sophistication. The project provided several items of useful procedural detail that can be added to the existing information pool and benefit of the composting industry. This will be equally useful for the composting of green waste as for seafood waste but will be most useful in the context of composting of high nitrogen materials.
Such procedural information, generated from this work confirmed that a regime of attaining 70°C for the first four days (or of one hour for all the material according to EU guidance) followed by controlled temperature composting at the optimum 55°C would be ideal. The material should be conditioned and matured using forced air (low volume) ventilation to maintain aerobic conditions through the cooling phase. Use of a fully enclosed system in order to establish verifiable and uniform temperature control does not mean that process management can be reduced and appears to be essential.
Initiating the composting process using high moisture content material is clearly beneficial, and the policy of restricting exhausted air in order to retain moisture within the system was found to be very important. Premature drying out is a problem as it leads to early cessation of composting and a dusty, potentially hazardous end product. Adding water late in the composting process is not ideal due to the cooling effect and risk of increased leachate generation. Leachate recirculation during the later stages is not recommended for similar reasons, plus the risk of reinoculation with pathogens.
Barley responded less than tomatoes to the higher rates of compost nitrogen application. This probably reflects tomatoes having a higher nitrogen requirement. Compost application improved barley establishment. This can probably be attributed to the increased organic matter providing better moisture retention in the surface of the soil and therefore better germination. Poorer vigour, with the higher rates of application of the mackerel compost was noted for the tomato crop. It was apparent that there was some inhibition from the mackerel based compost materials, and this may have been due to the combined effect of the oil content and the relative immaturity of the compost. The presence of blue mold growth on the soil surface of the mackerel based compost pots was a possible visual indicator of some phytotoxic effect.
Overall, the results of the growing trial have shown that the composts produced from a blend of green waste with seafood waste can have beneficial effects on plant growth. This was under controlled glasshouse conditions, and it is expected that the benefits will be confirmed by carrying out field trials.
CONCLUSIONS BASED ON COMPOST TRIALS
The work has shown that all the seafood waste types were high in nitrogen and that mixes of feedstocks that contain seafood in excess of a ratio 1:3 (seafood to green waste), are liable to experience problems during composting and will produce high nitrogen compost materials that are not suitable for general use. The trials have shown that composting at temperatures of around 70°C for four days will provide the necessary levels of pathogen destruction in seafood and green waste based mixed feedstock.
This work has also indicated that a period of around six weeks in an “in-vessel” format composter should lead to stabilized material at exit, but that foreshortening the process is likely to leave the material in an active state that may give rise to uncontrolled emissions and ill-defined product.
The green waste contained higher levels of contaminants compared to the seafood waste. Some types of shell waste were found to contain slightly elevated levels of heavy metals but, after composting with green waste, the composted products were well within the acceptable range for agricultural usage. The work has shown that subject to Good Agricultural Practice limits for organic manure application, there are unlikely to be adverse affects and some benefits from using seafood waste/green waste based compost. Confirmation from field scale trials is required.
Some issues have been raised in regard to the use of mackerel based compost. Circumstantial evidence would suggest that this is related to the high oil content and the possible phytotoxic mold growth.
It is evident that composting need not entail expensive infrastructure and that management of the process is more important than the structure or container within which the process is carried out. The key management aspects are: Feedstock preparation; Adequate moisture content (and avoidance of in-process drying); Ventilation control (recirculation with minimal fresh air aspiration controlled by oxygen concentration measurement); Ease of loading/unloading; Air exhaust control; and Adequate temperature monitoring and logging through the process.
Michaela Archer is with the Seafish Industry Authority at the St. Andrews Dock in Hull, England. Her e-mail is m_archer@seafish.co.uk. David Baldwin is with ADAS Environment in Wolverhampton, England. His e-mail is adas@environment.co.uk.
COMPOSTING WHELK WASTE IN A CONTINUOUS FLOW OPERATION
IN 2005, the Sea Fish Industry Authory commissioned a project in northwest England to evaluate commercial composting for cooked whelk waste. AM Seafoods provided the feedstock comprising flesh and shell, while TEG Environmental did the composting at its commercial site near Preston.
The TEG silo-cage is a continuous-flow operation, where each silo holds 20 metric tons of compostable material at a time and receives approximately two metric tons of a mixed feedstock each day. Feedstock is delivered by an overhead feeder system, with each layer staying in the silo for about 14 days. The bottom is unloaded onto a conveyor and taken away for maturation.
Initially, one metric ton of whelk waste was mixed with 1.8 metric tons of amendment (i.e. shredded green waste, spent grain, broiler litter and recycled compost.) After mixing, 1.7 metric tons of material was loaded into a silo cage each day over a six week period. Additional green waste was added to achieve a 70°C temperature.
Regarding temperature, composted by-products must reach 70°C for at least one hour. Mixed material “struggled to reach this temperature” as the green waste was too woody and the whelk waste contained little flesh. Following addition of extra green waste, temperatures quickly rose to over 70°C which lasted approximately 36 hours.
Composted animal by-products must pass tests for the indicator pathogens Salmonella and Enterobacteriaceae. Results show that the compost passed the legal limits.
Compost was analyzed to see whether it was safe and suitable in land-based applications and found to be a good material with satisfactory nitrogen, phosphorous and potassium levels. High calcium levels could restrict some uses. Conversely, this could be beneficial in producing alkaline conditions.
Fees charged by composting companies vary according to the type, quantity and difficulty of the waste to be treated. It is estimated that animal by-product composting gate fees are currently about £40-60 per metric ton. TEG indicated that the gate fee for them to compost whelk waste would be within this range.
All composting facilities require capital expenditure on land and equipment. It is estimated that it would cost around £1 million for a new 10,000 per metric ton per year in-vessel facility which conforms to current legal standards. Operating costs and gate fees for the receipt and treatment of other materials are variable but the unit cost of composting is estimated at about £50 per metric ton in an in-vessel system.