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Literature Review

 Waste Generated by the Aquaculture Industry

According to department of environmental conservation (1987), the discharge volume of wastewater from an aquaculture facility approximately 140,000 cubic meters per year. Aquaculture wastewater may contain chemicals that are harmful to the development of plants. It may also contain bacteria and other organisms that are detrimental for plants and animals. Wastewater may even contain bacteria and other organisms which, when eaten by animals, may in turn infect the people who eat the contaminated meat. Managing waste generated from aquaculture wastewater represents a tremendous problem; Kristiansen, (1996) stated that “Handling wastewater is a major problem in all animal agriculture systems, but there are substantial differences between aquaculture wastewater and manure from dairy or hog systems. The latter are typically in the range of 5 – 15 % suspended solids, while fish wastewater can be anywhere from 0.2 – 4.0 % suspended solids”. Typically, suspended solids concentrations from drum filters used in intensive aquaculture operations are around 0.5 %. Waste production from aquaculture systems will be from 0.2 to 0.5 kg of waste per kg of feed (Drennan et al., 1995; Chen et al., 1997.

Possible Methods of Wastewater Treatment

The two most common methods used to recycle solid wastes from aquaculture facilities are land application and composting. Aquaculture sludges are good for use in both crop and created wetland. Nevertheless, if transportation costs make sludge disposal on crop land uneconomical, disposing of the sludge on-site within created wetlands might be the next best alternative.
Due to an increasing environmental concern regarding the aquaculture wastewater, various methods for treating wastewater have been implemented. Four main stages associated with aquaculture wastewater management are waste collection, fish-fed loss reduction, suspended particle separation, and sludge treatment. The later phase has rarely been considered because of the large increase in sludge volume originating from such sources (Bergheim et al., 1993a), however, it is becoming an increasingly significant problem.
Loehr et al. (1984a) studied the feasibility of using earthworms to stabilize wastewater treatment sludge and similar wastes. The study concluded that both excessive and inadequate moisture content can adversely impact earthworm growth. Worm growth at high and low total solids contents (7.9, 18.4, 18.6, 20.5, and 25.1% solids) was statistically different from worm growth occurring in the middle range of solids (9.3 to 17.1% solids). The best worm growth occurred over a range of total solids, wet basis (w.b.), of about 9 to 17% solids. Hence, the identified appropriate range of moisture content (91 to 83%) is the total solids content to which the worms were exposed. Loehr (1984) extended the experiment to determine whether the vermistabilization process could be self supporting, e.g., whether the aerobic condition was maintained by worms (E. fetida) over a long period of time. Results showed that at the application rate (200 grams/week) after about 12 months, volatile solids reduction of 10- 15% was achieved. No apparent adverse effects were observed as worms and cocoons were distributed throughout the accumulation. No major difference in the performance of the vermistabilization was observed as a function of time. High concentrations of nitrate-N in the effluent in the worm reactors imply the aerobic conditions were maintained.
Edward et al. (1988) based a study on Loehr’s which researched the potential of earthworms to manage sewage sludge with a 10% solid content. Results showed that E. fetida can increase the Volatile Sludge Destruction (VSD) rate when present in aerobic sludge (Table 2-1). The increase in the VSD rate reduces putrefaction due to anaerobic conditions. The worms aid in the more rapid degradation of organic matter through increased aeration as they move through the sludge.
The use of sand filter beds in wastewater treatment has been studied with regards to municipal wastewater, sewage sludge, and industrial wastewater. Very few studies have focused on aquaculture wastewater. Currently, there is very little information available on treating the higher solid content (11 % solid) of the fish sludge and separating fish sludge from the higher moisture content (98.5 %) of aquaculture wastewater from a drum filter.
Kristiansen and Cripps (1996) conducted a study on the treatment of fish wastewater using sand filtration. This study’s objective was to evaluate the feasibility of sand as a renovation, stabilization, and drying system for sludge derived from the first stage treatment of aquacultural wastewater. Feasibility was assessed in terms of hydraulic capacity and treatment efficiency. The pilot study comprised coarse sand-filled infiltration beds loaded with either an artificial fish farm wastewater (AFW), backwash water from a micro sieve (BW), or sediment micro sieve backwash water (SBW) collected daily from a settling chamber. Results obtained from the study showed that the hydraulic conductivity of all the columns was reduced from an initial 2000 cm / day, measured as infiltration rate, to < 100 cm / day after 40 days of usage. A large reduction in hydraulic conductivity was caused by the establishment of a clogging mat on the sand filter surface. Kristiansen stated that “About 60 % of the sludge total organic C was removed by the filters. Nitrogen in the effluent from the SBW loaded filter was predominately organic, and nitrate concentrations were significant (< 0.03 mg NO3N L-1). Effluent ammonium concentration decreased from 97 % of the effluent total nitrogen (TN) after 1month of loading, to 10 % after 2 to 3 months, with an attendant increase in nitrate to about 65 % of the TN. The P binding capacity of the test sand volume was exceeded after 1 to 2 months of SBW loading. This capacity was not exceeded during the experiment, using the two other effluent types (BW and AFW). Filter effluent P concentrations were about 1.4 mg / l. At a SBW loading of 1 cm / day, to coarse sand, with a hydraulic head of > 10 cm, it was expected that 2 to 3 months loading could occur before maintenance or change of filter surface sand would be required.” The use of sand infiltration for treating salmon farm sludge was therefore shown to be feasible.
Wridge et al., 1996 conducted a study on intermittent sand filtration for domestic wastewater treatment, focusing on the effects of filter depth and hydraulic parameters. Wridge stated that “The objective of the study was to operate a series of pilot-scale intermittent sand filters for the treatment of domestic wastewater, and evaluate their performance based on current discharge standards for wastewater quality in the state of Ohio. Specific objectives were to relate sand filter depth, hydraulic conductivity, and infiltration rate to treatment performance. Results show that the 60 cm filter constantly produced an effluent that met the Ohio department of health regulations for BOD5 of 20 mg/L after nine weeks of operation. Filter depth also influenced total suspended solids removal, but to a lesser degree than for BOD5 removal. The effect of filter depth was also clearly evident for ammonia-N removal. Estimates were made of saturated hydraulic conductivity and infiltration rate for the laboratory filters. In general, as filter run increased, both the hydraulic conductivity and infiltration rate decreased”.
While many studies have examined the effects of vermicomposting process on domestic wastewater, municipal wastewater, sewage sludge, and industrial wastewater, few have examined effects on aquaculture wastewater. Scientific studies have helped establish the technical basis for vermistabilization. Research (Kristiansen et al., 1996; Wridge et al., 1996; Edwards et al., 1988; Loehr et al., 1984) has shown that sand filtration method with worms can be effective for separating solids from aquaculture wastewater. In addition, studies have identified the effect of filter media, depth of filter-bed, hydraulic conductivity, and various variable concentrations such as ammonia, biological oxygen demand (BOD) and nitrate for separating solids and treating wastewater. Therefore; these studies may be helpful in designing an experiment for treating aquaculture sludge with higher percentage of solid (10 – 20%) and wastewater from drum filter at higher moisture content (98.5 to 99.5).


Composting is a biological decomposition of organic matter under controlled aerobic conditions into humus-like stable products. The processing of organic wastes into organic fertilizers via composting is a technique that has been used to address the issues of environmental pollution, reliance on chemical fertilizers, sustainable natural fertility, and minimizing the development of new landfills. Vermicomposting uses worms to convert animal, agricultural, and industrial wastes to useful fertilizers.
Raymond, 1988 reported that “Earthworms have been used for waste stabilization for many years, especially in the Philippines and other countries in Southeast Asia. The process is also being used in Italy, England, and the Netherlands not only to stabilize wastes, but also to produce castings for horticultural purposes. In the United States, feasibility studies (Camp et al., 1981; Pincince et al., 1980) evaluated the process for sludge management and indicated that the operating costs of a practical system may be competitive with other sludge management options for certain communities”.
Worms maintain aerobic conditions in the waste mixture, ingest solids, convert a portion of the organic media into biomass and respiration products, and expel the remaining, partially stabilized matter as discrete material (castings). Worms and microorganisms act symbiotically to accelerate and enhance the decomposition of the organic matter. Degradation is a function of the portion of waste that is biodegradable, maintenance of aerobic conditions, and avoidance to toxic conditions. Earthworms perform physical/mechanical and biochemical actions through substrate aeration, mixing, and grinding as they process waste. Thus, vermicomposting lowers operational costs, making it a very economical method for waste treatment. Hand et al. (1988) defined vermicomposting as “a low cost technology for the processing or treatment of organic wastes”.
Edwards (1988) noted that “At the same time earthworms promote microbial activity since the fecal material or ‘casts’ that earthworms produce is more fragmented and microbially active than what earthworms consume”. During this process, the important plant nutrients in the material, particularly nitrogen (N), potassium (K), phosphorus (P), and calcium (Ca) are released and converted through microbial action into forms that are much more soluble and available to plants than those in the parent compounds.
However, Ndegwa and Thompson, 2000 similarly reported that “The major drawback in the vermicomposting process is that…vermicomposting processes must be maintained at temperatures below 35oC. Exposure of worms to temperatures above this is lethal. During the vermicomposting process therefore, the temperatures are not high enough for acceptable pathogen kill and hence the process does not meet EPA rules for pathogen reduction. In some cases, depending on the feed substrate, some forms of quick composting will be needed for the vermicompost to meet EPAs process to further reduce pathogens (PERPs) guidelines for pathogen destruction for class-A compost (Class-A compost is said to be satisfactory for general distribution)”.

Efficiency of Eisenia fetida in Vermicomposting

Many earthworm species have potential in vermicomposting (Neuhauser et al., 1979b; Kaplan et al., 1980a, b). Edward (1988) studied five earthworm species (Dendrobaena veneta, E. fetida, E. eugeniae, Perionyx excavatus and Pheretima hawayana) for their use in vermicomposting of sludge, evaluating growth and reproduction as indicators of their fitness. Of these species, E. fetida produced more live worms per cocoon than any of the other species tested. E. fetida thus appears to be an appropriate species to use in vermistabilization studies since it produced the greatest number of young worms per initial parent worm. Environmental Resource Systems at the University of Arkansas also reported that “The most beneficial worm to soil is Eisenia foetida—also known as the red worm, brandling worm, red wiggler, or manure worm. Most worms live and die within the same year, but in culture can live up to four and a half years.”
The growth of E. fetida and other species shows that litter-dwelling earthworms can be easily bred in vermiculture (Loehr et al., 1985). Dominguez and Edwards (1996) concluded that E. andrei, a close relative of E. fetida, could be cultured in pig manure grew and matured between 65% and 90% moisture content, the optimum being 85%. There is a direct relationship between the moisture content and the growth rate of earthworms, and E.fetida can survive in moistures between 50% and 90% (Edwards et al., 1985; Sims and Gerard, 1985), growing more rapidly between 80% and 90% in animal wastes (Edwards et al. 1985). Reinecke and Venter (1985) concluded that the optimum moisture content for E.fetida is well above 70% in cow manure, making it perfect for vermicomposting of sludge with high liquid contents.
Many researchers have therefore focused on E. fetida as a suitable species for vermicomposting sludge. Graff (1974), Watanabe and Tsukamoto (1976), Hartenstein (1978), Loehr et al. (1984) and Hartenstein and Bisesi (1989) suggested using E.fetida for managing labile organic wastes on soil. Neuhauser et al. (1980a) demonstrated that “biological sludge derived from municipal wastewater treatment facilities is a very favorable substrate and growth medium for E.fetida”.
It is clear, however, that E. fetida survives best within a certain temperature range. Kaplan et al. (1980) reported that “E.fetida grew most rapidly on biological sludges when incubated at fixed-temperatures of 20oC, 25oC or 28oC depending on the moisture content of the sludge substrate”. Loehr et al. (1984) indicated that the “maximal rate of growth for E. fetida was observed when incubated at fixed-temperatures of 25oC”. Temperature must be taken into consideration for any waste treatment operation that wishes to make use of vermicomposting as a possible treatment strategy.

Treatment of Wet Fish Sludge with Vermicomposting

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