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9.2.3 Culture systems

Wastewater should never be reused without prior treatment if the produce (fish or aquatic vegetables) is intended for direct human consumption.

Conventional designs for wastewater reuse in aquaculture incorporate complete treatment of the effluent because it is fed to fish ponds. Thus, there is a series of anaerobic, facultative and maturation ponds before the effluent is reused in a fish pond. While the treated effluent would conform to the WHO tentative guideline of 1 x 103 faecal coliforms/100 ml (WHO, 1989), there would be only minimal reuse of nutrients in the wastewater because of the high degree of treatment before the effluent enters the fish ponds.

An improved design has been proposed (Mara et al., 1993; Mara, 1997) which provides minimal (but adequate) treatment of wastewater and maximal production of microbiologically safe fish (box 5). This was developed to unify the approaches of sanitary engineers and aquaculturists to wastewater-fed aquaculture design. Less than 10% of the total pond area is used for pretreatment, with far more of the nutrients contained in the wastewater used to produce fish than in the conventional design. The design criterion for the fish pond is a nutrient surface loading rate of 4 kg total nitrogen/ha/day, with the number of faecal coliforms estimated in the pond water to be <_1 x 10³ / 100 ml. Thus, the microbiological quality of fish pond water rather than the pretreated wastewater influent to the fish pond complies with the tentative WHO guideline for faecal coliforms. This takes into consideration the extremely rapid die-off of faecal coliforms, which are indicators of pathogenic bacteria, in fertile wastewater-fed fish ponds. There is still a sufficiently long retention time in pretreatment to eliminate human trematode eggs.

Most wastewater reuse is direct to produce fish or aquatic vegetables without any intervening steps for human consumption (Figure 2). In societies in which direct reuse of wastewater is socially unacceptable, it may be appropriate to promote indirect reuse to increase social acceptance of the practice. Wastewater may be used to produce either fish seed, or fish or aquatic plants as animal feed. Such systems incorporate an extra step in the food chain. Although they are likely to be less ecologically and economically efficient than direct reuse, by "lengthening the food chain" wastewater reuse becomes indirect and perhaps feasible in societies in which direct reuse is socially unacceptable.

Wastewater may be used to culture fish seed (fingerlings) which can then be on-grown for human food in separate culture systems that do not involve wastewater reuse. The major source of tilapia seed in Vietnam is a series of ponds fertilized with contaminated surface water in the suburbs of Ho Chi Minh City.

Alternatively, wastewater may be used to culture fish or aquatic plants as animal feed. Wastewater-fed fish could be used as an alternative high-protein source for diets of livestock and high-value carnivorous fish and shrimp. A significant percentage of the protein in formulated pelleted diets is fish meal. Marine capture fisheries are the main source of fish meal but production is static or declining because of overfishing. Fish cultured in wastewater may play a role in the future to augment the supply of fish meal. This is particularly true for tropical developing countries, many of which import significant quantities of fish meal to culture large amounts of penaeid shrimps. Research has already demonstrated the feasibility of production and reuse of tilapia raised on septage as a high-protein animal feed (Box 6).

A second indirect strategy to reuse wastewater to produce high-protein animal feed is cultivation of duckweed (Iqbal, 1999). Cultivation of duckweed in ponds fed with nightsoil or contaminated surface water is a traditional Chinese practice. In China duckweed is raised to feed the fingerlings of grass carp before they are large enough to consume grass. There has been considerable experimentation, and establishment of pilot projects in several countries to cultivate duckweed on wastewater to feed to Indian major carps and tilapias as well as poultry and pigs. Duckweeds have several favourable attributes for culture as high-protein animal feed (Box 7) but their continuous cultivation over extended periods of time is not easy (Edwards, 1990b).

There are duckweed-based, wastewater treatment systems (Alaerts et al., 1996; Poole, 1996). A complete cover of weed serves to remove nutrients from the wastewater stream and also leads to an effluent with a low suspended solids content because of shading of the water column. A USA based company has patented a duckweed based wastewater treatment system although the duckweed plant is not reused. Similar systems have been established by the NGO PRISM at Mirzapur, Tangail district in Bangladesh and duckweed from these systems are harvested and fed to fish in adjacent fish ponds (Case Study 3).

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