Human wastes are an extensively used resource in many parts of the world. With the rapid growth in the population of developing countries which are also becoming increasingly urbanized, wastewater reuse in aquaculture may have a role to play in closing the nutrient cycle and reducing eutrophication of surface waters. As a substitute for fresh water in irrigation or aquaculture, waste water has a significant role in water resources management. By releasing fresh water sources for potable water supply and other priority uses, waste water reuse contributes to water conservation and has economic advantages (Anon, 1989, Edwards, 1985,1990, Mara and Cairncross, 1989).
The most common productive uses of human waste are:
Aquaculture-waste water systems have two purposes: treating waste water and producing a useful product. This means that the system must be able to produce an acceptable effluent as well as a product that does not contain unacceptable levels of harmful chemicals, pathogenic bacteria or viruses (Hejkal et al, 1983)
Fertilization of aquaculture ponds with human and animal wastes has been practised for thousands of years in Asia; today at least two-thirds of the world production of farmed fish comes from ponds fertilized in this way. Fish can also be successfully farmed in the maturation ponds of a series of waste stabilization ponds; annual yields of up to 3000 kg/ha have been obtained. The use of untreated excreta to fertilize fish ponds is becoming less common in many parts of the world, and in China excreta are now used only after storage for four weeks in closed containers. Recently crustaceans such as crayfish and shrimps as well as microalgea have been grown in treatment ponds. (Mara and Cairncross, 1989). However, the promotion of organic fertilisation is constrained by sociological and public health considerations for human excreta reuse (Edwards, 1990).
Waste water, although sufficient as a source of nutrients, can present problems such as toxicity to fish, accumulation of heavy metals and toxic substances in the muscles of fish, and the potential danger of transmission of pathogens from waste water to handlers and consumers (Buras et al, 1987, Edwards, 1985, Mara and Cairncross, 1989).
The nutrient status of an aquatic ecosystem regulates its productivity. In sewage-fed fish ponds, a large amount of organic and inorganic nutrients enters the sediment and water, and enhances the productivity of the water bodies (Edwards, 1985, Edwards and Sinchumpasak, 19981 Edwards et al, 1981 Olah et al, 1986).
Normal domestic and municipal waste water is composed of 99.9% water and 0.1% suspended, colloidal and dissolved solids. These contain major plant nutrients (nitrogen, phosphorus and potassium) and trace nutrients (copper, iron and zinc). Total nitrogen and phosphorus concentrations in raw wastewater are usually in the ranges 10–100 mg/l and 5–15 mg/l respectively, and potassium is in the range of 10–40 mg/l (Mara and Cairncross, 1989). The principle in waste-loaded fish ponds is to add enough waste to provide enough food for the aquatic organisms which the fish eat, but not enough to lead to dangerously low dissolved oxygen levels which could risk fish survival (Anonymous, Undated, Edwards, 1985).
The most suitable fish to rear in a waste-fed pond are those that are tolerant to low levels of dissolved oxygen, which may be caused by unpredictable algal blooms. They should also be filter feeders, so that they can utilize plankton growth. A wide range of fish species is reported to have been cultivated in excreta-loaded systems: common carp, crucian carp, Indian major carps, Chinese carps, mullet, milkfish, catfish, tilapia, and under specific conditions trout and salmon. Tilapia are generally not as common as carps in these systems although these are better able to tolerate harsh environmental conditions than other species. (Anonymous, Undated, Edwards, 1985).
Fish mortality in a waste-fed pond can result from at least three possible causes. First, the depletion of oxygen due to bacterial oxygen demand caused by an increase in organic load. Second, the depletion of oxygen overnight due to the respiratory demand of too large a concentration of phytoplankton (the latter having grown in response to an increase in nutrients caused by an organismic imbalance or by a large inflow of nutrients). Thus in this second case it is not the BOD of the excreta itself that causes the greatest reduction of dissolved oxygen in a waste-loaded fish pond. The third possible cause is high ammonia concentration in the waste feed (Anonymous, Undated, Edwards, 1985).
The reported yields from sewage-fed aquaculture vary. Mara and Cairncross (1989) report yields up to 3 tons/ha/yr while Edwards (1985) found that yields of finfish in excreta-loaded fish pond systems in the tropics should be at least 5–6 tons/ha/yr, and with good management could be as high as 10–12 tons/ha/yr.
Some aquatic macrophytes are cultivated as vegetables for human consumption in aquaculture ponds; duckweeds are also cultivated, mainly for fish feed. Among the aquatic plants grown for use as vegetables are water mimosa (Neptunia oleracea), water cress (Rorippa nasturtium-aquaticum), water spinach (Ipomoea aquatica) and Chinese water chestnut (Eleocharis dulcis). The duckweeds Lemna, Spirodela and Wolffia are cultivated in some parts of Asia in shallow ponds fertilized with excreta, mainly as feed for Chinese carps but also for ducks, chickens and edible snails (Anonymous, Undated).
Excreta-associated diseases are very common in developing countries, and excreta and waste water contain correspondingly high concentrations of excreted pathogens such as bacteria, viruses, protozoa and helminths. Fish apparently do not suffer from infections caused by enteric bacteria and viruses that cause disease in warm-blooded animals -- humans and livestock -- but they may carry pathogens passively in the gut, mucous and various tissues (Buras et al, 1987, Edwards, 1985, Mara and Cairncross, 1989).
The potential health risks associated with the use of excreta and waste water for aquaculture are (Edwards, 1985, Mara and Cairncross, 1989):
Excreta-related infections are communicable diseases in which the pathogen leave the bodies of infected persons in their excreta, eventually reaching other people, whom they enter via either the mouth (for example through contaminated food) or the skin (as in schistosomiasis and hookworm). Mara and Cairncross have grouped these diseases into five categories according to environmental transmission characteristics and pathogen properties (Table 1.).
ENVIRONMENTAL CLASSIFICATION OF EXCRETED ORGANISMS
(from Mara and Cairncross, 1989):
Category 1. Caused by excreted virus (e.g. Hepatitis A and enteroviral infections), protozoa (amoebiasis and giardiasis) and the helminths Enterobius vermicularis (pinworm or thread worm) and Hymenolepis nana (dwarf tape worm). They are immediately infective after excretion (“non-latent”) and have a low median infective dose. Transmission of the disease occurs predominantly in the immediate domestic environment especially when low standards of personal hygiene prevail, although survival time of excreted virus and protozoa may be long enough to pose a health risk in excreta and wastewater use schemes.
Category 2. Excreted bacteria, infective immediately on excretion. Moderately persistent and can multiply outside their host. Commonly transmitted in the immediate domestic environment, but can survive longer transmission route, and therefore they can and do pose real health risks in excreta and waste-water use schemes. Examples are cholera, salmonellosis and typhoid.
Category 3. Soil-transmitted initial nematode that require no intermediate host. Most important are the human roundworm (Ascaris lumbricoides), the hookworm (Ancylostoma duodenale and Necator americanus), and the human whipworm (Trichuris thrichiura). All readily transmitted by the agricultural use of raw or insufficiently treated excreta and waste water; causes the greatest public health concern.
Category 4. Cow and pig tapeworm, Taenia saginata and T. solium, respectively. Viable eggs must be ingested by a cow or pig; potential route of transmission is irrigation of pasture with waste water.
Category 5. Water-based helminthic infections. Require one or two intermediate aquatic hosts, the first of which is a snail, in which a huge asexual multiplication takes place, and the second (if existing) either a fish or an aquatic macrophyte. Many have a limited geographical distribution, and it is only in endemic areas that their transmission is promoted by the aquaculture use of raw or insufficiently treated excreta and waste water, together with the practice of eating raw or inadequately cooked fish and aquatic vegetables.
There is little danger of disease from eating well-cooked fish or vegetables since the heat destroys pathogens. But it is important to emphasize that the consumption of raw, partially cooked, or improperly preserved products can be a serious health hazard. It is possible that the most significant health hazard is the risk from handling and preparing contaminated products (Edwards, 1985).
It is difficult to prevent the contamination of domestic sewage with toxic chemicals such as heavy metals and synthetic organics from domestic nonfeacal substances or from industrial sewage. These substances may accumulate in aquatic organisms and make them unsuitable as sources of human or animal feed. Toxic chemicals may be present in such concentrations in sewage that they even constitute a threat to fish growth in maturation ponds. Algae are known to accumulate in various heavy metals but, with the possible exception of mercury, fish raised in sewage-fed ponds have not been observed to accumulate high concentrations of these toxic substances (Anonymous, Undated, Edwards, 1985 Hejkal et as, 1983).
Epidemiological evidence for transmission of diseases caused by aquaculture use of sewage is limited. Only when it comes to the transmission of certain trematode diseases, mainly those caused by Clonorchis (oriental liver fluke) and Fasciolopsis (giant intestinal fluke) in Asia is there clear epidemiological evidence. There is no evidence for transmission of schistosomiasis, which is none the less a major potential risk to those who work in excreta-fertilized ponds. There is no conclusive evidence of bacterial disease transmission by passive transfer of the pathogens by fish and aquatic vegetables, although this too remains a potential risk (Mara and Cairncross, 1989).
The concentration of micro-organisms in the water determines their presence in fish tissues, and there appears to be a threshold concentration in pond water below which micro-organisms do not penetrate into fish muscle. The threshold concentration for viruses appears to be an order of magnitude less than for bacteria. The intraperitoneal fluid can have high levels of bacteria and viruses, an important discovery from a public health point of view since the fluid comes into direct contact with the handler when the fish is gutted, creating a potential source of infection (Buras et al, 1987, Edwards, 1985).
Invasion of fish muscle by bacterial pathogens is very likely to occur when the fish are raised in ponds that contain concentrations of faecal coliforms and salmonellae of >104 and >105 per 100 ml respectively; the potential for muscle invasion increases with duration of exposure of the fish to the contaminated water (Mara and Cairncross, 1989).
Edwards (1985) suggests that by “lengthening the food chain”, i.e. using organisms raised in excreta-fed ponds for animal feed rather than directly for human consumption, an extra step is added to the food chain. This eliminates direct human consumption of organisms raised in excreta-fed systems, and should decrease risk to public health.
The waste water used in aquatic macrophyte culture should be treated to the quality of 0 trematode eggs/l (for example in waste stabilization ponds) and in fish culture it should be additionally treated in maturation ponds or by disinfection to a level of less than 1000 faecal coliforms per 100 ml. Excreta should be treated to the same quality as waste water. Storage at ambient temperatures renders trematode eggs unviable, and the minimum storage period is four weeks for Schistosoma spp. To achieve the quality of less than 1000 faecal coliforms per 100 ml, excreta should be treated by composting or digestion. The control of snails can be achieved by keeping the pond embankments free of vegetation. In macrophyte ponds mosquito breeding should be controlled by the introduction of larvivorous fish. Tentative microbiological guidelines are given in Table 2.
TENTATIVE MICROBIOLOGICAL QUALITY CRITERIA FOR THE AQUACULTURE
USE OF WASTE WATER AND EXCRETA
(from Mara and Cairncross, 1989).
|Reuse process||Viable trematode eggs|
(arithmetic mean number per litre or kg)
(geometric mean number per 100 ml or per 100 g)
|Aquatic macrophyte culture||0||<104|
* This guideline assumes that there is a one log10 reduction in faecal coliforms occurring in the ponds, so that in-pond concentrations are <1000 per 100 ml.
Cultural beliefs vary widely in different parts of the world. Consequently it is not possible to assume that excreta and waste water use practises that have developed in one area can easily be moved to another. In many countries where these practices are not traditional there may be deep-rooted cultural prejudices against the consumption of fish reared on human wastes. It is mostly the less developed countries of Africa, Asia and Latin America that were not densely populated until relatively recently, and which now are expanding the most rapidly, that do not have a tradition of excreta use (Edwards, 1985,1990, Mara and Cairncross, 1989). However, some religious authorities have ruled that the use of well-purified and treated waste water is an acceptable practice. It is also important to note that the readiness of people to accept traditional values is often underestimated (Anon., 1989). A thorough market study should be carried out to see if the excreta reuse products are saleable in the current cultural context (Edwards, 1985).
Properly planned and managed excreta and waste water use schemes can have a positive environmental impact, as well as increasing agricultural and aquaculture yields. Environmental improvement results from several factors, including (from Mara, D. and S. Cairncross, 1989):
The recommended system for waste water treatment in developing countries, provided that land is available at reasonable cost, is the stabilization pond system. This consists of a series of shallow man-made lagoons with the earlier ponds (anaerobic and facultative ponds) in the series loaded with waste water at too high a rate for fish to survive due mainly to oxygen depletion during the night. Fish culture can take place in later ponds in the series (maturation ponds) which are aerobic and function principally in pathogen removal. However, the production in these ponds may be low because most of the nutrients will have been removed from the water in earlier ponds in the series. The nutrient concentration reaching the maturation ponds may not be enough for adequate natural feed production for fish (Edwards, 1990).
One treatment option for aquaculture is to connect ponds in series and avoid harvesting from the first pond. A minimum of three ponds with a minimum total detention time of 25 days will produce an effluent that is either completely pathogen-free or with only low concentrations of enteric bacteria and viruses; pathogen helminths and protozoa will have been completely destroyed (Edwards, 1985 Mara and Cairncross, 1989).
Depuration is a cleaning process in which fish are kept in clean, running water for various lengths of time. It should be incorporated into excreta reuse schemes in which fish are produced directly for human food, also in systems with adequate treatment, to eliminate the possibility of pathogens being present in the fish intestines or on the outside of the fish body. Keeping fish in clean water for at least 2 to 3 weeks before harvest will remove any residual objectionable odours and reduce contamination with faecal micro-organisms. However, depuration does not guarantee complete removal of pathogens from fish tissue and digestive tracts unless the contamination is very slight. Fish which have been grown in water containing high concentrations of micro-organisms may have pathogens present in muscle, organs, and intra-peritoneal fluid, and depuration would then be ineffective (Anonymous, Undated, Buras et al, 1987, Edwards, 1985, Mara and Cairncross, 1989).
Schistosomiasis is best controlled by treatment and snail control. Regular chemotherapy would be beneficial in endemic areas. Provision of adequate sanitation and clean water supplies is also an important factor in limiting human exposure (Mara and Cairncross, 1989).
In aquatic systems used to raise macrophytes, mosquitoes may be controlled. The mosquito fish Gambusia affinis can be introduced into a water hyacinth system to eat the miracidia. This fish can tolerate low dissolved oxygen levels occurring in these ponds. The more or less complete cover of duck weed on the surface inhibits mosquito breeding since the larvae would be prevented from surfacing for air. Open water areas should be maintained in fish ponds since certain types of mosquitoes breed in association with aquatic macrophytes. Tilapia species, which are highly suitable for excreta reuse systems, consume mosquito larvae (Edwards, 1985).
It is very important to promote good hygiene in all stages of handling and processing excreta-raised fish to minimize potential bacterial and viral contamination. The final step which makes waste-grown fish safe for human consumption is adequate cooking; the consumption of raw, undercooked or improperly preserved or processed fish should be strongly discouraged (Edwards, 1985).
Waste water and excreta are valuable resources which should be utilized as far as possible, keeping in mind the possible health risks. These health risks can be minimized or eliminated completely through proper management of the waste, hygienic handling of the produce and thorough cooking of the food produced. Especially in countries where droughts occur, waste water is a valuable resource, since although the flow of sewage may decrease during a drought, it will never cease entirely. Also, the available water will be utilized more efficiently -- clean water for human consumption, and waste water for agricultural and aquaculture use.
Through the recycling of nutrients from urban areas fish can be produced at the same time as reduction of the effects of cultural eutrophication. The problem is that many developing countries which are now rapidly expanding do not have any tradition of excreta reuse, and the cultural prejudices against consuming fish raised in ponds fed with human waste can be strong.
ALCOM could help to introduce waste water reuse systems through research on the growth of fish in sewage water, taking into consideration possible health risks. If the results show good growth and no health risks the system should be promoted in SADC countries where a sewage treatment system exists. The effect of such promotion would be reduced negative environmental impacts from the waste and increased protein production. However, it is important to carry out a careful market study, taking socio-cultural beliefs into account to make sure that the produce will actually be accepted for consumption before any large project is initiated.
Anonymous. Undated. Waste water treatment and use in agriculture. FAO irrigation and drainage paper No. 47.
Anonymous. 1989. Health guidelines for the use of waste water in agriculture and aquaculture. World Health Organization Technical Report Series 778.
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Edwards, P. 1985. Aquaculture a component of low cost sanitation technology. UNDP Project Management Report No. 3., World Bank Technical Paper No. 36.
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Edwards. P and O.A. Sinchumpasak. 1981, The harvest of microalgae from the effluent of a sewage fed high rate stabilization pond by Tilapia nilotica, Part 1. Description of the system and the study of the high rate pond. Aquaculture Vol 23 pp 83–105.
Edwards, P. Sinchumpasak, O.A. and M. Tabucanon. 1981. The harvest of microalgae from the effluent of a sewage fed high rate stabilization pond by Tilapia nilotica. Part 2 Studies of the fish ponds. Aquaculture Vol 23 pp 107–147.
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