About UNEP
United Nations Environment Programme
Division of Technology, Industry and Economics
top image
space space space
Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

9.2. Towards sound wastewater reuse practice

9.2.1 Public health

As discussed earlier most wastewater-fed aquaculture systems have been developed by farmers to produce aquatic produce without adequate attention to public health. Most wastewater is usually not treated prior to reuse.

There are no currently accepted international health standards for excreta reuse in aquaculture. The WHO has a tentative guideline (WHO, 1989) but it is generally considered to be unduly restrictive, thereby encouraging the continued use of unregulated wastewater reuse. Furthermore, there is a need to base health standards on epidemiological rather than microbiological guidelines i.e., on actual rather than potential risk (Strauss, 1996).

Figure 2.52: Schema of wastewater reuse strategies. Source: Edwards (2000)

Largely anecdotal evidence does not indicate a significantly increased risk to public health from consumption of wastewater-fed fish. However, scientifically based data to support this are almost entirely lacking. There is also a need to assess the occupational risk of workers in, and handlers of fish from, wastewater-fed systems. Contamination of fish with heavy metals and organics is likely unless there is separation of industrial wastewater from domestic wastewater reused in aquaculture.

A subjective assessment of health risks from pathogens and toxic substances is presented in Table 2.8. Microorganisms particularly viruses and bacteria, die rapidly in wastewater-fed ponds which explains in part why fish from such systems do not appear to significantly increase insanitary disease. Trematodes pose a significant risk in East Asia if fish harvested from nightsoil fed ponds are consumed raw. Schistosomiasis does not pose a risk to the health of pond workers because it is rare in Asia.

Table 2.8: Health risks

Health risk  Nightsoil


Domestic Industrial Agroindustrial Mixed

Viruses + + - +/- +
Bacteria + + - +/- +
Protozoa + + - +/- +
Trematodes +++ + - - +
Heavy metals - - ++ +/- ++
Organics - - +++ +/- +++

Toxic substances (heavy metals and organics) probably pose a major but poorly documented threat to public health in Asia because of widespread contamination of domestic with industrial wastewater which is often reused without prior treatment.

Guidelines for domestic wastewater reuse in aquaculture have been published (Pullin et al., 1992) (box 3).

Box 3 Public health guidelines for wastewater-fed aquaculture
  • minimum retention time of 8-10 days for raw wastewater
  • in wastewater-fed fish pond water :
    • a tentative maximum critical density of 105 total bacteria/ml
    • absence of viable trematode eggs
  • suspension of wastewater loading to the system for 2 weeks prior to fish harvest
  • hold fish for a few hours after harvest to facilitate evacuation of gut contents
  • fish muscle quality :
    • < 50 total bacteria /g muscle
    • no Salmonella
  • good hygiene in handling and processing :
    • gut the fish
    • wash
    • cook well

The guidelines relate to microorganisms and parasites. Total bacteria does not refer to total coliforms or to faecal coliforms but to SPC/ml (Buras, 1990). Although E. Coli was present in high concentrations in fish pond water in ponds overloaded with wastewater, it was not always recovered from fish organs and very rarely from their muscles. It seems as though it was not performing the role of an indicator organism so it was suggested that E. coli should not be used as an indicator for the microbiological quality of fish grown in wastewater-fed fish ponds (Buras, 1990). Fish and other aquatic organisms cultured in wastewater should be monitored to ensure that concentrations of contaminants are within international and national safety limits.

      Main Menu


  • Brochure
  • IETC Brochure

  • International Year of Forests
  • International Year of Forests

  • World Environment Day
  • ??????

  • UNEP Campaign
  • UNite to Combat Climate Change