Predominantly with domestic sewage the issue of contamination with bacteria or viruses is extremely important. However, also with industrial wastewater pathogens might occur and should at least once in the beginning be analyzed. Total coliform and fecal coliform organisms are often used as indicators for microbiological contamination of wastewater. Nematode eggs are used as an indicator for parasite microbiological standards for wastewater reuse in agriculture (see table 1). They are often set in conjunction with specified requirements for treating wastewater. There are currently several alternative approaches to establishing microbiological guidelines for reusing wastewater (see textbox below). These have different outcomes as their objectives: the absence of fecal indicator bacteria in the wastewater, the absence of excess cases of enteric disease in the exposed population and a model generated risk which is below a defined acceptable risk.
The absence of fecal indicator bacteria in the wastewater
This approach has led to guidelines that require zero fecal coliform bacteria/100 ml for water used to irrigate crops that are eaten raw in addition to a requirement for secondary treatment, filtration and disinfection. The United States Environmental Protection Agency (US EPA) and the US Agency for International Development (USAID) have taken this approach, and consequently have recommended strict guidelines for wastewater use .
No measurable excess cases in the exposed
population: epidemiological perspective
The objective of this approach is that there should be no actual risk of infection—that is, there should be no measurable excess risk of infection attributable to the reuse of wastewater as evaluated using scientific evidence, especially from epidemiological studies. This was the approach adopted in the 1989 WHO guidelines, for which epidemiological evidence was used (when available);
However, results from any given study are generally specific to the time and place of that study. Extrapolation of the results to other times and other locations — as is necessary when they are used for regulation—depends on making assumptions about the changes to variables, such as contact with wastewater, which might affect the outcome.
In scientific terms, assessment of actual health risks continues to be a controversial matter; there are either too few epidemiological studies available to permit any precise weighting of risks or the studies are not sufficiently practice-orientated to permit the results to be translated into concrete policy.
A model-generated risk that is below a defined acceptable risk
In this approach an acceptable risk of infection is first defined — for example, for the microbial contamination of drinking-water supplies. The US EPA has set annual risk of 10-4 per person. Once the acceptable annual risk has been established by the regulator, a quantitative microbial risk assessment (QMRA) model is used to generate an estimated annual risk of infection. A microbiological quality guideline limit would then be set so that the model produces an estimate of an annual risk which is below the regulator’s acceptable annual risk.
Table 1: Different approaches to set microbiological standards for wastewater reuse
Presently, researchers are divided between two schools of thought on the question of the appropriate level of nematodes and fecal coliform in wastewater that should be used for irrigation. The two schools of thought are: the less stringent epidemiological evidence school led by the WHO and the "no risk school" led by the US. The "no risk" philosophy cannot be adopted by many countries, especially developing countries, which cannot find financial resources for expensive treatment systems, but badly require wastewater for irrigation. Under the "no risk" scenario, the only options left for these countries would be, either no wastewater reuse or wastewater reuse (illegal) without any regard for the tough (and thus impractical) guidelines.
Differentiating between the potential risk and actual risk of contracting a disease is another issue in developing appropriate guidelines. The actual health risk depends on three more factors namely:
time of survival of pathogens in water or soil,
infective dose, and
host immunity.
The risks to populations are dependent on the irrigation method used. Health risks from irrigated crops are greatest when spray or sprinkler irrigation is used, and the risk to field workers is greatest when flood or furrow irrigation is used. However, other potential sources of crop contamination should also be considered such as crop handling, transportation and the sale of products in unhygienic markets. Consumers can themselves make an important contribution to minimizing risks by, for instance, complying with sanitary standards in processing and using wastewater, i.e. by handling it on the basis of the information available.
Important Criteria
Important Criteria