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Health Protection Measures and Health Safeguards in Water Re-Use for Non-potable Applications

Dr. G. Goldstein
Coordinator, Healthy Cities Programme, Department of Health Promotion, WHO

Introduction

Land applications of wastewater and sewage sludge have been common practices around the world for centuries. Three common practices are:

use of wastewater for crop irrigation
use of excreta for soil fertilization and soil structure improvement
use of wastewater and excreta in aquaculture

Efforts to ensure the safe use of wastewater must consider the protection of the health of workers involved, crop handlers, those living near treated fields, and the public at large. I will focus on the re-use of wastewater and excreta in agriculture and aquaculture. The term wastewater refers to domestic sewage and municipal waste waters that do not contain substantial quantities of industrial effluent; by excreta I refer to nightsoil and to excreta-derived products such as sludge and septage. I shall propose that health protection considerations will generally require that some treatment be applied to these wastes to remove pathogenic organisms. Apart from treatment of waste water and excreta, other control measures considered will include crop restriction, waste application techniques and human exposure control.

Until recently the focus in re-use of waste water has been the prevention of transmission of communicable diseases and the control of microbiological contamination. The health hazards of chemical pollution have been considered comparatively of minor importance. However Chang et al (1995) reported that large-scale irrigation of crops with mostly untreated municipal wastewaters - containing industrial wastewater - could be harmful to crops and cause injuries to humans. Poorly controlled toxic and hazardous waste dumping and discharges into drains is an increasing problem that leads to significant amounts of chemical pollutants in municipal wastewater and sewage sludge, and land application may be restricted because of the potential for food chain transfer of pollutants.

Urbanization fundamentally involves a change in the mode of production, from agriculture to industry, and in our rapidly urbanizing world industrialization proceeds unchecked. In many cities in developing countries, estimates are:

that 50% of industrial production are small-scale or informal enterprises

that in many slums and houses in peri-urban settlements, 50% or more of thouseholds may have domestic cottage industries, involving chemicals and a risk of toxic discharges or spills.

If we consider a peri-urban settlement with crowding and lack of organized protection of water sources, it seems likely that the greatest toxic chemical hazard to humans is direct contamination of water sources from indiscriminate dumping, and not the food chain transfer of pollutants via the waste – soil – plant - human route. Wastes from improperly managed informal enterprises may directly contaminate groundwater and wells, however the focus of this paper is the health hazards from chemicals that result from re-use of waste water.

Disorders caused by chemical toxicants are harder to identify than water-borne diseases, which have short latency, unique clinical symptoms, readily identifiable exposure route and unambiguous dose-response relationship. The prevalence of diseases of chemical aetiology is not well documented. Some diseases may result from a single chemical compound, eg methyl mercury and Minamata disease. More often chemical agents act as one of the co-factors in a multi-causal relationship , eg the Itai-Itai disease in which Cd toxicity is only one of the causal factors.

Hundreds of chemical toxicants known or suspected to be carcinogenic, mutagenic or teratogenic appear in the priority pollutant lists of the US and the Council of European Communities, and they are ubiquitous in the environment. Because of the long latency period, and because chemicals are common in the environment, the exposure due to a given pathway cannot always be separated from the background exposure.

Given the ambiguous cause-effect relationship and the lack of epidemiological evidence, a dose-response relationship for chemical toxicants must be derived from animal bioassays or other means. There is a major challenge to develop criteria that are not overly restrictive to beneficial use of wastewater and sewerage sludge, and yet protect human health that is vulnerable to hundreds of toxic chemicals that may be present in municipal wastewater and sewerage.

Health Risks from microorganisms in waste water

An actual risk to public health is present only when all the following conditions are met in the re-use of waste water:

either an infective dose of an excreted pathogen reaches a field or pond, or the pathogen multiplies in the field or pond to form an infective dose

the infective does reaches a human host

the host becomes infected

the infection causes disease or further transmission

A review of studies on wastewater irrigation has concluded that:

(a) when untreated wastewater is used to irrigate crops there is a high actual health risk from intestinal nematodes and bacteria, but little or no risk from viruses, and (b) treatment of wastewater is highly effective in safeguarding health. In particular:

crop irrigation with untreated wastewater causes excess intestinal nematode infection in crop consumers and field workers. Field workers, especially those who work barefoot, are at higher risk of hookworm infection than those not working in waste-water irrigated fields

irrigation with adequately treated wastewater does not lead to excess intestinal nematode infection in field workers or crop consumers

cholera and probably typhoid can be effectively transmitted by irrigation of vegetable crops with untreated wastewater

cattle grazing on pasture irrigated with raw wastewater may become infected with beef tapeworm but there is little risk to humans

some health impacts have been observed on the health of people living near fields irrigated with raw wastewater, due to direct contact with the soil or indirectly by contact with farm workers

sprinkler irrigation with treated wastewater may promote aerosol transmission of excreted viruses, although in practice transmission is rare

In the aquacultural use of excreta and wastewater, there is evidence of transmission of trematode diseases (Clonorchis, oriental liver fluke, and Fasciolopsis, giant intestinal fluke). Also, schistosomiasis is a potential risk for those who work in excreta-fertilized ponds.

Microbiological Guidelines

The very strict microbial standards developed by the California State Health Department and others some 50 years ago, (2 coliforms per 100 ml for effluent irrigation of vegetables and salad crops eaten uncooked) was based on a "zero risk" concept. This standard was not feasible with common wastewater treatment technologies, even in developed countries, and as well was not based on epidemiological evidence. Nonetheless the California standard became the most commonly accepted guideline around the world.

The irrational application of unjustifiably strict microbial standards for waste water irrigation of treated wastewater for crop irrigation contributed to a situation where standards were not enforced, and the hazardous irrigation of salad crops with raw wastewater was widely practiced in many developing countries.

The Engelberg standard was based on a critical evaluation of the massive amount of epidemiological data reviewed and analysed by the World Bank study (Shuval et al, 1986) and the IRCWD/WHO Study (Blum and Feachem, 1995) on credible health effects associated with wastewater and excreta use in agriculture. They concluded that:

the risks of irrigation with treated wastewater were minimal and that the California standard was unduly restrictive

the main risk in many developing countries was associated with helminthic diseases and a high degree of helminth removal was necessary.

Waste stabilization ponds are particularly effective in achieving the helminth goal, and are usually the wastewater treatment method of choice in warm climates wherever land is available at reasonable cost. It was understood that the strict helminth standard recommended is an indicator for all of the large pathogens (that settle readily) including the protozoa Amoeba and Giardia.

Intestinal nematodes include Ascaris, Trichuris and hookworms, and are expressed in arithmetic mean no. of viable eggs per liter
Fecal coliforms are expressed as geometric mean no. per 100 ml.

The "requirements for treatment in stabilization ponds to meet standard" may vary from the indicative figures given, but the guidelines note that a minimum degree of treatment for restrictive use is equivalent to at least a 1-day anaerobic pond followed by a 5-day facultative pond or its equivalent. Mara and Silva (1986) note that in many cases the most appropriate treatment option for restrictive use will be a waste stabilization pond system comprising a 1-2 day anaerobic pond followed by a facultative pond (5 days) and a maturation pond (5 days).

For aquacultural use, guidelines for the microbiological quality of treated excreta and wastewater were:

zero viable trematode eggs per litre or per kilogram (on an arithmetic mean basis) and less than 10 000 fecal coliform bacteria per 100 millilitres or 100 grams (on a geometric mean basis)

The stringent trematode guideline is necessary as these pathogens multiply greatly in their first intermediate host. The value for fecal bacteria assumes a 90% reduction of these bacteria in the pond, so that fish and aquatic vegetables are not exposed to more than 1000 fecal coliforms per 100 millilitres.

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