Chapter 3 - Implementing health protection measures for wastewater use

Wastewater treatment to lower health risks
Lowering risk of direct human exposure in areas using wastewater
Lowering risk to consumers through crop restrictions

The discussion to this point has centred on defining the risk of disease transmission. All wastewater contains pathogens and these pathogens do pose a risk. As discussed in Chapter 2, that risk can be defined. The focus now shifts to evaluating what can be done to minimize or eliminate that risk.

The water is the means that allows an infectious pathogen to move to a new host. The intermediate step in this process is crop production which can provide a route of infection.

There are two approaches to developing a regulatory programme for health protection. The first is to focus on lowering the risk from the water. This is normally done by wastewater treatment or treatment and disinfection. Where the treated water does not meet health protection standards for unrestricted irrigation, the focal point for risk reduction shifts to the point of water use (irrigation). Here agricultural restrictions can lower the potential health risks. The point of water use is usually where the route of infection shifts to the soil and crop; therefore, these become the primary focus of management or regulatory strategies.

There are numerous agronomic practices that can assist in lowering the risk from wastewater use but most of these are individual site decisions that are normally made by the farmer to increase agricultural production and not to lower the overall disease infection risk. Farmers cannot be expected to implement a programme that focuses on individual cultural practices since the farming goal is agricultural production. Any regulatory approach must be institutional and have a primary focus on the type of crop grown. Such an approach avoids the regulation process being involved with the way a particular crop is grown.

The following sections briefly describe the two levels of approach: wastewater treatment and control at the field level. The latter is divided into the steps needed to prevent worker safety problems and those needed to prevent infection of the consumer of the crop.

Wastewater treatment to lower health risks

The water is the vehicle for movement of any pathogenic organism in wastewater. Any regulatory programme must first focus on intercepting these pathogens and rendering them harmless. The first option is to provide treatment of the wastewater. There is no perfect treatment process but the long-term goal should be to reduce the risk from the wastewater by meeting the guidelines adopted by WHO (1989). If the treatment process is capable of consistently meeting the WHO Guidelines then the effluent wastewater should be safe for unrestricted irrigation. It should be remembered that the wastewater is still a vehicle for transmission of pathogens as it is not a pathogen-free environment but it should pose an insignificant risk of disease infection when used properly for crop irrigation (WHO, 1989).

TABLE 7: Qualitative comparison of various wastewater treatment systems

	Criteria	Package plant	Activated sludge plant	Extended aeration activated sludge	Biological filter	Oxidation ditch	Aerated lagoon	Waste stabilization pond system
Plant performance	BOD removal	F	F	F	F	G	G	G
	FC removal	P	P	F	P	F	G	G
	SS removal	F	G	G	G	G	F	F
	Helminth removal	P	F	P	P	F	F	G
	Virus removal	P	F	P	P	F	G	G
Economic factors	Simple and cheap construction	P	P	P	P	F	F	G
	Simple operation	P	P	P	F	F	P	G
	Land requirement	G	G	G	G	G	F	P
	Maintenance costs	P	P	P	F	P	P	G
	Energy demand	P	P	P	F	P	P	G
	Sludge removal costs	P	F	F	F	P	F	G

Key: FC = Faecal coliform; SS = Suspended solids: G = Good: F = Fair: P = Poor
Source: Arthur (1983).

TABLE 8: Expected removal of enteric pathogenic micro-organisms in various wastewater system

Treatment process		Removal (log10 units) of (i.e., 4 log10 units, equivalent to = 10-4 = 99.9 percent removal)
		Bacteria	Helminths	Viruses	Cysts
Primary sedimentation
	Plain	0-1	0-2	0-1	0-1
	Chemically Assisteda	1-2	1-3g	0-1	0-1
Activated sludgeb		0-2	0-2	0-1	0-1
Biofiltrationc		0-2	0-2	0-1	0-1
Aerated lagoonc		1-2	1-3g	1-2	0-1
Oxidation ditchb		1-2	0-2	1-2	0-1
Disinfectiond		2-6g	0-1	0-4	0-3
Waste stabilization pondse		1-6g	1-3g	1-4	1-4
Effluent storage reservoirsf		1-6h	1-3h	1-4	1-4

^a Further research is needed to confirm performance.
^b Including secondary sedimentation.
^c Including settling pond.
^d Chlorination or ozonation.
^e Performance depends on number of ponds in series and other environmental factors.
^f Performance depends on retention time, which varies with demand.
^g With good design and proper operation the recommended guidelines are achievable.

Source: Mara and Cairncross (1989).

The most appropriate wastewater treatment is that which will produce an effluent meeting the recommended microbial guidelines both at a low cost and with minimal operational and maintenance requirements (Pescod and Arar, 1988). Table 7 gives a general qualitative comparison of various types of wastewater treatment systems in use today. Good reviews of wastewater treatment processes are found in FAO (1992), Shuval et al. (1986a) and WHO (1989).

The degree of removal of micro-organisms from wastewater by a treatment process is best expressed in terms of log₁₀ units (e.g., a reduction of 4 log₁₀ units = 10^-4 = 99.9% removal). To achieve the recommended WHO Guidelines for unrestricted irrigation, a reduction in the bacterial concentration of at least 4 log₁₀ units is required along with the need to achieve a reduction in the helminth egg concentration of 3 log₁₀ units (WHO, 1989). Table 8 gives the expected removal of various pathogens from typical wastewater treatment systems using the log₁₀ units.

Lowering risk of direct human exposure in areas using wastewater

In the area where wastewater is used directly or indirectly for crop production, three groups are at risk of disease infection:

· agricultural workers and their families;
· crop handlers; and
· those living near the areas irrigated with wastewater.

These rural groups carry the same relative risk of exposure as was shown in the epidemiological model of Shuval et al. (1986b). The greatest risk is from helminth infections and, as a result of repeated exposure, these rural groups likely build up high infection doses that are then transmitted to others in their community (Mara and Cairncross, 1989).

Lowering the potential for disease transmission must first focus on the source of contamination - the water. Wastewater treatment for helminth control must be a priority. In addition to treatment, all wastewater use schemes should have a control programme aimed at the rural population, especially the agricultural worker. The goal is to either prevent direct contact with the pathogens, or prevent any contact leading to disease. Measures to protect the agricultural field worker and the crop handlers include wearing protective clothing especially gloves and shoes (to prevent contact with pathogens), maintaining high levels of hygiene (to remove any pathogens present) and possibly immunizations (to prevent infection leading to disease). Local residents should be kept fully informed about where wastewater is used so they may avoid the area. Of equal importance is to educate workers, residents and others not to use canals or other wastewater facilities for drinking or domestic purposes. A broader discussion of these preventative measures is presented in WHO (1989).

FIGURE 4: Generalized model to show the level of risk to human health associated with different combinations of control measures for the use of wastewater in agriculture

(Source: Blumenthal et al. (1989) and Mara and Cairncross (1989))

Lowering risk to consumers through crop restrictions

The first approach to lowering the health risk from wastewater use in agriculture is by adequate wastewater treatment. In reality, in most developing countries, wastewater treatment to the levels proposed in the WHO Guidelines (Table 5) is a long-term goal. In the interim, until treatment facilities are operating, widespread unrestricted use with untreated or partially treated wastewater will continue. Temporary steps need to be taken to improve the existing situation. Along with building treatment facilities, an equal importance needs to be given to the second level of approach which is managing the wastewater use area to ensure this is not the source of infectious diseases.

Once the wastewater is applied, the field and the crop become the vehicle of infectious exposure. The field is the route of exposure to the agricultural worker (see previous section) and the crop becomes the route of exposure to the consumer of that crop. The generalized model used by WHO (Figure 4) demonstrates the relative risk to human health when using wastewater and shows that cropping restrictions can be an effective measure to protect the consumer. Strauss (1991) reviewed the application of the control measures model to several wastewater use areas worldwide and concluded that the model was an effective planning tool.

Crop restriction is the most widely used measure to protect public health because it provides protection for both the general population and population groups that may have a lower resistance to infection. This latter group includes those not part of the indigenous population such as tourists or persons outside the country when produce is being exported to other countries or regions. The focus of crop restriction has been on salad or vegetable crops that are normally eaten raw. Recently, however, other crops have come under concern because of the introduction of pathogens into the home from wastewater irrigated fields. Many of the root crops and crops grown in contact with the ground (melons) are suspect (Shuval et al., 1986a).

Many feel that crop restrictions are administratively unattainable (Shuval et al., 1986a) and need a strong institutional framework along with a capacity to monitor and control compliance with the regulations (WHO, 1989). Mara and Cairncross (1989) discuss the strengths and weaknesses of a crop restriction programme and point to five factors that will contribute to a successful programme:

a law-abiding society or strong law enforcement exists;

a public body controls allocation of waste and has legal authority to enforce crop restrictions;

the irrigation in the wastewater use area has strong central management;

there is adequate demand for the unrestricted crops and they fetch a reasonable price; and

there is little market pressure in favour of the excluded crops.

The success of a crop restriction programme depends greatly on how many users there are and whether the wastewater use occurs within a defined area. Crop restriction is relatively simple to implement where the wastewater is used by a small number of large farms, whether they are private farms, cooperatives, state farms, or operated by the wastewater authority. Such an arrangement allows regulation to occur within a specified area. Knowing where the water is being used is a key factor in an effective programme. Enforcement of crop restrictions on a large number of small farms will be more difficult but, if they are within a defined area, control is much easier.

Crop restriction is easiest to implement when the wastewater use scheme, or at least the distribution of the wastewater, is centrally managed by an irrigation association or the wastewater authority. Centralized control of the wastewater distribution makes control much simpler regardless of the number of farmers utilizing the water and it implies that the wastewater is being used in a defined area. Centralized control removes many of the unknowns that make field level enforcement difficult.

Reality in many developing countries, however, is that the wastewater - treated, partially treated and untreated - is discharged directly to surface waters and these are again diverted downstream for irrigation purposes. This unrestricted discharge leads to widespread distribution of the wastewater and makes crop restriction extremely difficult. In this situation, it is essential to enforce strict control over the effluent quality being discharged but, as discussed previously, most treatment systems in developing countries are not capable of producing a consistent effluent quality or there are no treatment works in place. This means raw sewage or partially treated wastewater is being discharged, diluted and distributed throughout the irrigation network.

In addition to large unrestricted discharges occurring from the urban centres, secondary discharges are also having an impact on irrigation water quality. Secondary discharges are those that occur into irrigation canals after the irrigation water is diverted from the main surface water supply.

Lack of adequate treatment works, continued unrestricted distribution of wastewater and the impact of secondary discharges in developing countries make a reduction in infection potential at wastewater use sites difficult without some control on cropping. Without controls on cropping, the Agricultural Ministry in developing countries, in the near-term, may not be able to provide sufficient, safe, vegetable products to meet national needs or for export. In addition, as the world population becomes increasingly aware of the need for clean water and clean food products, the ability of the Agricultural Ministry to meet this demand will be greatly diminished.

In the interim, until adequate and reliable wastewater treatment facilities are completed or well defined use areas established, a national programme needs to be established to identify large irrigated areas that can be safely used to meet national and export vegetable production goals without having to implement a large surveillance and enforcement programme that is usually associated with crop restrictions. This programme should be a joint programme between the Health and Agricultural Ministries with the goal to:

assess the extent of contamination of existing irrigation water supplies;

define an approach to certify safe production areas; and

provide a database to assist in developing a national strategy to contain the source of the contamination of water used by irrigated agriculture.

Chapter 4 discusses an approach to promoting such safe production areas.