2.1 Introduction
2.2 Human exposure control
2.3 Effluent quality guidelines for health protection
2.4 Water quality guidelines for maximum crop production
2.5 Health protection measures in aquacultural use of wastewater
Health protection measures which can be applied in agricultural use of wastewater include the following, either singly or in combination:
- Wastewater treatment
- Crop restriction
- Control of wastewater application
- Human exposure control and promotion of hygiene
In the past, wastewater treatment has been widely adopted as the major control measure in controlled effluent use schemes, with crop restriction being used in a few notable cases. A more integrated approach to the planning of wastewater use in agriculture will take advantage of the optimal combination of the health protection measures available and allow for any soil/plant contraints in arriving at an economic system suited to the local sociocultural and institutional conditions.
A WHO (1989) Technical Report on 'Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture' discusses the integration of the various measures available to achieve effective health protection. Limitations of the administrative or legal systems in some countries will make some of these approaches difficult to apply, whereas shortage of skilled technical staff in other countries will place doubt upon reliance on wastewater treatment as the only control mechanism. To achieve greater flexibility in the use of wastewater application as a health protection measure, irrigation systems must be developed to be capable of delivering low quality wastewater and restrictions on irrigation technique and crops irrigated must become more common.
Of the health protection measures mentioned above, only human exposure control is not dealt with in greater depth in later chapters of the Manual. The objective with this approach is to prevent the population groups at risk from coming into direct contact with pathogens in the wastewater or to prevent any contact with the pathogens leading to disease. Four groups are at risk in agricultural use of wastewater:
- agricultural workers and their families
- crop handlers
- consumers of crops, meat and milk
- those living near the areas irrigated with wastewater
and different methods of exposure control might be applied for each group.
Control measures aimed at protecting agricultural field workers and crop handlers include the provision (and insistence on the wearing) of protective clothing, the maintenance of high levels of hygiene and immunization against (or chemotherapeutic control of) selected infections. Examples of these measures are given in the WHO (1989) Technical Report mentioned. Risks to consumers can be reduced through cooking the agricultural produce before consumption and by high standards of food hygiene, which should be emphasized in the health education associated with wastewater use schemes. Local residents should be kept fully informed on the use of wastewater in agriculture so that they, and their children, can avoid these areas. Although there is no evidence to suggest that those living near wastewater-irrigated fields are at significant risk, sprinklers should not be used within 100 m of houses or roads.
Special care must always be taken in wastewater use schemes to ensure that agricultural workers or the public do not use wastewater for drinking or domestic purposes by accident or for lack of an alternative. All wastewater channels, pipes and outlets must be clearly marked and preferably painted a characteristic colour. Wherever possible, outlet fittings should be designed/selected so as to prevent misuse.
Following several meetings of environmental specialists and epidemiologists, a WHO Scientific Group on Health Aspects of Use of Treated Wastewater for Agriculture and Aquaculture arrived at the microbiological quality quidelines for wastewater use in agriculture shown in Table 8. These guidelines were based on the consensus view that the actual risk associated with irrigation with treated wastewater is much lower than previously thought and that earlier standards and guidelines for effluent quality, such as the WHO (1973) recommended standards, were unjustifiably restrictive, particularly in respect of bacterial pathogens.
Table 8: RECOMMENDED MICROBIOLOGICAL QUALITY GUIDELINES FOR WASTEWATER USE IN AGRICULTUREa
|
Category |
Reuse condition |
Exposed group |
Intestinal nematodesb (arithmetic mean no. of eggs per litrec |
Faecal coliforms (geometric mean no. per 100 mlc) |
Wastewater treatment expected to achieve the required microbiological quality |
|
A |
Irrigation of crops likely to be eaten uncooked, sports fields, public parksdd |
Workers, consumers, public |
£ 1 |
£ 1000d |
A series of stabilization ponds designed to achieve the microbiological quality indicated, or equivalent treatment |
|
B |
Irrigation of cereal crops, industrial crops, fodder crops, pasture and treese |
Workers |
£ 1 |
No standard recommended |
Retention in stabilization ponds for 8-10 days or equivalent helminth and faecal coliform removal |
|
C |
Localized irrigation of crops in category B if exposure of workers and the public does not occur |
None |
Not applicable |
Not applicable |
Pretreatment as required by the irrigation technology, but not less than primary sedimentation |
a In specific cases, local epidemiological, socio-cultural and environmental factors should be taken into account, and the guidelines modified accordingly.b Ascaris and Trichuris species and hookworms.
c During the irrigation period.
d A more stringent guideline (<200 faecal coliforms per 100 ml) is appropriate for public lawns, such as hotel lawns, with which the public may come into direct contact.
e In the case of fruit trees, irrigation should cease two weeks before fruit is picked, and no fruit should be picked off the ground. Sprinkler irrigation should not be used.
Source: WHO (1989)
The new guidelines are stricter than previous standards in respect of the requirement to reduce the numbers of helminth eggs (Ascaris and Trichuris species and hookworms) in effluents for Category A and B conditions to a level of not more than one per litre. Also implied by the guidelines is the expectation that protozoan cysts will be reduced to the same level as helminth eggs. Although no bacterial pathogen limit is imposed for Category C conditions where farm workers are the only exposed population, on the premise that there is little or no evidence indicating a risk to such workers from bacteria, some degree of reduction in bacterial concentration is recommended for any effluent use situation.
The WHO Scientific Group considered the new approach to effluent quality would increase public health protection for the large numbers of people who were now being infected in areas where crops eaten uncooked are being irrigated in an unregulated, and often illegal, manner with raw wastewater. It was felt that the recommended guidelines, if adopted, would achieve this improvement and set targets which are both technologically and economically feasible. However, the need to interpret the guidelines carefully and modify them in the light of local epidemiological, sociocultural and environmental factors was also pointed out.
The effluent quality guidelines in Table 8 are intended as design goals for wastewater treatment systems, rather than standards requiring routine testing of effluents. Wastewater treatment processes achieving the recommended microbiological quality consistently as a result of their intrinsic design characteristics, rather than by high standards operational control, are to be preferred. In addition to the microbiological quality requirements of treated effluents used in agriculture, attention must also be given to those quality parameters of importance in respect of groundwater contamination and of soil structure and crop productivity.
Although heavy metals may not be a problem with purely domestic sewage effluents, all these elements are potentially present in municipal wastewater.
Traditionally, irrigation water is grouped into various quality classes in order to guide the user to the potential advantages as well as problems associated with its use and to achieve optimum crop production. The water quality classifications are only indicative guidelines and their application will have to be adjusted to conditions that prevail in the field. This is so because the conditions of water use in irrigation are very complex and difficult to predict. The suitability of water for irrigation will greatly depend on the climatic conditions, physical and chemical properties of the soil, the salt tolerance of the crop grown and the management practices. Thus, classification of water for irrigation will always be general in nature and applicable under average use conditions.
Many schemes of classification for irrigation water have been proposed. Ayers and Westcot (FAO 1985) classified irrigation water into three groups based on salinity, sodicity, toxicity and miscellaneous hazards, as shown in Table 9. These general water quality classification guidelines help to identify potential crop production problems associated with the use of conventional water sources. The guidelines are equally applicable to evaluate wastewaters for irrigation purposes in terms of their chemical constituents, such as dissolved salts, relative sodium content and toxic ions. Several basic assumptions were used to define the range of values in the guidelines and more detailed information on this is reported by Ayers and Westcot (FAO 1985).
The effect of sodium ions in irrigation water in reducing infiltration rate and soil permeability is dependent on the sodium ion concentration relative to the concentration of calcium and magnesium ions (as indicated by SAR) and the total salt concentration, as shown in the guidelines. It is graphically illustrated in Figure 4 which clearly indicates that, for a given SAR value, an increase in total salt concentration is likely to increase soil permeability and, for a given total salt concentration, an increase in SAR will decrease soil permeability. This illustrates the fact that soil permeability (including infiltration rate and surface crusting) hazards caused by sodium in irrigation water cannot be predicted independently of the dissolved salt content of the irrigation water or that of the surface layer of the soil.
Table 9: GUIDELINES FOR INTERPRETATION OF WATER QUALITY FOR IRRIGATION
|
Potential irrigation problem |
Units |
Degree of restriction on use |
|||
|
None |
Slight to moderate |
Severe |
|||
|
Salinity |
|||||
|
Ecw1 |
dS/m |
< 0.7 |
0.7 - 3.0 |
> 3.0 |
|
|
or |
|
|
|
|
|
|
TDS |
mg/l |
< 450 |
450 - 2000 |
> 2000 |
|
|
Infiltration |
|||||
|
SAR2 = 0 - 3 and ECw |
|
> 0.7 |
0.7 - 0.2 |
< 0.2 |
|
|
|
3 -6 |
|
> 1.2 |
1.2 - 0.3 |
< 0.3 |
|
|
6-12 |
|
> 1.9 |
1.9 - 0.5 |
< 0.5 |
|
|
12-20 |
|
> 2.9 |
2.9 - 1.3 |
< 1.3 |
|
|
20-40 |
|
> 5.0 |
5.0 - 2.9 |
< 2.9 |
|
Specific ion toxicity |
|||||
|
Sodium (Na) |
|||||
|
|
Surface irrigation |
SAR |
< 3 |
3 - 9 |
> 9 |
|
|
Sprinkler irrigation |
me/I |
< 3 |
> 3 |
|
|
Chloride (Cl) |
|||||
|
|
Surface irrigation |
me/I |
< 4 |
4 - 10 |
> 10 |
|
|
Sprinkler irrigation |
m3/l |
< 3 |
> 3 |
|
|
Boron (B) |
mg/l |
< 0.7 |
0.7 - 3.0 |
> 3.0 |
|
|
Trace Elements |
|||||
|
Miscellaneous effects |
|||||
|
Nitrogen (NO3-N)3 |
mg/l |
< 5 |
5 - 30 |
> 30 |
|
|
Bicarbonate (HCO3) |
me/I |
< 1.5 |
1.5 - 8.5 |
> 8.5 |
|
|
pH |
Normal range 6.5-8 |
||||
1 ECw means electrical conductivity in deciSiemens per metre at 25°C
2 SAR means sodium adsorption ratio
3 NO3-N means nitrate nitrogen reported in terms of elemental nitrogenSource: FAO(1985)
Municipal wastewater effluents may contain a number of toxic elements, including heavy metals, because under practical conditions wastes from many small and informal industrial sites are directly discharged into the common sewer system. These toxic elements are normally present in small amounts and, hence, they are called trace elements. Some of them may be removed during the treatment process but others will persist and could present phytotoxic problems. Thus, municipal wastewater effluents should be checked for trace element toxicity hazards, particularly when trace element contamination is suspected. Table 10 presents phytotoxic threshold levels of some selected trace elements.
Table 10: THRESHOLD LEVELS OF TRACE ELEMENTS FOR CROP PRODUCTION
|
|
Element |
Recommended maximum concentration (mg/l) |
Remarks |
|
Al |
(aluminium) |
5.0 |
Can cause non-productivity in acid soils (pH < 5.5), but more alkaline soils at pH > 7.0 will precipitate the ion and eliminate any toxicity. |
|
As |
(arsenic) |
0.10 |
Toxicity to plants varies widely, ranging from 12 mg/l for Sudan grass to less than 0.05 mg/l for rice. |
|
Be |
(beryllium) |
0.10 |
Toxicity to plants varies widely, ranging from 5 mg/l for kale to 0.5 mg/l for bush beans. |
|
Cd |
(cadmium) |
0.01 |
Toxic to beans, beets and turnips at concentrations as low as 0.1 mg/l in nutrient solutions. Conservative limits recommended due to its potential for accumulation in plants and soils to concentrations that may be harmful to humans. |
|
Co |
(cobalt) |
0.05 |
Toxic to tomato plants at 0.1 mg/l in nutrient solution. Tends to be inactivated by neutral and alkaline soils. |
|
Cr |
(chromium) |
0.10 |
Not generally recognized as an essential growth element. Conservative limits recommended due to lack of knowledge on its toxicity to plants. |
|
Cu |
(copper) |
0.20 |
Toxic to a number of plants at 0.1 to 1.0 mg/l in nutrient solutions. |
|
F |
(fluoride) |
1.0 |
Inactivated by neutral and alkaline soils. |
|
Fe |
(iron) |
5.0 |
Not toxic to plants in aerated soils, but can contribute to soil acidification and loss of availability of essential phosphorus and molybdenum. Overhead sprinkling may result in unsightly deposits on plants, equipment and buildings. |
|
Li |
(lithium) |
2.5 |
Tolerated by most crops up to 5 mg/l; mobile in soil. Toxic to citrus at low concentrations (<0.075 mg/l). Acts similarly to boron. |
|
Mn |
(manganese) |
0.20 |
Toxic to a number of crops at a few-tenths to a few mg/l, but usually only in acid soils. |
|
Mo |
(molybdenum) |
0.01 |
Not toxic to plants at normal concentrations in soil and water. Can be toxic to livestock if forage is grown in soils with high concentrations of available molybdenum. |
|
Ni |
(nickel) |
0.20 |
Toxic to a number of plants at 0.5 mg/l to 1.0 mg/l; reduced toxicity at neutral or alkaline pH. |
|
Pd |
(lead) |
5.0 |
Can inhibit plant cell growth at very high concentrations. |
|
Se |
(selenium) |
0.02 |
Toxic to plants at concentrations as low as 0.025 mg/l and toxic to livestock if forage is grown in soils with relatively high levels of added selenium. As essential element to animals but in very low concentrations. |
|
Sn |
(tin) |
|
|
|
Ti |
(titanium) |
- |
Effectively excluded by plants; specific tolerance unknown. |
|
W |
(tungsten) |
|
|
|
C |
(vanadium) |
0.10 |
Toxic to many plants at relatively low concentrations. |
|
Zn |
(zinc) |
2.0 |
Toxic to many plants at widely varying concentrations; reduced toxicity at pH > 6.0 and in fine textured or organic soils. |
1 The maximum concentration is based on a water application rate which is consistent with good irrigation practices (10 000 m3 per hectare per year). If the water application rate greatly exceeds this, the maximum concentrations should be adjusted downward accordingly. No adjustment should be made for application rates less than 10 000 m3 per hectare per year. The values given are for water used on a continuous basis at one site.Source: Adapted from National Academy of Sciences (1972) and Pratt (1972).
Figure 4: Threshold values of sodium adsorption ratio and total salt concentration on soil permeability hazard (Rhoades 1982)
2.5.1 Special concerns in aquacultural use of human wastes
2.5.2 Quality guidelines for health protection in using human wastes for aquaculture
The measures which can be taken to protect health in aquacultural use of wastewater are the same as in agricultural use, namely wastewater treatment, crop restriction, control of wastewater application and human exposure control and promotion of hygiene. For the protection of workers in aquaculture ponds, the quality of the water is of paramount importance, as it is in respect of the contamination of fish or plants grown in excreta-fertilized or wastewater ponds. Transmission of pathogens can occur through persons handling and preparing contaminated fish or aquatic plants, which make human exposure control and hygiene important features of aquaculture programmes. Both the treatment applied to excreta, nightsoil or wastewater before introduction to an aquaculture pond and the rate of waste application will have an effect on the quality of water in the pond. In the past, these factors have not been controlled for health reasons but rather so as to ensure that a pond is not overloaded organically or chemically to the point where it will not support fish life or be suitable for the growth of aquatic plants. Reliance has been placed primarily on minimizing the risk of pathogen transmission through consumption by thorough cooking of the products. This has not always been satisfactory and, where the pond products are eaten uncooked, no health protection is provided. In some aquacultural practices, for example in rural Indonesia, depuration techniques are used in attempting to decontaminate fish in the period immediately preceding harvesting.
A number of human excreted helminthic pathogens, when released to aquaculture ponds, can involve fish or aquatic plants as intermediate hosts. Strauss (1985) has listed the following trematode infections as being capable of transmission in this way:
Clonorchis
Heterophys
Opistorchis
Metagonimus
Diphyllobothrium
However, he indicated that only clonorchiasis (liver fluke) and the closely related opistorchiasis have been transmitted through fish grown in excrete-fertilized or wastewater (freshwater) ponds. The first phase of development of these pathogens occurs in specific snails or copepods (minute crustaceans), with fish acting as a second intermediate host. These helminthic infections have significant public health importance in Asia, where fish are sometimes eaten raw. Strauss also pointed out that the helminthic pathogens Fasciola (sheep and cattle liver flukes) and Fasciolopsis (giant intestinal fluke) have the same pattern of life cycle but depend on aquatic plants, such as water chestnut, water cress and water bamboo, as secondary intermediate hosts onto which free-swimming cercariae become attached and where they encyst.
Aquatic snails also serve as intermediate hosts for the trematode-genus Schistosoma which is the causative agent of schistosomiasis (bilharzia). Transmission can occur when workers wade into aquaculture ponds in which infected snails are present and the larval schistosome penetrates the skin. This occupational hazard exists only where this disease is endemic and where snail hosts are present in aquaculture ponds. Schistosome infection, particularly Schistosoma japonicum, has been identified in excreta-fertilized fish ponds.
Fish grown in excreta-fertilized or wastewater ponds may also become contaminated with bacteria and viruses and serve as a potential source of transmission of infection if the fish are eaten raw or undercooked. Pathogenic bacteria and viruses may be passively carried on the scales of fish or in their gills, intraperitoneal fluid, digestive tract or muscle. Strauss (1985) reviewed the limited literature on excreted bacteria and virus survival in fish and concluded that:
- invasion of fish muscle by bacteria is likely to occur if the concentrations of faecal coliforms and salmonellae in the pond are greater than 104 and 105 per 100 ml, respectively;- the potential for muscle invasion increases with the duration of exposure of the fish to contaminated pond water;
- little accumulation of enteric microorganisms and pathogens on, or penetration into, edible fish tissue occurs when the faecal coliform concentration in the pond water is below 103 per 100 ml;
- even at lower pond water contamination levels, high pathogen concentrations might be present in the digestive tract and the intraperitoneal fluid of the fish;
- pathogen invasion of the spleen, kidney and liver has been observed.
Because only limited experimental and field data on the health effects of sewage-fertilized aquaculture are available, the WHO Scientific Group on Health Aspects of Use of Treated Wastewater for Agriculture and Aquaculture could suggest only a tentative bacterial guideline for the quality of aquaculture pond water. The tentative bacterial guideline suggested is a geometric mean number of faecal coliforms of £ 103 per 100 ml (WHO, 1989). Furthermore, in view of the dilution of wastewater which normally occurs in aquaculture ponds, this ambient bacterial indicator concentration could be achieved, the Scientific Group suggested, by treating wastewater fed to ponds to a level of 10³-104 faecal coliforms / 100 ml. Such a guideline should ensure that invasion of fish muscle is prevented but pathogens might accumulate in the digestive tract and intraperitoneal fluid of fish. This might then create a health risk, through cross-contamination of fish flesh or other edible parts and transmission to consumers, if standards of hygiene in fish preparation are inadequate. High standards of hygiene during fish handling and, especially, gutting are necessary and cooking of fish is an important health safeguard. Similar considerations apply to the preparation and cooking of aquatic plants.
Table 11: BACTERIOLOGICAL QUALITY OF FISH FROM EXCRETA-REUSE SYSTEMS
|
Total aerobic bacterial concentration in fish muscle tissue, bacteria/g |
Fish quality |
|
0- 10 |
Very good |
|
10- 30 |
Medium |
|
> 50 |
Unacceptable |
Source: Buras et al. (1987)
Buras et al. (1985, 1987) have questioned the value of faecal coliforms as bacterial indicators for fish muscle because, in their studies, they were not always detected, whereas total aerobic bacteria (standard plate count) were. They proposed that total aerobic bacteria should be the indicators on the grounds that, if they were detectable in the fish, there was a chance that pathogenic bacteria would also be present. Consequently, the bacteriological standards for fish raised in excreta-fertilized and wastewater ponds indicated in Table 11 were recommended by Buras et al. (1987). A more recent State-of-the-Art-Review of Reuse of Human Excreta in Aquaculture (Edwards, 1990) discussed this issue and suggested that it was unlikely that fish will be of an unacceptable bacteriological quality when raised in excreta-fed ponds that are well-managed from an aquacultural point of view to produce good fish growth. That is, fish ponds loaded with excreta at a level which leads to the development of a relatively large biomass of phytoplankton, serving as natural food for the fish, but with adequate levels of dissolved oxygen maintained in the water, for the fish, should produce fish with acceptable bacteriological quality.
Transmission of the helminthic infections clonorchiasis and fasciolopsiasis occurs only in certain areas of Asia and can be prevented only by ensuring that no trematode eggs enter the pond or by snail control. Similar considerations apply to the control of schistosomiasis in areas where this disease is endemic. The Scientific Group (WHO, 1989) recommended an appropriate helminth quality guideline for all aquacultural use of wastewater as the absence of viable trematode eggs.
