Rev. sci. tech. Off. int. Epiz., 1991, 10 (3), 847-866

Effects of wastewater irrigation of pastures on the health of farm animals and humans

H.I. S H U V A L *

Summary: Wastewater recycling and reuse through agricultural irrigation can provide an economical means of preventing pollution of surface water sources, while conserving water resources and valuable nutrients for increased crop production and grazing pastures. With proper methods of wastewater treatment, health risks to farmers and farm animals as well as to the public who consume agricultural produce can be prevented. The possibility of transmission of the beef tapeworm Taenia saginata, Salmonella sp. and Mycobacteria sp. to farm animals grazing on wastewater-irrigated pasture is evaluated. While such transmission is possible when raw, untreated wastewater is used for irrigation, the risk is greatly reduced and essentially eliminated with effluent treated to meet the new World Health Organisation (WHO) recommended microbial guidelines for effluent. These guidelines call for one or less helminth eggs per litre and a mean of 1,000 faecal coliform bacteria per 100 ml.

In warm climates and where land is available, one of the most economical methods of achieving this standard is the use of multi-cell stabilisation ponds designed for a 25-day detention period. In colder climates and where land is a limiting factor of wastewater treatment, conventional secondary biological treatment followed by disinfection can also be applied to meet this standard.

KEYWORDS: Animal health - Infectious diseases - Livestock - Public health -Stabilisation ponds - Wastewater irrigation - Wastewater treatment - Zoonotic diseases.

I N T R O D U C T I O N

In developed and developing countries alike, the disposal o f wastewater from urban areas can pose a serious threat to the env ironment . The surface and underground sources o f drinking water may become contaminated by pathogenic micro-organisms and toxic chemica l s . Recreational and fishing areas in rivers and lakes and a long coastal shores may b e c o m e pol luted, and odours and mosqu i to breeding may b e c o m e a problem if wastewater is d i sposed o f improperly . T h u s , the proper treatment and sanitary disposal o f wastewater should be of high priority in urban deve lopment programmes if they are to protect public health and preserve the amenit ies o f the env ironment .

H o w e v e r , m a n y deve loped and deve lop ing countr ies conce ive o f wastewater treatment and disposal as an unavo idable , yet nonproduct ive expense . A s a result,

* Lunenfeld Kunin Professor of Environmental Health, Division of Environmental Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel.

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funds for investment in technical solutions have often been limited or not readily available. In areas where water is in short supply, there is a strong economic incentive because recycling of wastewater for agricultural irrigation helps to conserve resources (including water and soil nutrients) and protect the environment by preventing river pollution and seawater intrusion in coastal areas, and maintaining water quality. In addition, recycling may often be the least-cost solution for wastewater disposal.

This article describes the health problems associated with wastewater irrigation and the technical solutions which have been developed to make it an economically attractive option. Special emphasis is given to the control of animal disease associated with grazing on wastewater-irrigated pastures.

The idea of recycling wastes in agriculture is not new. In China and other parts of Asia, night soil (human faeces and urine) has been used to fertilise crops and replenish depleted soil nutrients since ancient times. The earliest sewage farms documented in the literature appear to be those of Bunzlau, Germany, which were in operation in the sixteenth century.

With the increased use of water carriage sewerage systems during the nineteenth century, more countries became interested in wastewater farming or land application, particularly in Europe. The First Royal Commission on Sewage Disposal in England gave its official blessing to the practice in 1865, stating that, " T h e right way to dispose of town sewage is to apply it continuously to the land and it is by such application that the pollution of rivers can be avoided ." During the same period, the conservationist movement in Europe began advocating that land application should become part of a policy for resource recycling and returning nutrients to the soil. In 1868, Victor Hugo gave voice to this view in Les Misérables: "All the human and animal manure which the world loses... by discharge of sewage into rivers... if returned to the land, instead of being thrown into the sea, should suffice to nourish the world."

Thus, the use of wastewater in agriculture or in land application arose from the desire to prevent pollution in rivers and to conserve water and nutrients to improve agriculture. These early reasons for wastewater reuse remain valid to this day, although the emphasis has changed as a result of experience, scientific advances and economic considerations, as explained below.

E V A L U A T I O N O F E P I D E M I O L O G I C A L E V I D E N C E

Exist ing regulat ions lack an ep idemio log ica l basis

The strict health regulations governing wastewater reuse which have been developed in industrialised countries over the past sixty years have been based to a great extent on scientific data indicating that most enteric pathogens can be detected in wastewater and that they can survive for extended periods in wastewater-irrigated soil and crops (Fig. 1). Most health authorities have concluded that , because pathogens can survive long enough to contaminate crops, even if their numbers are very low, they pose a serious risk to public health. However, these regulations were formulated at a time when sound epidemiological evidence was rather scanty. As a result, policy-makers used the cautious "zero r isk" approach and introduced very strict regulations that they hoped would protect the public against the risks thought to be associated with

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wastewater reuse. Most industrialised countries were not concerned that these regulations were overly restrictive because the economic and social benefits of unrestricted wastewater reuse were of only marginal interest.

Organism

Campylobacter spp. Giardia lamblia Entamoeba histolytica Shigella spp. Vibrio cholerae Salmonella typhi Salmonella spp. Escherichia coli (path.) Enteroviruses Hepatitis A virus Ancylostoma duodenale Trichuris trichiura Taenia saginata Ascaris lumbricoides

* T y p i c a l average n u m b e r of o rgan i sms facces ** Es t ima ted average l i f e o f infective stage at 2 0 o C - 3 0 ' ( '

FIG. 1

Pers istence o f selected enteric p a t h o g e n s in water , was tewater , soil and crops

(based on data from ref. 10)

One of the primary goals of our original study carried out for the World Bank (27) was to reevaluate all the credible, scientifically valid and quantifiable epidemiological evidence of the real human and animal health effects associated with wastewater irrigation. Such evidence was needed to determine the validity of current regulations and to develop appropriate technical solutions for existing problems.

The following evaluation is based on scientific papers published in recognised journals and on numerous unpublished government reports, university theses and private papers obtained during an intensive worldwide search carried out with the help of international and national agencies and individuals. Over 1,000 documents , some more than 100 years old, were examined in the course of this study, but few offered concrete epidemiological evidence of health effects. Most of them based their conclusions on inference and extrapolation. Nonetheless, about fifty of these reports provided enough credible evidence based on sound epidemiological procedures to make a detailed analysis useful. Those studies are reviewed in detail in the UNDP-World Bank report on which this paper is based (27). The general conclusions of some of the more pertinent studies are presented below.

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Populat ion groups consuming vegetables and salad crops irrigated with raw wastewater

In areas of the world where helminthic (worm) diseases caused by Ascaris and Trichuris are endemic in the population, and where raw, untreated wastewater is used to irrigate salad crops a n d / o r other vegetables generally eaten uncooked, the consumption of such crops may lead to infection. In 1931 Khalil (15) demonstrated the importance of this route of transmission in his pioneering studies in Egypt. Similarly, a study in Jerusalem (26) provided strong evidence that massive infections of both Ascaris and Trichuris may occur when salad and vegetable crops are irrigated with raw wastewater. The disease almost totally disappeared from the community when raw wastewater irrigation was stopped (Fig. 2). Two studies from Darmstadt, Germany (16, 2) provided additional support for these conclusions (Fig. 3).

F I G . 2

Relat ionship between Ascaris-positive s too l samples in the populat ion of western Jerusalem

and supply of vegetables and salad crops irrigated with raw wastewater in Jerusa lem, 1935-1982

(3, 14, 26)

Supply of was tewa te r -irr igated vegetables cut off

Supply of was tewa te r -irr igated vegetables re int roduced

Was tewa te r i r r igat ion of vegetables s topped

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F I G . 3

Was tewater irrigation of vegetables and Ascaris preva lence in Darmstadt , Berlin and other G e r m a n c i t ies , 1949

Darmstadt used raw wastewater for irrigation. In Berlin, wastewater underwent biological treatment and sedimentation

(2, 16, 23)

These studies also indicate that regardless of the level of municipal sanitation and personal hygiene, irrigation of vegetables and salad crops with raw wastewater can serve as a major pathway for continuing and long-term exposure to Ascaris and Trichuris infections. Both of these infections are of a cumulative and chronic nature, so that repeated long-term reinfection may result in a higher worm load and increased negative health effects, particularly among children.

Cholera can also be disseminated by vegetable and salad crops irrigated with raw wastewater if it is carrying cholera vibrio (9). This possibility is of particular concern in non-endemic areas where sanitation levels are relatively high, and the common

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routes of cholera transmission, such as contaminated drinking water and poor personal hygiene, are closed. Under such conditions, the introduction of a few cholera carriers (or subclinical cases) into a community could lead to massive infection of the wastewater stream and subsequent transmission of the disease to the consumers of vegetable crops irrigated with the raw wastewater, as occurred in Jerusalem in 1970 (8).

Similarly, evidence from Santiago, Chile strongly suggests that typhoid fever can be transmitted by fresh salad crops irrigated with raw wastewater. As Figure 4 illustrates, the number of typhoid fever cases in Santiago would rise rapidly at the beginning of the irrigation season (October), after 16,000 ha of vegetables and salad crops (usually eaten uncooked) were irrigated with raw wastewater (24). The relatively high socio-economic level, good water supply and good general sanitation in the city support the hypothesis that wastewater irrigation can become a major route for the transmission of such bacterial disease.

F I G . 4

Seasonal variation in typhoid fever cases in Sant iago and the rest o f Chi le

(average rates, 1977-1981) (based on ref. 24 and other published reports)

Heal th effects a m o n g sewage farm workers

Sewage farm workers exposed to raw wastewater in areas of India where Ancylostoma (hookworm) and Ascaris infections are endemic have much higher levels of infection than other agricultural workers (Fig. 5; 17). The risk of hookworm

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infect ion is particularly great in areas where farmers customari ly work barefoot because the broken skin o f their feet is readily penetrated by the mot i l e h o o k w o r m larva. Sewage farm workers in this study also suffered more from anaemia (a s y m p t o m o f severe h o o k w o r m infestat ion) than the controls . T h u s , there is ev idence that cont inuing occupat ional exposure to irrigation with raw wastewater can have a direct effect o n h u m a n product iv i ty and o n the e c o n o m y .

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F I G . 5

Intensity o f parasitic infect ion in sewage farm workers and contro ls in various regions o f India

(based on ref. 17)

S e w a g e farm workers are a lso l iable to b e c o m e infected with cholera if the raw wastewater being used for irrigation is obta ined from an urban area experiencing a cholera ep idemic . This s i tuat ion is particularly likely to arise in an area where cholera normal ly is no t e n d e m i c and where the level o f immuni ty a m o n g the sewage farm

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workers is low or non-existent. The 1970 cholera outbreak in Jerusalem provides an illustration of this case (8).

Studies from industrialised countries have thus far produced only limited, and often conflicting, evidence of the incidence of bacterial and viral diseases among wastewater-irrigation workers exposed to partly or fully treated effluent, or among workers in wastewater treatment plants exposed directly to wastewater or wastewater aerosols. Most morbidity and serological studies have been unable to give a clear indication of any excess prevalence of viral diseases among such occupationally exposed groups.

With the severe epidemic of cholera currently underway in some South American countries, special precautions should be taken in the case of farmers irrigating large areas of vegetables and salad crops with untreated wastewater in the immediate vicinity of cities. Although this practice is usually frowned upon by the authorities, it is tolerated in some of the continent 's largest and most important cities. There is ample evidence that cholera can be transmitted by vegetables and salad crops eaten uncooked which have been irrigated with raw wastewater derived from communities where cholera transmission is in progress. Those cities now facing the imminent threat of cholera would be wise to take drastic measures to prevent the growing and marketing of such crops.

As proved by the 1973 Naples epidemic, cholera can also be transmitted by clams, mussels and other bivalves and possibly by fish grown in coastal waters polluted by wastewater flows from cholera-infected cities (25). The same may apply to fish harvested from sewage-polluted rivers and aquaculture ponds . There is no evidence that farm animals grazing on wastewater-irrigated pasture can serve as biological vectors of cholera but the udders of milk cows, sheep and goats can become contaminated mechanically and infect milk and milk products which are consumed unpasteurised.

It is hypothesised that at a very young age many sewage farmers or treatment plant workers have already acquired relatively high levels of permanent immunity to most of the common enteric viruses endemic in their communit ies. Thus, by the time they are exposed occupationally, the number of susceptible workers is small and any excess disease which does develop is not statistically significant. Presumably this is also the case among infants and children in developing countries because they are exposed to most endemic enteric viral diseases by the time they reach working age. Although this is not the case for some bacterial and protozoan pathogens, multiple routes of concurrent infection with these diseases may well mask any excess among wastewater-irrigation workers in developing countries.

Heal th effects a m o n g popula t ion groups residing near wastewater- irrigated fields

There is little evidence linking disease a n d / o r infection among populat ion groups living near wastewater treatment plants or irrigation sites with pathogens contained in aerosols of wastewater. Most studies have shown no demonstrable disease resulting from aerosols caused by sprinkler irrigation and aeration processes. Researchers agree, however, that most of the early studies were inadequate.

Our own studies in Israel suggest that aerosols from sprinkler irrigation with poor microbial-quality wastewater can, under certain circumstances, cause limited infections among infants living near the fields. The studies, however, also concluded that the

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health effects were negligible and could be controlled by better treatment (7, 28, 29). Similar findings have been reported in the United States.

These findings support the conclusion that the relatively high levels of immunity against most viruses endemic to the community generally block additional environmental transmission by wastewater irrigation. Therefore, the additional health burden is not measurable. The primary route of transmission of such enteroviruses, even under good hygiene conditions, is through contact infection in the home at a relatively young age. As already mentioned, such contact infection is even more common in developing countries, so that a town's wastewater would not normally be expected to transmit viral diseases to rural areas using it for irrigation.

Transmiss ion of disease to h u m a n s by meat or dairy products from cattle or sheep grazing on wastewater- irrigated fields

The primary route of infection of humans with Taenia saginata, the beef tapeworm, and Taenia solium, the pork tapeworm, is through the consumption of raw or undercooked meat infected with the larval stage of these worms. Although there are no confirmed reports of human infections related to the consumption of beef or pork from animals grazed on wastewater-irrigated pastures, a number of authorities consider that route of infection to be a potential risk (30, 12, 13).

Numerous reports have shown that cattle become infected with the parasite by grazing on pasture irrigated with raw or partly treated wastewater. As early as 1937, Penfold et al. (19, 20) reported accumulation of the eggs of T. saginata, the beef tapeworm, in the soil near raw wastewater outlets at the Werribbee Farm in Melbourne, Australia. Forty-six percent of the cattle that had been on the farm for six months were infested with cysts of Cysticercus bovis, caused by infections of the animal muscle by T. saginata.

These researchers also reported that 100 cases of human tapeworm infection had been treated in Melbourne during the six-month period of their study. They found that the oxen recovered from the infestation after long-term exposure of two-and-a-half to three years and acquired immunity to further infection, but the beef muscles were left with the distinctive cysts of scar tissue. Subsequently, the Australian government banned the sale for human consumption of cattle raised on the Melbourne sewage farm (33). However, it is impossible to draw any conclusions from these reports as to whether the human cases of the disease in Melbourne were a direct result of consuming meat from cattle grazed on wastewater-irrigated pasture at the Werribbee Farm. There was undoubtedly a sufficient concentration of T. saginata eggs excreted by the population into the sewage system to cause the massive infections detected among the cattle exposed to pastures irrigated with raw wastewater. It is not clear, however, that the full cycle of infection (man-sewage-cattle-man) was in operation. No information is provided about tapeworm infection rates in other Australian cities which did not practice sewage irrigation of pastures.

Since that time all wastewater at the Melbourne sewage farms has been treated in ponds or in sedimentation basins prior to irrigation of pastures. This treatment effectively settles out the eggs of T. saginata. A recent report from the manager of Werribbee Farm indicates that some 22,000 head of cattle and 20,000-50,000 sheep are grazed on sewage-irrigated pastures (J .B. McPherson, personal communication). Cattle sales are strictly regulated by law. All cattle must be slaughtered at a registered abattoir controlled by the Department of Agriculture and carcasses are subject to

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rigorous inspection. With effluent treated in ponds prior to pasture irrigation, condemnation of carcasses of Werribbee Farm animals from all causes has run at about 0.02 percent in recent years, or half the average for inspected cattle from the rest of the state of Victoria. This statistic provides important indirect evidence of the effectiveness of pond treatment in removing the settleable eggs of helminths such as T. saginata.

Other reports (14, 30) have confirmed that cattle can be infected with the disease by grazing on raw wastewater-irrigated fields or by drinking wastewater from open canals. The eggs of T. saginata, for example, are known to survive in the soil for six months under cool moist conditions, although under hot dry conditions survival is unlikely to exceed two months (10). Thus, the potential for transmission by wastewater irrigation cannot be overlooked. However, such transmission may be primarily a veterinary problem owing to the serious economic losses resulting from the infection of animals.

The possibility that salmonellosis may be transmitted to cattle grazing on wastewater-irrigated pasture has also been studied. Salmonella organisms are normally present in wastewater, often in high concentrations, and can survive for long periods in moist soil. Although some experimental epidemiological evidence from studies in Switzerland, Germany and the United Kingdom strongly suggests that cattle can become infected with salmonellae after grazing on pastures fertilised with wastewater sludge (20), there is no specific indication of further transmission to humans as a result of the infection of cattle. Salmonellosis is generally endemic as a zoonotic disease among cattle and other domestic animals, and the disease is transmitted between them by a number of different routes. Feachem et al. (10) concluded that there is no clear evidence that cattle grazed on pastures fertilised with sludge, night soil or wastewater are more at risk from salmonellosis than other cattle. Pike (21) has likewise concluded that although transmission of salmonellosis to cattle by the use of sludge on grazing land cannot be ruled out, it would appear to be a minor cause except in cases of highly dense animal populations on pastures heavily fertilised with untreated sludge containing high concentrations of salmonellae.

These findings suggest that the problem of Salmonella transmission to cattle and subsequently to humans as a result of wastewater irrigation of pasture land can indeed be considered a marginal problem, although it cannot be completely ignored. Thus, pre-treatment of effluent in order to provide some measure of inactivation or removal of Salmonella organisms would be prudent .

The third potential public health problem associated with wastewater irrigation of pastures is that effluent may contain Mycobacterium tuberculosis, which can cause bovine tuberculosis. Humans , in turn, can become infected by consuming contaminated milk or meat. The pathogen has been found in wastewater, and although much has been written about the possibilites of such transmission, there is no concrete evidence supporting it (11). Feachem et al. (10) have concluded that it remains doubtful whether transmission of either human or bovine tuberculosis is significantly affected by wastewater irrigation of pasture lands. However, we believe that in areas where tuberculosis is a major public health problem, this issue should be carefully evaluated in any plans to irrigate pastures with wastewater.

There are no published reports on disease transmission due to dairy products derived from milk animals grazed on wastewater-irrigated pastures. Physical contamination can occur when milk is taken from cows or sheep whose udders have

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become contaminated with human pathogens from the wastewater-irrigated pastures. Enteric bacterial pathogens from wastewater such as Salmonella can multiply rapidly in uncooled milk and create a possible source of foodborne disease. If the milk is boiled or pasteurised before humans consume it and before cheese and other dairy products are prepared, this problem can be overcome. Regardless of whether pastures are irrigated with wastewater, pasteurisation of all milk for direct consumption or for the preparation of dairy products is an essential public health measure.

Reduct ion in negative health effects by wastewater treatment

Epidemiological studies have provided evidence that negative health effects can be reduced when wastewater is treated for the removal of pathogens. For example, Baumhogger (2) reported that in 1949 residents of Darmstadt who consumed salad crops and vegetables irrigated with raw wastewater experienced a massive infection of Ascaris; but the residents of Berlin, where biological treatment and sedimentation were applied to wastewater prior to the irrigation of similar crops, did not (Fig. 3).

Another study on intestinal parasites was conducted on school-children near Mexico City (22). The prevalence of intestinal parasites in children from villages which used wastewater irrigation did not differ significantly from that in children from the control villages, which did not irrigate with wastewater. The lack of significant difference between the two groups may have resulted from storage of the wastewater in a large reservoir for weeks or months prior to its use for irrigation. Presumably, sedimentation and pathogen die-away during long-term storage were effective in removing the large, easily settleable protozoa and helminths, which were the pathogens of interest in this study.

The absence of negative health effects in Lubbock, Texas (5) and Muskegon, Michigan (6) appears to be associated with the fact that well-treated effluents from areas of low endemicity were used for irrigation.

The data from Werribbee farm in Melbourne indicate the total elimination of Cysticercus bovis from cattle grazing on pastures irrigated with well-treated stabilisation pond effluent. Data from these field studies strongly suggest that pathogen reduction by wastewater treatment can have a positive effect on human and animal health. In all of the above studies, this positive effect was achieved despite the use of effluent which had not been disinfected and which contained many thousands of faecal coliform bacteria per 100 ml. These data agree with water quality data on pathogen removal and suggest that appropriate wastewater treatment resulting in effective but not necessarily total removal of the principal pathogens and indicator organisms can provide a high level of health protection.

A P P R O P R I A T E L O W - C O S T M E T H O D S O F W A S T E W A T E R T R E A T M E N T F O R I R R I G A T I O N

Goals of wastewater treatment for irrigation

As shown by epidemiological studies, one of the most effective ways of protecting human and farm animal health when utilising wastewater for crop and pasture irrigation is appropriate wastewater treatment which achieves significant pathogen

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reduction. In areas with plentiful rainfall, wastewater has traditionally been disposed of or diluted in large bodies of water, such as rivers and lakes. High priority has been given to maintaining the oxygen balance of these bodies of water to prevent serious odour and other environmental nuisances resulting from wastewater pollution.

Most of the conventional processes used to treat wastewater in industrialised countries have been designed primarily to remove the suspended and dissolved organic fractions which decompose rapidly in natural bodies of water. The organic matter in wastewater, usually measured as biochemical oxygen demand (BOD), provides rich nutrients to the natural micro-organisms of the stream, which multiply rapidly and consume the limited reserves of dissolved oxygen (DO) in the streams. If oxygen levels drop too far, serious odours may develop and fish may die.

A secondary goal of conventional wastewater treatment has been to reduce pathogenic micro-organisms in order to protect the quality of the sources of drinking water used by downstream communities. However, most conventional treatment systems are not particularly efficient in removing pathogens. Thus, communities which draw their drinking water from surface sources cannot depend upon upstream wastewater treatment to reduce pathogens to a safe level; they must remove the pathogens with their own water treatment systems using a series of highly efficient, technical and costly processes (e.g. coagulation, sedimentation, filtration and chemical disinfection). The most effective conventional wastewater treatment system is activated sludge, which removes 90-99% of viruses, protozoa and helminths and 90-99.9% of bacteria. Conventional processes cannot achieve higher levels of pathogen removal without great additional expense for chemical disinfection, such as chlorination, or for additional sand filtration. Further research and development are needed to improve the removal of helminths by conventional methods. As yet, little effort has been made to develop new and more effective methods.

In contrast, stabilisation ponds are low in cost, easy to operate (and thus highly suitable for developing countries) and very effective against pathogens. Well-designed, multi-cell pond systems can remove 99.99-99.9999% (4-6 log10) of the bacteria and helminths from raw wastewater, and in warm climates can achieve an effluent quality of about 1,000 faecal coliform bacteria per 100 ml.

When wastewater is to be used to irrigate crops for human consumption or animal pasture the goals of treatment are the reverse of the goals of conventional treatment. The primary goal for treatment of wastewater to be used for irrigation must be removal of pathogenic micro-organisms in order to protect the health of the farmers, animals and consumers. (Actual microbial standards and guidelines will be discussed below.) Removal of the organic material, however, which contains valuable agricultural nutrients is neither necessary nor desirable, although aerobic conditions should be maintained because a black, highly odorous, anaerobic wastewater effluent would probably be an environmental nuisance to farmers and nearby residents.

If wastewater is to be disposed of by means of a conventional system and discharged into a lake or river, then the primary goal is removal of the organic material (that is, lowering of the BOD) in order to prevent pollution of the natural waterways. Removal of pathogens is only a secondary goal of conventional treatment systems. They are not designed to , and do not, remove pathogens as efficiently as the stabilisation ponds which are used to treat wastewater prior to irrigation.

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For sprinkler or drip irrigation systems, suspended particles must be removed to prevent clogging of the orifices in the irrigation equipment. In all cases, the large rapidly settleable solids must be removed to prevent sedimentation in irrigation canals.

To summarise, wastewater that is to be used for irrigation must be treated for the following reasons (in descending order of priority):

1) to remove pathogens, including helminths, bacteria, viruses and protozoa

2) to maintain aerobic conditions and prevent serious odours

3) to ensure that nutrients of agricultural importance are not lost

4) to remove large settleable solids to prevent irrigation channels from clogging and, in the case of sprinkler or drip systems, to remove suspended solids as required.

Stabil isat ion p o n d s - a highly eff icient wastewater treatment process

It is beyond the scope of this paper to provide a full review of the science and technology of wastewater treatment which might meet all the criteria of agricultural irrigation. For further information, the reader is referred to standard texts on the subject and the full UNDP-World Bank report (27).

The main emphasis of this paper is on stabilisation ponds because they are suitable for many situations in developing countries, cost little to use, require little or no mechanical equipment, and are robust and easy to operate. What is most important , they provide an exceptionally high degree of pathogen removal, better than that achieved by most conventional treatment processes.

Stabilisation ponds are simple, natural waste treatment systems consisting of large, open, earthen lagoons or ponds, usually 1.5-2 m deep. Typically, they hold the sewage flow for 20-25 days. Although many factors — for example, wastewater quantity and quality, climatic conditions and the degree of treatment required — must be taken into account in the actual design of stabilisation ponds, the pond area required for warm countries is about 3 m 2 per person; that is, 30 ha for a city with a population of 100,000 connected to the sewerage system.

The wastewater in stabilisation ponds is treated by a number of natural biological and physical processes without the need for the expensive concrete tanks, mechanical equipment, energy and chemicals that are usually required in conventional treatment plants. Sunlight, which serves as the main source of external energy, stimulates the natural biological processes, which in turn stabilise the organic wastes. Bacteria and other micro-organisms break down the complex decomposable organic matter (which can become a nuisance if not removed) in the wastewater, and the nutrient components are taken up by the stable algae biomass. The bacteria also produce the C O 2 required by algae. The algae in turn produce oxygen by photosynthesis which is used by the growing biomass of aerobic bacteria at work purifying the wastewater. The effluent from stabilisation ponds is usually biologically stable, not objectionable in smell, green in color owing to the algae biomass and rich in nutrients of value to agriculture.

The main physical process at work in wastewater stabilisation ponds is sedimentation. Since wastewater is retained in the pond system for long periods of time, all of the rapidly settleable solids are removed, along with a majority of the slower, more settleable suspended particles. Most of the helminth eggs settle out as well. These eggs vary in size from 5 to 150 pm and their specific gravity is greater

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t h a n w a t e r (it r a n g e s f r o m 1.055 to 1.18). T h e eggs of t h e la rges t (Schistosoma sp.) h a v e a se t t l ing ve loc i ty of 12.5 m / h a n d t h o s e of t he smal les t (T. saginata) se t t le at a r a t e of 0 .26 m / h , which m e a n s t h a t , t h e o r e t i c a l l y , t o t a l r e m o v a l f r o m a 1.5 m deep s t ab i l i s a t i on p o n d will be a c h i e v e d in a b o u t six h o u r s .

Field s tudies h a v e s h o w n t h a t wel i -des igned mult i -cel l s t ab i l i sa t ion p o n d s a l lowing 10-20 d a y s of r e t e n t i o n can r e m o v e essent ia l ly 1 0 0 % of t h e h e l m i n t h eggs (10, 18, 34).

Bac t e r i a , v i ruses a n d p r o t o z o a a r e of ten a t t a c h e d t o l a rge r faecal pa r t i c l e s that set t le ou t in p o n d s y s t e m s . At bes t , h o w e v e r , o n l y 9 0 % c a n b e r e m o v e d by s e d i m e n t a t i o n . T h e m o s t ef fect ive p r o c e s s for r e m o v i n g b a c t e r i a a n d v i ruses in s tab i l i sa t ion p o n d s is d i e -away , which increases with t ime, p H a n d t e m p e r a t u r e . M a n y d e v e l o p i n g c o u n t r i e s h a v e ho t c l ima te s in wh ich s t ab i l i s a t i on p o n d s a r e exposed to t h e d i rec t r ay s of t h e sun a n d m a y r e a c h t e m p e r a t u r e s u p t o 4 0 ° C . T h e p H at m i d d a y is c o m m o n l y 9 or h ighe r o w i n g t o t he p h o t o s y n t h e t i c ac t iv i ty of the a l g a e . P r e d a t o r y o r c o m p e t i n g m i c r o - o r g a n i s m s m a y a l so affect d i e - a w a y by a t t a c k i n g o r d a m a g i n g p a t h o g e n s d i rec t ly or ind i rec t ly . E x p o s u r e to t he u l t r av io l e t r ays of t h e sun m a y also p lay a ro le in ki l l ing p a t h o g e n s in p o n d s .

L o n g re ten t ion t imes , h o w e v e r , a p p e a r to be the mos t i m p o r t a n t fac tor in reducing bac t e r i a l c o n c e n t r a t i o n s in p o n d s y s t e m s .

in w a r m c l imates wi th t e m p e r a t u r e s in excess of 2 0 ° C , a p o n d sys tem with 4-5 cells a n d a 20-25 d a y r e t e n t i o n t i m e usua l ly r e d u c e s t h e faecal c o l i f o r m c o n c e n t r a t i o n by 4-6 log10 o r d e r s of m a g n i t u d e - t h a t i s , by 9 9 . 9 9 - 9 9 . 9 9 9 9 % . T h u s , if t he initial c o n c e n t r a t i o n of faecal c o l i f o r m s in t h e r aw eff luent is a p p r o x i m a t e l y 1 0 7 / 1 0 0 ml, t he ef f luent will c o n t a i n 1 0 - 1 , 0 0 0 / 1 0 0 m l . T h e s a m e p o n d will r e d u c e e n t e r i c viruses by 2-4 log10 ( i . e . f rom a n ini t ia l c o n c e n t r a t i o n o f a b o u t 1 ,000 /100 ml to 10 or f e w e r / 1 0 0 ml ) . H e l m i n t h s will b e r e m o v e d comple t e ly , whi le t he B O D will be reduced by a b o u t 8 0 % . F igu re 6 s h o w s t h e gene ra l i s ed r e m o v a l c u r v e s for B O D , h e l m i n t h eggs , b a c t e r i a a n d vi ruses in a mul t i -ce l l s t ab i l i s a t i on p o n d sys t em in a w a r m c l ima te .

S t a b i l i s a t i o n p o n d s a r e , t h e r e f o r e , h igh ly su i t ab l e for t r e a t i n g w a s t e w a t e r for i r r i g a t i o n . T h e y a r e m o r e eff icient in r e m o v i n g p a t h o g e n s , p a r t i c u l a r l y h e l m i n t h s , t h a n a r e c o n v e n t i o n a l w a s t e w a t e r t r e a t m e n t s y s t e m s . In a d d i t i o n , t hey p r o d u c e a b io log ica l ly s t a b l e , o d o u r l e s s , n u i s a n c e - f r e e e f f luen t w i t h o u t r e m o v i n g t o o m a n y of t he n u t r i e n t s . T h u s , p o n d s s h o u l d be t h e sys tem of c h o i c e fo r w a s t e w a t e r i r r iga t ion in w a r m c l i m a t e s , especia l ly if l a n d is a v a i l a b l e a t a r e a s o n a b l e p r i ce .

Fo r d e v e l o p i n g c o u n t r i e s , p o n d s a r e pa r t i cu l a r l y a t t r a c t i v e b e c a u s e they cost little t o m a i n t a i n , a r e r o b u s t a n d fa i l - sa fe . T h e y s h o u l d neve r be c o n s i d e r e d a cheap subs t i t u t e . In real i ty they a re s u p e r i o r t o c o n v e n t i o n a l m e t h o d s of t r e a t m e n t in almost all respects . A l t h o u g h p o n d s r equ i re relatively large l and a r ea s , l and costs a re generally no t a s e r ious o b s t a c l e .

The Engelberg Report and the new W H O guidelines

In Ju ly 1985 a g r o u p of e n v i r o n m e n t a l expe r t s a n d e p i d e m i o l o g i s t s m e e t i n g at E n g e l b e r g , Swi t ze r l and , u n d e r t he ausp ices of t he U N D P , W o r l d B a n k , W H O , U N E P a n d t he I n t e r n a t i o n a l Re fe rence C e n t r e for W a s t e s D i s p o s a l f o r m u l a t e d t en t a t i ve new m i c r o b i o l o g i c a l gu ide l ines for t r e a t e d w a s t e w a t e r r euse in a g r i c u l t u r a l i r r i ga t i on (1). T h e g r o u p rev iewed t he e p i d e m i o l o g i c a l ev idence g a t h e r e d in t he U N D P - W o r l d Bank s t u d y (27) a n d the e p i d e m i o l o g i c a l ana lys i s p r e p a r e d for t he W H O by B l u m and

861

Treatment phases

F I G . 6

General ised removal curves for B O D , he lminth eggs , excreted bacteria and viruses in was te stabi l isat ion p o n d s

at temperatures a b o v e 2 0 ° C

Feachem (4). The group accepted the main findings and recommendations of the UNDP-World Bank study and concluded that "current guidelines and standards for human waste use are overly conservative and unduly restrict project development, thereby encouraging unregulated human waste u s e " . The new, tentative guidelines recommended in the Engelberg Report and later recommended by the W H O meeting of experts (32) are presented in Table I. Since the possibility of transmitting helminth disease by wastewater irrigation of even non-edible crops was identified as the top health problem, a new, stricter approach to the use of raw wastewater was developed. The Engelberg guidelines recommend effective water treatment in all cases to remove helminths to a level of one or fewer helminth eggs per litre.

The main innovation of the Engelberg guidelines is: for crops eaten uncooked (and on pasture land where farm animals are grazed), an effluent must contain one or fewer helminth eggs per litre, with a geometric mean of faecal coliforms not exceeding 1,000/100 ml. This is a much more liberal coliform standard than the early California requirement of 2 total coliforms/100 ml or even the 1973 W H O guideline of 100 total coliforms/100 ml (31).

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In November 1987, an official W H O meeting of scientists reviewed the earlier W H O wastewater irrigation microbial guidelines in light of the vast array of new evidence. After careful consideration, it adopted the Engelberg guidelines as the basis for the authoritative new 1989 W H O guidelines (32). Thus , the highest international public health body has given its s tamp of approval to this new approach.

An attractive feature of the new W H O effluent guidelines (32) is that they can be readily achieved with low-cost, robust stabilisation pond systems which are particularly suited to developing countries. High levels of pathogen removal can be achieved by such low-cost systems (Fig. 6).

* * *

UTILISATION DES EAUX USÉES POUR L'IRRIGATION DES PÂTURAGES : RÉPERCUSSIONS SUR LA SANTÉ H U M A I N E ET ANIMALE. - H.I . Shuval.

Résumé: Le recyclage et la réutilisation des eaux usées pour l'irrigation des cultures peuvent être des moyens économiques de prévention de la pollution des eaux de surface, qui préservent les ressources hydriques et les éléments nutritifs utiles à l'agriculture et à l'élevage. L'utilisation de méthodes adaptées pour le traitement des eaux usées permet de protéger la santé des exploitants, du bétail et des consommateurs des produits agricoles. L'auteur évalue les risques de transmission de Taenia saginata, Salmonella sp. et Mycobacteria sp. aux animaux conduits sur des pâturages irrigués par des eaux usées. Si l'irrigation par des eaux usées brutes, non traitées, peut favoriser la transmission de ces agents pathogènes, le traitement des eaux conformément aux nouvelles normes de l'Organisation Mondiale de la Santé (OMS) sur les effluents permet de réduire considérablement et même d'éliminer pratiquement ces risques. La limite maximale admise est d'un œuf d'helminthe par litre et de 1 000 bactéries fécales conformes (en moyenne) pour 100 ml.

Dans les climats chauds, et à condition de disposer de vastes superficies, l'une des techniques les plus rentables pour respecter ces normes est le recours à des bassins de stabilisation multi-cellulaires conçus pour une période de retenue de 25 jours. Dans les pays froids, et lorsque la superficie disponible constitue un facteur limitant pour le traitement des eaux usées, l'application d'un traitement biologique secondaire classique, suivi de désinfection, permet également d'atteindre cet objectif.

MOTS-CLÉS : Bassins de stabilisation - Bétail - Eaux résiduaires d'irrigation -Maladies infectieuses - Santé animale - Santé humaine - Traitement des eaux usées - Zoonoses.

* * *

CONSECUENCIAS DEL RIEGO DE PASTOS CON A G U A S RESIDUALES P A R A LA SALUD DEL HOMBRE Y LOS ANIMALES. - H.I . Shuval.

Resumen: El reciclaje y la reutilización de las aguas residuales para el riego de cultivos puede ser un medio económico de evitar la contaminación de las aguas superficiales y de preservar tanto los recursos hídricos como las sustancias nutritivas indispensables para el desarrollo de la producción agrícola y ganadera.

864

El empleo de métodos adecuados para el tratamiento de las aguas residuales permite proteger la salud de los agricultores, del ganado y de los consumidores de productos agrícolas. El autor calcula la posibilidad de transmisión de Taenia saginata, Salmonella sp. y Mycobacteria sp. a los animales criados en pastos regados con aguas residuales. Esa posibilidad, que existe efectivamente cuando se riegan los pastos con aguas residuales sin tratamiento previo, disminuye notablemente e incluso desaparece práticamente cuando se aplican las nuevas normas de la Organización Mundial de la Salud (OMS) relativas al tratamiento de efluentes. Los limites máximos establecidos por estas normas son: un huevo de helminto por litro y un promedio de 1.000 bacterias conformes por ¡00 ml.

En climas cálidos, y siempre que se disponga de suficiente superficie, uno de los métodos más económicos de cumplimiento de estas normas es la utilización de embalses de estabilización multicelulares con un período de contención preestablecido (25 días). En climas más fríos, y allí donde la superficie disponible constituye un factor limitante para el tratamiento de las aguas residuales, las normas pueden cumplirse aplicando un tratamiento biológico secundario clásico seguido de una desinfección.

PALABRAS CLAVE: Embalses de estabilización - Enfermedades infecciosas -Ganado - Riego con aguas residuales - Salud pública - Sanidad animal -Tratamiento de las aguas residuales - Zoonosis.

* * *

REFERENCES

1. A N O N . (1985). - Health aspects of wastewater and excreta use in agriculture and aquaculture. Report of review meeting of environmental specialists and epidemiologists, Engelberg, Switzerland, 1-4 July. Sponsored by the World Bank/WHO. International Reference Centre for Wastes Disposal, Dubendorf, Switzerland.

2. B A U M H O G G E R W. (1949). - Ascariasis in Darmstadt and Hessen as seen by a wastewater engineer (in German). Z. Hyg. Infektionskr., 129, 488-506.

3. B E N - A R I J. (1962). - The incidence of Ascaris limbricoides and Trichuris trichuria in Jerusalem during the period of 1934-1960. Am. J. trop. Med. Hyg., 11, 336-338.

4. B L U M D . & F E A C H E M R.G. (1985). - Health aspects of night soil and sludge use in agriculture and aquaculture: an epidemiological perspective. International Reference Centre for Wastes Disposal, Dubendorf, Switzerland.

5. C A M A N N D.E. , N O R T H R O P R.L., G R A H A M P.J. , G U E N T Z E L M . N . , H A R D I N G H . J . ,

K I M B A L L K.T. , M A S O N R.L. , M O O R E B . E . , S O R B E R C . A . , B E C K E R C M . &

J A K O B O W S K Y W. (1983). - An evaluation of potential infectious health effects from sprinkler application of wastewater to land. Lubbock, Texas. Second interim report to the US Environmental Protection Agency, Cincinnati.

6. C L A R K C.S., B J O R N S O N H.S. , H O L L A N D J.W., E L I A V.J. & M A J E T I V.A. (1981). -Evaluation of the health risks associated with the treatment and disposal of municipal wastewater and sludge. US Environmental Protection Agency, Cincinnati, EPA-600/S-1 81-030.

7. F A T T A L B . , W A X Y . , D A V I E S M . & S H U V A L H.I. (1986). - Health risks associated with wastewater irrigation: an epidemiological study. Am. J. publ. Hlth, 76, 977-979.

8. F A T T A L B., Y E K U T I E L P. & S H U V A L H.I. (1986). - Cholera outbreak in Jerusalem 1970 revisited: the case for transmission by wastewater irrigated vegetables. In Environmental epidemiology: epidemiological investigation of community environmental disease (J.R. Goldsmith, ed.). CRC Press, Boca Raton, Florida.

865

9. F E A C H E M R . G . (1982). - Environmental aspects of cholera. Epidemiology III: transmission and control. Trop. Dis. Bull., 79, 1-47.

10. F E A C H E M R . G . , B R A D L E Y D . J . , G A R E L I C K H. & M A R A D . D . (1983). - Sanitation and disease: health aspects of excreta and wastewater management. John Wiley and Sons, Chichester, UK.

11. G R E E N B E R G A .E . & K U P K A E. (1957). - Tuberculosis transmission by wastewater: a review. Sewage and industrial wastes, 29, 524-537.

12. G R E E N B E R G A .E . & D E A N B.H. (1958). - The beef tapeworm, measly beef, and sewage: a review. Sewage and industrial wastes, 30, 262-269.

13. J E P S O N A . & R O T H H. (1949). - Epizootiology of Cysticercus bovis - resistance of the eggs of Taenia saginata. In Report of the 14th international veterinary congress, Vol. 2. His Majesty's Stationery Office, London.

14. J J U M B A - M U K A B U O . R . & G U N D E R S E. (1971). - Changing patterns of intestinal helminth infections in Jerusalem. Am. J. trop. Med. Hyg., 20, 109-116.

15. K H A L I L M . (1931). - The pail closet as an efficient means of controlling human helminth infections as observed in Tura prison, Egypt, with a discussion on the source of Ascaris infection. Ann. trop. Med. Parasitol., 25, 35-62.

16. K R E Y W. (1949). - The Darmstadt ascariasis epidemic and its control (in German). Z. Hyg. Infektionskr., 129, 507-518.

17. K R I S H N A M O O R T H I K . P . , A B D U L A P P A M . K . & A N W I K A R A . K . (1973). - Intestinal parasitic infections associated with sewage farm workers with special reference to helminths and protozoa. Proceedings of symposium on environmental pollution. Central Public Health Engineering Research Institute, Nagpur, India.

18. M A R A D .D. & S I L V A S . A . (1986). - Removal of intestinal nematode eggs in tropical waste stabilisation ponds. J. trop. Med. Hyg., 89, 71-74.

19. P E N F O L D W.J. , P E N F O L D H.B. & P H I L I P S M. (1937). - The criteria of life and viability of mature Taenia saginata ova. Med. J. Aust., 2, 1-5.

20. P E N F O L D W.J., P E N F O L D H.B. & P H I L I P S M. (1937). - Taenia saginata and cysticercosis bovis. J. Helminthol., 15, 37-48.

21. P I K E E.B. (1980). - The control of salmonellosis in the use of sewage sludge on agricultural land. In Characterisation, treatment and use of sewage sludge (P. L'Hermite & H. Ott, eds.). D. Reidel, Dordrecht, Netherlands.

22. S A N C H E Z L E V Y A R . (1976). - Use of wastewater for irrigation in Districts 03 and 88 and its impacts on human health (in Spanish). Master's thesis, School of Public Health, Mexico City.

23. S C H L I E P E R C. & K A L I E S W. (1949). - Quantitative investigations on the infection by roundworms of the inhabitants of the Darmstadt rural district. Zentralbl. Bakteriol. (Orig.), 154, 78-86.

24. S H U V A L H.I. (1984). - Health aspects of irrigation with sewage and justification of sewerage construction plan for Santiago, Chile. Unpublished internal report, World Bank, Washington DC.

25. S H U V A L H.I. (1986). - Thalassogenic diseases. UNEP regional seas reports and studies, No . 79. United Nations Environment Programme, Nairobi, Kenya.

26. S H U V A L H.I., Y E K U T I E L P. & F A T T A L B. (1984). - Epidemiological evidence for helminth and cholera transmission by vegetables irrigated with wastewater: Jerusalem, a case study. Water Sci. & Tech., 17, 33-42.

27. S H U V A L H.I. , A D I N A . , F A T T A L B. , R A W I T Z E. & Y E K U T I E L P. (1986). - Wastewater irrigation in developing countries: health effects and technical solutions. World Bank Technical Paper No . 51. World Bank, Washington, DC.

866

28. S H U V A L H.I. , W A X Y . , Y E K U T I E L P. & F A T T A L B. (1989). - Transmission of disease associated with wastewater irrigation: a prospective epidemiological study. Am. J. publ. Hlth, 79, 850-852.

29. S H U V A I . H.I. , W A X Y . , Y E K U T I E L P. & F A T T A L B. - Prospective epidemiological study of enteric disease transmission associated with sprinkler irrigation with wastewater: an overview. Proceedings of Water Reuse Symposium IV (1987), Am. Water Works Ass., Denver (accepted for publication).

30. S I L V E R M A N P.H. & G R I F F I T H S A. (1955). - A review of methods of sewage disposal in Great Britain with special reference to the epizootiology of Cysticercus bovis. Ann. trop. Med. Parasitol., 46, 436.

31. WHO (1973). - Reuse of effluents: methods of wastewater treatment and health safeguards. Report of a WHO meeting of experts. WHO technical report series No. 517, Geneva.

32. WHO(1989). - Health guidelines for the use of wastewater in agriculture and aquaculture. Report of a WHO scientific group. WHO technical report series No. 778, Geneva.

33. W i l S O N H. (1944). - Some risks of transmission of disease during the treatment, disposal and utilisation of sewage, sewage effluent and sewage sludge. Inst. Sewage Purif. J., 214-238.

34. YANEZ F . , Rojas R., C A S T R O M . L . & M A Y O C . (1980). - Evaluation of the San Juan stabilisation ponds: final research report of the first phase. Pan-American Center for Sanitary Engineering and Environmental Sciences, Lima, Peru.