liquid waste or sewage discharged into a river or the sea". Effluent in the artificial sense is in general considered to be water pollution, such as the outflow from a sewage treatment facility or the wastewater discharge from industrial facilities.
Sewage effluents, discharged into freshwater, estuarial and coastal waters present a risk to the user of those waters, should they contain viable (oo)cysts. The densities of (oo)cysts in receiving sewage effluent can be significantly greater than in those that do not receive such discharges (e.g. Parker, 1993; Medema and Schijven, 2001). During periods of prolonged or high rainfall, the volumetric design capacity in the sewerage network or at the works may be exceeded. in such circumstances, excess sewage is pumped straight to the receiving watercourse having undergone minimal (i.e. preliminary) treatment only, either at the sewage treatment works or within a verflow (CSO) system. Gibson III et al. (1998b) found up to 40 000 oocysts/100 l and up to 283 000 cysts/100 l in effluent discharged from CSOs.
Effluent irrigation and crop contamination.
Although reuse of sewage effluent for crop irrigation is commonplace in arid regions of the world, few studies have assessed levels of contamination on irrigated crops such as fruit and vegetables, which are consumed raw. Field trials undertaken in Morocco revealed the presence of on (254 cysts/kg), mint (96 cysts/kg), carrots (155 cysts/kg) and radish (59.1 cysts/kg) on crops irrigated with raw wastewater. Potatoes taken from a field irrigated with raw wastewater contained 5.1 cysts/kg (Amahmid et al., 1999). Earlier Mexican studies revealed the presence of Giardia cysts and other protozoan parasites on various fruit and vegetables irrigated with raw wastewater (Kowal and Pahren, 1982; Felix et al., 1996). A Costa Rican study (Monge and Chincilla, 1996) found that, of eight different fresh vegetables, commonly consumed raw, coriander roots and, radish, tomato, cucumber and carrot were contaminated with oocysts. Giardia was detected on coriander (cilantro) leaves and roots (Monge and Arais, 1996). Samples from vegetables collected at several small markets in a periurban slum in Peru harboured Cryptosporidium oocysts (Ortega et al., 1997). Of the vegetables examined, 14.5% contained C. parvum oocysts suggesting that washing vegetables does not completely remove Cryptosporidium oocysts. Such studies indicate the potential for fruit and vegetables to become contaminated and subsequently act as vehicles for the transmission of foodborne protozoan disease. A survey of lettuce and sold in a number of Chilean markets (Franjola and Guttierez, 1984) revealed the presence of Iodamoeba (Pseudolimax) butschlii and , indicative of faecal contamination (probably untreated wastewater used for irrigation). For vegetables and fruit that are eaten raw, the presence of infectious presents an unacceptable risk to the consumer.
Based upon available epidemiological evidence, WHO promulgated bacterial and parasitological guidelines for the unrestricted use of effluent for irrigation, i.e. including the irrigation of fruit and vegetables eaten raw (WHO, 1989). These require a faecal count of less than 1000/100 ml and not more than 1 egg/l. Several developing countries have adopted these into their national standards. The state of California, USA requires sewage effluent to be treated (coagulation, filtration and chlorination) and comply with more stringent effluent standards based upon TU), total coliform count (median < 2.2/100 ml) and virus (5 log10 reduction) criteria before permission is granted for reuse. Interestingly, neither the WHO guidelines nor the California regulations make reference to protozoan pathogens, in contrast to the state of Arizona (Giardia < 1 cyst and < 1 virus per 40 l for unrestricted irrigation), which also considered a similar standard for Cryptosporidium