Health risks associated with wastewater used for irrigation in urban agriculture, in Nairobi Kenya

Abstract

Wastewater can be defined as any water that has been adversely affected in quality by anthropogenic influence. It is used extensively for irrigation in Kenya, and in other countries where water is scarce. Risks involved in re-use of wastewater have necessitated the World Health Organisation (WHO) to formulate guidelines for irrigation water for example: total coliform counts not to exceed lXl04 cfullOO ml for leaf crops. The study a survey evaluated levels of contamination of the wastewater, the irrigated vegetables, and respective soils and associated health risks. Comparison was also made between vegetables obtained from the irrigated farms and those bought in selected market outlets. The areas covered were two urban slums located in Nairobi, Kenya: Maili Saba and Kibera. The markets studied included: Gikomba, Wakulima, Kibera and Korogocho. Emphasis was on bacteriology: total coliform counts and presence of Vibrio cholera and Salmonella Typhi; and parasitology: helminth larvae and eggs, and protozoa .. This was done using standard bacteriological and parasitological procedures. Benefits and risks of wastewater farming (including respective awareness), as well as mitigation strategies, were identified through focus group discussions and questionnaire survey. Results from a questionnaire survey indicated food security aocording to the farmers: ability to grow crops throughout the year and nutrition as important benefits that the farmers could attribute to usage of wastewater. On the contrary, there, seemed to have been general lack of awareness towards risks involved in usage of wastewater. Of the 232 farmers interviewed, only 28% identified health risks as a constraint, while 22.4% were not aware of any respective risks. Most farmers engaged in risky behaviours that could easily result in disease transmission: Eighty two percent (190) of respondents ate sugarcane among other crops while in the wastewater-irrigated farms, while 37.9% (88) admitted to not wearing protective clothing while working on their farms. Thirty (30) samples taken from selected manholes and canals used for irrigation, others taken from vegetables from farms and markets (182), soil (64) and faecal samples (174).Water and vegetable samples were analysed for bacteriology and parasitology. The faecal and soil samples were analysed for parasitology. Wastewater yielded average coliform counts of 1.89x107 per 100milliliters, which was statistically significantly (p<O.02) above the WHO guidelines. Parasite larvae' (13%), Balantidium coli (86%) and Entamoeba coli (6.6%) were isolated. One sample yielded Vibrio cholera. Kibera farm vegetables (55) also had high contamination with faecal coliforms, averaging 3.78xl05 per 100 milliliters; and yielded parasites (64) that included: Entamoeba hystiolytica (14%), Entamoeba coli (14%), Balantidium coli (6%). One vegetable sample was found to have an egg of Schistosoma haematobium. There were statistically significant differences in average contamination levels between wastewater irrigated farm vegetables (3.78x 105 per l Ouml) and those from Gikomba (5.l8x 106 per l Oflml), Wakulima (4.0xl06 per lOuml) and Korogocho (5.2xl08 per lOurnl) markets (p=O.OOO,p=O.OOl and p=O.OOO,respectively). Soil contamination was at an average of 46 parasitic larvae per kilogram, and 12.5 Ascaris lumbricoides eggs per gram. Comparing faecal analyses, wastewater farmers were shown to contain higher intensity of parasite infestation compared to the non wastewater users. Faecal sample results of the wastewater farming community (149) showed Trichuris trichura (18%); Ancylostoma (24%); Strongyloides (2%); Ascaris lumbricoides (16%); Entamoeba coli (14%); and Entamoeba hystiolytica (1.3%). One faecal sample showed eggs of Schistosoma mansoni. On the other hand, from the 24 faecal samples from non-wastewater farming community, Trichuris trichura was isolated (8%); Ancylostoma (41%); and Ascaris lumbricoides (12.5%). The Maili Saba men (n=51) had a higher variety of parasitic infestations Trichuris trichura (13.7%), Strongyloides (2%), Entamoeba coli (17.6%), Anchylostoma spp (15.6%), Entamoeba hystiolytica (4%) and Ascaris lumbricoides (13.7%). One case of Schistosoma mansoni was noted in a boy, compared to their counterparts, the Kibera Men (n= 10) (Anchylostoma spp (10%), Ascaris lumbricoides (40%) and Entamoeba coli (20%)). There was no significant difference between parasite infestation rates in the women in the two study sites. This study has shown that, while there are benefits to wastewater farming in Kenya, there are also risks involved. These are indicated by the parasite burden found in wastewater users, as compared to the non-users. The isolation of Vibrio cholerae organisms from wastewater highlights the risk of wastewater as a source of these pathogenic organisms for humans. The total coliform count in wastewater was not only higher than the recommended level WHO, but also an indicator that the farmers were using almost raw sewage for irrigation. This study also indicated lack of awareness among the wastewater users, with regard to the respective risks. Thus, awareness campaigns need to be initiated so as to educate the farmers on how to safeguard themselves. The observation that market vegetable coliform counts were higher than farm vegetable ones has introduced another aspect of vegetable contamination; one that originates from the vegetable handlers: the middle-people and traders. Consumers are, therefore, advised to cook there vegetables well before eating. Domestication of the WHO guidelines to the Kenyan scenario and their enforcement is highly recommended.