VEGETABLE PRODUCTION WITH WASTEWATER FROM AKAKI RIVER IN ADDIS ABABA: PERCEPTIONS, RISKS, AND LOW-COST REMEDIATION

Abstract

The use of wastewater to produce food crops particularly vegetables is very prevalent within and outside the urban centers of developing countries. In the capital city of Ethiopia, Addis Ababa, where irrigation water for cultivation of vegetables is commonly derived from the polluted Akaki River, the practice has often been blamed for causing negative externalities to public health and the environment. To address these issues, the studies of this doctoral dissertation mainly focused on five aspects of wastewater and dried faecal matter biochar use in vegetable cropping system of Addis Ababa. In the first study, data were collected on farm through 263 individual interviews and 12 focus group discussions. The difference in perception to quality consideration of Akaki River/irrigation water is highlighted by the result of Kruskal-Wallis H test analysis which shows significant mean value (1.33) of positive perception towards the water quality by male farmers. Among the perceived health risks, skin problems were top-rated health risk while eye burn, sore feet and abdominal pains were rated low across the farming sites. Irrespective of the farming site and gender differences, the most accepted health risk reduction measures were health promotion programs and cessation of irrigation before harvesting. The requisite quantitative data on contamination levels of irrigation water and selected leafy vegetables and potential health implications were covered in study II and III. Lettuce was used as a test crop in study II compared to additional two leafy vegetables (Swiss chard, and Ethiopian kale) in study III. At this stage, an assessment was done to determine the faecal coliform, helminth eggs and heavy metal levels of irrigation water and vegetables harvested from 10 urban vegetable farming sites. Attempts were also made to assess the efficacy of common green salads washing methods and potential health risk associated with the consumption of the analyzed vegetables via computing estimated daily intakes (EDIs) and target hazard quotients (THQs) of heavy metals. The mean faecal coliform levels of irrigation water ranged from 4.29-5.61 log10 MPN 100 ml−1, while on lettuce, the concentrations ranged from 3.46-5.03 log10 MPN 100 g−1. Helminth eggs and larvae were detected in 80% of irrigation water and 61% of lettuce samples. The helminth eggs identified included those of Ascaris lumbricoides, Hookworm, Enterobius vermicularis, Trichuris trichiura, and Taenia. Compared with the WHO recommendations and international standards, the faecal coliform and helminth eggs levels in irrigation water and lettuce samples exceeded the recommended levels. Irrespective of the tested washing methods, faecal coliform and helminth eggs levels were significantly (p<0.05) reduced. However, the heavy metal concentrations in irrigation water and irrigated soils did not exceed upper threshold limits. Moreover, Cd, Co, Cr, Cu, Ni and Zn concentrations in all analyzed vegetables were lower than the recommended maximum permissible limits. Results of two way ANOVA test showed that variation in metals xiii concentrations were significant (p < 0.001) across farming site, vegetable type and site x vegetable interaction. The EDI and THQ values showed that there would be no potential health risk to local inhabitants due to intake of individual metal. To assess the efficacy of seven treatments including biochars produced from dried faecal matter and manures as stabilizing agents of Cd in Cd-spiked soils, lettuce was grown in glasshouse on two contrasting soils in study IV. Ammonium nitrate (NH4NO3) extraction results indicated that faecal matter biochar, cow manure biochar and lime significantly reduced bioavailable Cd by 84–87, 65–68 and 82–91 %, respectively, as compared to the spiked controls. The immobilization potential of faecal matter biochar and lime were superior to the other treatments. On the basis of a preceding study, the effect of dried faecal matter biochar, N fertilizer and their interaction on biomass production and nutrient contents of lettuce was tested. Lettuce was grown over two growing cycles under glasshouse on two contrasting soils amended once at the start with factorial combination of faecal matter biochar at 4 rates (0, 10, 20 and 30 t ha−1) with 0, 25 and 50 kg N ha−1 in randomized complete block design. For both soils, maximum fresh yields were recorded with biochar and combined application of biochar with N treatments. It was also observed that faecal matter biochar application resulted in noticeable positive residual effects on lettuce yield and tissue nutrient concentrations. Most nutrients analyzed (N, P, K, Mg, Cu and Zn) were within or marginally above optimum ranges for lettuce under biochar amendment. In conclusion, the study indicated that faecal contamination level of lettuce irrigated with contaminated irrigation water is above the threshold of safe consumption, but in a range which can be addressed through relatively simple and low-cost mitigation measures. Nevertheless, it is clear that heavy metals pose relatively no risk to local inhabitants through the consumption of leafy vegetables grown on polluted water- irrigated vegetable farming sites. On the other hand, application of faecal matter biochar enhances Cd immobilization, yield and tissue nutrient concentrations of lettuce, suggesting that faecal matter biochar could be used as an effective heavy metal stabilizing agent and fertilizer for lettuce production.