Risk Analysis of Wastewater use in Crop Production: A case of Glen Valley Irrigation Scheme, Botswana

Abstract

The Glen Valley Irrigation Scheme uses treated sewage effluent for crop production, however the potential public health risks associated with wastewater use is of major concern to workers and consumers. This research was conducted in Glen Valley in Gaborone, Botswana from February to May 2011. The study aimed at analyzing the health risks of wastewater use for crop production in the Glen Valley Irrigation Scheme. Samples of effluent, soil and vegetables (spinach, tomatoes, and green pepper) were collected from critical points and analyzed for selected quality parameters of health significance following recommendations by the American Public Health Association (effluent), Food and Agriculture Organisation (soil) and ATSDR (vegetables). Effluent samples were collected 7 times from the filter and 3 times from the drip pipe. There were 3 soil sampling campaigns from the spinach, green pepper and tomato plots. Vegetables were collected from each of the above mentioned plots on 3 sampling campaigns. The mean values of pH were 9.09±0.15 for effluent and were within the FAO range of 6.5–8.5 for irrigation. The electrical conductivity ranged from 710 μS/cm to 760 μS/cm which was less than the guideline value of 2000 μS/cm. For effluent, cadmium and lead concentrations were lower than the 0.01 mg/l and 0.05 mg/l respectively which was within the long-term threshold limit for irrigation. Total coliforms in the effluent were within the WHO limit of 1000 CFU/100 ml ranging between 0 and 470 CFU/100ml. The pH values from all soil samples were above the FAO recommended limit of 6.5. No pathogens were detected in vegetables, but coliforms were detected. Heavy metals were detected in all the vegetables but cadmium in green pepper exceeded minimal risk levels by ATSDR. It can be concluded that at present the health risks for the consumption of the vegetables is low except for cadmium in green pepper which is above the recommended Minimum Risk Levels. Therefore it is recommended that regular monitoring of the effluent, soil and vegetables be done to protect the health of workers and consumers.