Heavy metal concentrations in soil and vegetables irrigated with sewage effluent: A case study of Embu sewage treatment plant, Kenya

As a result of the increasing constraint in the availability of fresh water for irrigation, wastewater especially sewage effluent is being used for irrigation of agriculture fields, particularly in urban and peri‑urban centers. However, there is increasing concern over the associated potential health risks due to the dietary intake of contaminated vegetables. This study was conducted to analyze the levels of copper, zinc, cadmium and lead in sewage effluent, and in the vegetables and soil irrigated using this sewage effluent. Sewage effluent, soil and plant samples were collected and subjected to acid digestion to extract the heavy metals from the samples. Thereafter, concentration levels of the heavy metals were determined using Atomic Absorption Spectrophotometer (AAS). The mean concentrations of 0.484–1.834 mg/L, 1.432–4.612 mg/L, 0.015–0.353 mg/L, 0.011–2.123 mg/L for copper, zinc, cadmium and lead, respectively, were obtained in the sewage effluent which were above the WHO permissible levels in wastewater for irrigation. Due to continuous use of sewage effluent for irrigation, gradual accumulation of heavy metals in the soil could occur which could eventually lead to increased uptake of the heavy metals by the growing vegetables. Therefore, to ensure food safety and the use of sewage effluent for irrigation, we suggest that it is important to conduct continuous monitoring and pollution control.