Sludge characterization at Kadahokwa water treatment plant, Rwanda

A study was carried out to characterize the sludge produced at Kadahokwa Water Treatment Plant (KWTP) in Butare to assess the effectiveness of the sludge treatment and potential impacts of sludge disposal on the environment. Parameters analyzed were chromium, nickel, cadmium, lead, copper, zinc, iron, manganese, aluminium, total nitrogen, total phosphorus, potassium and cation exchange capacity (CEC). The results showed that 450±244.5 tons (dry weight) of sludge are produced annually. The concentrations of heavy metals in the sludge were below the standard limits for land application set by different countries. The high concentrations of nickel (42.3±2.5 ppm), chromium (29.9±6.2 ppm), cadmium (1.1±0.3 ppm) and lead (31.6±3.7 ppm) in the dried sludge posed a pollution risk for the wetland. The CEC was 28.4–33.3 cmol (+)/kg and pH was 6.50–7.45. It was concluded that the KWTP sludge is a poor source of total carbon, a moderate source of nutrients (NPK), and an important source of micronutrients, making it generally suitable for reuse for crop production. The CEC showed that the sludge could improve soil nutrient and water holding capacity. The higher concentration of aluminium (280 ppm) in the sludge creates an opportunity for recycling.