Suitability of waste water sludge for agricultural use: a case study of Ruai sewage treatment plant, Nairobi, Kenya
Mbugua, P. K.
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Information on heavy metals in sewage sludge is important as it forms a vital baseline for among others the detection of undesirable effects when used in agriculture as a fertilizer. The aim of this study was to determine the suitability of waste water sludge from Ruai waste water treatment plant for agricultural use. The study was carried out over a period of four months from November 2012 to February 2013. The objective of the study was to find out the levels and variations of heavy metals between and within anaerobic and aerobic ponds from Ruai Waste Water Treatment Plant and whether the sludge conforms to standards recommended by European Union for application as a fertilizer. Sampling was done during the day and samples analyzed within four hours of sample collection. Sludge samples were collected every two weeks from anaerobic and aerobic ponds. Samples were collected from the four corners, entrance, middle and at the exit of each pond using a PVC pipe. The samples were analysed for heavy metals: Zinc, Chromium, Copper, Cadmium, Lead and Nickel using an Atomic Absorption Spectrophotometer (APHA). The study revealed that mean values of Zinc in anaerobic pond sludge were A3(1582.82mg/kg), A5(1619.80mg/kg), A7( 5937.61mg/kg), while in aerobic pond were F3(1688.45mg/kg), F5(1387.72mg/kg), F7(3191.73 mg/kg); In anaerobic ponds the mean levels of copper were A3(1319.92 mg/kg), A5(1665.92mg/kg), A7(895.49mg/kg) while in aerobic ponds, the mean values were F3(1788.70mg/kg), F5(1127.95mg/kg), F7(1171.48mg/kg); The levels of Cadmium in anaerobic ponds were A3(33.28mg/kg), A5(8.92mg/kg), A7(6.82mg/kg), while in aerobic pond were F3(12.85mg/kg), F5(7.95mg/kg), F7(5.90mg/kg); Mean Values Chromium in anaerobic ponds were A3(7714. 00mg/kg), A5(15175.88mg/kg), A7(6998.72mg/kg) and F3(12026.88mg/kg), F5(9642.387.95mg/kg), F7(7929.23mg/kg) in aerobic ponds; The mean levels of Lead was highest in aerobic pond 3 F3(4967.01mg/kg) and lowest in anaerobic pond 7 A7(1771.73mg/kg). Levels of Nickel varied during the study period with a mean of A3(47.93mg/kg), A5(116.87mg/kg), A7(188.58mg/kg) in anaerobic ponds and in F3(185.58 mg/kg), F5(6.46mg/kg), F7(4.50mg/kg) in aerobic ponds respectively. At p≤ 0.05, there was a significant difference in the levels of heavy metals between anaerobic ponds but no significant difference between anaerobic and aerobic ponds except anaerobic pond 3 compared with Aerobic 3, and Anaerobic 7. The levels of Cd, Cu, Zn and Ni in both anaerobic and aerobic ponds were lower than the European Union limits for sludge intended for land application as a fertilizer, while concentrations of Pb in both anaerobic and aerobic ponds was above the limits. In conclusion, the levels of Pb being higher than recommended limits restrict the possible use of Ruai waste water sludge as a fertilizer. It is recommended that pre-treatment of effluent by industries should be enforced to conform to NEMA standards and guidelines. This will reduce the levels of heavy metals in sludge as most of the heavy metals originate from industrial effluent discharges. Further study on other toxic heavy metals like mercury, indicator microorganisms and pathogen levels in sludge is recommended.