A study on the efficiency of Nakuru town sewage treatment works
Ngari, Josephine M.
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A study to evaluate the efficiency of Nakuru town sewage treatment plant (TSTP) was carried out between August 2003 and March 2004. TSTP is located within Nakuru National Park and discharges its final effluent into Lake Nakuru via a small stream or a trickle. The treatment plant has two wastewater treatment systems. One system is a combination of both conventional treatment units and wastewater stabilization ponds in series (trickling filter line). The second system comprises wastewater stabilization ponds only (anaerobic pond line). This study aimed at establishing the efficiency of the various treatment units within the plant and whether the final effluent meets the minimum requirement for discharge to the environment. This was achieved by determining changes in the levels of selected physico-chemical and biological properties of the wastewater at different stages along the two treatment systems. There were significant differences (P < 0.05) in the concentration of most of the selected physico-chemical and biological properties between equivalent treatment units and of the two treatment lines. All the treatment units did not meet the designer's specification in terms of BOD reduction. For example, a mean BOD of 299.4 mg OZ L- I observed at the anaerobic pond treatment unit was much higher than the expected value of 206 mg 02 L-1. The nutrient levels in the final effluent from the anaerobic pond line (APL) were within the set limits while those from trickling filter line (TFL) were above the set limits for discharge to the environment. There were significant reductions in microbial numbers by most treatment units along both APL and TFL. Mean densities of faecal coliforms (261 coliforms /100m1) observed in the final maturation pond effluent of APL was significantly lower (P = 0.007, n = 22) than that recorded in the same pond along TFL (6500 coliforms/100 ml). A mean of 261 faecal coliforms/100 ml recorded in the final effluent from APL was lower than the set limit of 1000 faecal coliforms 100/ml recommended by the designers of the plant for discharge to the lake. Generally APL achieved higher organic and microbial load reductions. Identification of algal species in the facultative and maturation ponds revealed presence of potentially toxic algae such as Microcystis in the ponds along both APL and TFL. The results generated from this study indicate that; various treatment units do not meet their expected performance standards, there is a danger of algal toxin poisoning to wildlife, and that there is a danger of polluting the environment with human pathogens. Corrective measures should be taken to improve the performance of the treatment units and the quality of the final effluent.