Assessment Of Efficiency Of Wastewater Treatment Based On Physico-Chemical And Biological Parameters Of Kisii Town Wastewater Treatment Plant, Kenya

Abstract

Knowledge of the nature and composition of wastewater is critical in wastewater treatment, re-use, and disposal. Kisii municipality wastewater treatment plant (WWTP) is a lagoon system that treats wastewater, and discharges its effluent into river Riana. The river serves as a source of water for domestic, agricultural, and industrial uses downstream. The WWTP does not have adequate capacity to fully treat all the wastewater from the municipality. The discharge of partially or untreated wastewater into river Riana particularly during system breakdown is of great concern due to the potential health risks it poses to the environment, human and animals. This study aimed at assessing the efficiency of the WWTP in treating wastewater based on analysis of selected physical, chemical and biological parameters, of health concerns. This was done both on the initial and current wastewater treatment plant design during the period 2019 and 2021 respectively to establish whether there was an improvement in wastewater polishing. Monthly samples for physical, chemical and biological parameters were collected for analysis. Temperature, pH, dissolved oxygen (DO), electrical conductivity (EC), and total dissolved solids (TDS) were measured in situ using YSI multi-parameter probe model 35C. Total suspended solids (TSS), chlorophyll-a, nutrients and Total and Fecal coliforms (TC and FC) were analyzed ex situ following standard procedures described in APHA, 2014. Atomic Absorption Spectrophotometry (AAS) was used to determine heavy metals concentrations of Cd, Cu, Pb and Zn in wastewater, sediments, and plankton samples. Inverted microscope (model: Zeiss Axiovert 35) was used to identify and enumerate phytoplankton while a simple light compound microscope (Model: Olympus, Japan) was used to identify and enumerate zooplankton using standard identification keys. Microsoft Excel version 2010 and SPSS version 22 software were used to analyze physical, chemical and biological data while PAST software was specifically used to determine the biodiversity diversity indices of the plankton. The physical, chemical, heavy metals, and biological (coliforms) parameter levels of the effluent were compared with NEMA, WHO, and EPA standards. The mean DO, EC, TSS, TDS, SRP, NO2-N, NO3-N, TP and TN differed significantly among the sampling stations (ANOVA; p < 0.05) both spatially and monthly before and after renovation of the lagoon. 126 phytoplankton species were identified belonging to 6 families: Euglenaphyceae, Bacillariophyceae, Dinophyceae, Cyanophyceae, Chlorophyceae, and Zygnematophyceae. The total phytoplankton biovolume recorded was 1066.14 mm3 L -1 . For zooplankton, 15 species were identified and they belonged to three major groups: Cladocera, Rotifera, and Copepoda. The total zooplankton abundance recorded was 5745 IndL-1 . The means of TC and FC for the initial WWTP were 76.3 ± 10.98 and 55.66 ± 9.89 counts/100ml respectively while for the current WWTP were 37.64 ± 3.3 and 17.94 ± 2.3 counts/100ml. The heavy metals identified in the WWTP were copper (Cu), lead (Pb), and zinc (Zn) but cadmium (Cd) was below detection level throughout the study period. The parameters pH, temperature, TDS, NO3-N, Cu, and Zn were within NEMA standards while others were above, showing that the plant did not efficiently polish the wastewater. Polishing efficacy of the WWTP was below 70% for the majority of the parameters assessed, of major IX concern it was observed that coliforms (TC and FC) counts, TP, and TN concentrations did not meet the required standards. The two nutrients are responsible for eutrophication and poor water quality of river Riana and the main river Kuja that flows to Lake Victoria. Plankton further contributed in wastewater polishing by incorporation of nutrients and heavy metals into their biomass. Lastly, renovation of the lagoon must have contributed to its improvement in efficiency of wastewater polishing but the design still has challenges dealing with nutrients and coliforms. The current study findings form a baseline for further studies in the lagoon. The Gusii Water and Sanitation Company can use this information to improve on their wastewater treatment processing meet the laid down guidelines for effluent discharge into the environment. The study recommends construction of a wetland for further polishing of effluent discharged in the removal of nutrients and heavy metals.