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.