Kariobangi Sewerage Treatment Plant in Nairobi City County is a conventional mechanical
wastewater treatment plant with an installed capacity of 32,000 m3/day and has an in built
methenation unit. It is the only available conventional mechanical wastewater treatment plant in
the city and its efficiency is therefore, of paramount importance. A total of 48 samples from three
sites within the plant namely the Inlet (I), the Clarifier(C) and the Outlet (O) were analyzed for
various parameters. Development activities within the treatment plant’s catchment area were
assessed with view to establish their effects on the performance of the plant. Physico-chemical
parameters, chemical parameters, heavy metals, oil & grease and bacteriological presence
analyses were carried out so as to evaluate the efficiency of the plant. The physico-chemical
parameters analyzed included settleable solids (SS), chemical oxygen demand (COD), and
biochemical oxygen demand after five days (BOD5). The chemical parameters analyzed were
nitrates and phosphates. The heavy metals analyzed included cadmium (Cd), lead (Pb), and
chromium (Cr). Oil & grease was also analyzed while Total coliforms were analyzed as the
bacteriological parameter. Imhoff cone was used to determine the level of settleable solids.
Potassium dichromate titration method was used to determine the COD. Sample incubation
process which gave daily readings of BOD for five days was used to determine the
concentrations of BOD5. Ascorbic acid method was used for phosphate determination. Flame
Atomic Absorption Spectrometry was used to analyze the samples for cadmium, chromium and
lead presence. Solvent extraction was used to determine the concentration of oil & grease. The
nutrient-culture method was used to establish the concentrations of total coliforms.
The results for various parameters, for I, C and O were: settleable solids; 6.5-15.7 ml/l, 0.2- 0.9
ml/l, 0.25- 1.9 ml/l respectively; COD; 600- 4000 mg/l, 80- 120 mg/l, 70-115 mg/l respectively;
BOD5 ; 310-650 mg/l, 60-290 mg/l, 60-210 mg/l respectively; nitrates; 0.158-1.437 mg/l, 0.1671.178
mgl, 0.132-1.00 mg/l respectively; phosphates; 76-200 mg/l, 82.5-179 mg/l, 86-127 mg/l
respectively; Cd; 0.505- 0.723 mg/l, 0.1543- 0.2893 mg/l, 0.8500-1.4465 mg/l respectively; Cr;
4.625- 58.568 mg/l, 0.0743- 0.9432 mg/l, 10.16 mg/l respectively; oil & grease; 1.012- 1.901
mg/l, 1.1246-1.349 mg/l, 0.923- 1.032 mg/l respectively; Total coliforms; 253,000- 590,000
counts/100 ml, 180,000 -561,000 counts/100 ml, 160,000- 783,000 counts/100 ml respectively.
Percentage reduction efficiency for settleable solids was found to be in the range of 74.7- 96.7 %
while COD and BOD5 had percentage reduction efficiency ranging from 88.3- 98 % and 43.6-

84.5 % respectively. Nitrates and phosphates had percentage reduction efficiency of the range 17-36.5
% and -13.2- 36.5 % respectively. Oil & grease had percentage reduction efficiency in
the range of 14.4 -92.6 % while total coliforms had percentage reduction efficiency of -32.7 66.9
%. With the average outflow volume of 109.3 litres/second the pollution loadings were of
139,357 mg/s, 35,303.9 mg/s, 20,493.76 mg/s, 22.36 mg/s, 111.78 mg/s, 82.78 mg/s,
375,718,750 counts /s for settleable solids, COD, BOD5, Cd, Cr, Oil & grease and total coliform
respectively. Cottage industries involving paint making, metal works and construction of
residential and commercial buildings due to population upsurge were some of the development
aspects established in the plant’s catchment area. In conclusion, the quality of the effluent that
emanated from the plant was wanting and the pollution load discharged into Nairobi River was
in large figures.



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