| dc.description.abstract | Evaluation of treatment efficiency of Nairobi wastewater at Dandora Estate and Sewerage
Treatment Works (DESTW) and possible applications of the effluent was undertaken.
A study of selected organochlorines and organophosphates insecticides in supply water and
reclaimed wastewater samples was conducted. It was believed that serious chemical
contamination and nutrient enrichment of the receiving water body (Nairobi River) and
groundwater occurred as a result of discharge of insufficiently treated effluents from DESTW and
leakage of ponds and that reuse of these effluents as they were, limited to maintenance of the
river flow and recharge of aquifers.
Treatment efficiency was evaluated in terms of Contaminant Mass Removal Percent and the
selected quality parameters monitored included Biochemical Oxygen Demand (BODs), Chemical
Oxygen Demand (COD), Total Solids (TS), Total Suspended Solids (TSS), pH, Electrical
Conductivi (EC), Ammonia-Nitrogen (NHrN) , Nutrients (NOrN and PO/-), Sulfates, and
~ fVle t,-r.~ ..s;;; --yeavy
)vfetals. Other parameters determined included Sedimentation Rate, Evaporation Rate,
Relative Humidity and Air Temp ature. '
S'a"'pl-e.c Water samples were ~ from Nairobi catchment reservoirs (Thika, Sasumua, and Ruiru) and
Ruai Dispensary borehole. Treated water samples were obtained from Nairobi City and Kampala
City water taps espectively. Sludge samples were taken from the waste stabilization ponds of
0:-~ DESTW whfle sediments from Nairobi River at DESTW for heavy metal analysis. Effluent
samples were taken from DESTW and only final effluent was obtained from Bogolobi Sewerage
Treatment Works (BSTW) in Kampala Uganda.
Analytical techniques (Spectro~hotometric, Turbidimetric and Gravimetric) given by Standard
Methods for the Examination:Water and Wastewater and the methods (Liquid-Liquid Extraction,
Gel Permeation Chromatography (GPC), and Gas Chromatographic (GC)) outlined in the
Environmental Monitoring 0 f Pesticide Residues Guidelines for the S ADC Region were used.
Sedimentation and Evaporation Rates were determined using improvised equipment while Air
Temperature and Relative Humidity was 0 btained using T hermohygrometer. Linear regression
analysis was performed on the data obtained.
The results showed the overall removal of 88.96 % BODs, 69.39% COD, 65.91% TSS, 75.26%
NH -N, -11.29% NOrN, 48.65% ro.'. and 82.58% SO/". Raw wastewater, Final effluent,
Upstream and Downstream water samples had BODs of}85 ± 8.8!., 51 ± 4.50,34 ± 5.23,62.00 ±
8.14; COD of 814 ± 3.72, 299 ± 15.94, 186 ± 3.85,186.00 ± 6.40; TSS of 418 ± 8.09,171 ±
14.37, 79 ± 4.52,171.0 ± 4.02; NH3-N of 28.70 ± 7.50, 8.52 ± 3.13,1.35 ± 0.01, 2.35 ± 0.05;'/
NOrN of 5.92 ± 3.57, 7.90 ± 0.87, 2.1 5 ± 0.03, 5.30 ± 0.05; NOTN of 3.27 ± 2.09, NID, 1.63 ±
9 ± 0.09,8.48 ± 1.43; sot of 32.28 ± 10.90,
6.75 ± 1.51,10.05 ± 1.96, 13.20 ± 2.76; mg/l respectively; pH of 7.36 ± 0.36, 8.98 ± 0.55,7.85 ±
0.24,8.06 ± 0.01; and EC (mS/m) of, 79.30 ± 0.08, 100.10 ± 13.06, 50.60 ± 1.44, 68.00 ± 0.25;
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respectively. Raw wastewater ~ contained 5.866 ± 0.020 mg/l Fe, 2.570 ± 0 .011 mg/l Mn,
0.613 ± 0 .011 mg/l Cr, 0.145 ± 0.008 mg/l Cd, 0.558 ± 0 .005 mg/l Pb, 0.096 ± 0 .003 mg/l Cu,
and 1.255 ± 0 .007 mg/l Zn while Anaerobic pond sludge and effluent indicated 6!.~7~ ± 0.110
g/kg Fe, 2.4il ± 0.007 mg/l Fe; 13.382 ± 0.245 /kg Mn, 0.624 ± 0.007 mg/l Mn; 53.430 ± 0.004 mg/kg
Cr, 0.205 ± 0.003 mg/l Cr; 71.750 ± 0.003 mg/kg Cd, 0.029 ± 0.004 mg/l Cd; 76.450 ±
0.039 mg/kg Pb, 0.113 ± 0.004 mg/l Pb; 43.280 ± 0.005 mg/kg Cu, 1.030 ± 0.004 mg/l Cu; and
295.000 ± 0.025 mg/kg Zn, 0.256 ± 0.004 mg/l Zn while in Upstream water and Downstream
sediment samples, 2.941 ± 0.109 mg/l Fe, 30.325 ± 0.082 g/kg Fe; 1.864 ± 0.033 mg/l Mn, 2.785
± 0.497 g/kg Mn; 0.02 ± 0.007 mg/l Cr, 36.850 ± 0.003 mg/kg Cr; 0.003 ± 0.002 mg/l Cd, 0.300
± 0.003 mg/kg Cd; 0.006 ± 0.003 mg/l Pb, 85.790 ± 0.006 mg/kg Pb; 0.022 ± 0.005 mg/l Cu,
33.375 ± 0.003 mg/kg Cu; and 0.149 ± 0.003 mg/l Zn, 384.46 ± 4.429 mg/kg Zn was obtained.
Borehole water Estimated sedimentation rates were 2.06 m3/h in anaerobic pond, 18.65 and 28.35
m3/h in facultative ponds respectively which yielded pond lifespans of 2.6,23.5, and 35.7 years
respectively. Since commissioning of these ponds in 1980 and 1992 respectively, the current
lifespans were estimated at 0, 10.5, a d 10.7 years respectively .
.Total evaporation rate was about 52,30'0 m3/d with a mean air temperature of 24.24° C (range: 17~
30) and mean relative humidity of 55.84% (range: 33-85). Some pesticide residues were detected
in samples and the total levels per sample in mg/l were: treated Nairobi tap water, 0.0008; treated
Nairobi effluent, 0.0012; Ruiru catchment reservoir, 0.0017; Thika catchment reservoir; 0.0016;
Sasumua catchment reservoir, 0.0018; treated Kampala tap water, 0.0012; and treated Kampala
effluent, 0.0017. Regression analysis gave significant positive correlations (at p = 0.05 or p =
0.01) between BOD/COD, BOD/TS, and TS/EC but none between pH/EC. Regression equations
for heavy metal contents of s elected ponds effluents and sludges 0n s elected physicochemical
parameters also revealed significant correlations. Removal of about 10 mg/l Suspended Solids
(SS) resulted in an average BODs reduction of 3.10 mg/I.
The observed effluent quality may continue to restrict the possible reuse of Nairobi wastewater
and 1ead tor apid loss 0 f biodiversity, extinction 0 f natural watercourse, impairing t he aquifer ?\ - suitability for water supply, accumulation of toxic compounds in food chain apart from
maintenance of the river flow and recharging of aquifers. Possible presence of pesticide residues
in supply waters may generate serious decline in product quality and their market values. | en |
