Assessment of Heavy Metals and Nutrients in Dandora Estate Sewerage Treatment Works

The project undertakes to establish the levels of heavy metals and nutrients from Dandora estate sewerage treatment works (DESTW) which is located in Ruai-Nairobi County. This is through analyzing the influent, effluent and Nairobi River where the treatment works discharges its water. Sampling was done in February 2015, 18 water and sediment samples were analyzed, and the parameters studied include nitrates, phosphates, heavy metals – lead, cadmium, nickel, copper and zinc. Conformity to NEMA standards for wastewater was also determined.

Results obtained were as follows: phosphates concentration of the influent was 9.69±0.71 mg/l, which was within NEMA standard of 30g/l and effluent range between 4.42 – 6.88 mg/l. There is reduction in the levels. The concentration upstream (4.70 mg/l) and downstream (4.66 mg/l) Nairobi river was approximately the same due to dilution.

Nitrates concentration for the influent was 4.27 mg/l, which was within NEMA standard of 20 mg/l, the effluent ranged between 6.87 – 107.91 mg/l, and NEMA does not give a specific standard value. Downstream a higher concentration value of 6.07 mg/l compared to upstream, which gives 1.04 mg /l. this, could be attributed to pollution from the treatment works.

For heavy metals in the water samples, nickel and lead were below the detection limit of the instrument of 0.02 and 0.01, respectively. Zinc ranged between 0.01- 0.08 mg/l, copper <0.01- 0.02 mg/l, cadmium 0.01-0.07 mg/l. The sediment samples had very high concentration levels of the heavy metals. Zinc 64.65-356.91 mg/kg; lead 16.50- 78.50 mg/kg, copper 3.84-43.69 mg/kg, cadmium 1.97-4.02 mg/kg and nickel ranged between 1.29 to 8.35 mg/kg. There are no NEMA standard guidelines for heavy metals in sediments.

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