Performance evaluation of Bamag and Canadian water treatment plants at Weija waterworks in Accra

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This is an engineering study report on performance evaluation of two conventional water treatment plants, Bamag and Canadian Plants, at Weija Water Works in Accra. The study ‘as carried out from April 1st to July 30th 2001. The study provides information on the capabilities and limitations of the various unit processes and operations to contribute towards engineering solutions to the problem of inadequate water supply in Accra. On-site water quality and filtration operation analyses were carried out to evaluate final water quality removal efficiencies, production efficiencies of filters, backwash, filter media, and coagulant dosage of each plant. Operating records were also reviewed to evaluate plant performance between 1997 and 2001. Drinking Water Quality Standard and GWCL plant performance targets were used as performance criteria for the evaluation. Results of overall removal efficiencies of Colour, Turbidity, Iron, Manganese and Coliform for Barnag Plant ranged 92-96°/s. 89-91%. 67-100%. 100% and 100°/o respectively, and for Canadian Plant, the range were 9 1-94%. 75-92%, 83-100°/o. 100% and 100°/o respectively. Production efficiencies (percentage of filter design capacity) of Barnag and Canadian filters ranged 82-93% and. 116-157.8% respectively whereas water production by the plants as percentage of rated plant capacity ranged 84.8-99% and 57.4-78.8% respectively. Two filters were in use in Canadian instead of four hence the high filter production efficiencies. Percentage backwash water for Barnag and Canadian individual filters ranged 3.4-11.4% and 0.4%-2.5% respectively. The effective size of sand media for Bamag and Canadian were 0.7mm and 0.82 mm with uniformity coefficient of 2.1 and 2.1 respectively. The effective size of Bamag sand media was lower than the design size of 0.95 mm. Bamag bed expansion was as low as 6%. Bamag and Canadian filtration rates ranged 5.68-6.12 m/h and 8.06-11.19 m/h respectively. Conclusions reached were that a comparison of the two plants indicated that turbidity removal by Bamag was considerably higher than that of Canadian: Bamag actual alum dosage was less than both the nominal and Canadian alum dosages: final water qualities of both plants were within the Drinking Water Quality Standard but did not meet GWCL plant performance target at times. Bamag final water quality and water production after the rehabilitation, in 1 998 were excellent. The performance now indicates deterioration in both effluent quality and water production. It is recommended that the Bamag sand media he replaced with sand of effective size of 0.95 mm and the depth reduced from 0.9m to 0.8m. The design of Bamag plant is recommended for new water treatment works however the filter designs especially the limited water head need to be modified to increase the water head over sand.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfilment of the requirement for the award of Degree of Master of Science in Water Supply and Environmental Sanitation, 2001