Assessment of the performance of a direct dual media (anthracite and sand) filtration system in drinking water treatment using filter columns

Abstract

Uganda is naturally endowed with an abundance of surface water resources. These include rivers, lakes, streams and wetlands. Pollutants/contaminants are commonly present in water, and require removal. Rapid and slow sand filtration, bio-sand water filtration and roughing filtration are commonly used for water treatment. The study was carried out in Kampala, it involved a series of analysis on the water filtration system in Ggaba 1 Water Treatment Plant. The plant makes use of both direct flow and conventional filtration systems. Whereas the conventional filter attains the required water quality, it has high treatment costs due to the large space requirements needed for the clarification process. Additionally, the filtrate from the direct flow filter (Patterson filters) contains some parameters that exceed the WHO standards, i.e. color (15 PtCo), turbidity (5 NTU), TSS (0 mg/l) and TOC (2 ppm) and always requires further treatment. There is therefore need for a process that can cope with both high flows and deteriorating water quality of IMB to meet the growing water demand in Kampala Metropolitan Area. This study therefore aimed at evaluating the performance of a direct dual media filtration system at Ggaba 1 water treatment plant. Specifically, the study evaluated the performance of Patterson filters at the plant, characterized dual filter media and assessed the performance of the direct dual media filtration system. The dual media filter was designed to have Anthracite and Sand. To optimize the process, the filter media depths were varied in a range of 0.3 m – 1.2 m and the filter was run at flow rates of 5 m/h, 7.2 m/h and 8.5 m/h. Poly Aluminium Chloride (PAC) was added to the raw water mixing tank as a coagulant. The existing direct flow filter at Ggaba 1 water treatment plant yields a filtrate of the following characteristics; 8 NTU turbidity, 80.6 PtCo color, 9.5 mg/l and 4.7 ppm, TSS and TOC respectively. The optimal dual media filter was run at a filtration rate of 8.5 m/h, filter media depth of 0.9 m of anthracite and 0.3 m of sand, with filtrate of the following characteristics; 0.7 NTU turbidity, 7.6 PtCo color, 0 mg/l and 1.0 ppm, TSS and TOC respectively. This would in lead to increased water production at the plant by 23.6%, and improve water coverage and service reliability in the Greater Kampala Metropolitan Area (GKMA). However, further studies should be carried out to investigate possible anthracite losses during backwash and its implication on the cost of the filter media and water treatment.