Study and treatment for surface water for Sylhet city

Abstract

Safe drinking water is used as a barrier against many water borne diseases. Sylhet, one of the six divisional cities of Bangladesh is suffering from shortage of domestic water supply. At present, water supply system of Sylhet City Corporation (SCC) mainly dependent on ground water fulfils only 48.8% demand of its 0.5 million population. The rest of the people are deprived from the facilities and mainly depend on hand pump tube well. Unfortunately the iron and arsenic problem in and around the city has made the use of ground water risky for drinking purpose. Obviously water utilities in Sylhet have to think of surface water sources as an alternative to ground water. This study attempts to evaluate the quality of surface water bodies around Sylhet city and the degree of treatment required to make these water sources suitable for domestic water supply. The physical, chemical and biochemical characteristics of water of the river Surma and the river Kushiyara were determined through extensive laboratory tests. Except suspended solids and turbidity all other parameters have higher concentrations in the dry season comparing to the wet season for both the rivers as high flows in wet season provide better dilution of pollutants. Huge amount of surface and agricultural runoff and bank erosion increases suspended solids and turbidity in the wet season. It is evident from experimental results of water quality parameters that both the rivers can be used as source of water for fishing, industrial and irrigation purpose according to Bangladesh standards of inland surface water. However, the test results of coliform exceed the limit of Bangladesh standard for recreational use. Comparing to the surface water bodies around Dhaka city it can be concluded that the degree of pollution of the Surma and the Kushiyara is not yet high and can be used as an alternative source of water supply instead of ground water for SCc. Between the two rivers, the Surma has some advantages over the Kushiyara with respect to distance and water quality. Before using as potential source of water supply, the water of the Surma should be treated to reduce turbidity, suspended solids, faecal pollution, and organic matter. Physico-chemical treatment methods such as sedimentation, coagulation, sedimentation followed by coagulation, sand filtration alone and in combination with sedimentation and coagulation were analyzed to treat the water of the Surma. The removal efficiency of turbidity, suspended solids, BODs and fecal coliform was taken as indicating parameters for assessing the effectiveness of different treatment units. Among these treatment options sand filtration in combination with 10-mg/l coagulation as well as sand filtration in combination with sedimentation (4-hr) and coagulation (2.5 mgll) can be effectively used for the treatment of solids content, turbidity, BODs and fecal coliform from the water of the Surma. Both the two options are found satisfactory for treating the above parameters to keep them within the acceptable limit of Bangladesh standards for using the water of the Surma for drinking purpose.