Quantitative risk assessment for urban water supply in selected communities

Abstract

In Dhaka, water supply systems vary among communities from conventional piped water system to many improvised systems, such as hand pump fitted with tubewell, flexible pipes carrying water from main supply line, etc. In addition, there are also some shallow tubewells in slum areas. It has been found that in slum areas, piped water supply (38%) and shallow tubewells (31%) are two major water supply options. In low income areas, major options are hand pump connected with DWASA line (48%) and piped water supply (41%). Almost all connections in low come communities are individual house connections, while those of slum areas are community type. Middle and high income communities generally have house connections with reservOIrs. Microbial as well as some physico-chemical water quality parameters were tested for a total of 80 water points covering all communities. Sanitary Inspection was carried out for each water point. Sanitary condition of piped water supply without reservoir was found to be poor compared to other options. Shallow tubewell and piped water supply with reservoir, which are predominant in slum and middle / high income communities, respectively have relatively better sanitary condition. Boiling practice is common among middle and high income communities (nearly 79%). Among low income communities, the practice of boiling water is relatively low (44%); slum people usually do not boil their water. Water quality determined through laboratory analysis was found to vary from community to community as well as from option to option. Ammonia concentration of 40% of supplies in low income community areas exceeded the Bangladesh drinking water standard, while for slum and middle/high income communities this percentage was 33% and 7%, respectively. This is probably because of the fact that a major portion of low-income community area under this study (Sabujbagh) is served by Saidabad surface water treatment plant, which is known to have high ammonia concentration during the dry season. Fifty percent of the water samples collected from different communities had sufficient chlorine at delivery end. About 46% of samples showed presence of FC. Among the supply systems, shallow tubewells were found to suffer least from microbial contamination. Hand-pumps fitted with DWASA line also suffer less from this problem. This is probably because of the fact that here water comes directly from the DWASA main, which contains some residual chlorine that acts against the microbial contamination. Piped supply without reservoir, which mostly serves the slum and the low-income areas, suffers from significant microbial contamination because here long, flexible, PVC pipes often carry water over wastelands, increasing the risk of pollution. ..In house connection with reservoir, if reservoirs are not cleaned frequently this can cause deterioration of microbial water quality. Microbial water quality data were used in a Quantitative Health Risk Model to estimate and compare health risk burden of different water supply options in different communities. The upper confidence limit of DALY for middle/high income communities was found to be less (4.11 yrs /1000 py), in comparison to that of slum (15.91 yrs/1000 py) and low-income communities (15.57 yrs/ 1000 py). Manifestation of waterborne diseases among different communities was estimated from a questionnaire survey, which showed higher prevalence of diarrhoea (8.5%) and typhoid (1.6%)