Assessment of water quality at DND conveyance canal and its implications on treated water quality at Saidabad water treatment plant

Abstract

The Saidabad Water Treatment Plant (SWTP), commissioned by DWASA in July 2002, serves as one of the major potable drinking water sources for the people of Dhaka. DWASA utilizes a section of the DND open conveyance canal to convey raw water from Sarulia intake on the Sitalakhya river to the SWTP. During dry season each year, water quality of the Sitalakhya river deteriorates significantly, resulting in poor quality of treated water at the SWTP. In this study, water quality of Sarulia intake, DND canal and the SWTP were intensively monitored during the dry season (January to March) of 2007. For this purpose water samples were collected from the Sarulia intake, 6 locations along DND canal, and 5 locations within the SWTP and analyzed for a wide range of parameters. The water quality of the DND canal was also monitored during the wet season (June to September, 2007). The water quality of Sitalakhya river and the DND canal appears to deteriorate progressively from January to March as dry season progresses. The water quality during the dry season is characterized by high concentrations of BODs, COD, TOC, Ammonia, Phosphate and Chlorophyll-A (algae); and low concentrations of DO. Water quality within the DND canal does not appear to change significantly, with the exception of Chlorophyll-A, whose concentration increases by a factor 2 to 3 within the canal. Removal of Ammonia or conversion of Ammonia to Nitrate is insignificant along the canal. BOD reduction within the canal by biodegradation appears to be insignificant. Suspended solids reduction (up to - 40%) within the canal during dry season is not accompanied by BOD reduction, which possibly suggests that any reduction in BOD due to settling of particulate BOD (and also biodegradation) is probably accompanied by addition of BOD of similar magnitude from the surrounding areas and from the sediment. During the wet season (June to September), water quality of Sitalakhya river and DND canal improves significantly due to rainfall and freshwater inflow from upstream, which results in significant reduction of BODs, COD, Ammonia, Phosphate and Chlorophyll-A concentrations and increase in DO concentration. Suspended Solids (SS) concentration in the Sitalakhya river increases significantly during the wet season; however, the DND canal appears to function as a sedimentation basin and significant reduction in SS concentration (up to 80%) occurs within the DND canal. The potential sediment oxygen demand (pSOD) of sediments of DND canal was found to be very high and thus could exert significant oxygen demand on the water, especially during re-suspension events. With deterioration of raw water quality, the treated water quality at the SWTP also suffers during the dry season. In dry season, higher alum doses are also required for removal of SS and algae. Relatively high concentrations of Aluminum (AI), exceeding the Bangladesh drinking water standard, were also detected in the treated water, which most likely comes from high doses of alum (up to 60 mg/L) used during the dry season, suggesting need for optimization of the coagulation process. During dry season almost the entire applied Chlorine is likely to be converted into Monochloramine. The residual chlorine at SWTP appears to exceed the available standard (USEPA) and guideline (WHO) of total Chlorine and Monochloramine during the dry season. During wet season the Chlorine to Ammonia ratio at the SWTP may become very high (> 4), which may promote formation of Dichloramine and Nitrogen Trichloride and give rise to taste and odor problems. Break point chlorination (BPC) does not appear to be a feasible option during the dry season, but BPC could be considered as an option to ensure effective chlorination during the wet season. Total ammonia concentration of treated water at the SWTP is slightly lower than that in the raw water, possibly due to oxidation of some Ammonia to Nitrate. A significant fraction of total Ammonia in the treated water is likely to exist as Chloramines. In the assessment of THM formation potential, the principal uncertainty is the amount of free chlorine that would be available for reaction with organic matters and bromide, leading to THMs formation. The raw water at the SWTP has been found to be very susceptible for THM formation during dry season because of high concentration of DOC and bromide, and relatively higher pH and temperature. Presence of high concentration of Ammonia, however, may significantly reduce TTHM formation by limiting the concentration of free chlorine. The predicted Total THMs (TTHMs) concentration for the wet season was relatively low, satisfying the Bangladesh standard (for chloroform) and USEPA and EU drinking water standards; but the predicted TTHMs concentration during the dry season exceeds these standards by a large margin. Limited laboratory measurements of THMs during the dry season (April 2008) showed that. the values are well within the available drinking water standards. THM precursors (e.g., DOC, bromide), THMs, and applied Chlorine dose should be included in the regular monitoring program at the SWTP. Therefore, the quality of the raw water drawn through the intake structure varies significantly over the year and this affects the efficiency of treatment at the plant and the quality of treated water. Dry season (January to March) is the critical period for the SWTP, during which raw water quality deteriorates significantly, putting strains on the treatment processes. Doses of alum and chlorine used at the plant also depend on the quality of raw water. Possible presence of non-biodegradable organic materials in raw water from industrial discharges may contribute to the high COD and TOC values along the DND canal and at the SWTP. The high concentrations of ammonia, organic matter and algae appear to be the major water quality problem ofraw water at the SWTP.