Assessment of the ipmact of overflows from special sewage diversion structures on the water quality of Hatirjheel

Abstract

Hatirjheel, which is now the largest surface water body within Dhaka, serves very important hydrologic functions of draining and detaining storm water from a large area of Dhaka city. As a part of a restoration project, eleven especially designed sewage diversion structures (SSDSs) have been constructed at eleven major storm sewer outfall locations around Hatirjheel. The SSDSs are connected to a peripheral “diversion sewer” network. Although designed to carry storm water, the storm sewers discharging into Hatirjheel carry both storm water and domestic sewage. During the dry season, the entire flow (consisting of sewage) carried through the storm sewers up to Hatirjheel is diverted downstream along the peripheral “diversion sewers”. The SSDSs are designed to allow overflow of storm water into Hatirjheel during wet season. However, since the storm sewers carry both storm water and sewage, the overflows discharging into Hatirjheel during wet season cause deterioration of water quality of Hatirjheel. In this study, assessment of the capacity of the main diversion sewers to accommodate future flows has been made; and water quality of Hatirjheel has been assessed through a year-long water quality monitoring. The combined total catchment area of all 11 SSDSs of Hatirjheel is about 23.73 km2. The SSDS-1 and SSDS-10 have the highest catchment areas; 6.1 km2 and 4.87 km2, respectively. It has been found that the main diversion sewer system running along the periphery of Hatirjheel might not be able to accommodate the increased sewage flow in the near future (beyond 2015). The water quality of Hatirjheel is poor throughout the year, but is especially poor during the wet season when rainwater-sewage mixture overflows into Hatirjheel through the SSDSs. Water quality of Hatirjheel is particularly poor in near the SSDSs through which rainwater-sewage overflows during the wet season (SSDSs-1, 2, 7, 9). For example, during the wet season of 2014 (June to October 2014), the highest BOD5 and COD values recorded for water samples collected in front of SSDS-1 were 90 mg/l and 175 mg/l, respectively. Total ammonia nitrogen (TAN) (and free ammonia) concentration in Hatirjheel water increased during the wet season due to entry of rainfall-sewage mixture; ammonia concentration continued to increase after the end of wet season (up to January 2015), most likely due to release of ammonia into water column from the decomposition of organic matter present in overflow water. Ammonia concentration in Hatirjheel water exceeds the USEPA standards for natural water by a huge margin. Ammonia concentration in Hatirjheel water is so high that fish species are unlikely to survive in this environment. The BOD5 and Ammonia concentration of Hatirjheel water is much higher than the corresponding Bangladesh standards, and Hatirjheel water appears to be unsuitable for any purpose including fisheries, recreational and irrigation. Nitrate concentration in Hatirjheel water increased at the end of wet season, possibly due to oxidation of ammonia to nitrate; subsequent reduction in nitrate concentration is possibly due to incorporation into algal mass. Sulfate and phosphate concentrations in Hatirjheel water are relatively high throughout Hatirjheel throughout the year. Sulfide concentration was relatively higher but reduced significantly during the dry season. Due to increase in wastewater/sewage flow, the overflow of sewage-storm water mixture into Hatirjheel during the wet season will continue to increase in the future; in fact, in the future, overflow of sewage from SSDSs could take place even during the dry period. This is likely to cause significant pollution of Hatirjheel throughout the year. In order to improve water quality of Hatirjheel, the domestic sewer connections to the storm sewer system must be disconnected gradually by DWASA. In this regard, catchment of SSDS-1 should be given priority. If domestic sewer connections to storm sewer could be disconnected within this catchment, then pollution load discharging into Hatirjheel (through SSDS-1) would be reduced significantly and water quality of Hatirjheel is expected to improve considerably. Efforts should be made to assess possible diversion of sewage flows from certain sections of the Hatirjheel catchment in order to avoid overloading of the main diversion sewer system of Hatirjheel.