Evaluation of water quality restoration of Hatirjheel

Abstract

The Hatirjheel lowlands serve very important hydrological functions of draining and detaining storm water from a large area (about 30 km2 ) of Dhaka city. These lowlands receive storm water discharges primarily through a number of major storm sewer outfalls. However, illegal connections of both domestic and industrial wastewaters to the storm sewer network are rampant in Dhaka. As a result, during dry season, the storm sewers mainly carry domestic sewage as well as industrial wastewater. The untreated domestic sewage and industrial wastewater drains through this lowland via the Begunbari khal-Norai khal into the Balu river. This has significantly degraded the environmental condition of Hatirjheel and reduced its capacity to work as a retention basin for storm water. In order to prevent pollution, remove water logging, and to improve water quality condition of the entire Hatirjheel lowlands, a project has been initiated (referred to here as "Hatirjheel Restoration Project") by the Government. This study provides an assessment of the waste load that is discharges into Hatitjheel and presents a simple modeling framework for prediction of Hatirjheel water quality after implementation of the restoration project. As a part of the present study, the major storm sewer outfalls discharging into the Hatirjheel were identified through analysis of available of maps and field reconnaissancc. Discharge measurements were carried out at all major outfalls discharging into Hatirjheel during the dry season. Characteristics of wastewater coming through the storm sewers were assessed through collection and analysis of wastewater samples during both dry and wet seasons. As a part of the "Hatirjheel restoration project", the entire dry weather flow (consisting of mainly domestic and industrial wastewater) discharging into Hatirjheel through the major storm sewer outfalls will be diverted through especially designed sewage diversion structures (SDSs) and diversion sewers, while during wet season a part of the mixed flow of storm water and wastewater will be allowed to discharge into Hatirjheel. This study prescnts a simple model for predicting the water quality of Hatitjheel after implementation ofHatirjheel restoration project. Nine major storm sewer outfalls were identified that discharge into Hatirjheel. These include 3 box culvert outlets and larger diameter (up to 96 inch) storm sewers. The combined dry weather flow (consisting of primarily domestic sewage and some industrial wastewater) through these 9 outfalls was estimated to be about 147 cfs. The box culvert outlets behind Sonargaon Hotel and at Niketon were found to be the major contributor, accounting for about 41% and 16% of total flow, respectively. The dry weather flows discharged though the major outfulls were found to contain high concentrations of BOD, COD, ammonia, phosphate and very low DO. The characteristics of the dry weather flow have been found to be comparable to those of medium- and high-strength domestic sewage. The high COD to BOD ratios (up to 8.16) of wastewater at some of the outfalls are indicative of the presence of nonbiodegradable industrial effluent. Treatment of the mixed sewage, comprising both domestic and industrial wastewater, will be major challenge in the design and operation of the proposed Dasherkandi sewage treatment plant. The characteristics of wastewater improve to some extent during the wet season, due to dilution with rainwater. Estimated TSS, BOD and COD waste loads into Hatitjhcel were estimated to be 209 ton/day, 36 ton/day and 125 ton/day, respectively. Thus, if the entire dry weather flow is diverted to a treatment plant to be built at Dasherkandi, it would not only improve the water quality of Hatirjheel, but would also improve the water quality of Bengunbari khal-Balu river-Sitalakhya river system. The simple model developed in this study, though has a number of limitations, and provides some preliminary estimates of the water quality (DO and BOD) of Hatitjheel after implementation of the SDSs and diversion sewers. The model simulations show that if the SDSs and diversion sewers work properly, the water quality of Hatirjheel would remains within acceptable level (that is DO would be above 4 mgtl) during the wet season (July to September). The DO of Hatirjheel appears to be sensitive to a number of parameters, most notably the initial DO of Hatirjhecl, that is the DO of Hatirjheel at the beginning of July. In order for the DO to be high at the beginning of July, it must be ensured that the SDSsdiversion sewers work properly and wastewater does not get into Hatirjheel from other point or non-point sources. For better understanding of water quality of Hatirjheel, additional data on flow and characteristics of water/wastewater and application of an appropriate numerical model are suggested.