Modeling the impact of wasteload allocation on the water quality of the Sitalakhya river

Abstract

The surface water pollution is considered as one of the most serious environmental problems in most developing countries. Most of the rivers in the urban areas of the developing world are the end points of effluents discharged from the industries and domestic sources. In Dhaka, huge quantities of untreated domestic and industrial wastes are being released everyday in the Sitalakhya river, flowing along the eastern side of the city. The water quality of this river is deteriorating rapidly, especially during dry season at certain reaches of the river. The Saidabad water treatment plant, which draws water from the Sitalakhya river near Sarulia (around 400m downstream of its confluence with heavily polluted Balu river), is facing serious problems in treating water due to poor raw water quality, especially due to the presence of excessive ammonia and algae at the intake areas during the dry season. In this study, the present status of water quality of the Sitalakhya river, Balu river, Tongi khal and Norai khal have been assessed through field tests and laboratory analyses of water samples from selected locations during the dry seasons of 2008 and 2009. To compare the dry season water quality with wet season, river water samples were also collected and analyzed from some of the selected sampling locations during the rainy/flood season in August 2008. To assess the depth variation and tidal effects on the water quality, river water samples were also collected and analyzed from some of the selected sampling locations at 0.6 m and 2.5 m below the water surface and during both ebb tide and flood tide in April 2009. The average pollution loading rates from the major point sources were assessed by measuring flows and concentrations of selected parameters at major outfalls during the dry season of 2008 and 2009. A one-dimensional quasi-steady state water quality model has been developed using the finite segment approach under the modeling framework of the Water Quality Analysis Simulation Program (WASP7.3), developed by U.S. Environmental Protection Agency. The water quality model was calibrated and verified using the water quality and the hydrodynamic data of the rivers during the dry period of 2008 and 2009, respectively. Sensitivity of the model was analyzed to determine the effects of different parameters such as dispersion coefficient, phytoplankton settling velocity, deoxygenation coefficient, sediment oxygen demand and input loading on the concentration profiles of the key water quality parameters. A number of load reduction scenarios were developed to assess their impact on water quality of Sitalakhya river. A preliminary assessment of the effects of increasing temperature due to climate change on water quality were also assessed using the predicted weather (temperature) data of years 2030, 2050, and 2070 from a regional climate model PRECIS. Dissolved oxygen concentration of the Sitalakhya river from Tarabo to Siddirganj, and the entire length of the Balu river and the Norai khal has been found to be close to anoxic level due to huge amount of pollution load in these areas. Even the wind-induced natural aeration together with the mixing/dispersion effects of the river are not sufficient enough to raise the dissolved oxygen even above 2 mg/L along the major portion of its reach. Progressive increases in the dissolved oxygen concentrations measured in the upper portion of the Sitalakhya river have been captured by the model results. Ammonia-Nitrogen concentrations varied from 0.05 mg/L to 13.42 mg/L along the Sitalakhya river in dry season. Highest ammonia concentration was found at Sarulia near Saidabad WTP intake in March 2009. Higher concentration of ammonia was observed during flood tide than ebb tide near Saidabad WTP intake as DND drainage canal (located about 2.7 km downstream of the WTP intakes) discharges huge amount of ammonia to Sitalakhya river. No significant variation in ammonia concentration with depth was observed. The Balu River is highly polluted and high ammonia concentrations were observed throughout its length with relatively higher ammonia concentration found near the confluence of Norai khal. The spatial and temporal patterns simulated by the model matched the field data very well. The 5-day Biochemical Oxygen Demand varied from 0.4 mg/L to 28.8 mg/L along the Sitalakhya river in dry season. Highest BOD5 was also found at Sarulia near Saidabad WTP intake in February and March 2009. Spatial and temporal variations of BOD5 were found similar to those of ammonia-nitrogen. The spatial trend predicted by the model for orthophosphate matched reasonably well with the observed data. Orthophosphate concentrations varied from 0.08 mg/L to 2.8 mg/L along the Sitalakhya river, 0.8 to 7.2 mg/L along the Balu river, 3.58 mg/L to 10.2 mg/L along the Norai khal in dry season. The verified model with the pollutant loadings of 2009 has been used to assess the impacts of different pollutant loading scenarios (e.g. diversion of wastewater from the Hatirjheel-Begunbari lowlands to a proposed WWTP at Dasherkandi, setting much stringent national effluent standard, etc). Following each run, model output of 25 March of 2009 on spatial basis has been compared with the verified model results as it has been observed that during the last week of March water quality of the Sitalakhya river becomes worst. Model results have also been compared on temporal basis from February to April at Sarulia near intake point of Saidabad WTP. The model results indicate that the Sitalakhya river water quality near intake point of Saidabad water treatment plant may not improve appreciably through reduction of waste load from a single major point source like Norai khal. Even if all the major point source discharges into the river system satisfy the national effluent standard, the water quality improvement will not be significant during the dry season. For maintaining minimum water quality level in Sitalakhya river, it would be necessary to set much stringent and location-specific (especially depending on characteristics of receiving water body) effluent standard. For example, maximum permissible limits for industrial effluent discharges into inland surface water for BOD5 may be reduced from 50 mg/L to 20 mg/L, and NH3-N discharge limit from 5 mg/L to 3 mg/L. No new polluting industry (even with ETP) should be allowed to be set up along the bank of the Sitalakhya river as this river is used as a raw water source for drinking water supply. The developed water quality model predicted progressive decrease of ammonia-nitrogen and BOD5 with the increase of temperature. Model predicts progressive decrease of nitrate due to higher uptake of nitrate by phyto-plankton as growth rate of phyto-plankton increases with the increase of temperature and solar radiation. DO concentrations have been predicted to increase to some extent due to higher algal photosynthetic DO production.