Abstract:
Dhaka city is situated on the bank of the river Buriganga, which at present is one of the most polluted rivers in Bangladesh. The river receives wastewater from numerous pollution sources along its way, which are discharged as industrial effluents, municipal sewage, household wastes, clinical wastes and oil. A large share of pollution load of the river Buriganga comes from the tannery industries in the Hazaribagh and Rayer Bazar area through Kamrangir Char khal and Rayer Bazar khal. In order to protect the river from pollution, government has already started the process to relocate the tannery industries from Hazaribagh to Savar. Out of 154 tanneries, 106 tanneries have moved to Savar from Hazaribagh till March 2018. The main focus of this research was to assess the possible impacts of the relocation of tannery industries on the water quality of Buriganga river through field measurement, laboratory analysis and modeling.
In this study, a portion of Buriganga river from Boshila Bridge to Postagola Bridge has been selected as the study reach to assess the existing water quality during the dry season of the year 2017 and 2018. Water samples were collected from nine monitoring locations along this reach during that period. Kamrangir Char khal and Rayer Bazar khal have been taken as point sources that contribute pollution loads towards the river mainly from the tannery industries. From the field measurements and laboratory analyses, some improvement in water quality of Buriganga river has been found in March 2018 compared to March 2017. Appreciable reduction in BOD, electrical conductivity, ammonia, nitrate and orthophosphate concentrations has been observed along the river in March 2018 (compared to March 2017). For example, COD values along the Buriganga river ranged from 65 to 140 mg/L during the dry season of 2017, while it ranged from 55 to 85 mg/ L during the dry season of 2018; Ammonia concentration in river water varied from 13.8 to 16.18 in March 2017, while it was found to vary from 10.8 to 13.25 in March 2018. This improvement in water quality could be attributed to the shifting of a large number of tannery industries from Hazaribagh during this period.
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 (WASP 7.3), 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 river during the dry period of 2017 and 2018, respectively. The verified model with the pollutant loading condition of 2018 has been used to investigate the impacts of relocation of tannery industries on Buriganga river water quality by considering four load reduction scenarios in the model. According to the model prediction, the DO level of the Buriganga river may not improve significantly due to tannery relocation; this is due to the significant waste load that the river carries from upstream (i.e., upstream of tannery industries) locations. But considerable reduction of BOD, ammonia, nitrate and phosphate are expected as a result of the tannery relocation. It appears that reduction in waste load from other upstream locations would be needed in order to achieve substantial improvement in the water quality of the River.
