Mathematical model for ammonia pollution along the Balu-Lakhya river system and assessing its impacts on dissolved oxygen

Abstract

Balu–Lakhya river system is situated in the eastern side of Greater Dhaka city. Many industries are situated on the bank of the rivers and are discharging effluent directly into the rivers. Domestic sewage of Dhaka and Narayangonj city is finally discharged into Balu river through Norai khal and Lakhya River through DND khal, Majheepara khal, Tanbazar khal, Killarpul khal, Kalibazar khal and B.K Road khal. These waste materials enormously deteriorate the water quality in these rivers. The improvement of river water quality is crucial for its sustainability as well as to have a portable source to meet the demand of city water supplies. Previous studies developed water quality model due to Biochemical Oxygen Demand (BOD) pollutants for river systems around Dhaka city. High concentration of ammonia becomes a major problem in the river system, which is hampering the efficiency of Sayedabad Water Treatment Plant. This study principally aims to develop Water Quality (WQ) model for BOD, Ammonia, Nitrate pollutants and assesses its impact on dissolved oxygen (DO) along with different scenarios, considering different waste loading patterns. The hydrodynamic (HD) model required for developing WQ model is calibrated with observed discharge and water level. A number of trials were accomplished to develop the water quality model, which has been calibrated with observed DO and Ammonia level. Results of the base simulation show that the DO level at present is below the critical DO level (4 mg/l) and ammonia level is above 3 mg/l at down stream of Lakhya River. Projection simulations, for year 2015 and 2025 show that, the problem would be increased in future. The problem will be more acute if water level increases due to climate change. In future, the up stream of the Lakhya River, which is maintaining DO level above 4 mg/l, would cross the critical value. Eight other options have been considered for identifying the major source of pollution near Sarulia and assessed its impact on the water quality of the Lakhya and Balu River. No single source is responsible for deteriorating the Lakhya River water quality. Norai khal and Tongi khal are responsible for deteriorating Balu River water quality. Waste water discharging from Norai khal, Ghorasal, Narayangonj and Tarabo industrial area have to be prohibited for improving the water quality in the Lakhya-Balu river system. Augmentation of water may require for this improvement, which is not analyzed in this present study. Data used for calculating waste load and calibration is based on secondary sources. The projected pollutants estimation is based on population growth rate which may not be the single factor.