Modeling thermal effluent of a power plant and its effect on water quality of the Sitalakhya river

Abstract

The River Sitalakhya receives pollutants from various sources. It is a source of water for domestic and industrial water supplies and a centre of numerous activities in the region. The river receives thermal discharges from Globeleq and Siddhirganj Power Plants and is the designated recipient of the additional thermal discharges from proposed power plants in the area. Thermal discharges produce thermal plumes of elevated temperature and greatly influence the major water quality parameters mainly through alteration of dissolved oxygen level, microbial activity and kinetic coefficients of reactions in the river. This study is aimed to investigate the effects of thermal discharges in the River Sitalakhya from the existing and proposed thermal power plants in the area. The CORMIX model has been used for prediction of thermal plume for discharges of Siddhirganj and Globeleq Power Plant under ~ifferent tidal conditions of the river during the critical dry period of the river. Sensitivity analysis of the model was conducted for variable model parameters such as Manning's roughness coefficient, . . wind speed, heat loss coefficient, river discharge, excess, temperature and channel geometry within practical range of variations. The predicted values and the trend of excess temperature of the thermal plume by model simulation fairly agreed with the measured values of temperature at different stages of tidal cycle. The highest excess temperatures were found to occur at I hour after Low Water Slack (LWS), which can be considered as the critical time of the tidal cycle. The excess average predicted and measured surface temperature of the thermal plume for submerged discharge by Globeleq Power Plant was found much lower as compared to surface discharge near the bank by Siddhirganj power plant under existing conditions of thermal discharges. The excess temperatures for Globeleq Power plant in all cases were much lower than the critical temperature at the Regulatory Mixing Zone (RMZ). Although the discharge of Siddhirganj Power Plant exceeded the RMZ criteria, it conforms to the existing ECR'97 for disposal of thermal effluent. The model has been simulated for increased thermal discharges of proposed power plants at Siddhirganj for critical period of the tidal cycle. The excess temperatures at RMZ were found to be about two times higher than the critical temperatures at RMZ and high temperatures regimes were found to be extended long distances far beyond the RMZ. It is also anticipated that an additional 210 MW power plant disposing themlal effluent in the Sitalakhya may cause heat entrapment because of tidal effects leading to temperature build up. Water quality simulation of some major water quality parameters such as BOD, DO, ammonia, nitrate, phosphate was done under existing conditions of the river and fair agreement between simulated and measured values was found. Excess temperature by thermal discharges caused a small decrease in BOD and ammonia, a small increase in nitrate and a significant decrease in DO (Approx. 0.5 mg/L). These changes are due to increased bacterial activity, alteration of kinetic coefficients at elevated temperatures under present condition of discharges. The increase in temperature due to thermal discharges from the existing and proposed power plants in the area as predicted by the model is likely to cause about 50% increase in bacterial activity resulting in significant deterioration of DO content of the river in future.