Seasonal variation of water quality index at raw water collection point of Saidabad water treatment plant and its impact on plant operations

Abstract

This study was conducted to determine the seasonal variations of the water quality index at the raw water collection point of the Saidabad Water Treatment Plant (SWTP) and its impact on plant operations. Firstly, monthly water quality and plant operational data were collected from the SWTP and the Department of Environment (DoE). Commonly used water quality parameters i.e., Chemical Oxygen demand (COD), Biochemical Oxygen Demand (BOD), Dissolved Oxygen (DO), Alkalinity, Ammonia, Total Dissolved Solid (TDS), pH, Electrical Conductivity (EC), Temperature, and Fecal Coliform data have been collected and used for the study. Variations of water quality parameters and Water Quality Index (WQI) were analyzed for the raw water collection point of SWTP and the Demraghat point (Upstream of the raw water collection point of SWTP) of the Shitalakhya River. Widely used WQI methods (WAWQI, CCMEWQI, and NSFWQI Methods) were used for the determination of the WQI at the raw water collection point of the SWTP and Demraghat from the year 2017 to 2021. Multiple regression analysis was conducted for identifying statistically significant variables affecting the WQI values at the SWTP. In this study, the WQI values were found between 91 to 214 (as per the WAWQI rating, water quality is very bad and unsuitable for drinking), 34 to 72 (as per the CCMEWQI rating water quality is poor, marginal, and fair), and 24 to 41 (as per the NSFWQI rating water quality is bad and very bad), by using WAWQI, CCMEWQI, and NSFWQI methods, respectively at the raw water collection point of SWTP from the year 2017 to 2021. The ranges of WQI values were found between 49 to 224, and 42 to 90 byusing WAWQI, and CCMEWQI methods, respectively at Demraghat point in pre-monsoon, monsoon, and post-monsoon periods from the year 2017 to 2021. It was found that WQI values were very high at the raw water collection point of SWTP using the WAWQI method compared with the Demraghat point of the Shitalakhya River due to higher concentration of Ammonia and lower DO values. DO level and Ammonia concentration were found exceeded the permissible limits set by DoE for most cases from the year 2017 to 2021.Overall, the condition of the waterbody of the Shitalakhya River was found very poor from the year 2017 to 2021. The water level (WL) values of the river during the pre-monsoon, monsoon and post-monsoon were found inversely correlated with the WQI values, as the WL increased, the WQI values were found decreasing. Statistical analysis revealed that four water quality parameters i.e., DO, Ammonia, TDS, and Turbidity are statistically significant for estimating the WQI values. However, from the plant operational perspective, it has been found that Ammonia concentration significantly impacted the pre-treatment processes for the plant. Maximum DO was found 5.00 mg/l (in the monsoon period of the year 2018) and the minimum value of DO was 0.01 mg/l (in pre-monsoon period of 2021) at Demraghat point. DO was high in the monsoon period due to the high monsoon flow of the river. The mean value of DO was found 2.2 mg/l and 2.11 mg/l from the year 2017 to 2021 at Demraghat and the raw water collection point (Sarulia) of SWTP, respectively which is not suitable for aquatic ecosystem of the river and indicates the water was unsuitable for any use without special treatment. Ammonia concentration was found 0.65 ppm to 12.65 ppm at the location of the raw water collection point of SWTP in the pre-monsoon and post-monsoon periods, respectively from the year 2017 to 2021. The mean value of Ammonia concentration was found 4.80 ppm which crossed the permissible limit of Environmental Quality Standards (EQS). It has been found that when Ammonia concentration is more than 2 ppm, SWTP requires special pretreatment processes. The pretreatment unit (Nitrification) isrequired in the pre-monsoon and post-monsoon period from the year 2017 to 2021 which increased the water treatment cost of the SWTP substantially.The unit cost of water treatment was found increasing from 19% to 151% during 2018 to 2021compared with base year unit cost of treatment unit cost (considering the year 2017 as the base year) due to the increasing pre-treatment, plant maintenance, and utility costs of the plant.