Spatio-temporal variation of microbial indicators of river water and treatment efficiencies of drinking water treatment plants along the upper Mahaweli river segment of Sri Lanka

Abstract

The microbial pollution status of river surface water is important to ensure a river-based quality drinking water supply for the public. The present study aimed to investigate bacterial contamination status in the upper Mahaweli River, the main drinking water supplier to the hill country of Sri Lanka. Both the raw surface water and treated water, taken at 14 drinking water treatment plants (DWTPs) along the river segment of 60 km between Kotmale and Victoria reservoirs, were tested for total bacterial counts (TBC), total coliform counts (TCC) and faecal coliform counts (FCC). Irrespective of the flow, the selected river segment showed significant contamination variations of TBCs, TCCs and FCCs between dry and wet seasons due to point source discharges and drains of septic waste. The level of pollution was enhanced in the wet season due to the diffusion of pollutants with the runoff. Locations of DWTPs could be clustered into 3 categories according to microbial contamination as very low (Kotagala, Thalawakelle-Galkanda, Kandy South, Pundaluoya, Ulapane, Greater Kandy), moderate (Nawalapitiya, Paradeka, Elpitiya, Nillambe, Balagolla, Haragama, University Plant, Polgolla) and high (Thalawakelle-Nanuoya). Although the bacterial reduction efficiencies of many DWTPs (10 out of 14) were higher than 77.9%, treated water samples from 10 DWTPs exceeded the maximum TBC recommended by the US Environmental Protection Agency (2.70 log₁₀ CFU/mL) for drinking water, during wet season due to high bacterial load in the surface water. Isolates from treated water were Gram-positives with Staphylococci (74.3%) and Micrococci (25.7%). Total coliforms and faecal coliforms were absent in treated water satisfying the SLS 614:2013 guidelines for drinking water. The disinfection process of DWTPs should be improved to ensure the safety of drinking water, particularly during the wet season. Incorporation of more microbiological parameters into routine testing practices is important in understanding bacterial load and diversity to improve treatment strategies.