Watershed Characterization of Demodara Reservoir with Special Attention on Soluble Manganese Concentration

Abstract

To ensure smooth operation and to manage water quality of the Demodara Water Treatment Plant (DWTP), better knowledge of its watershed is required. In line with this evolving response to water quality management; the National Water Supply & Drainage Board is currently piloting the development of a Water Safety Plan for the Demodara water supply scheme. For the purpose, watershed assessment as a baseline study in a border context for the study area is addressed in this paper. The DWTP is fed by the Demodara reservoir. Water quality tests were conducted representing its catchment and none of its water quality parameters except manganese (Mn) shows critical levels in the catchment. The catchment was analysed to capture the Mn source. Results show that the geological formation of the catchment consists of MnO₂; especially in the Charnockitic gneisses layers. With rainfall, Mn⁺⁴ converts to dissolved Mn⁺². It then comes to the Demodara reservoir from surface runoff and, it remains as Mn⁺² at the bottom levels of the reservoir due to anaerobic, reducing conditions. Raw water is pumped to the DWTP from low levels of the reservoir, and therefore Mn enters in to DWTP system. Manganese chemistry is very complex and consists of multiple oxidative states. At the DWTP, Mn is converted to insoluble manganese by oxidation using KMnO₄ followed by the removal of oxidized, insoluble manganese through filtration. However, manganese treatment does not completely remove all manganese from the treated water. It will allow some manganese to enter the distribution system, particularly in high rate water treatment plants. Literature reports emphasize that Mn has no negative health impacts at the levels commonly found in drinking water. However, aesthetic problems caused by Mn are a continuing source of concern for customers and DWTP operators. Therefore, treating Mn in raw water requires continuous monitoring by plant operators. For the smooth functioning of the DWTP, a number of recommendations are discussed in this paper.