Management of chlorine decay and formation of disinfectant by-products in drinking water: A case of Gaborone City, Botswana

Chlorine is used in most countries as a disinfectant to control multiplication of pathogenic bacteria in potable water. However, water suppliers have two major challenges in management of chlorine disinfection in water distribution systems (WDS): decay and disinfectant byproducts (DBPs) formation. This is a preliminary study to investigate the occurrence of chlorine decay and DBPs formation and how they are affected by water quality factors after water treatment in Gaborone city WDS. The study was conducted by water analysis for chlorine residual, DBPs and water quality factors (natural organic matter, iron, manganese, heterotrophic plate counts, temperature and pH) that may influence chlorine decay and DBPs formation in a WDS. Water samples analyzed were from treatment plants, reservoirs and customers’ taps. The results demonstrate that there is a high chlorine dosage at treatment plants, low chlorine residual at remote areas of the network, high rate of chlorine decay and presence of DBPs in the WDS. Water quality factors that are suspected to be contributing to chlorine decay and formation of DBPs are organic compounds, high temperature and biofilm growth. However, other factors related to hydraulic conditions can also influence chlorine decay in a WDS. Therefore, the study recommends (1) further investigation of the effects of organic compounds, temperature/seasons and growth of biofilms on chlorine decay, (2) assessment of DBPs level and compliance to regulations in the WDS, (3) modelling and evaluation of the effects of hydraulic performance on chlorine decay in the WDS, and (4) optimization of booster locations and dosages to maintain chlorine residual within the standard limits across entire WDS.