Treatment and Distribution System Effects on Chloramine Decay, pH, Nitrification, and Disinfection By-Products: Case Study1

Abstract

The Massachusetts Water Resources Authority (MWRA) supplies wholesale unfiltered surface water to 2.2 million people in 43 communities, including Boston. Current MWRA treatment consists of adding hypochlorite for primary disinfection, soda ash and carbon dioxide for corrosion control, hydrofluosilicic (hexafluosilicic) acid for fluoride treatment, and hypochlorite and ammonia for maintaining distribution system residuals. The new Walnut Hill water treatment plant, scheduled to come on line in early 2005, will consolidate current treatment facilities under one roof and use ozone instead of hypochlorite as the primary disinfectant. It was observed from distribution system monitoring and bench-test results that the decay of chloramines was affected by prechlorination. The decay of chloramines released ammonia for subsequent nitrification. Complete nitrification and subsequent denitrification at current target alkalinity and combined chlorine levels affected pH. Increased microbiological activity associated with nitrification also decreased haloacetic acid concentrations. Bench and pilot tests indicate that the use of ozone will increase the rate of chloramine decay and will affect pH stability. Ozonation of natural organic matter produces organic acids, which typically decreases the pH in bottle tests. Results from a pilot plant, set up to simulate the new treatment plant with old cast-iron pipes simulating the distribution system, indicate that corrosion and biological activity inside the pipe have major impacts on water quality and that pipe surface reactions often cause pH to increase.