Critical Elements in the Manufacture and Application of Preformed Chloramines

摘要

Concerns about chlorinated disinfection by-products (DBPs) formed in the presence of free chlorine have led many water utilities to use chloramines for disinfection in distribution systems. Chloramines, which are formed by a reaction between free chlorine and ammonia, can also be used to control biofouling during filtration by chlorine intolerant membranes in water or advanced wastewater treatment. However, the traditional method of creating chloramines in the main process flow has significant disadvantages in certain situations. For example, free chlorine contact time before ammonia can create chlorinated DBPs. Additionally, poor mixing during the production of chloramines forms more dichloramine, which is known to produce more N-nitrosodimethylamine (NDMA) than monochloramine. Manufacturing a high-concentration solution of preformed chloramines in a well-designed sidestream process is one approach to minimizing these negative effects of chloramine disinfection. Adding preformed chloramines to drinking water will reduPs and NDMA produced from dichloramine. Water reuse facilities are also concerned with NDMA in the plant effluent, which highlights another pntial application. Preformed chloramines could also be used for biofouling control during reverse osmosis (RO) desalination of seawater. When seawater is chloraminated using separate addition of chlorine and ammonia bromamines are formed because free chlorine will rapidly oxidize bromide to bromine, which can react with ammonia to form bromamines. Preformed chloramines avoid bromamine formation because the bromide is never exposed to free chlorine. The application of preformed chloramines for biofouling control of RO membranes has been demonstrated at the pilot- and demonstration-scale facilities of West Basin Municipal Water District. Additionally, bench-scale testing has been performed to explore specific issues related to manufacturing preformed chloramines. Experience in these applications has identified a number of challenges with the production of a stable preformed chloramine solution. This discussion will explore critical factors such as solution concentration, mixing parameters (time, method, reaction rates, etc.), sidestream water quality, chlorine:ammonia ratio, and pH. Two different methods for producing preformed chloramines, batch manufacturing and inline mixing, will be compared. Additionally, techniques for verifying the quality of the resulting preformed chloramines will be discussed. Operational experience with preformed chloramines suggests this approach could be adapted to improve chloramine efficacy in other drinking water and wastewater treatment applications.