Water Research 38 (2004) 2375–2383 Formation of haloacetic acids during monochloramination Yinan Qi, Chii Shang*, Irene M.C. Lo Department of Civil Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 12 May 2003; received in revised form 15 December 2003; accepted 7 February 2004 Abstract Factors that affect the formation of haloacetic acids (HAAs) during monochloramination, such as monochloramine application techniques, the initial chlorine (Cl) to ammonia-N (N) ratios, the bromide concentrations, and the wastewater quality, were studied. Aqueous humic acid solutions and undisinfected wastewater effluent samples obtained from two Hong Kong Sewage Treatment Works (STWs) were monochloraminated under various conditions. HAA formation was strongly affected by the monochloramine application techniques. The formation of trichloroacetic acid (TCAA) and total HAAs was reduced by adding preformed monochloramine. A higher initial Cl:N ratio indicated a higher chlorine demand and consequently led to higher HAA yields. Increasing the bromide concentration shifted HAAs from chlorinated species to brominated species and increased the yields of total HAAs, concurrent with decreases in the yields of dichloroacetic acid (DCAA) and TCAA but with increases in those of the other HAAs measured. Variations in the patterns of HAA formation were observed in monochloraminated wastewater effluent samples. The variations cannot be simply explained by the chlorine chemistry involving ammonia and/or bromide but are likely attributable to the combining effects of the water quality and the characteristics of the organics in the wastewater. r 2004 Elsevier Ltd. All rights reserved. Keywords: Bromide; By-products; Disinfection; Haloacetic acids; Monochloramination; Monochloramine 1. Introduction Chlorination has been utilized in water and waste- water disinfection for nearly 100 years since the importance of disinfecting drinking water was recog- nized in the early Twentieth Century. Chlorination remains the predominant disinfection means due to its broad-spectrum germicidal potency, low cost, and well- established practices. However, when water or waste- water is chlorinated, chlorine reacts readily with a wide variety of organics to form disinfection by-products (DBPs), such as the well-known trihalomethanes (THMs) and haloacetic acids (HAAs). These DBPs have been identified as cancer-causing reagents in the last three decades [1]. Since then, attempts have been made to switch to other disinfection methods, including ozonation and UV-irradiation. These alternatives were later found to have their own drawbacks, such as the formation of other DBPs and their higher operational costs. Removing DBP precursors prior to chlorination has been proposed as a remedial action; however, this idea is sometimes technologically or economically impracticable in wastewater treatment. Under such circumstances, monochloramination is a good alterna- tive to realize DBP reduction while the main goal of inactivating pathogens can still be achieved. As a matter of fact, combined chlorine (generally, monochloramine) is posited to be the primary residual contributing to the inactivation in wastewater chlorination, if the waste- water is not nitrified or not well nitrified. Many studies have investigated the formation of DBPs during water chlorination and monochloramina- tion. In general, monochloramination, compared to chlorination, has been reported to yield significantly ARTICLE IN PRESS *Corresponding author. Tel.: +852-2358-7885; fax: +852- 2358-1534. E-mail address: cechii@ust.hk (C. Shang). 0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.02.007