N 2 yields from monochloramine conversion by granular activated carbons are decisive for effective swimming pool water treatment Bertram Skibinski a, * , Eckhard Worch b , Wolfgang Uhl a, c, d, * a Technische Universit€ at Dresden, Chair of Water Supply Engineering, 01062, Dresden, Germany b Technische Universit€ at Dresden, Chair of Hydrochemistry, 01062, Dresden, Germany c Norwegian Institute for Water Research (NIVA), 0349, Oslo, Norway d Norwegian University of Science and Technology (NTNU), Institute of Civil and Environmental Engineering, 7491, Trondheim, Norway article info Article history: Received 28 May 2018 Received in revised form 21 November 2018 Accepted 26 November 2018 Available online 28 November 2018 Keywords: Swimming pool water Monochloramine Combined chlorine Granular activated carbon Catalytic reaction Surface reaction abstract Inorganic chloramines (mono-, di- and trichloramine) are formed in swimming pool water from the unintended reaction of free chlorine with ammonia that is introduced by bathers. Monochloramine is of particular interest as it is known to react further in pool water forming harmful DBPs, such carcinogenic N-nitrosodimethylamine (NDMA). During pool water treatment with granular activated carbon (GAC) filters, monochloramine is transformed by chemical reactions on the carbon surface to N 2 and ammonia. As ammonia is led back into the pool where it is chlorinated again under the renewed formation of inorganic chloramines, it is recommended to use GACs with a high N 2 yield for monochloramine transformation in pool water treatment. In this study, yields of N 2 and ammonia from monochloramine conversion by commercially available GACs were determined using a fixed-bed reactor system under conditions that are typical for swimming pool water treatment. The N 2 yields remained constant with on-going exposure of the GAC to mono- chloramine and ranged from 0.5% to 21.3%, depending on the type of GAC used. Correlation analyses were conducted to identify carbon properties that can determine the N 2 yield for monochloramine conversion, such as the amount of oxygen groups, the elemental composition and the trace metal content. It was found that the N 2 yield significantly correlates with the copper content of the tested carbons. Model calculations combining pool hydraulics with formation/abatement of inorganic chloramines and NDMA as well as chloramine transformations in GAC filters showed that the concentration of inorganic chlo- ramines and carcinogenic NDMA can be decreased by a factor of ~2, if the tested GACs could be modified to convert up to ~50% of the monochloramine to N 2 . © 2018 Elsevier Ltd. All rights reserved. 1. Introduction and objectives In public swimming pools, chlorine is usually the primary disinfectant. According to national and international regulations (e.g., DIN, 19643-1, 2012; ANSI/APSP-11-2009, 2009; WHO, 2006), the concentration of free chlorine has to be kept within a certain concentration range. Chlorine reacts with organic and inorganic substances introduced by bathers (Keuten et al., 2012, 2014), to form organic and inorganic disinfection by-products (DBPs) (Zwiener et al., 2007). The reaction of free chlorine with ammonia (Weil and Morris, 1949; Isaac and Morris, 1983; Qiang and Adams, 2004) and urea (Blatchley and Cheng, 2010) leads to the formation of inorganic chloramines (mono-, di- and trichloramine) in swim- ming pool water. Among the variety of known disinfection by- products, the occurrence and toxicological relevance of di- and trichloramine is frequently studied and discussed in the literature (Isaac and Morris, 1983; Blatchley and Cheng, 2010; Schmalz et al., 2011). Especially trichloramine has been found to cause respiratory problems, and there is evidence that it increases the risk of asthma for children during adolescence (Bernard et al., 2003, 2008). At the neutral pH of swimming pool water, monochloramine is known to be the most dominant species among the inorganic chloramines (Weaver et al., 2009) that are monitored in swimming pool water on a regular basis as a component of the sum parameter “combined chlorine”. Concentrations of monochloramine in pool water are * Corresponding authors. Norwegian Institute for Water Research (NIVA), 0349 Oslo, Norway. E-mail addresses: bertram.skibinski@tu-dresden.de (B. Skibinski), wolfgang. uhl@niva.no (W. Uhl). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres https://doi.org/10.1016/j.watres.2018.11.068 0043-1354/© 2018 Elsevier Ltd. All rights reserved. Water Research 152 (2019) 74e86