Effects of feed water NOM variation on chloramine demand from chloramine-decaying soluble microbial products during rechloramination

Abstract

Earlier, we reported on soluble microbial products-mediated chloramine decay in nitrifying waters. However, we neither separated the agent(s) nor identified the factors that enhanced the production of chloramine-decaying soluble microbial products (cSMPs). Experiments were conducted by feeding reactor sets (each consisting of five reactors connected in series) with treated water (3–8 mg-DOC.L−1) obtained from a water treatment plant. The reactors simulated various nitrifying conditions that are experienced in a chloraminated system. In unfiltered samples obtained from nitrified reactors, about 89–93% of the dosed chloramine decayed within 40 h. The cSMP-mediated decay accounted for 21–39% of all chloramine decay in the samples from 0 to 5 mg-C.L−1 fed reactors and 15% in the samples from 7 to 8 mg-C.L−1 fed reactors. Microbial processes (mediated by nitrifiers and/or heterotrophs) and biomass-associated microbial products (BMPs) in insoluble form accounted for 13–21% for the reactors fed with 0–5 mg-C.L−1 and 34% for those fed with 7–8 mg-C.L−1. The cSMPs were separable with a 30 kDa cut-off membrane but not with 50 or 100 kDa membranes, i.e., they were above 30 kDa but below 50 kDa in size, and were confirmed to be a protein(s). The protein(s) accelerated chloramine decay by accelerating chloramine auto-decomposition and nitrite oxidation. As opposed to the traditional belief, unknown factors accounted for approximately 34–53% in commonly encountered re-chloraminated nitrifying waters (2–5 mg-DOC.L−1). Understanding the identity and role of these factors – such as cSMPs, BMPs, heterotrophs - will lead to a better control of chloramine.