Modeling and assessing chlorinated and brominated THM formation for water treatment adaptation against hydroclimatic impacts.

Hydroclimatic impacts affect natural organic matter (NOM) in surface water, the precursor of disinfection by-products (DBPs) including four regulated trihalomethanes (THM4). Treatment adaptation analysis on the impact is hindered by a lack of mechanistic models that quantify competitive reactions of chlorine, bromine, and other oxyanions with NOMs in disinfection of treated waters. Here we propose a THM model using competitive reaction kinetics and analyzed THM formation of treated waters at the Miller water plant in Cincinnati, USA during a flash flood in the Ohio River. Determined specific THM4 formation potential (FP) for the river water at plant intake showed an increase by 6.45 % in mean and nearly ∼2 times in standard variation during the flood event. This source water impact survived in conventional treatment and remained detectable after advanced GAC treatment, while UV and chlorine disinfection further modified THM compositions. Specifically, raw and processed waters of limited NOM removal are characteristic of rapid and large chlorine decay, subdued THM formation, and high fractions of brominated THMs in the early-time time of chlorination. This phase of fast reactions is followed by a main THM formation stage. The second type of water, after high degrees of NOM removal, follows NOM-limited reaction kinetics in decreasing bromoform and enriching chloroform formation. These observations on THM FP and bromine incorporation are represented well by the modeling equations. Thus, water treatment and chlorine disinfection can be adapted against source water changes by model-simulating and managing NOM reactivity, THM FPs and composition of the treated waters.