Cometabolism of trihalomethanes by nitrifying biofilters under drinking water treatment plant conditions
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This research studied the feasibility of THM cometabolism in laboratory-scale biofilters under conditions that reflect drinking water treatment practice. Initially, batch kinetic studies were conducted to determine whether nitrifying bacteria could reliably cometabolize all four THMs at a sufficient rate to make the process attractive to utilities. The kinetic experiments showed that nitrifier communities likely to be seen in drinking water treatment facilities can degrade THMs at a sufficient rate by themselves, without seeding a pure culture. These results also indicated that temperature sensitivity and product toxicity could be concerns if THM cometabolism by nitrifying bacteria was implemented as a treatment option in treatment facilities. In particular, as bromine substitution increases, both THM degradation kinetics and product toxicity increase. A series of laboratory-scale biofilter experiments was conducted to investigate the feasibility of THM cometabolism in the envisioned process configuration. The operating conditions of the mixed culture biofilters scaled to typical full-scale rapid filtration operating conditions seen in drinking water treatment practice. Overall, the biofilter experiments suggest that, for a 2 mg N/L TOTNH3 (the sum of ammonia-nitrogen and ammonium-nitrogen) removal, total THM removals might initially approach 32-38% (25- 31 μg/L total THMs). This initial removal might decline to 11-12% (9-10 μg/L total THMs) over time as bacteria are selected from THM product toxicity. Even if this decreased performance occurs, the 11-12% removal is potentially attractive in drinking water treatment practice. The allowable influent monochloramine concentration resides between 1 mg/L to 2.5 mg/L as Cl2, with the use of 1 mg/L as Cl2 being conservative. A simple kinetic model for THM cometabolism was incorporated into AQUASIM to describe biofilter performance under conditions where by-product toxicity is not a concern. Overall, total THM removal of 9 to 54% was projected in the full-scale simulations, which illustrates the potential of THM cometabolism to have a significant impact on treated water quality for utilities where their water quality will likely see a benefit from the proposed process. Even though these removals are modest, drinking water treatment plants might only require removals in this range to maintain compliance with existing and future regulations.