Disinfection byproduct formation resulting from settled, filtered, and finished water treated by titanium dioxide photocatalysis

This study evaluated strategies targeting disinfection byproduct (DBP) mitigation using TiO₂ photocatalysis with varying influent water quality. A Purifics Photo-CAT Lab reactor was used to assess total trihalomethane (TTHM) and haloacetic acid (HAA) formation as a function of photocatalytic treatment using water from a conventional coagulation/flocculation/sedimentation process, granular activated carbon filtration, and a DBP hot spot in the water distribution system. Regardless of influent water quality, photocatalysis reduced DBP precursors; however, low-energy limited photocatalysis (<5kWhm^-3), exacerbated the production of TTHMs and HAA5s beyond initial levels. Accordingly, limited photocatalysis is not a suitable option when TTHMs and HAA5s are a concern, regardless of the level of pretreatment. Limited photocatalysis yields incomplete oxidation, wherein larger, more aromatic, humic organic compounds are broken into smaller molecular weight, less aromatic, and less humic moieties, which have considerable potential to produce DBPs. More complete mineralization of DBP precursors is obtained using extended photocatalysis (80-160kWhm^-3), which substantially decreases DBP precursors as well as TTHM and HAA5 concentrations. In order to balance DBP mitigation, energy, and chemical usage, targeted use of TiO₂ photocatalysis is necessary in a water treatment train (e.g., extended photocatalysis at a distribution system hot spot, where the volumetrically high energy requirements may be justifiable).