Toilet Bowl Cleaning Tablets as Sources of Chlorine, Bromine, and Disinfection Byproducts in Wastewater

Commercial toilet bowl cleaning tablets were examined in laboratory systems to characterize their release of active halogens and their potential to form trihalomethanes (THMs) when combined with synthetic sewage. Active halogens (e.g., HOCl, HOBr, and reactive halamines) were quantified via derivatization with 1,3,5-trimethoxybenzene prior to analysis by liquid chromatography. The effects of several variables on halogen release profiles were examined, including pH, ionic strength, temperature, tank solution volume, flushing frequency, and tablet brand. Changes in pH resulted in modest or no appreciable changes in halogen release profiles. Release of active halogens increased as ionic strength decreased and as temperature increased. Tank volume, flushing frequency, and tablet brand had pronounced impacts on halogen release profiles. Maximum measured active chlorine and bromine concentrations in toilet tank water were 189 mg/L as Cl₂ and 164 mg/L as Cl₂, respectively. Active halogens persisted in toilet bowl water for >24 h. When toilet-tablet-treated water was combined with synthetic sewage, THMs formed at up to 219 ppb with bromine incorporation factors up to 2.86. Active halogens and highly brominated THMs released into wastewater from toilet tablets could have implications for downstream microbial ecology, septic system performance, and overall water quality.