Impacts of disinfection methods in a granular activated carbon (GAC) treatment system on disinfected drinking water toxicity

The efficacy of implementing granular activated carbon (GAC) treatment in combination with pre- or post-chlorination for mitigating disinfection byproducts (DBPs) in drinking water has been promising, yet its impact on water toxicity remains unclear, necessitating cost-effective and informative effect-based toxicity assessment. This study, using recently developed yeast toxicogenomic and human cell RT-qPCR assays targeting DNA and oxidative stress, compares the toxicity level and nature of water treated through a pilot-scale GAC system with post-chlorination (GAC/Cl₂) or pre-chlorination upstream of GAC (Cl₂/GAC/Cl₂), with water treated by chloramination (Cl₂/NH₂Cl). Experiments were conducted at environmentally relevant bromide and iodide levels across three GAC beds. The post-chlorination with GAC generally reduces genotoxicity and oxidative stress more effectively than Cl₂/NH₂Cl or Cl₂ treatment at ambient halide concentrations. However, pre-chlorination with GAC was inconsistent in lowering the effluent toxicity in comparison to the post-chlorination-GAC treatment, especially at high halide levels, where no toxicity reduction was observed compared to non-GAC-treated water. Correlation analysis of detected DBPs and toxicity quantifiers, along with maximum cumulative ratio analysis identifies top DBPs that contribute to the toxicity and their cumulative risks, pointing the iodinated DBPs (I-DBPs) and nitrogenous DBPs (N-DBPs) as the significant contributors to DNA and oxidative stress. The results highlight that unregulated DBPs play a critical role in water toxicity, and whole water toxicity monitoring in complement to regulated DBPs detection is needed for treatment strategies efficacy assessment to address unregulated DBPs and their health risks.