Controlling 1,4-dioxane and disinfection byproducts during ozone/hydrogen peroxide-granular activated carbon drinking water treatment while maintaining ozone residuals for pathogen inactivation credit

Declining water quality and the need to control multiple contaminants with existing infrastructure present challenges for drinking water treatment plants. Retrofitting existing ozonation systems (O₃) to ozone/hydrogen peroxide (O₃/H₂O₂) can accelerate hydroxyl radical (^•OH) generation and potentially enhance contaminant removal, while lowering measurable O₃ residuals required for pathogen inactivation credit. We evaluated 1,4-dioxane removal, bromate control, and formation potential of 28 regulated and unregulated disinfection byproducts (DBPs) in pilot-scale O₃-granular activated carbon (GAC) and O₃/H₂O₂-GAC treatment trains while maintaining O₃ residuals. At the 1.7 mg/L O₃ dose needed to maintain ≥ 0.3 mg/L O₃ residual, 44.7 ± 1.9% of 10 μg/L 1,4-dioxane was removed, exceeding previously reported removal. Modeling results indicated that •OH formation during O₃ decay (20°C) at pH ∼8.0 and dissolved organic carbon ∼4 mg-C/L drove removal. H₂O₂ addition increased 1,4-dioxane removal by <14% and mitigated up to 56 ± 11% bromate, although mitigation was constrained by low H₂O₂/O₃ ratios required to maintain O₃ residual. O₃/H₂O₂ treatment did not improve DBP and chloramine-associated DBP formation control, but reduced chlorine-associated DBP formation by 9-20% while increasing calculated additive toxicity (CAT) by 16-41% due to elevated haloacetaldehyde and halonitromethane formation. Subsequent GAC treatment reduced DBP formation after chlorination by 4-17% and CAT by 20-47%. Thiol reactivity and CAT showed different trends, indicating the need for complementary toxicity assessments. These results highlight the challenge of simultaneously controlling chemical contaminants within the current pathogen crediting framework.