H₂O₂ residuals from advanced oxidation processes (AOPs) may have critical impacts on the microbial ecology and performance of subsequent biological treatment processes, but little is known. The objective of this study was to evaluate how H₂O₂ residuals influence sand systems with an emphasis on dissolved organic carbon (DOC) removal, microbial activity change and bacterial community evolution. The results from laboratory batch studies showed that 0.25 mg/L H₂O₂ lowered DOC removal by 10% while higher H₂O₂ concentrations at 3 and 5 mg/L promoted DOC removal by 8% and 28%. A H₂O₂ dosage of 0.25 mg/L did not impact microbial activity (as measured by ATP) while high H₂O₂ dosages, 1, 3 and 5 mg/L, resulted in reduced microbial activity of 23%, 37% and 37% respectively. Therefore, DOC removal was promoted by the increase of H₂O₂ dosage while microbial activity was reduced. The pyrosequencing results illustrated that bacterial communities were dominated by Proteobacteria. The presence of H₂O₂ showed clear influence on the diversity and composition of bacterial communities, which became more diverse under 0.25 mg/L H₂O₂ but conversely less diverse when the dosage increased to 5 mg/L H₂O₂. Anaerobic bacteria were found to be most sensitive to H₂O₂ as their growth in batch reactors was limited by both 0.25 and 5 mg/L H₂O₂ (17-88% reduction). In conclusion, special attention should be given to effects of AOPs residuals on microbial ecology before introducing AOPs as a pre-treatment to biological (sand) processes. Additionally, the guideline on the maximum allowable H₂O₂ concentration should be properly evaluated.

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