The influence of backwashing strategy on floc accumulation and organic carbon content in full-scale biofilters

This study investigates backwashing strategies under full scale conditions with analysis focused on pairing washed filter media associated turbidity with dissolved organic carbon (DOC) removal. A core sampling monitoring program was implemented on full-scale biofilters at two water treatment plants to assess these monitoring tools under varying seasonal and operational conditions. Despite processing the same water source with similar pre-treatment methods (flocculation, coagulation, sedimentation), notable differences were observed between biofilters, showing the importance of site-specific evaluations. The findings indicate that backwashing that included air scouring was more effective than hydraulic backwashing alone in floc removal, with reduced air scour durations maintaining efficiency. Ineffective backwashing led to high floc retention turbidity across filter depths, particularly in systems relying solely on hydraulic wash. Interestingly, filters using hydraulic backwashing, despite failing to meet floc retention guidelines, achieved higher DOC removals suggesting that current floc retention guidelines may be too low for biofilters. Additionally, Fluorescence Excitation-Emission Matrix (FEEM) analysis indicated higher soluble microbial by-product fluorescence in specific filter groups, correlating with higher effluent DOC levels. These findings highlight the distinct nature of biofilters compared to conventional filters, and the importance of additional considerations for backwashing practices and effective DOC removal emphasizing the need for tailored monitoring tools with biofiltration.