Effect of backwashing biologically activated carbon on coagulability of organics in surface water

This study shows if biologically activated carbon (BAC) is backwashed at the correct frequency, a number of benefits can be derived in addition to aiding the subsequent coagulation process. Previous studies have shown that the BAC improves the removal of dissolved organic carbon (DOC) by subsequent coagulation by decreasing non-coagulable dissolved organic carbon (NC-DOC). However, the actual mechanism of such observation or optimising strategies of NC-DOC removal is unknown. The impact of backwashing on BAC reactor performance in terms of NC-DOC removal and microbial community structure was investigated. A laboratory scale BAC column was operated for more than five months with backwashing at once every five days, but in one cycle it was operated without backwashing for 14 days and the effluent collected at different times from the last backwash was subjected to enhanced coagulation (EC). All the effluent of BAC collected at different days depicted better floc forming characteristics than the feed water which is raw surface water. The effluent collected on day three from the last backwash (BAC-3d) contained the least amount (1.64 mg/L) of NC-DOC despite the highest DOC (3.89 mg/L) of all effluents. The coagulant requirement (5 mg-Fe³⁺/mg-DOC) was minimal for BAC-3d effluent among raw water and all other BAC effluent water samples. This is remarkable given the raw water contained 2.76 mg/L of NC-DOC. The microbial community on BAC granules on day three contained a higher abundance of biodegradable organic matter (BOM) removing microorganisms and low abundance of opportunistic pathogens. Similar performance in terms of DOC removal with the backwash was also observed in the continuous operation of other BAC columns. Possible backwash frequency that optimises the BAC/EC and derives many other benefits is proposed. The BAC/EC combination could help solve many emerging issues cost-effectively hence, needs further investigation.