Effects of hydraulic backwash load on effluent quality of upflow BAF

Using an upflow biological aerated filter (BAF) with treatment capacity of 100 ton/day, various hydraulic loads of backwashing water and influent were investigated to determine how they affect effluent quality of BAF during normal operation. The BAF packed with expanded polypropylene media of 3-5 mm diameter was operated over a five-month period for paper wastewater treatment. The average removal efficiencies of 78% for chemical oxygen demand (COD) and 88% for suspended solids (SS) were maintained, but the effluent CODs varied between 5-60 mg/l depending on the biodegradable fraction of influent wastewater. During normal BAF operation with cyclic backwash, the effluent SS concentrations showed initial peaks after backwash and gradually decreased to a steady state, while the soluble CODs of effluent did not significantly changed. The effluent SS concentrations and the required time intervals to reach steady state after backwash were related with hydraulic loads of backwashing water and influent wastewater applied to the BAF. A higher load of backwashing water gave larger deviation of initial SS peaks from the steady state, the extent of which was more significant under the higher load of influent wastewater.