Inert Soluble Organic Matter in Return Flow from Sludge Treatment Process and Its Control by Coagulation

In the wastewater reclamation plants, one of the important parameters is inert soluble organic matter for controlling fouling in the membrane processes. We studied the fate of inert soluble organic matter, S_I, by field studies in municipal wastewater treatment plants including a sludge treatment train and a lab-scale experiment on coagulation of the sludge and the return flow. The filed survey showed: (1) the thermal treatment unit produced the S_I load, about 60% of the total S_I load to the water treatment train; (2) the sludge treatment trains consisted of thickening + chemical conditioning + filter press or the train with thickening + anaerobic digestion + filter press produced about 20% of the total S_I load to the water treatment train; (3) a remarkable decrease in S_I concentration occurred in chemical conditioning and the dewatering process of digested sludge, but in the conditioning of raw thickened sludge, the S_I concentration was increased. The jar test using thickener overflow and digester supernatant showed: (4) in the coagulation of the thickener overflow, low pH condition gives higher performance in S_I reduction, and a higher dose of Poly Silicate Iron (PSI) gives the higher removal percentage; (5) as to the digester supernatant, S_I reduction strongly depends on pH rather than coagulant dosage and the removal performance of S_I is far smaller than the removal of total organic matter.