Dead-end forward osmosis as an alternative for deep sludge dewatering: Evaluation method and characteristics analysis

Abstract

Sludge deep dewatering technologies that are effective and energy-saving are urgently required. Forward osmosis (FO) technology was recently used for dewatering sludge. However, the final moisture content was too high—above 95.0%—to meet the treatment and disposal requirements. In this study, a novel dead-end forward osmosis (DEFO) technique was proposed and verified for deep dewatering of excess sludge. In DEFO, the feed side is in dead-end mode, while the draw side is in cross-flow mode. Further, when 30.8 g/L excess sludge, 2.0 M NaCl draw solute, 3.0 cm/s cross velocity on the draw side, and 3‰ cationic polyacrylamide dosage was used, the moisture content of the filter cake was as low as 69.2%, equating to 90.0% sludge reduction. A proposed evaluation model for DEFO sludge dewatering behavior includes critical characteristics, such as water flow osmosis resistance and water permeability coefficient of the FO membrane. The sludge dewatering process continued regardless of whether the filter cake broke. In contrast to the conclusions endorsed in traditional dead-end pressure filtration, the DEFO filter cake exhibits an abnormal structure with a dense upper cake and a loose structure for the lower cake, resulting in a permeable filter cake. The photographs, moisture content distribution, and filter cake frozen scan verified this. The solid compressive pressure and hydraulic pressure in the DEFO filter cake were negligible, resulting in low water flow resistance of the filter cake. These findings provide further insights into the use of DEFO technology for deep sludge dewatering.