Feasibility of applying forward osmosis to the simultaneous thickening, digestion, and direct dewatering of waste activated sludge

► Forward osmosis (FO) was innovatively applied to sludge thickening and digestion. ► FO was applied to the dewatering of raw sludge without thickening or additives. ► Flux/salt reversal/digestion efficiency of sludge thickening were characterized. ► The factors influencing the dewatering performance were studied. The feasibility of applying forward osmosis (FO) to the simultaneous thickening, digestion, and dewatering of waste activated sludge was investigated. After 19days of operation, the total reduction efficiencies of the simultaneous sludge thickening and digestion system in terms of mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) were approximately 63.7% and 80%, respectively, and the MLVSS/MLSS ratio continuously decreased from 80.8% to 67.2%. The MLSS concentration reached 39g/L from an initial amount of 7g/L, indicating a good thickening efficiency. In using FO for sludge dewatering, two major factors were verified, namely, initial sludge depth and draw solution (DS) concentration. A sludge depth of 3mm, where a dry sludge content of approximately 35% can be achieved in approximately 60min, is recommended for future applications. In addition, the present study proved the feasibility of using seawater reverse osmosis concentrate as the DS.