Enhancement in Dewatering Efficiency of Disrupted Sludge through Ultrasonication and Re-Flocculation—Sustainable Sludge Management

The solids in sewage sludge are primarily composed of organic matter and offer new possibilities for sustainable sludge management, if considered as a stable biomass source in terms of quantity and quality. Reducing the volume of sludge with an extremely high moisture content is challenging, and enhanced dewatering through mechanical treatment is crucial from an environmental and sustainability perspective because it alleviates the reliance on thermal treatment. This study employed ultrasonication to enhance the dewatering efficiency of activated sludge. The disruption of sludge induced by ultrasonication notably facilitated the elimination of intracellular water during mechanical expression. Additionally, the ultrasonicated sludge was verified to be re-flocculated by introducing inorganic electrolytes such as Ca 2+ (divalent cations), Al 3+ (trivalent cations), and polyferric sulfate. Conversely, no re-flocculation of disrupted sludge was observed upon applying organic polymer flocculant. Under optimized conditions, the sludge re-flocculation progressed to form large flocs, leading to a decreased suspended solids (SS) value from 1423 to 73 mg/L and reduction in capillary suction time (CST) from over 2000 to 18 s. Following pretreatment, the moisture content of the mechanically expressed cake at 500 kPa decreased significantly from 76 wt% (untreated sludge) to less than 60 wt% (treated sludge) due to the elimination of intracellular water.