Effect of inorganic coagulant addition under hydrothermal treatment on the dewatering performance of excess sludge with various dewatering conditions

A study for producing low moisture content sludge cake was performed by adding NaCl, CaCl2, Ca(OH)(2), or FeCl3 to excess sewage sludge under hydrothermal treatment (HT) and varying dewatering conditions from low to high pressure (0.04-12 MPa). FeCl3 addition under HT resulted in cakes with significantly lower moisture content than HT cakes, and moreover, than raw sludge cakes, under all dewatering conditions. The addition of CaCl2 produced similar effects to FeCl3 addition but at weaker extents. The addition of Ca(OH)(2) produced much weaker effect than the addition of CaCl2 under the low-pressure dewatering. However, the effects were comparable under the medium- and high-pressure dewatering. The effect of NaCl addition was only prominent under the high-pressure dewatering. Comparison on the moisture content of sludge cakes with the treatments of HT, coagulant-added, and their combination showed that synergy between HT and coagulant addition in producing low moisture content sludge cake was evidenced.