A study for producing low moisture content sludge cake was performed by adding NaCl, CaCl 2 , Ca(OH) 2 , or FeCl 3 to excess sewage sludge under hydrothermal treatment (HT) and varying dewatering conditions from low to high pressure (0.04-12 MPa). FeCl 3 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 CaCl 2 produced similar effects to FeCl 3 addition but at weaker extents. The addition of Ca(OH) 2 produced much weaker effect than the addition of CaCl 2 under the low-pressure dewatering. However, the effects were comparable under the medium-and highpressure 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.
High moisture content along with poor dewaterability are the main challenges for sewage sludge treatment and utilization. In this study, the effect of hydrothermal treatment at various temperature (120-200 ˚C) on the properties of sewage sludge derived solid fuel was investigated in the terms of mechanical dewatering character, drying character, calorific value and heavy metal distribution. Hydrothermal treatment (HT) followed by dewatering process significantly reduced moisture content and improved calorific value of sewage sludge with the optimum condition obtained at 140˚C. No significant alteration of drying characteristic was produced by HT. Heavy metal enrichment in solid particle was found after HT that highlighted the importance of further study regarding heavy metal behavior during combustion. However, it also implied the potential application of HT on sewage sludge for heavy metal removal from wastewater.
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