Calcium compounds have been known to be important for enhancing aggregation and dewaterability of activated sludge. In this study, the effect of calcium compounds (CaO, Ca(OH)2, CaCl2, CaSO4 and CaCO3, respectively) on sludge dewaterability and calorific value was investigated. Upon addition of CaO, Ca(OH)2, CaCl2, CaSO4 and CaCO3 (112 mg/g dry sludge) into activated sludge, after compressed filtration the moisture content of sludge was found to be 49.7%, 55.7%, 57.1%, 65.3% and 56.5%, respectively. The addition of calcium compounds altered the structure of sludge by CaCO3 and Ca(OH)2 production, which improved sludge filterability and provided frameworks and water passages during compression. Furthermore, sludge conditioned by CaO addition had better dewaterability than other calcium compounds. The heat generated from the CaO hydration and high pH might damage flocs' structure and improve dewaterability of sludge. Moreover, it was found that excess addition of calcium compounds led to low calorific value in combustion process. The study concluded that CaO is the optimal additive for sludge further dewatering by compressed filtration.
Keywords: Activated sludge; calorific value; combustion gas; compressed filtration; dewaterability.