Improvement Of Activated Sludge Dewaterability By Chemical Conditions

The dewatering plays a crucial role in activated sludge treatment and disposal. The sludge conditioning is widely applied to improve the sludge dewaterability prior to mechanical dewatering, which is the most commonly used dewatering method. Compared to the physical and the biological conditioning, the chemical conditioning prevails in the aspect of operation, cost and application. This study investigated the effects of different chemical conditioning agents and their combination on the dewaterability of activated sludge, analyzed the influencing mechanism, and established economic analysis preliminarily based on the pilot test.The activated sludge dewaterability was significantly improved by Fenton (Fe2+/H2O2), Chitosan (CTS), Polyacrylamide (PAM), FeCl3, CaO, Cetyl Trimethyl Ammonium Bromide (CTAB), and further enhanced by the following chemicals combinations:Fenton+PAM, CTS+PAM, FeCl3+CTS, CTS+CTAB, FeCl3+PAM, PAM+CTAB, FeCl3+CaO. Among these combinations, CTS+PAM, FeCl3+CTS, FeCl3+PAM and FeCl3+CaO showed better performance in reducing the capillary suction time (CST) at dosage of2.0+2.0,30.0+3.0,40.0+3.5and30.0+150.0mg/g dry solid (DS), respectively. In the pilot test, the corresponding cake solid content was increased from20.13%to23.76%,23.62%,27.15%and37.57%respectively after the activated sludge was conditioned with the four chemicals combinations mentioned above and dewatered by the frame filter under0.6MPa for7.0h. The highest cake solid content was obtained after FeCl3+CaO (30.0+150.0mg/g DS) conditioning, and the chemical cost is180.0RMB/t DS.The optimal activated sludge dewaterability was achieved by conditioning with Na2S2O8and FeSO4·7H2O, among the combination of FeSO4·7H2O with Na2S2O8, K2S2O8, K2Fe04and H2O2. The suitable dosage for FeSO4·7H2O+Na2S2O8was62.5+50mg/g DS and the activated sludge time to filter (TTF) was3.5min after conditioning. The TTF could be further reduced to3.0min with an addition of100mg/g DS CaCl2. Since the TTF were15.0,10.5,10.0and4.0min, respectively, for the CTS+PAM (2.0+2.0mg/g DS), FeCl3+CTS (30.0+3.0mg/g DS), FeCl3+PAM (40.0+3.5mg/g DS) and FeCl3+CaO (30.0+150.0mg/g DS) conditioning, it is obvious that FeSO4·7H2O+Na2S2O8+CaCl2improved the activated sludge dewaterability best. In the pilot test, the cake solid content was increased from10.57%to24.37%after the sludge was conditioned with FeSO4·7H2O8+CaCl2(62.5+50.0+100.0mg/g DS) and dewatered by the frame filter under0.6MPa for16.0h, and the chemical cost is338.0RMB/t DS.The activated sludge was aggregated and the particle size and soluble chemical oxygen demand (SCOD) were increased after conditioning with CTS+PAM, FeCl3+CTS, FeCl3+PAM, FeCl3+CaO, FeSO4·7H2O+Na2S2O8and FeSO4·7H2O+Na2S2O8+CaCl2. The maximum particle size was obtained with CTS+PAM conditioning and the micrograph also showed the densest floes agglomeration. The strongest increase of SCOD was resulted from FeSO4·7H2O+Na2S2O8+CaCl2conditioning. These phenomena indicate that the conditioning agents could aggregate the activated sludge floes, and thus improve the dewaterability. The role of FeSO4·7H2O+Na2S2O8mainly lies in decomposing the microorganism and releasing bound water.The results also showed that TTF and cake solid content had a negative correlation, while they had no apparent connection with CST. Therefore, TTF was more suitable than CST to characterize the activated sludge dewaterability.The chemical conditioning agents FeCl3+CaO (30.0+150.0mg/g DS) and FeSO4·7H2O+Na2S2O8+CaCl2(62.5+50.0+100.0mg/g DS) showed the best performance in improving the activated sludge dewaterability, and the increase of the cake solid content in the pilot test were13.80% and9.38%, respectively. Their chemical costs are180.0and338.0RMB/t DS respectively.