Study On The Mechanism Of Chemical Conditioning And Dewatering Of Sewage Sludge In Urban Sewage Treatment Plant

Wastewater treatment processes produce large quantities of sludge commonly containing over 99% water. Development of high-performance dewatering process is the important way to relieve the pressure of rapid increase in waste activated sludge production and also is a hot issue in the field of environmental engineering in China. Currently,the mainstream technology for activated sludge deep dewatering is high-pressure compression filtration process, and the final drying performance is mainly dependent on selection of chemical pre-conditioning technologies.This study mainly studies several coagulant conditioning effect on sludge dewatering performance,Using ferric chloride（FeCl₃）, polyaluminium chloride（PAC） and cationic polyacrylamide（PAM） as coagulant to study the effects of different chemical coagulant dosage on sludge dewatering.Analysis of the vario us effect of polymerization aluminum speciation on sludge dewatering of differ ent alkaline PAC, and the influence mechanism of the conditioning process on the physical and chemical properties of sludge was analyzed emphatically. Th e main research contents and results are as follows:（1）Chemical conditioning required PAC and ferric chloride sludge dosage is much higher than that of PAM, and the conditioning effect is poor. PAM conditioning of sludge particle size is very large, up to a maximum of 845.561 m, is the original sludge particle size of 8.6 times, far greater than PAC andFeCl₃ conditioning of sludge particle size. Adding coagulant after the solubilization of sludge extracellular polymeric substances（SEPS）, loose bound extracellular polymeric substances（LB-EPS） and tightly bound extracellular polymers（TB-EPS） in organic matter content decreased, the solubility of extracellular polymeric substances（SEPs） in organic matter content of the most obvious.There was no significant correlation between the change of the size of sludge and capillary water absorption time（CST） in the process of sludge conditioning.Micro particles play a more important role in the process of sludge dewatering, and the effect of particle size on the dewatering property can be neglected when the size of the sludge is over a certain range.In the process of chemical conditioning, the water content of sludge decreased with the increase of coagulant dosage.After flocculation, the electrostatic repulsion between the sludge particles is reduced, and the internal binding water between the sludge particles is released from each other, so that the water quantity of the mechanical dewatering can be changed to improve the dewatering performance of the sludge.At the same time, the combination of water on the part of EPS, which was originally based on hydrogen bonding or hydrophilic interaction, was transferred to the solution.In addition, through the EPS of the inclusions embedded in the internal clearance of the water also due to the flocculation of the double layer structure of the electric double layer is compressed to release EPS, the joint effect of these factors lead to a reduction in water content.（2）Morphology of the PAC, early in 1931 is people first proposed concept of morphology（Jander 1931）, the polymeric chloride aluminium, these forms is according to a variety of experimental approaches have been proposed to prepare PAC adding molar ratio of alkali solution and the total amount of aluminum can represent soluble liquid is the important parameters of the hydroxyl polymer solution, commonly known as the basicity. Different basicity of PAC. The polymerization aluminum forms are also different, low basicity of PAC is mostly monomeric aluminum and oligomeric state aluminum（Al_a）, alkalization degree more high poly（Al_c） state of aluminum（Al_b） and poly state aluminum content is higher, and the higher the basicity of PAC pH is higher.Series of PACl coagulants with the alkalinity of the increase of sludge dehydration effect is better, namely poly state of aluminum（Al_b） and poly state aluminum（Al_c） ratio oligopeptides aluminum（Al_a） is more beneficial for sludge dewatering and in the sludge conditioning poly high charge state and aluminum poly state charge replacement of sludge flocs and EPS that the compression of electric double layer and the EPS cracking of water released.Poly aluminum（Al_b） and poly state aluminum（Al_c） formed flocs is not but floc structure closely, floc strength is relatively high, and oligomeric state of aluminum（Al_a） formed flocs is relatively large, but floc structure loose, floc strength is relatively low.The AlCl₃ is conditioning the oligomeric state of aluminum（Al_a） after the formation of floc structure intensity is minimum, and the formation of flocs size is also the largest, so the floc structure is loose and are therefore more likely to be compressed, instead of the PACl_2.5 and Al₁₃ poly state of aluminum（Al_b） and high poly state of aluminum（Al_c） after conditioning floc structure strength larger, the formation of flocs feet small, floc structure closer, is not easy to be compressed, the formation of dewatered sludge cake has many honeycomb structure, which is conducive to sludge dewatering, namely Al_b and Al_care better than Al_a for sludge dewatering.With the same dosage of 0.05gAl/gTSS, the EPS components of the sludge treated with different degree of alkalinity and Al₁₃ were significantly changed.After PAC2.5 and Al₁₃ conditioning of the sludge, the concentration of dissolved EPS is the lowest, respectively, 5.99mg/gTSS and 4.06mg/gTSS.