Synthesis Of Polydimethyldiallyl Polymeric Conditioning Agents And Enhancement Of Sludge Dewatering Performance

The activated sludge method is the most widely used wastewater treatment technology and with the increase in wastewater collection and treatment capacity,residual sludge production is also increasing.The conditioning and dewatering treatment of sludge has become an important research direction in this field.In this study,a new poly（dimethyldiallylammonium chloride）（PDMDAAC）was prepared and the optimum preparation parameters were obtained.On this basis,the effect and influencing factors of the synergistic conditioning of sludge with PDMDAAC and Fe Cl₃ to improve the dewatering performance were investigated and the possible mechanisms were explored,using the sludge from a wastewater plant thickener as the target.The main findings are as follows:（1）The optimum preparation conditions for the one-step synthesis of PDMDAAC were obtained using monomeric dimethyldiallylammonium chloride（DMDAAC）solution as raw material and ammonium persulphate as initiator.When the mass fraction of monomer was 50%,the amounts of initiator and complexing agent（tetrasodium EDTA）were 1.25%and 0.007%respectively,the polymerization temperature was 68℃and the reaction time was 3.5 h,the characteristic viscosity of the prepared PDMDAAC reached 2.79 d L/g and the monomer conversion rate was 94.72%.（2）The sludge conditioning compound（SCC）consisting of PDMDAAC,ferric chloride（Fe Cl₃）and HCl conditioned the sludge better than the sludge conditioned by a single PDMDAAC,Fe Cl₃ and HCl conditioner.The optimum dosage of PDMDAAC was 3.75 g/g DS,p H was adjusted to 3.0 and the optimum dosage of Fe Cl₃ was 20 g/g DS.Under these conditions,the water content of the sludge was reduced from 98.00%to 59.40%,the capillary absorption time was reduced from 84.20 s to 20.85 s,and the S-EPS and p H of the sludge was reduced to 20.85 s.and the S-EPS and TB-EPS of the sludge could be used as the main response indicators of the sludge dewatering performance.（3）The effect of SCC on storage time and residual sludge dewatering performance of different treatment processes was investigated and it was found that when the storage time reached 12 h with continuous aeration,it was not conducive to dewatering and the moisture content dropped to 61.58%;studies on the residual sludge dewatering performance of multi-stage multi-stage A/O,oxidation ditch and MBBR processes showed that SCC,a compound conditioner,would reduce the sludge moisture content to 60.00%or less,which is widely applicable.（4）The pattern of changes in the composition of organic matter in the supernatant was investigated and it was found that SCC caused a large amount of protein（PN）from TB-EPS in the extracellular polymer（EPS）to be dissolved into S-EPS and the hydrophilic polar groups to be detached,thus allowing a large amount of water to be removed,and the correlation between the PN content in TB-EPS and sludge dewatering was determined.The mechanism of sludge conditioning by SCC was deduced as follows:firstly,the flocculation structure of the sludge was broken by the action of PDMDAAC and hydrochloric acid,resulting in the solubilisation of EPS and the release of a large amount of water encapsulated in EPS.Secondly,the ferric chloride flocculates and binds the finely dispersed sludge particles together,further facilitating the release of free water and improving the dewatering performance of the sludge.（5）A simple economic cost analysis of the PDMDAAC polymer produced showed that the cost per tonne of PDMDAAC produced was$5996.50,which is much less than the market price of$10,500.A simple economic analysis of the sludge conditioning using both PDMDAAC+Fe Cl₃ and PDMDAAC+Fe Cl₃+HCl combinations was also carried out and the operating costs per tonne of dry sludge（sludge moisture content from 98%to 65%and from 98%to 58%）were$624.10 and$634.02 respectively.This is not only lower than the conventional conditioning method which costs$665.60,but the method is also effective in reducing the investment costs and operating costs of subsequent sludge disposal.