Sludge dewatering performance of a cationically modified lignin coagulant
Coagulation conditioning is a sludge pretreatment method with the characteristics of low cost and simple operation.However,the traditional inorganic salt coagulants and synthetic organic polymeric coagulants may have some serious potentials in health risks and cause some environmental problems due to their residues of metal ions and toxic organic monomers in practical applications.Lignin is a kind of renewable resources widely available in nature.In this paper,a series of cationically modified lignin-based coagulants(LN-CD)with different charge density(CD)were prepared by grafting copolymerization using lignin as raw material while acrylamide and methacryloxyethyl trimethyl ammonium chloride as co-monomers.The activated sludge obtained from a municipal sewage plant at Xianlin of Nanjing was used as the target.The filter cake moisture content(FCMC),specific resistance of filtration(SRF),time to filter(CST)and compression coefficient(s)of sludge after conditioned by various LN-CD coagulants were determined.The dewatering mechanisms of LN-CD were discussed in detail by combination of the apparent dewatering performance and the changes in the contents and distributions of the extracellular polymeric substance(EPS)fractions and components,sludge flocs,and microstructures of sludge cakes.The results show that LN-CD has a good dewatering performance,and the CD is an important structural factor of this lignin-based coagulant.Generally,the higher the CD is,the better the sludge dewatering performance is obtained.After the conditioning of LN-CD 1 with the highest CD in this series of lignin-based coagulants,FCMC,SRF,CST and s reach to 68.91％,0.89×1012 m·kg-1,15.0 s and 0.81,respectively;the obtained sludge floc also showed the largest size and the most compact structure;and the contents of different EPS fractions and their components including protein and polysaccharide in sludge were effectively reduced also.These findings are ascribed to that LN-CD can effectively aggregate sludge particles through charge neutralization and bridging effects,compress and destroy EPS structure,and thus release the bound water.In addition,the unique aromatic ring structure of lignin can play the role of skeleton builder due to its rigid structural characteristics,which is conducive to the improved compressibility and permeability of sludge cakes;besides,its partially hydrophobic property can combine the hydrophobic fragments in EPS and effectively compress EPS through the hydrophobic association,further improving the dewatering performance.In summary,LN-CD has the advantages of simple preparation,environmental friendliness,good sludge dewatering performance,and thus exhibits a good application prospects in sludge treatment.
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