| Sludge disposal is an important part of sewage treatment systems,and its cost accounts for more than half of the total cost.With the strict implementation of environmental laws and regulations in various countries and the increasing awareness of environmental protection of people,sludge treatment and disposal has become one of the hot issues in the water treatment industry.In order to improve the efficiency of treatment and disposal and reduce the cost of sludge in practical treatment,efficient solid-liquid separation is a crucial step.However,sludge is a complex system consisting of stably suspended inorganic/organic colloidal particles with poor settleability.In addition,sludge contains a large amount of extracellular polymeric substances(EPS),so that the water is tightly bound to the surface of the particles or adsorbed to the organic matter and its aggregates.Moreover,sludge is compressible.Therefore,it is difficult to carry out effective solid-liquid separation by the mechanical method alone,and sludge should be pre-treated.Coagulation/flocculation has the characteristics of low cost and strong operability,which is one of the most commonly used sludge conditioning methods.At present,the commonly used coagulants/flocculants mainly include inorganic coagulants composed of aluminum and iron salts,and synthetic organic polymeric flocculants represented by polyacrylamide(PAM).They have a good practical performance and are therefore widely used.However,those traditional coagulants and flocculants might result in secondary pollution and entail many health risks due to residual metal ions or the release of noxious monomers into the target water,therefore,it is urgent to develop an efficient and economical green flocculant.At present,natural polymeric flocculants are environmentally friendly and praised as "green flocculants in the 21st century".Moreover,due to its rigid chain structure,natural polymeric flocculants can be used as skeleton builders to improve the compressibility of the sludge cake also.Moreover,polysaccharide-based flocculants,such as starch and cellulose,have the sugar-ring structure similar to that of polysaccharide(PS)in EPS,which is beneficial for their tightly binding to each other,destruction of the solvated layer of EPS,efficient release of bound water,and improvement of sludge dewatering performance.However,the application of natural polymer-based flocculants in sludge conditioning has not been studied so far.There were few reports related to the influence of the molecular structure of natural polymer-based flocculants on sludge conditioning performance,and their combinations with other coagulants or other conditioning methods were even more limited.In this thesis,a series of cationic starch-based flocculant(St-WH)with different degrees of substitution were prepared by using 3-chloro-2-hydroxypropyl trimethyl ammonium chloride(CTA)as the etherifying agent.The target waste activated sludge was taken from a municipal sewage treatment plant in Nanjing,and the conditioning performance of traditional inorganic coagulant FeCl3 and cationic organic polymeric flocculant PAM were also determined for comparison.The effects of dose and charge density of St-WH on the sludge dewatering performance were discussed in detail.Furthermore,St-WH4,a more effective one,was selected to be used in combination with FeCl3.EPS fraction and composition,water distribution,and sludge cake microstructure were studied.Combined with the practical sludge dewatering process,the mechanism of sludge dewatering was discussed in detail based on the molecular structure of natural polymeric flocculant.In addition,the applications and mechanisms of acidification,oxidation(H2O2,NaClO and KMnO4),and flocculation used solely or in combinations for dewatering of sludge were studied systematically.The response surface methodology(RSM)was also used for optimizing the combined process.The specific work of this thesis was as follows:(1)A series of St-WHs with different degrees of CTA substitution and charge densities were successfully prepared.The effects of CTA content of St-WH on sludge dewatering were systematically investigated.Also,the influences of dose and charge density were discussed in detail.A pilot experiment of sludge dewatering process using St-WH4 with a higher degree of CTA substitution was carried out and compared with the cationic PAM currently used in this wastewater treatment plant.St-WH4 showed better dewatering performance than PAM.(2)Then St-WH4 was combined with FeCl3 to condition sludge through a coagulation-flocculation process,wherein FeCl3 acted as coagulant and St-WH4 was flocculant.The combined conditioning method was cost-effective,the dewaterability of which was significantly better than that of using St-WH4,FeCl3 or PAM currently used by this wastewater treatment plant.FeCl3 had good charge neutralization effect and could effectively compress the double electric layer on the surface of sludge particles.St-WH not only had strong bridging effect,but also contained strong cationic quaternary ammonium salt groups on its backbone,therefore,the residual negative charge on the surface of the sludge floc could be further neutralized.In addition,FeCl3 and St-WH4 could be both used as skeleton builders to reduce the compressibility of the sludge.And FeCl3 and St-WH4 could efficiently bind to protein(PN)and polysaccharide,respectively,for further enhancing the sludge dewatering rate.PAM with high molecular weight has strong bridging effect,so it could achieve better dewaterability when used alone.However,its effective flocculation window was narrow,and it would increase the viscosity of sludge to deteriorate its compressibility.Moreover,PAM was difficult to destroy the EPS of the inner layer,which was not conducive to practical applications.(3)Coagulation/flocculation could effectively improve the sludge dewaterability,but it was difficult to destroy the EPS in the sludge to efficiently release the bound water.In order to further improve the dewatering efficiency,the sludge was conditioned by a combined process of pre-oxidation and re-coagulation/flocculation.First,the pre-oxidation effects of three different oxidants(H2O2,NaClO and KMnO4)on the sludge were compared.It was found that the pre-oxidation alone could effectively destroy the sludge flocs and EPS,but worsened the dewatering performance.The deterioration of sludge dewaterability caused by oxidation alone might be due to the release of a large amount of organic matters and fine floc particles to the outer layer.In addition,it was speculated that the deterioration effect of the three oxidants roughly followed the order of NaClO>>KMnO4>H2O2Re-coagulation/flocculation conditioning could improve the dewaterability of the pre-oxidized sludge.The order of dewatering efficiency of coagulants/flocculants used was St-WH4>PAM>FeCl3,which further proved that this starch-based flocculant had good sludge dewatering performance.In addition,the experimental results indicated that acidic condition was more favorable for sludge dewatering than alkaline condition.(4)Based on the above-mentioned investigation,the acidification was further introduced into the sludge conditioning.HCl acted as acidifier,KMnO4 was oxidant,and St-WH4 used as flocculant.The dewatering performance of sludge conditioned with acidification,oxidation and flocculation used solely or in various combinations was studied.The RSM method was used to verify and optimize the combined method.The study found that the combined method of acidification,oxidation,and re-flocculation had better dewatering performance than the other processes,and the moisture content of the sludge cake could be reduced to 79.6%.On the one hand,acidification and oxidation had similar effect to destroy the sludge flocs and EPS structure to release the bound water.While under acidic conditions,the oxidation potential of KMnO4 is higher.The subsequent flocculation process could conduct effective charge neutralization and bridge effects.The dual function of St-WH effectively aggregated the fine flocs and organic fragments to improve the sludge filtration performance,to reduce the bond water content,and to improve the sludge dewatering performance.On the other hand,acidification had a protonation effect on the released organic fragments,so that EPS could be effectively compressed,especially the PN component,which was beneficial to improve the dewatering performance.Therefore,unlike oxidation,acidification used alone could obtain a good dewatering performance.In addition,the study also found that the destruction of sludge flocs and EPS was irreversible while protonation effect was reversible by acidification conditioning.Therefore,when the sludge pH was changed to neutral or alkaline,the dewatering performance of acidic sludge was significantly deteriorated.In summary,St-WH is a simple and low-cost etherified natural polymeric flocculant with good flocculation and sludge dewatering performance,especially combined with FeCl3 and other pretreatment processes,such as acidification and oxidation.Compared with traditional inorganic coagulants and organic synthetic polymeric flocculants,St-WH exhibits high cost performance in practical sludge dewatering and this starch-based flocculant have wide application potentials. |
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