Study On The Effects Of Chemical Conditioning On Sludge Dewatering Performance Based On Extended DLVO Theory And Floc Structure Characteristics

Municipal sludge is a by-product of sewage treatment.Its extracellular polymeric substances（EPS）contain a large amount of hydrophilic organic matter.Sludge is difficult to be dewatered,which brings difficulties to subsequent treatment and disposal.Ma ny studies have found that chemical conditioning methods such as coagulants,flocculants,and advanced oxidation process can effectively improve sludge dewatering performance.However,different sludge conditioning processes will change the mutual attraction and repulsive force between the sludge particles,and affect the sludge floc structure.In view of the above problems,the main contents include:1.Effect of different valence cations,polymer and Fenton’s reagent on sludge dewatering performanceThe sludge was conditioned by different valence metal salts(Na⁺,K⁺,Ca²⁺,Mn²⁺,Fe³⁺,Al³⁺),high molecular polymer flocculant（polyaluminum chloride:PAC）and advanced oxidizing oxidant method（Fenton’s reagent）.Capillary suction time（CST）and specific eesistance to filtration（SRF）were used as indicators for dewatering performance evaluation.The particle size distribution,zeta potential and sedimentation performance of the treated sludge were also measured.The results showed that Fe³⁺,Al³⁺and Fenton’s reagent could greatly improve sludge dewatering performance.As the dosages were 1.2mmol Fe³⁺/g VS,1.2 mmol Al³⁺/g VS,2 mmol Fe²⁺/g VS and 2.5 mmol H₂O₂/g VS,the CST of the conditioned sludge decreased from 220.0 s to 33.5,54.1,24.4 s,and the SRF decreased from 2.0×10¹³ m/kg to 0.4×10¹³,0.5×10¹³,0.4×10¹³ m/kg.2.Study on the sludge de watering performance based on s urface thermodynamic analysis of extended DLVO（Derjaguin-Landan-Venvey-Overbeek）theoretical methodThe contact angles of the sludge conditioned by different valence cations,high molecular polymer and Fenton’s reagent were determined under the optimal dosage.The surface thermodynamic analysis was carried out to obtain the extended DLVO curves of different conditioning sludges.The results showed that after conditioning with Fe³⁺,Al³⁺,and Fenton’s reagent,the interfacial adsorption free energyΔG_adhdh of the sludge decreased from 9.5 mJ/m² of the raw sludge to-33.4,-10.8,and-38.0 mJ/m²,indicating that the hydrophilicity of the surface of the sludge floc s changed to hydrophobic(△G_adh<0).The energy barrier of the conditioned sludge was reduced from 7.7×10⁵ KT（K is the Boltzmann constant,which is 1.38×10^-2323 J/K,T is the absolute temperature,which is 298K）to less than 0 KT,which showed that the energy barrier between the sludge flocs disappeared.The sludge particles could spontaneously collide and squeeze,greatly reducing the interstitial water of the sludge,and improving the dewatering performance.For comparison,the energy barriers for the sludge conditioned by Na⁺,K⁺,Ca²⁺,Mn²⁺,and PAC were 7.6×10⁵ KT,6.6×10⁵ KT,7.2×10⁵ KT,1.1×10⁵ KT,and 1.1×10⁵ KT,respectively.The results indicating that the aforementioned chemical conditioning agents did not change the hydrophilic character of the sludge flocs,and the results were consistent with the dewatering performance.3.Discussion on the influence mechanis m of different chemical conditioning methods on the structure changes of sludge flocsThe metal elements distribution in solid-liquid phase,EPS content distribution,bound water content,fractal dimension（D_f）,dynamic morphology were characterized in different conditioning sludge.The results showed that most of the mono valent elements and divalent Ca element were distributed in the liquid phase.The divalent Mn,trivalent Fe and Al and the Fe in Fenton’s reagent were mostly distributed in the solid phase,which compressed electric double layer and acted as coagulation in sludge pretreatment.The fractal dimension results further indicated that the sludge flocs conditioned by Fe³⁺,Al³⁺and Fenton’s reagents were more compact,and the D_f values were 2.67,2.61 and 2.63,respectively,which were much larger than the raw sludge（2.45）and Na⁺（2.46）,K⁺（2.47）,Ca²⁺（2.46）conditioning sludge.The content of protein and polysaccharide in the Bound-EPS was significantly reduced after conditioning with Fe³⁺,Al³⁺and Fenton’s reagent.The bound water content of the sludge conditioned by Fe³⁺,Al³⁺and Fenton’s reagent was reduced from 2.92 g/g DS of the raw sludge to 2.13,2.22 and 2.22 g/g DS,respectively.The compacted sludge flocs could effectively reduce the Bound-EPS content of the sludge and release the bound water binded with Bound-EPS,thereby improving the dewatering performance of the sludge.The Na⁺,K⁺,Ca²⁺,Mn²⁺,Al³⁺reagents did not change the sludge particle size in the first 15 s during the conditioning process,and the particle size of Fe³⁺conditioning sludge increased significantly at the 15th s.The particle size of Fenton reagent conditioning sludge showed a fluctuating change in the first 15 s,which might be resulted by a phenomenon of dissociation-reconstruction of sludge flocs.The correlation analysis results showed that there was a significant positive correlation between the bound water content and the sludge flocs adsorption free energy△G_adh（r=0.81,p<0.01）and energy barrier（r=0.9,p<0.01）,which further explained that the hydrophilic property of the sludge flocs would increase the repulsive force between the sludge flocs and increase the bound water content of the sludge.Principal component analysis showed that the most common result of the main component of the sludge was its first common factor（F₁）,which accounted for 68.3%of the principal components.The first common factor（F₁）is mainly to characterize the thermodynamic properties of the sludge surface（interface free energy,Lewis acid-base hydration free energy,particle size）.It showed that changing the hydrophilic properties of sludge flocs is a key factor to improve the dewatering performance of sludge.