Study On Dewatering Mechanisms And Enhancing Dewaterability Of Paper Sludge

In the paper industry,environment pollution includes waste water,waste gas and solid waste.China has focused on the waste water and gas pollution in recent decades,issued a series of relevant measures and got the good results.In previous years,the disposal of sludge was mainly for landfill with secondary pollution and so now the resource treatment of sludge has been in process.However,the complex colloidal nature and strong hydrophilicity hamper its dewatering without pretreatment and influence the sludge treatment and disposal.Based on the influencing factors of the sludge dewaterability,it is a hot spot to find the appropriate conditioning methods to improve the sludge dewaterability and reduce the sludge moisture content.Therefore,it is necessary to improve the sludge dewaterability and then study the dewatering mechanisms,so as to provide the theoretical basis for developing new conditioning methods and industrialization.In this paper,waste paper sludge was subjected to potassium permanganate（KMnO₄）oxidation pretreatment,abandoned polyster textile fiber（APTF）as the sludge skeleton and the combination of ferric chloride（FeCl₃）and KMnO₄ to analyze the roles of various physicochemical properties,the rheological behavior and microscopic structure;infrared（IR）,X ray diffraction（XRD）and inductively coupled plasma spectrum（ICP-AES）analysis were also performed to clarify the dewatering mechanisms in-depth.Finally,the feasibility of combined conditioning of KMnO₄,APTF and FeCl₃ with the optimum adding conditions was investigated to take the six typical paper sludge as object and then the mechanism model was proposed.The main conclusions of this paper are identified as follows:（1）Based on the principle of advanced oxidation processes,waste pulping activated sludge was subjected to potassium permanganate（KMnO₄）oxidation pretreatment and then the influence and mechanism on sludge dewatering were comprehensive investigated in the present study.At lower KMnO₄ dosage,changes of the physicochemical and rheological characteristics were that:as the sludge disintegration degree increased,the total extractable extracellular polymeric substance（EPS）increased and then the bound water were released;the rheological behavior showed that the yield stress and viscosity decreased while flowability increased and the positive thixotropic behavior weakened;the particle size and microscopic structure were changed insignificantly.the intensity of characteristic peak decreased in FT-IR analysis.These changes resulted in damaging the gel-like environment and making the surface of sludge floc stripped but did not completely induce the floc breakage as a result of KMnO₄ with lower redox potential.Thus,sludge dewaterability was improved with the optimum dosage of 16 g/kg dry solid（DS）.After sludge oxidation,KMnO₄ mainly transferred to the sludge solids with the states of MnO₂and Fe_xMn_yO_z.（2）Waste pulping activated sludge was subjected to APTF as the exogenous fiber skeleton and then the feasibility in sludge dewatering was discussed.The results showed that the optimal size of APTF was 0.78±0.01 mm and the optimal dosage was related to the sludge concentration.The optical dosage of FeCl₃ was 120 g/kg DS.Under the combined conditioning of KMnO₄ and FeCl₃,compared with the raw sludge,the specific resistance,the water content of sludge cake and the filtration time decreased by 77.48%,7%and 57.94%,respectively and the net sludge yield increased by 68.87%;the compressibility of the sludge cake decreased by 65.22%and some big cracks and pores were seen in the sludge cake profile,in which the fibers were embedded.The zeta potential was insignificantly changed after adding APTF conditioning,indicating that the APTF did not exchange the ions with the sludge flocs.Therefore,APTF can improve the sludge dewaterability as the skeleton.（3）The combined conditioning of FeCl₃ and KMnO₄ as the oxidation and coagulation methods was used to enhance the improvement of sludge dewatering.Compared with KMnO₄conditioning alone,the water removal rate of the combined conditioning increased by 9.80%and10.91%respectively after centrifugation at 730 g and 2500 g,indicating that FeCl₃ can enhance the improvement of sludge dewatering.It was also showed that the addition of KMnO₄ can reduce the dosage of FeCl₃ with the same water removal rate.The rheological behavior showed that with the combined conditioning the hysteresis area increased by 236%while the yield stress and viscosity increased and then flowability decreased,indicating the enhancement of the gel characteristics of sludge and resisting strain.Under the combined conditioning,the particle size were increased significantly,the total EPS content decreased and the bound water content decreased by 48.39%,indicating that the combined conditioning can promote the transformation of the bound water into free water.There was no difference of the characteristic peak in IR analysis under KMnO₄ conditioning with or without the adding of FeCl₃,indicating that FeCl₃did not react with EPS but inhibited the extraction of EPS from sludge.The combined conditioning showed the double effects of the oxidation destruction and flocs reconstruction while FeCl₃ and KMnO₄ mainly transferred to the sludge solids with the states of MnO₂,Fe_xMn_yO_z,C₄₈H₄₄Fe_14.01N₁₅O_35.68S_2.25,Fe₂O₃,Fe₃O₄ and K₆Fe₂O_5.（4）The same previous sludge was subjected to the combined conditioning of KMnO₄,APTF and FeCl₃ with the optical dosage.The improvement of sludge dewatering was better than that in previous conditioning,that is,the sludge specific resistance decreased by 84.02%,filtration time decreased by 43.70%and the net sludge yield increased by 108.59%.It is also feasible for the combined conditioning on the six typical paper sludge dewatering.The mechanism of the combined conditioning was proposed as follows.The sludge colloidal environment was destroyed by KMnO₄,resulting in the decrease of viscosity and the release of bound water;APTF formed a water channel in sludge as a skeleton and reduce the compressibility of sludge;FeCl₃ reconstructed into the larger sludge flocs with the effect of charge neutralization and compression of two electrical layers.Therefore,the sludge dewaterability was enhanced.