Sewage Sludge-based Activated Carbon Preparation And Pyrolysis Kinetics Of Sewage Sludge From Wastewater Treatment Plant

With the increasing of waste water treatment plant and discharge standards weremore and more stern, more and more sewage sludge were produced. In consideration ofthe fact that the sewage sludge with high moisture content was hard to be disposed,Sanitary landfills, sludge farm application and ocean disposal et al existed problems.Sewage sludge treatment and disposal had become a focused issue in global. Pyrolysiswas one kind of effective techniques in sewage sludge treatment, which had a lot ofadvantages comparing with normal method. Preparation of sewage sludge-basedactivated carbon was a new method of utilization sewage sludge.The sewage sludge-based activated carbons were prepared through chemicalactivating by zinc chloride （ZnCl2） as activating agent. And the affecting factors ofsewage sludge-based activated carbons are also discussed. The effect of affecting factorssuch as activation temperature, activation time, activator concentrations and liquid-solidratio was systematically studied. Effect of procedure parameters on the pore structure andabsorbability of sewage sludge-based activated carbon were also discussed.The sewage sludge-based activated carbons were prepared through chemicalactivating by zinc chloride （ZnCl2） as activating agent. And the affecting factors ofsewage sludge-based activated carbons are also discussed. The effect of affecting factorssuch as activation temperature, activation time, activator concentrations and liquidliquid-solid ratio was systematically studied. Comprehensive factor score was regarded asevaluating index, the optimum preparation parameters were obtained as follows:activation temperature was550℃, activation time was60minutes, activator concentrations4mol.L^-1liquid-solid ratio2.5:1, and the iodine adsorption capacity was358.68mg.g^-1,and the methylene blue adsorption reached45.9mg.g^-1.The effects ofvarious factors on properties of activated carbon are in the following descending order:activation time> activation time> activator concentrations>liquid-solid ratio. Sewagesludge-based activated carbons were studied by SEM, and mainly Mesoporous in nature.The thermal decomposition kinetics of dry sewage sludge was investigated bymeans of non-isothermal TG at different heating rates of5K·min^-1,10K·min^-1,15K·min^-1,20K·min^-1and30K·min^-1. The non-isothermal kinetic parameters andmechanical functions were analyzed by means of Flynn-Wall-Ozawa equation andCoats-Redfern equation. Before and after DTG peak, Flynn-Wall-Ozawa andCoats-Redfern were used to determine the kinetic parameters. Before and after DTG peak,Eαâ†'0=102.17kJ·mol^-1,88.17kJ·mol^-1respectively. Before DTG peak, the most probablekinetic function was No.3G-B Equation, and the corresponding mechanism wascontrolled by Three-Dimensional Diffusion （cylindrical symmetry）.The apparentactivation energy and the pre-exponential constant（A） were Eβâ†'0=107.44KJ·mol^-1，lnAβâ†'0=12.64respectively. After DTG peak, the most probable kinetic function was No.13Avrami-Erofeev Equation （n=3）, and the corresponding mechanism was controlled byrandom nuclear producing and growing process. The apparent activation energy and thepre-exponential constant （A） were Eβâ†'0=88.34KJ·mol^-1, lnAβâ†'0=14.25respectively.The thermal decomposition kinetics of activated sewage sludge was investigated bymeans of non-isothermal TG at different heating rates of5K·min^-1,15K·min^-1,20K·min^-1,30K·min^-1and50K·min^-1. The non-isothermal kinetic parameters andmechanical functions were analyzed by means of Flynn-Wall-Ozawa equation andCoats-Redfern equation. Four step pyrolysis models of sewage sludge activated by3mol·L^-1were as follow, two dimensional diffusion equation, cylindrical symmetrymechanism, lnAβâ†'0=8.4529; chemical reaction mechanism, the reaction orders forchemical reaction was1.5, lnAβâ†'0=3.6824; chemical reaction mechanism, the reactionorders for chemical reaction was3, lnAβâ†'0=23.019; Nucleation and growth mechanism,n=1/3, lnAβâ†'0=-4.5689. Four step pyrolysis models of sewage sludge activated by4mol·L^-1were as follow, chemical reaction mechanism, the reaction orders for chemicalreaction was2, lnAβâ†'0=0.0232;nucleation and growth mechanism,（n=3）, lnAβâ†'0=28.395;chemical reaction mechanism, the reaction orders for chemical reactionwas3,lnAβâ†'0=30.643; two dimensional diffusion equation,lnAβâ†'0=7.5558.Four steppyrolysis models of sewage sludge activated by5mol·L^-1were as follow,Mampel-Powerlaw（n=1/3），lnAβâ†'0=4.3671ï¼›chemical reaction mechanism, the reaction orders forchemical reaction was3, lnAβâ†'0=3.9417ï¼›chemical reaction mechanism, the reactionorders for chemical reaction was2,lnAβâ†'0=19.103ï¼›Mampel-Power law（n=3/2），lnAβâ†'0=17.133。The sorption action of sewage sludge-based activated carbon absorbing waste watercontaining Cr （VI） has been studied in this paper. sludge activated carbon was preparedby pyrolysis process using excess sewage sludge as raw material and ZnCl2as activatingagent． The optimum conditions for Cr（Ⅵ）adsorption using the prepared sewagesludge-based activated carbon are as follows：adsorption time10min，Cr（Ⅵ）initial massconcentration10mg/L，activated carbon dosage5g/L，solution pH2．Under theseconditions the removal rate of Cr（Ⅵ） higher than99.6％．Factors affecting the adsorbability of sewage sludge-based activated carbons on waste water containing Cr（VI）decreased in the order of activated carbon dosage>pH>Cr（Ⅵ）initial massconcentration>adsorption time. The adsorption of Cr （Ⅵ） follows the Pseudosecond-order kinetic model.Preparations of sewage sludge-based activated carbon, as a kind of reduction,recycle and safe treatment was a new method of utilization sewage sludge. This showedthe extensive application of treatment of wastewater containing heavy metals in thefuture. Sewage sludge-based activated carbon was a cheap adsorbent. A possibleutilization method of sewage sludge was introduced.