PCDD/Fs And PAHs Emission In Sewage Sludge Thermal Treatment

The amount of sewage sludge grows rapidly these years, as the wastewater treatment process improves. Thermal treatment technologies are preferred for their advantages of energy recovery, volume reduce and harmlessness. However emission characteristics of persistent organic pollutants (POPs) are not clear, which hinders the promotion of thermal treatment technology. Funded by National High Technology Research and Development Key Program of China(2009AA064704), evolution of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)and Polycyclic-aromatic hydrocarbons(PAHs)during thermal treatment of sewage sludge were studied. The characteristics of microwave pyrolysis of sewage sludge and pyrolysis/incineration of sewage sludge were investigated. PCDD/Fs and PAHs emission during sewage sludge thermal treatment were studied and finally a system of sewage sludge thermal treatment was designed.(1) Sewage sludge with active carbon heated up rapidly and pyrolysis took place under microwave power of more than 800W. The combustible pyrolysis gas contained mainly H2, CO and CH4. Active carbon ratio of 17% was enough for rapid pyrolysis of sewage sludge.From Thermogravimetric analysis-Fourier Transform Infrared Spectroscopy (TG-FTIR) analysis of sewage sludge pyrolysis and combustion, the pyrolysis process mainly consisted of dehydration, volatile release and decomposition of inorganics, while combustion of carbonaceous was added for the combustion process. The pyrolysis process released great amount of CO, CO2, NH3 and organic gase, while combustion process mainly released CO2. Moisture put off the dehydration peak while it had little influence on pyrolysis and combustion peaks. Smaller dry sewage sludge particle tended to have a faster weight loss rate at delayed temperature.(2) PCDD/Fs distribution in three resulting three product phases during pyrolysis of wet sewage sludge was dominated by the distillation and dechlorination effect. Toxic PCDD/Fs in the product had lower concentration yield than original sewage sludge however higher toxicity. PCDD/Fs mainly existed in liquid phase. PCDD/Fs yield in gas phase and liquid phase increased along with microwave power, while it peaked at 700℃ for electric furnace. Combustion of wet sewage sludge generated considerable PCDD/Fs at 750℃ and they were largely decomposed at 800℃.PAHs were generated during pyrolysis of sewage sludge for both of microwave and electric furnace. PAHs yield peaked at 700℃. Distillation had much effect on PAHs distribution between gas and oil product. For dry sewage sludge, PAHs yield peaked at 950℃ for different temperature, and was also influenced by gas flow, sample mass, residence time and particle size.According to gasification and combustion of dry sewage sludge using fluidized bed, the gasification gas had heating value of 15MJ/m3, which was a good fuel. Total toxic PCDD/Fs yield in gasification of sewage sludge was much lower than combustion, and they were both lower than the PCDD/Fs concentration in original sewage sludge.The global kinetic rate parameters for formation of PAHs during dry sewage sludge pyrolysis were obtained using one-stage pseudo-unimolecular reaction kinetic model. Activation energy for PAHs formation and decomposition were separately around 150 kJ/mol and 200kJ/mol. The calculated value corresponded well with the experiment value.(3) A system of continuous microwave pyrolysis of sewage sludge was designed on base on above research, which was appropriate for commercial application. The cost was estimated to 285.63 yuan/ton wet sewage sludge, while the resulting product valued 590.68 yuan/ton wet sewage sludge.