Influence Of Thermal And Alkaline Pretreatment On Dewatered Excess Activated Sludge And High-solid Anaerobic Digestion

With the development of urban construction,the production of dewatered excess activated sludge is more and more year by year.Thererfore,the reduction of sludge and energy recycling are very important.Mesophilic temperature anaerobic digestion process has been considered as the best process to treat excess activated sludge by Ministry of Environmental Protection.However,the sludge is difficultly decomposed by microbiology,resulting in the longer time and larger bulk of conventional anaerobic digestion.To slove these problems,the paper studied the thermal-alkaline pretreatment of dewatered excess activated sludge and explored the mechanism to improve biodegradability and gas production of anaerobic digestion.Meanwhile,the changes of substrates and methane yield were studied during anaerobic digestion.The thermal pretreatment of dewatered excess activated sludge was conducted to investigate the effect of thermal pretreatment on the methane production.The results showed that the concentrations of SCOD,soluble protein and soluble carbohydrate increased with the increase of pretreated temperature.Their concentrations after 120 ? pretreatment enhanced by 819.8%,496.3% and 2666.1%,respectively,compared to the untreated sludge.Daily methane yield of the pretreated sludge had one more peak during anaerobic digestion,compared to the untreated sludge.The cumulative methane yield obviously increased after pretreatment,which was improved by 71.28% after 120 ? pretreatment,compared to that of untreated sludge.The results of alkaline pretreatment showed that the concentrations of SCOD,soluble protein and soluble carbohydrate increased with the increase of alkaline dose.The results of high-solid anaerobic digestion showed that cumulative methane yield decreased with alkaline dose increasing.Compared to the untreated sludge,cumulative methane yield increased by 35.34% for 20 mgNaOH·g-1TS sludge alkaline treatment,while decreased by 6.99% for 80 mgNaOH·g-1TS sludge alkaline treatment.On the basis of the above results,the combined pretreatment of thermal and alkaline was designed by CCD of RSM.The results showed that the thermal-alkaline pretreatment could greatly increase the soluble component,which resulted in higher cumulative methane yield,and the time of methane-producing prolonged with the increasing temperature and alkaline dose.After optimized by design expert,the optimal conditions were temperature of 134.95 ? and alkaline dose of 23.77 mgNa OH·g-1TS.The R2 of the model was higher than 0.99,which indicated that the model was significant.After verification test under the optimal conditions,it was found that the pretreated temperature played a leading role during the thermal-alkaline pretreatment,and cumulative methane yield was of a little difference with the simulation value.During high-solid anaerobic digestion after thermal-alkaline pretreatment,it was found that SCOD and VFAs(volatile organic acids)concentration first increased and followly decreased.The results of cumulative methane yield were consistent with the verification test.Thermal-alkaline synergy resulted in 7.78% increase of cumulative methane yield,indicating thermal-alkaline synergy was obvious.It was found at the same time,the correlation curve of daily methane yield and VFAs presented the annular shape,and the annular area increased with the increase of pretreatment severity.The dynamics of cumulative methane yield were tested,the results of thermal pretreatment and thermal-alkaline pretreatment fit Gompertz model better than first-order kinetic model,whose R2 was higher than 0.99.