Production Of Volatile Fatty Acids By High Temperature Hydrolysis And Liquid Fermentation Of Sludge

Anaerobic fermentation for volatile fatty acids(VFAs)production has been widely considered to be a promising approach for the high value-added utilization of waste activated sludge(WAS).However,there exist some inhibitions,resulting in low substrate conversion rate of sludge.Thermal hydrolysis,an important way in promoting sludge utilization,is widely used as the pretreatment to improve the efficiency of sludge anaerobic digestion.In this study,the organic matter in sludge was released into the liquid phase and the dewatering performance of sludge was improved via thermal hydrolysis.Meanwhile,the yields of volatile fatty acids was increased and the fermentation period was shortened by liquid fermentation.The main research contents and results were listed as follows:(1)Thermal hydrolysis can promote the release of organic matter from sludge and improve dewatering performance of sludge.The influences of temperatures on sludge thermal hydrolysis were tested at 140,160,180 and 200 °C.Results showed that the maximum hydrolysis rates of TSS and VSS could reach 45.17% and 71.64%,respectively.However,by analyzing the compositions of dissolved organic matter(DOM)in sludge hydrolysate,it was found that excessive hydrolysis temperature would affect the quality of sludge carbon source.DOMs in raw sludge were mainly dominated by protein-like substances and soluble microbial by-products,accounting for 52.30% and 20%,respectively,both of which were regarded as biodegradable organics.However,excessive hydrolysis temperature would lead to Maillard reaction,and induce the formation of refractory organic compounds,such as humic acid.By evaluating the quality of the carbon source in sludge hydrolysate,it was found that the amount of released organic matter reached the highest at 180 ℃,but the proportions of the organic matter in the forms of humic and fulvic acids were also the highest.When the temperature increased to 200 ℃,the available organic carbon sources began to decrease.Moreover,excessive-high temperature(≥200 ℃)was not conducive to the hydrolysate fermentation for acids production.In addition,the dewaterability of sludge can be greatly improved by thermal hydrolysis,and the viscosity and capillarysunction time(CST)of sludge after thermal hydrolysis can be reduced to 14.40 mpa·s and 15.90 s,respectively.(2)Liquid fermentation can improve acids yield and the efficiency of sludge fermentation.The results shown that the period of sludge liquid fermentation can be shortened to 5 days and the acidification rate was about 0.22 g VFA-COD/g SCOD.The acidification rate during ordinary sludge fermentation process was 0.20 g VFA-COD/g SCOD.The temperature of thermal hydrolysis is the key factor affecting liquid fermentation.The optimal temperature was about 180 ℃.Excessively high temperature druring thermal hydrolysis will adversely affect acids production by anaerobic fermentation.The degradation of protein was low and the low activity of enzyme were the main reasons lead to the low conversion rates of liquid fermentation By added alkaline protease into the reaction system at the end of the primary fermentation,the degradation rate of protein can be increased from 18.89% to28.34%,the degradation rate of polysaccharide can be increased from 20.42% to 30.63%.The final concentration of volatile fatty acids(VFAs)can be increased by 52.19%,and the enzyme activity can be stably maintained at about 18.70 U/m L.At the same time,co-fermentation was also an effective way to improve acid production by liquid fermentation.When the ratios(calculated in COD)of sludge hydrolysate to molasses wastewater were 50:50,30:70 and20:80,the yields of VFAs can be increased from 21.09% to 53.29%,45.05% and 40.94%,respectively.(3)Combined additions of activated carbon and protease can alleviate the inhibition of refractory organic matter of liquid fermentation.The results of adding carbon materials to hydrolysate shown that adding both kinds of carbon materials(activated carbon and biochar)can promote acids production from sludge by liquid fermentation,and increase the acidification rate to 0.24 and 0.23 g VFA-COD/g SCOD respectively.The results of adding carbon materials to fermentation system shown that added activated carbon and biochar can increased the acidification rates of sludge to 0.23 and 0.20 g VFA-COD/g SCOD,respectively.In addition,by adding activated carbon to adsorb harmful substances before fermentation,and then added 50 mg/L alkaline protease,the acidification rate of sludge liquid fermentation can be increased to 0.30 gVFA-COD/gSCOD.