Volatile Fatty Acids Production Performance And Mechanism From Waste Activated Sludge Fermentation Boosted By Enzymolysis

The waste activated sludge(WAS)produced from urban(town)wastewater treatment plants(WWTPs)become a main way of conversion and storge of aqueous contaminants to clean the wastewater.Thus,a great amount of organic carbon source(organic proportion accounted for 60%)was riched in the WAS solid phase with considerable recovery potentials.However,the current WAS treatment cost accounts for 20-50% of the total wastewater treatment cost.How to achieve the WAS fast treatment with organics resource/bioenergy recovery from WAS biosolids was a hot topic currently in the field of solid waste treatment with the maximum economic,social and environmental benefits.Aimed at the problems in the traditional WAS treatment process,such as the long hydrolysis time and low resource recovery efficiency,the biological enzymes were used to disintegrate and pre-hydrolyse the solid particles for boosting WAS solubilization in this study.The performance of enzymolysis pretreatment for enhancing volatile fatty acids(VFA)production and microbial mechanism in WAS anaerobic fermentation process were inveatigated.The VFA accumulation affected by the impact factors and the corresponding relationship between microbial community composition and impact factors in the fermentation process were demonstrated.The enhanced VFA production performance by WAS conditioning of external carbon source in the anaerobic fermentation process was also investigated in this study.The biological enzymes of lysozyme,protease,amylase,cellulase and their mixture(compound enzymes)were adopted to pre-hydrolyse and disintegrate the WAS solids in order to boost the solid organics of WAS solubilization into liquid phase.The result showed that the lysozyme lysis presented a better WAS solids disintegration efficiency compared with other single enzymes(protease,amylase and cellulase).Under the condition of enzymes adding dosage ratio of 5%-15%(w/w),the SCOD content increased to 3950-5660 mg/L in the WAS supernant within 240 min reaction time.The compound enzymes(four single enzymes blend with 1:1:1:1 mixing ratio)promoted the WAS solubilization by increasing the SCOD dramatically from initial 200 mg/L to the levels of 6600-9100 mg/L in the WAS supernant within 180 min,which was higher than the sum of SCOD increase by the four single enzymes for WAS solubilization individually.It indicated that the compoumd enzymes displayed a better WAS solubilization performance than the single enzymes.Meanwhile,the sludge particle size tended to be enlarged due to the protein-aided bioflocculation effect with the distributive spread index(DSI)increase.The EEM spectra results indicated that the tryptophan-protein and simple aromatic protein substances were the main proteins in the liquid phase after the enzymolysis pretreatment of WAS,which played important roles on boosting WAS biodegradability by enzymolysis.Microbial diversity decresed and community distribution tending to be even in WAS solid phase affected by the enzymolysis pretreatment.The VFA production performance and microbial mechanism in the enzymolysis-pretreated WAS fermentation were investigated.The VFA production performance affected by various fermentation temperatures was disclosed within 10-d fermentation cycles.Results indicated that the VFA production in thermophilic fermentation was slightly higher than that in the normal and mesophilic fermentations.The VFA production was accumulated to 3200-3500 mg COD/L on day 4 to day 7 in the thermophilic fermentation process with the dominant VFA of acetate(1000-1200 mg COD/L)and butyrate(800-1100 mg COD/L).The VFA production of 4000-4200 mg COD/L was higher at 8 d-SRT than 11 d-and 6 d-SRTs(VFA production levels of 3500-4100 mg COD/L).The prolonging SRT wasn't conducive to boost the VFA further accumulation.The fermentation impact factors of temperature and biosolid retention time played important roles on microbial community dynamics.The biodiversity increase and low community evenness favored VFA rapid accumulation.The pH and ORP played dominant roles on microbial community in the fermentation prophase,while VFA accumulation tended to be the main restrictive factor in the terminal fermentation phase.Eliminating the VFA accumulation would facilitate the VFA further production.The main functional microbes of Proteiniborus?Pelotomaculum?Gallicola?Bacteroidales ? Synergistaceae ? Lactobacillus ? Anaerotruncus,Clostridium IV,Oscillibacter and Firmicutes emerged in the preated WAS fermentation,which played important roles on WAS organics degradation and various VFA generation pathways.Aimed at the low C/N ratio(6/1-9/1)of organics in WAS solid phase,the corn stalk and pig manure were used for WAS conditioning to optimize substrate C/N ratio,which led to comsiderable VFA accumulation in the fermentation process.When only the corn stalk was adopted with the fermentation substrate C/N ratio increasing to 20/1,the VFA accumulation arrived to 4500-4700 mg COD/L in the fermentation process.When the corn stalk and pig manure were adopted together for increasing the fermentation substrate C/N ratio to 20/1,the maximum VFA accumulation value arrived to 5100-5200 mg COD/L approximately with only 3400-3500 mg COD/L of VFA production in the control experiment.The dominant microorganisms of Lactobacillus?Clostridium ? Firmicutes ? Bacteroidales and Anaerotruncus were responsible for organics degradation and VFA accumulation in the fermentation process.Meanwhile,the WAS conditioning by external carbon source improved the microbial biodiversity with VFA accumulation in the fermentation system.Besides,the microbial community distribution tended to be uneven with climax community formation,which benefited the VFA production in the fermentation process.