Enhanced Production Of Short-chain Fatty Acids From Excess Sludge By Combined Free Ammonia With Alkyl Polyglucoside Treatment

With the gradual increase of the amount of urban sewage,the output of excess sludge is also increasing,and the problem of sludge treatment and disposal is becoming more and more important.Anaerobic digestion is a common method of sludge treatment.It can not only reduce sludge,but also produce short chain fatty acids(SCFAs).SCFAs produced by anaerobic fermentation of sludge can be used as a carbon source for biological nitrogen removal(BNR)in wastewater treatment plants to reduce the cost of wastewater treatment.Free ammonia(FA)is a kind of substance which can be easily obtained from the fermentation broth of WAS tewater treatment plant.Alkyl polyglycosides(APG)is a kind of biodegradable and environment-friendly biosurfactant,which can promote the dissolution and hydrolysis of particulate organic matter in sludge.In this study,APG and FA pretreatment methods were combined to promote the SCFAs production of excess sludge.The effects of FA pretreatment time,APG dosing method,APG dosage and FA concentration on anaerobic fermentation of sludge were studied;the effects of FA and APG synergism on dissolution,hydrolysis,acid production and methanogenesis were explored,and the mechanism of FA pretreatment synergism with APG to improve SCFAs production of sludge WAS clarified;the effect of FA pretreatment synergism with APG on microbial community structure WAS confirmed by high-throughput sequencing.The results are as follows:(1)The pretreatment time of FA pretreatment sludge for SCFAs production is 3 days,and the optimal APG dosing mode is adding FA at the same time.(2)When the concentration of FA was 140 mg/L,the dosage of APG was 0.150 g/g VSS,the maximum SCFAs production was 370.69 mg COD/g VSS,reaching the maximum in 11 days.When the dosage of APG was 0.125 g/g VSS,the maximum yield of SCFAs was360.83 mg COD/g VSS,and reached the maximum in 9 days(3 days of pretreatment and 6days of fermentation).When the dosage of APG was 0.125 g/g VSS,compared with 0.150g/g VSS,the fermentation time was shortened by 2 days,and the SCFAs yield had no significant difference.Considering the cost of APG and the land occupation of the reactor,0.125 g/g VSS was selected as the appropriate dosage of APG.(3)When the dosage of APG was 0.125 g/g VSS,adding a certain concentration of FA(20-80 mg/L)could promote the production of SCFAs,but when the concentration of FA was higher than 80 mg/L,the promotion degree decreased.When FA was 80 mg/L,the maximum SCFAs yield was 381.03 mg COD/g VSS in 8 d(3 days of pretreatment and 5days of fermentation).So the best concentration of FA is 80 mg/L.FA pretreatment combined with APG can promote the dissolution of ammonia nitrogen and phosphate in sludge.(4)FA pretreatment combined with APG promoted the dissolution of particulate organic matter;promoted the hydrolysis of macromolecular substances(carbohydrate and protein),and provided more substrates for the subsequent acidification process;inhibited the acidification process and methanogenesis process,but the inhibition degree of acidification process WAS far less than that of APG Inhibition of methanogenesis.Three dimensional fluorescence spectrum analysis showed that FA pretreatment combined with APG increased the biodegradability of dissolved organic matter(DOM)and released more biodegradable substances.(5)The results showed that the dominant flora in FA + APG group was Firmicutes and actinomycetes.The decrease of relative abundance of Proteobacteria and the increase of relative abundance of Firmicutes and Actinobacteria in FA + APG group were beneficial to the increase of SCFAs.The dominant microorganisms in FA+APG group were Clostridium(39.33%),actinomycetes(8.88%)and Bacillus(8.55%),which could promote hydrolysis and acidification,while the relative abundance of Alphaproteobacteria,Betaproteobacteria and Gammaproteobacteria decreased,resulting in the increase of SCFAs production.In the FA+APG group,Tissierella,Romboutsia,Staphylococcus,Proteiniclasticum and Proteiniborus,were the dominant bacteria,which were beneficial to hydrolysis and acid production.The relative abundance of Tissierella was as high as 17.28%.FA pretreatment combined with APG enriched the bacteria related to hydrolysis and acid production,and reduced the proportion of other bacteria consuming SCFAs,which made SCFAs accumulate better.In conclusion,considering the factors of sludge acid production,fermentation time,economy and so on,adding 80 mg / L FA and 0.125 g / g VSS APG to the sludge for 3 days of combined pretreatment,keeping p H at 9 for 3 days of pretreatment and fermenting for 5days can obtain the optimal acid production of anaerobic fermentation.