Short-Chain And Medium-Chain Fatty Acids Production Performance And Mechanism From Ferrate Pretreated Waste Activated Sludge

A large amount of waste activated sludge will be produced during the biological treatment of sewage.At the same time,about 60%of the initial energy in the sewage will be concentrated in the waste activated sludge,so this waste activated sludge is also considered as a kind of resource.Anaerobic fermentation is currently one of the most widely used technologies for the treatment and disposal of waste activated sludge.Short-chain fatty acids are intermediate products of anaerobic fermentation.They can be used as high-quality carbon sources for microorganisms to produce other higher value-added products such as medium-chain fatty acids.This subject is mainly aimed at the slow hydrolysis process in the anaerobic fermentation of waste activated sludge and the high solubility of the obtained short-chain fatty acids,which makes it difficult to separate them from the broth.The strong oxidant potassium ferrate is used to pretreat thewaste activated sludge.The pretreatment is expected to strengthen the hydrolysis efficiency of thewaste activated sludge and the subsequent production of short-chain fatty acids.The potassium ferrate in the solution can be stabilized by adjusting the pretreatment p H value to further improve the pretreatment efficiency.Finally,ethanol is used as an electron donor to convert the short-chain fatty acids in the fermentation broth into medium-chain fatty acids with higher value and lower solubility.The role of potassium ferrate pretreatment in promoting the hydrolysis of excess sludge and the release of organic matter was investigated first.The results showed that0.5 g/g VSS of potassium ferrate obtained the best pretreatment performance.SCOD increased from the initial 343 mg/L to 2206 mg/L.SCOD didn't increase much as the dosage of ferrate further increasing.The determination of proteins and polysaccharides in the supernatant showed that the substances released into the supernatant were mainly proteins.EEM spectrum further showed that tryptophan-like proteins are the main proteins in the supernatant after the pretreatment.Short-chain fatty acids were rapidly accumulated during the anaerobic fermentation of the waste activated sludge after pretreatment.The yield of short-chain fatty acids reached 2668 mg COD/L,which was much higher than the 475 mg COD/L of the control.The extractable EPS was linearly positively correlated with the final short-chain fatty acid yield,indicating that the destruction of the compact floc structure was an important reason for the enhancement of anaerobic fermentation performance.Inorganic phosphorus occupied the dominant position.The NMR spectrum showed that orthophosphate(Ortho-P)and polyphosphate(Poly-P)in the original sludge were two main forms of phosphorus.However,the Poly-P peak disappeared after anaerobic fermentation,indicating that the excessively uptaken Poly-P was degraded and released.The phosphorus content in fermentation broth afther ferrate pretreatment(the 0.5 g/g VSS test reached the lowest 30.58 mg/L)was much lower than the blank control group(113.87 mg/L),indicating that potassium ferrate pretreatment controlled the phosphorus release into the fermentation broth.Terminal restriction fragment length polymorphism(T-RFLP)analysis showed that the microbial diversity of was significantly reduced after pretreatment,indicating that hydrolytic and acid-producing bacteria dominate the system.Potassium ferrate is extremely unstable in acidic aqueous solution.In order to improve its stability,potassium ferrate pretreatment in alkaline environment was studied.The results showed that when the dosage of potassium ferrate is 0.5 g/g VSS under alkaline condition,SCOD increased from 170 mg/L to 3010 mg/L.When the same amount of potassium ferrate was added directly,the SCOD was 1550 mg/L,while the SCOD was only 850 mg/L when only an alkaline environment was provided.It showed that potassium ferrate still plays a major role in the pretreatment.The main components of the organic matter in the supernatant were still proteins and polysaccharides.EEM spectrum showed that the dissolved protein was mainly tryptophan-like proteins and a small amount of tyrosine-like proteins.The highest yield of short-chain fatty acids after potassium ferrate+alkaline pretreatment was 322.6 mg COD/g VSS,while it was only135.1 mg COD/g VSS in the control test.Adding potassium ferrate alone can obtain a higher yield of short-chain fatty acids than just providing an alkaline environment,indicating that the addition of potassium ferrate was the main reason for increasing the yield of short-chain fatty acids.Compared with the control(35.9%),potassium ferrate+alkaline conditions test also showed a much higher acetic acid abundance(57%),indicating that the pretreatment significantly promoted the production of acetic acid.The phylum Firmicutes and Bacteroidetes were the predominant microorganisms after pretreatment.At genus level,Macellibacteroides and Petrimonas became the most predominant genera.The abundances of Proteinlasticlasticum and Proteocatella also increased to some degree.All of those bacteria could produce short-chain fatty acids or secrete extracellular hydrolase,which play an important role in enhancing the hydrolysis and acid production performance.The short-chain fatty acids obtained by anaerobic fermentation of waste activated sludge had high solubility in aqueous solutions which made it difficult to separate.In order to obtain high-value products that are more conducive to separation and recovery.Electron donor ethanol was added to the anaerobic fermentation broth to obtain medium-chain fatty acids(mainly caproic acid)which has longer carbon chains and lower solubility.The results showed that the content of butyric acid and caproic acid in the system increased with time.The content of caproic acid stabilized at about 4 g/L.The serum bottle test showed that the mixed bacteria system can use 50 m M acetic acid and100 m M ethanol to obtain 2488 mg/L caproic acid and 1356 mg/L butyric acid.More than60%of the electrons in the substrate are finally distributed in caproic acid(43%)and butyric acid(19%).It showed that the system has a good performance in producing caproic acid.Methanogens must be inhibited when mixed culture system was used to produce medium-chain fatty acids.Therefore,the influence of chloroform,a classic methanogenesis inhibitor,on the microbial system was further explored.The results showed that the use of 0.05%(v/v)CHCl₃ can inhibit the chain elongation reaction to a certain extent,the production of butyric acid is not greatly affected,but the production of caproic acid is reduced to about half of the control experiment.The microbial community did not change significantly.When the dosage of CHCl₃ was higher than 0.1%(v/v),the chain elongation reaction hardly proceeded.Clostridium sensu stricto occupied an absolute dominant position in the control test and the 0.05%(v/v)CHCl₃ test,indicating that species in the Clostridium genus such as Clostridium kluyveri might be the core functional microorganisms in the chain elongation reaction.