Directional Optimization Of Odd-carbon VFAs Production From Hogwash Waste Grease By Anaerobic Fermentation And The Effect Of Long-Chain Fatty Acids

As the intermediate product of anaerobic digestion,odd-carbon volatile fatty acids(OCFA)are a kind of high-quality carbon source for the synthesis of polyhydroxyalkanoates(PHAs)and biological nitrogen and phosphorus removal(BNR).The production of OCFA,by anaerobic fermentation of hogwash waste grease,is conducive to the harmless and reduction of food waste and can realize the reuse of waste resources.Simultaneously,this technology also solves the problem of insufficient carbon sources in the treatment of food waste in cities and the sewage treatment process in the large areas of Rain Water,such as Guangdong,which has broad development prospects far-reaching practical engineering significance.However,at present,batch fermentation was mainly used in this kind of research,which is challenging to meet the continuous and stable carbon supply-demand of downstream processes such as PHAs and BNR.It is timely and necessary to explore the suitable type of sludge inoculation and steady and stable fermentation process in this context.Therefore,this research aims at the production of OCFA through the selection of a suitable type of sludge inoculation,steady fermentation process,and the effects of oleic acid and palmitic acid on the fermentation process,and explores the impacts of OCFA directional production process and long-chain fatty acids in two aspects of process optimization and mechanism exploration.Firstly,batch experiments were conducted to inoculate sludge from anaerobic tank,aerobic tank,residual sludge from secondary sedimentation tank,and hydrolytic acidification sludge respectively under the condition of MLSS=9.64±0.5 g/L.The indexes such as SCOD,ORP,pH,and VFAs were determined,respectively.And take the concentration of VFAs and the OCFA/VFAs(mass fraction)as the final reference index,combined with significant analysis to understand the inoculation effect,and then determine the best type of inoculation sludge.The results showed that the aerobic tank sludge was the best type of sludge inoculation for OCFA production by anaerobic fermentation of hogwash waste grease.The peak fermentation time was 4 d,the highest concentration of VFAs was 4219.2 mg/L,and the highest proportion of OCFA was 89.0%,respectively,which increased by 194%and 21.6%compared with the blank test group.In addition,combined with the significance analysis,it was found that the differences of VFAs concentration and OCFA ratio between the test group and the blank control group were at a significant level(P<0.01).Then,based on the working conditions of batch experiments and the type of sludge inoculation,the trial operation of the anaerobic sequence intermittent activated sludge(ASBR)process was started.The reaction cycle is five days in the experiment,and the hydraulic retention time(HRT)is 6.25 days.After the first cycle,the pH and NH4+were adjusted to the initial conditions of the first phase before each phase.By measuring the proportion of NH4+,PO43-and OCFA in the effluent,the stability of anaerobic fermentation by hogwash waste grease under the ASBR process operation was determined.The results show that during the trial operation of the ASBR process,the proportion of CODVFAs/SCOD and OCFA in the third and fourth phases fluctuates in the range of 46.7%～73.3%and 76.6%～90.9%,which is significantly lower than that in the previous two phases.The test results tend to be stable.Moreover,the concentration range of NH4+and PO43-in the effluent is small,and the concentration of PO43-is between 67.92～108.94 mg/L.In contrast,the attention of NH4+ is within the range of 63.01～131.96 mg/L,which belongs to the controllable range of the downstream process.Meanwhile,at the microbial level,it was confirmed that the main dominant bacteria in the ASBR process operation were Selenomonas,Prevotella,Propionispira,Megasphaera,Dialister,and Veillonellaceae.Finally,to explore the effect of long-chain fatty acids(LCFA)on the production of acidification and acidification type at the mechanism level,a simplified experiment was designed.Based on the research group’s previous research,oleic acid and palmitic acid,the main hydrolysates of hogwash waste grease,were added quantitative glycerol(the mass was 3.5g,accounting for about 10%of the group of hogwash waste grease)to simulate the ratio of hydrolysate.By adding oleic acid and palmitic acid,the concentration gradient of palmitic acid and oleic acid was set at 0～15 g/L and 0～12 g/L,respectively.The effects of different concentrations of oleic acid and palmitic acid on acidification were studied.The results show that oleic acid and palmitic acid will have a lag effect on the acidification process.The lag effect of palmitic acid is greater than that of oleic acid.When the dosage of oleic acid was 4 g/L,propionic acid was accumulated.When the oleic acid concentration increased gradually,the fermentation type changed from propionic acid fermentation to butyric acid fermentation,while the palmitic acid test group showed butyric acid fermentation.At the microbiological level,the bacteria with the most obvious difference among the test groups was Propriospina in the Veillonellaceae family.The relative abundance of Propriospina in the oleic acid test group was 239.5%higher than that of the blank control group and 16.2 times higher than that of the palmitic acid test group.