Research On Modeling Food Waste Fermentation And Polyhydroxyalkanoate Production

Food waste is hugely produced,resulting in an increasing demand for its recycling.Although anaerobic digestion is an effective way to recycle food waste,its high organic content and biodegradability make anaerobic digestors easier to fail due to souring.To avoid it,the organic matter can be alternatively converted into volatile fatty acids（VFAs）via fermentation,and then VFAs can be used to produce polyhydroxyalkanoate（PHA）.PHA,as a kind of biodegradable material,can help solve the pollution of petrolum-derived plastics.The mixed cultures with three stages-substrate preparation,enrichment of PHA-producers,and PHA accumulation in batch-are commonly used for PHA production.However,the stability and carbon source recovery rate of the process need to be improved.Therefore,this study focused on how to properly combine food waste fermentation with PHA production,how food waste fermentation liquid affects PHA production,especially the stability of enrichment,and how to improve the carbon recovery rate and the substrate utilization efficiency and included research on modeling,mechanisms and process improvement using kinetics and molecular biology.To solve the problem that food waste anaerobic digestion easily sours and fermentation needs the guidance of a model,based on ADM1 and CASADM models,a model suitable for both anaerobic digestion and fermentation of food waste was established,by adding the inhibitory functions of acetate,free hydrogen and long-chain fatty acids,and the metabolism of extracellular polymeric substance and soluble microbial products.By introducing proton conditions,p H calculation and chemical speciations were combined with biochemical reactions,and thus the model simulated the p H and alkalinity more accurately.Three groups of semi-continuously fed anaerobic digestors with different food waste loads were used to verify the model.Then,using the model to analyze the acidification process,the study found that the instant high organic load caused rapid degradation of organic matters and the accumulation of VFAs.Meanwhile,the bicarbonate alkalinity gradually declined.Ultimately,the bicarbonate could not buffer the acidity from VFAs accumulation,and p H dropped.The leading indicators of bicarbonate alkalinity to total alkalinity（ALK_B/ALK_T）<60%,bicarbonate alkalinity to SCOD（ALK_B/SCOD）<75 and propionate to acetate（Pro/Ace）decrease were proposed to ensure the stability of anaerobic digestion.In addition,the model was used to guide the real food waste fermentation.In order to maximize the VFAs content in the fermentation liquid and to fit PHA production,the p H should be adjusted to 6.0 and the hydraulic retention time（HRT）should not exceed 8 days.Food waste fermentation liquid usually has high organic content and salinity.To efficiently utilize the fermentation liquid,this study explored the influence of salinity and organic loading on the enrichment process.The results showed that the low loading（1350 mg COD/L-d）and the salinity of 5 g/L were the most suitable enrichment conditions.Paracoccus and Thauera with strong PHA synthesis ability accounted for more than 65%of the enriched cultures,and the maximum PHA content(PHA_max)reached more than 50.5%of dry cell weight.The salinity above 10 g/L inhibited PHA synthesis but significantly promoted the sedimentation of sludge by dispersing the sludge in granules and enlarging the size of the granules.Based on the conclusion that salinity had a positive impact on sludge’s settling,this study proposed a strategy that gradient increased salinity to solve the filamentous bulking during the enrichment.The results showed that the increasing salinity gradients（from 0 g/L to10 g/L）could significantly improve the settling ability of the sludge and thus keep the sludge concentration at a high level because high salinity could kill the filamentous bacteria（Meganema）.The salinity gradients enhanced PHA production and accelerated the screening of non-halotolerant bacteria.Paracoccus and Thauera became the dominant genera adapted to 5 g/L and 10 g/L,respectively.The PHA_maxreached over 70%after recovering.Current researches pay little attention to the carbon recovery rate and substrate utilization rate.Based on the optimization results from the model,the HRT of food waste fermentation was set for less than 8 days,and the p H was adjusted to 6.0.Both the results from the model and the test showed that the highest VFAs concentration was obtained with an HRT of 8 days.VFAs accounted for over 85%of soluble COD,and its conversion rate reached 0.434 g COD/g VS.Meanwhile,the proportions of butyrate and valerate which were the preference of PHA producers were about 80%.The test results showed that using the real fermentation liquid without filtration through a 0.45μm membrane could reduce the cost while reaching a high PHA_max（56.3%）.To further optimize carbon recovery,the pre-PHA accumulation strategy was proposed,in which the surplus VFAs in PHA accumulation batch assays were used for the enrichment.The study showed that this strategy could optimize the distribution of VFAs for the enrichment and PHA accumulation,and could increase the conversion rate of PHA from food waste by 49%,up to 0.073 g COD/g VS.In summary,this study modified the model and demonstrated the impact of salinity and organic loading on PHA production by optimizing parameters and processes.Then,this study proposed the method to quickly solve bulking by increasing salinity gradients,optimizing the distribution of carbon in food waste,and providing guidance for the application.