Study On Acidogenesis Stability Of Food Waste Fermentation And Extraction Optimization Of Polyhydroxyalkanoate

Food waste is a solid-liquid mixed waste with large environmental content and strong potential for resource utilization.Anaerobic fermentation is one of the most important technologies for food waste disposal.After anaerobic fermentation,macromolecular organic matter in food waste can be transformed into small molecular products with high added value.For example,polysaccharides,proteins and fatty acids in food waste can be converted into volatile fatty acids(VFAs)by a mixture of bacteria such as hydrolytic bacteria and acidifying bacteria,and VFAs can be further converted into biodegradable plastic(polyhydroxyalkanoate,PHA)by microorganisms such as halophiles.PHA is one of the best substitutes for petroleum-based plastics because of its high biocompatibility and environmental friendliness.Therefore,the synthesis of biodegradable plastic(PHA)from food waste has great economic and social benefits.At present,there are still many key scientific and technological issues to be further explored in relevant researches.For example,(1)The composition of different food wastes fluctuates greatly,and the co-regulation mechanism of hydraulic residence time and substrate concentration is not clear in the fermentation process,resulting in large differences in the composition of PHA synthetic products;(2)In the process of PHA synthesis by mixed bacteria,it is still difficult to use the food waste as carbon source to transform the microorganisms into high-value PHA;(3)After PHA is synthesized by microbial anaerobic fermentation,sugars and other components are utilized.However,the synthesis wastewater contains high concentration of proteins,amino acids and other substances,which can be processed and adsorbed together with urban sludge to remove heavy metals and reduce the risk of heavy metal pollution in urban sludge.It is necessary to study the adsorption mechanism of amino acids on heavy metal ions in synthesis wastewater,to guide the design and synthesis of efficient heavy metal adsorbents,and to ensure the maximum utilization of food waste resources.In view of the above problems,this paper takes food waste resource treatment as the research content,and systematically carries out the following three parts of the research:(1)The effect of the co-action of hydraulic residence time and substrate concentration on the operation stability of the reactor in the fermentation process was clarified,and the precise regulation of the product(butyric acid production)was realized.Under stable operation(hydraulic retention time of 10.6 days,fermentation substrate concentration of 30 g COD/L),the reactor can account for high VFAs yield(46.1%)and acid yield per day(1.93 g SCOD/L/d),which is suitable for the substrate preparation stage of"three-stage" PHA synthesis process.The results also showed that the prolongation of hydraulic retention time(from 5.3 d to 10.6 d)promoted the transformation from acetic acid fermentation to butyric fermentation at 30 g SCOD/L substrate concentration.The hydraulic retention time had no effect on butyric acid fermentation at low substrate concentration(12 g SCOD/L)and acetic acid fermentation at high substrate concentration(60 g SCOD/L).Meanwhile,the hydraulic residence time(HRT)shapes the diversity of microbial composition in the reactor with low substrate concentration.In general,regulating the hydraulic retention time and substrate concentration is simple and effective to guide the production and composition of VFAs,and to prepare high-quality substrates for PHA synthesis process.(2)The oxidant and ultrasonic-assisted PHA extraction method was innovatively designed to achieve a maximum recovery of 80%and a maximum purity of 95%PHA.In the production process of PHA,VFAs from kitchen waste was used as synthetic substrate.In order to further reduce the production cost of PHA,this paper explored the downstream process of PHA production(extraction process),and designed three oxidants and ultrasonic-assisted extraction methods.The results showed that NaClO had the best effect of breaking cell walls and the highest PHA recovery(about 80%),while H2O2 and Peroxydisulfate(PDS)had the worst wall breaking effect(37%and 18%respectively).When heating assisted H2O2 and PDS were used to break the wall for PHA extraction,the PHA recovery rate did not increase(about 38%)with the increase of H2O2 dosage,while the PHA recovery rate increased from 18%to 36%with the increase of PDS dosage.When ultrasonic technology was added to assist H2O2 and PDS to break the wall,the extraction effect was significantly improved(PHA recovery was increased to 65%—71%,product purity was up to 86%—94%),and the dosage of oxidant in sodium hypochlorite group was greatly reduced.As butyric acid type VFAs was used to synthesize PHA in this experiment,the content of HV monomer in PHA products in all extraction experiments in this chapter has been maintained at about 30%.Then the thermal properties and molecular weight of PHA products obtained by NaClO method and "NaCIO+ultrasonic" method were tested.The results showed that ultrasonic technology could extract PHA rapidly and with high quality with less oxidant from thallus,while maintaining good product performance of PHA.(3)The adsorption mechanism of common amino acids for heavy metals was elucidated,and the order of their adsorption capacity for heavy metals was obtained.The adsorption capacity of amino acids for heavy metal ions(Cd2+,Cu2+,Fe3+,Hg2+,Mn2+,Ni2+and Zn2+)was studied by Density Functional Theory(DFT)method.The calculated results at B3LYP-D3/def2TZVP level show that amphoteric ionic amino acids are the main forms of amino acids in aqueous solution due to hydrogen bonding and protonation of amino acids.For amino acids with different side chains,the contribution trend of side chains to heavy metal affinity is Carboxyl>Benzene ring>Hydroxyl>Amino.Amino acids are more attractive to metal ions with smaller radii.In addition,several amino acids with high affinity for heavy metals were Asp>Glu>Trp>Asn>Thr>Tyr>Ser>Phe>Ile>Cys.The results of theoretical simulation experiments show that the synthetic wastewater has stronger adsorption capacity for heavy metal ions such as Ni2+,Fe3+,Cu2+,Hg2+ and Zn2+ in real environment.Amino acids such as Asp,Glu,Trp,Asn,Thr and Tyr can be used as reference structures in the design of heavy metal ion adsorbents.