| Citric acid is a natural and multi-functional organic acid,which is mainly produced by submerged fermentation processes.As other bulk fermentation products,citric acid wastewater has characteristics of large water volume,high concentration,low pH and difficult to be treated.To reduce citric acid wastewater pollution to the environment,the domestic and foreign production enterprises mainly adopts"anaerobic digestion and aerobic digestion"to deal with citric acid wastewater.However,the governance model requires large area and high operation cost,besides the effluent is still unable to achieve the national discharge standards,which has brought heavy economic and environmental burden to the enterprises.To achieve the objective of green manufacture of citric acid,citric acid fermentation and anaerobic methane fermentation are integrated to form a circular ecological engineering system and according to the principle of"ecological nutrition chain"in this paper.Inhibitors,corresponding critical inhibitory concentration and inhibitory mechanism of the integrated ecosystem were studied.The combined membrane technology was then adopted to remove the key inhibitors from the anaerobic digestion effluent?ADE?.And the microbial community structure of the anaerobic methane fermentation in the ecosystem was analyzed.Finally,the mathematical model was established to simulate the operation rules of the integrated citric acid and methane fermentation recycling ecological engineering system to reveal the necessity of the stable operation.The integrated ecosystem plays an important role in resolving the problem of citric acid wastewater pollution,improving the comprehensive utilization efficiency of resources,realizing the industry's environment and economy coordinated development and promoting the transformation and upgrading of the whole fermentation industry.The main research contents and results were shown as follows:1.The integrated citric acid and methane fermentation recycling ecological engineering system was proposed.In this ecosystem,the citric acid fermentation wastewater is treated by methane fermentation in which the high concentration organics matter are degraded and biogas is produced.The biogas is then converted into steam and electric energy by cogeneration technology applied in citric acid production system.The ADE is further treated and recycled as the process water to produce citric acid.If the ADE is directly recycled to the citric acid fermentation,citric acid production was reduced by 9.1%compared with the fermentation with fresh water as process water.Studies showed that ammonia nitrogen,sodium ion,magnesium ion in ADE were main inhibitors for citric acid fermentation,and the corresponding critical inhibitory concentration were 100,200,40 mg·L-1,respectively.When ADE was treated by air stripping,the inhibition of ammonia and magesium ion on citric acid was basically eliminated,but the concentration of sodium ions was insignificantly unchanged.Therefore sodium ion became the main inhibitor in the integrated ecosystem and was used as a follow-up research object.2.The influencing mechanism of the key inhibitory sodium ion on the integrated citric aicd and methane fermentation recycling ecological engineering system were studied.It was found that high concentration of sodium ion?500mg/L?did not affect the growth and morphology of Aspergillus niger as well as the four key enzymes,pyruvate dehydrogenase,citrate synthase,aconitase and isocitrate dehydrogenase,in the TCA cycle.Further study found that the pH decrease rapidly in the early stage of citric acid fermentation?4-24 h?under the presence of high concentration of sodium ion.This leaded to the decrease of the activity of glucoamylase and isomaltase,thus causing the rise of residual total sugar and residual isomaltose concentration at the end of the fermentation and the decrease of the total amount of sugar available by Aspergillus niger,eventually leading to a decline in citric acid production.3.In order to eliminate inhibition of ADE on the citric acid fermentation in the integrated ecosystem,the application of ultrafiltration and nanofiltration coupled membrane technology to treat the ADE was proposed.Firstly,the molecular weight of the ultrafiltration membrane and the operation parameters was selected and optimized.An ultrafiltration membrane with the molecular weight of 100 kD was selected.The optimum pretreatment effect was obtained at the operating pressure of 0.45MPa,the operating temperature of 45?,and the flow velocity of 4m/s.In this condition,the removal rate of COD,colority and ammonia nitrogen were all above 30%,and the removal rate of turbidity was up to 98.5%.Then the nanofiltration membrane was screened and optimized.A nanofiltration membrane with intercept molecular weight of 150 D was obtained.As the operating pressure was 1.2 Mpa and the operating temperature was 45?,the removal rate of divalent metal ions and monovalent metal ions were100%and above 90%,respectively.The removal rates of COD,ammonia nitrogen,colority and alkalinity were between 85%-100%.Four batches of citric acid fermentation were performed using ADE as mixing material water treated by double membrane coupling technology,the results showed that the citric acid production,residual total sugar and citric acid yield were similar to those of the control group.Therefore,the double membrane coupling technique effectively remove the inhibitor from ADE for citric acid fermentation.4.The microbial community structure of the anaerobic digestion before and after the integrated citric acid and methane fermentation recycling ecological engineering system was analyzed by 454 pyrosequencing technology.It was found that the two kind of anaerobic sludge showed similar microbial community structure at the phylum level.The dominant phylum both contained Firmicutes,Proteobacteria,Chloroflexiand Bacteroidetes and Eurya Rchaeota.Euryarchaeota as the archaea was a specific methanogenic bacteria with acetic acid,whose abundance increased from 80.75%before the cycle to 98.74%after the cycle.Under the Euryarchaeota phylum,the two genera of Methanobacterium and Methanosaeta accounted for almost all of the abundance.This showed that the efficiency of the methane fermentation for high concentration of organics and the stability of the biogas fermentation are also strengthened after the coupled cycle.Therefore,the stability of the integrated citric acid and methane fermentation ecosystem is also guaranteed.5.According to the integrated citric acid and mathane fermentation recycling ecological engineering system,three mathematical models were established to simulate the accumulation of main inhibitors including total ions,organic compounds,volatile fatty acids and colorants.It was found that these inhibitory substances in the ADE reached relative stable equilibrium after 3-7 batches,which was consistent with the actual experimental results.Under the equilibrium state,the related inhibitors are below the critical inhibition concentration,and there was no obvious negative effect on citric acid fermentation.The production of citric acid,the fermentation time and the utilization ratio of starch were very close to that of the conventional process using deionized water in the integrated citric acid and mathane fermentation ecosystem of 15 batches of circulation experiment.During in the 15 batch cycle experiments,the relative fermentation parameters of integrated ecosystem remained stable,and the inhibitors were under the critical concentration and maintain equilibrium state,which proved the continuous and stable operation of the integrated ecosystems. |
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