Research On Technology And Effect Of Swine Wastewater Treatment Based On Anaerobic Fermentation And Microalgae Culture

The resource treatment of aquaculture wastewater has always been an important research field.Swine wastewater（SW）contains a lot of nutrients such as nitrogen and phosphorus,which has been treated by conventional technologies with certain effects,but the valuable substances have not been fully recycled.After anaerobic fermentation,SW can be more easily used by microalgae under heterotrophic conditions,and the harvested microalgae can be used for the development of high-value products.In this thesis,the anaerobic fermentation of SW was optimized,and the microalgae（Chlorella）cultivation using the fermented liquid as the substrate was enhanced in algae-bacteria symbiotic system which introduced the activated sludge as the origin of bacteria.This thesis focuses on the performance of the anaerobic fermentation reactor and microalgae cultivation reactor treating SW,and the operating mechanism was analyzed in terms of microbial community and nitrogen/phosphorus transformation,and the composition and biosafety analysis of harvested microalgal biomass were conducted.It provides support for the high-value utilization of microalgae biomass and the application of the technology system in the field of aquaculture wastewater resource.In this study,the fermentation time,temperature（25℃,35℃,55℃）and initial p H（5,7,9）of anaerobic fermentation of SW were optimized,and the optimum conditions for VFAs accumulation were analyzed.The results showed that the anaerobic fermentation time of 6 d,high temperature55℃and initial p H 9 were favorable to improve the production of VFAs.The VFAs accumulation was 2668.1 mg COD·L^-1,and acetic acid accounted for more than 60.0%of the total VFAs,followed by propionic acid.Using diluted fermentation broth as the substrate,Chlorella was mixotrophic cultured based on the algae-bacteria symbiotic system.Activated sludge was introduced in the process of exploring the ratio of algae to bacteria.By optimizing the key process parameters,it was found that under the condition of 1:0.2 algal/bacteria ratio（mass ratio）,the built-in light mode was more conducive to the increase of microalgae concentration and the removal of nutrients under the same light intensity of 8000 lux.Among them,with the growth of 43.9 mg·L^-1·d^-1,the maximum concentration of microalgae was 822.3 mg·L^-1,which was 4.1 times of the initial inoculation concentration.The removal rates of SCOD,TP,TN and NH₄⁺-N were 82.8%,84.9%,54.7%and43.2%.Based on the optimization of the above parameters,the anaerobic fermentation reactor and microalgae culture reactor were designed.The operation performance and mechanism of the two reactors for SW treatment were investigated under the semi-continuous condition.The results showed that Bacillaceae and Pseudomonadaceae affect each other and maintain the stability of the reactor during the 48 days of anaerobic fermentation.Among them,Pseudomonadaceae played a positive role in the production in VFAs production by anaerobic fermentation.VFAs accumulation increased,and remained at 2676.7 mg COD·L^-1 after stable operation,accounting for about 86.4%of SCOD,and p H value remained at 8.0-8.5.Bacillaceae promoted hydrolysis process,protein concentration decreased by 48.0%and polysaccharide concentration decreased by 41.5%,respectively.With the increase of humification degree of anaerobic fermentation effluent,the concentration of SCOD,TN and NH₄⁺-N increased.It was found that Pseudomnadaceae,Rhizobiaceae and Comamonadaceae were the main dominant bacterial families in semi-continuous operation of the microalgae culture reactor,and the correlation coefficient between the three and the microalgae concentration reached 0.92.In addition,the correlation coefficient of Caulobacteraceae,Moraxellaceae,Microbacteriaceae,Xanthomonadaceae and nutrients reached 0.75.These microorganisms jointly maintained the growth of Chlorella and the removal of nutrients during the semi-continuous flow operation of the reactor,so as to achieve the purpose of stable operation of the reactor.During this process,the highest concentration of microalgae reached 887.6 mg·L^-1,and finally reached a stable growth rate of 63.7 mg·L^-1·d^-1.The consumption of N and P remained stable at 36.9 and 3.1 mg·L^-1·d^-1.The final removal rates of SCOD,TP,TN and NH₄⁺-N were 81.9%,91.2%,62.8%and 64.4%,and DOM（mainly humic acid）in the system could also be effectively removed.It was found that the assimilation rates of N and P by microalgae were 31.2%and 3.4%respectively during the analysis of the N and P transformation,while the nitrification and denitrification rates of N and the precipitation rate of P accounted for more than 70.0%.It shows that while the assimilation is effective in this experiment,the nitrification and denitrification of N and the precipitation of P are also important ways of removing nitrogen and phosphorus.The component analysis of harvested microalgal biomass showed that the protein content was relatively high（52.4%）,indicating that the system could cultivate high-protein biomass.The analysis of biosafety indexes showed that all indicators meet the feed hygiene standard in GB13078-2017,and can be used for the development of mixed feed.The harvested microalgae were used to replace soybean in feed with 20%to synthesize microalgae mixed feed for feeding trial.The preliminary results showed that pigs in experimental group and blank group grew well.At the end of the trial,the pork quality analysis showed that the concentration of protein,reducing sugar,fat,and cholesterol in experiment groups was similar to that of the blank group,and the trace elements was higher in the former.It was indicated that microalgae with high protein content can be used to replace the original crude protein materials in the feed,which further reflects the potential of applying microalgae as protein resource.In conclusion,the combined treatment of SW by anaerobic fermentation and microalgae culture can not only remove pollutants,but also strengthen the accumulation of protein in microalgae,thus providing technical support for efficient resource treatment of aquaculture wastewater.