Research On Multi-organism Synergistic Wastewater Treatment And Energy Conversion Based On Microalgae

With excessive use of traditional fossil fuels,human beings are facing multiple crises including global warming,water pollution and energy shortage.In order to reduce the pressure on the environment and energy,it is urgent to seek a conversion technology that can realize clean energy production while treating wastewater.Microalgae can solve dual problems of water pollution and energy shortage by converting pollutants into bioenergy.However,unpretreated wastewater often contains high concentrations of organic matters and free ammonia nitrogen,which have toxic effects on microalgal growth and metabolism.In addition,microalgae have limited treatment efficiency of pollutants in wastewater,especially the conversion efficiency of macromolecular organic matter is low.Therefore,the biological interaction and synergy between microalgae and functional microorganisms for sewage treatment and energy-to-mass conversion is of great significance for the simultaneous and efficient removal and recovery of organic matter and inorganic salts in sewage.In view of the above problems,this paper firstly used Chlorella vulgaris for landfill leachate treatment and energy-to-mass conversion.Operating parameters such as landfill leachate concentration,light intensity and aeration CO₂ concentration were adjusted,to study the effect of operating conditions on the changes of landfill leachate composition,to obtain the inorganic salt absorption and transformation kinetics,growth kinetics and metabolic kinetics characteristics of microalgae in landfill leachate,and to clarify the energy conversion efficiency in the whole process of microalgae treatment for landfill leachate.Next,in order to overcome the low efficiency of pollutant energy-mass conversion in the process of treating landfill leachate with a single microalgal population,a dual-membrane cylinder photo-microbial fuel cell（DCP-MFC）was constructed by combining electricity-producing bacteria and microalgae.The power generation enhancement mechanism of the coupled system was explored by monitoring the microalgae growth and dissolved oxygen concentration changes in the reactor and the power generation output characteristics of the fuel cell;nitrogen transfer and transformation mechanisms was investigated in the coupled system by monitoring the nitrogen concentration changes in the reactor and microorganisms community analysis;the energy balance of the coupled system was subsequently assessed for potential.Finally,hydrogen gas was produced by bio-fermentation of hydrogen-producing bacteria using microalgae as biomass.By adding sodium citrate as a stimulus to enhance the hydrogen production efficiency during the fermentation process,the pH change,volatile fatty acid concentration change,hydrogen production rate and energy evaluation of the fermentation process were studied,and the hydrogen production performance when microalgae as substrates and the effect of sodium citrate on hydrogen production and volatile fatty acid production were clarified.The main conclusions of this thesis were shown as follows:（1）Changing the concentration of landfill leachate can greatly change the physical and chemical properties of landfill leachate,thereby affecting the growth and metabolic kinetics of microalgae.The results show that the 10%landfill leachate concentration,the160μmol/m²/s light intensity and the 7%CO₂ aeration concentration are the most suitable operating conditions for the growth of microalgae in landfill leachate.Through the study of nutrient absorption kinetics and growth metabolism kinetics of microalgae in landfill leachate,it is concluded that excessive light intensity and high carbon dioxide aeration concentration will trigger the protection mechanism of microalgae,causing microalgae to reduce the absorption of nutrients in landfill leachate by closing photosynthesis channels;insufficient nutrients in sewage,the higher light intensity and the higher carbon dioxide aeration concentrations favored the synthesis of carbohydrates and lipids.By energy potential analysis,it is found that the total energy yield of the whole process of microalgae treatment of landfill leachate is 37.13 kJ/L,which is higher than the calorific value of natural gas（35.59 kJ/L）.（2）In this paper,dual-membrane cylinder photo-microbial fuel cell（DCP-MFC）equipped with both anion and cation exchange membranes was constructed to realize the synergistic treatment of microalgae and electricity-producing microorganisms for wastewater.The results show that the energy output generation by the coupled system is155.21 kJ,which is higher than the sum of the energy of the single microbial fuel cell system MFC（2.33 kJ）and the single microalgae culture system（118.28 kJ）.The main reason for the improved energy output of DCP-MFC is that the cathode electrode can use dissolved oxygen in the cathode chamber to carry out the sunlight reduction reaction,which can improve the electron transfer efficiency and eliminate the toxic inhibitory effect of dissolved oxygen on microalgae,making the biomass of microalgae increased from 2.90g/L of the control group to 3.80 g/L of the experimental group,and the voltage output increased from 600 mV to 670 mV.In addition,DCP-MFC also achieved a nitrogen recovery rate of 92.6%;the nitrogen transfer mechanism study showed that NO₃^-was first converted into NO₂^-in the anolyte,and then NO₂^-was absorbed and converted by microalgae in the catholyte.（3）Microalgae itself contains a large amount of carbohydrates and proteins,which can be used as substrates for hydrogen production by biological fermentation to realize the deep energy conversion of microalgal biomass and produce high calorific value clean energy-hydrogen.Adding sodium citrate during the fermentation process can stabilize the pH of the fermentation broth,thereby maintaining the activity of hydrogen-producing bacteria and enhancing the production of biohydrogen.The results showed that the final hydrogen production was up to 15.36 mL/g biomass when the addition ratio of lemon microalgal biomass and sodium citrate was 5:1,while the final hydrogen production of the control group without sodium citrate was 13.44 mL/g biomass.The results of changes in the concentration of volatile fatty acids showed that the addition of sodium citrate could enhance the production of acetic acid,butyric acid and other fermentation products,thereby improving the energy conversion efficiency of the entire fermentation process.By analyzing the global energy balance,it was found that when the amount of sodium citrate added was microalgae:sodium citrate=5:1,the energy conversion rate was 34.4%,which was much higher than that of the control group without sodium citrate,which was 21.7%.