Study On The Nitrogen And Phosphorus Removal And Bioenergy Production Of Energy Microalgae Using Volatile Fatty Acids

The excessive development of fossil fuels poses a global energy and environmental crisis.As the raw material of the third generation bioenergy production,microalgae has attracted extensive attention.However,high cost and low yield are currently the main limitations of microalgae lipid production.In recent years,the combination of waste resource treatment and renewable energy development has become a beneficial attempt to break through this limitation.Utilizing wastewater as raw materials to produce microalgae biomass not only reduce the cost of microalgae cultivation while also treating wastewater but also promote the reduction of carbon emissions,which has both economic and environmental benefits.Therefore,selecting algae strains with high potential for biofuel production and finding cheap carbon sources to cultivate microalgae are the key to solving the problem.During the anaerobic fermentation process of treating organic waste,volatile fatty acids（VFAs）are generated,which not only inhibit the production process but also pose potential hazards to the ecological environment.In this paper,VFAs were selected as carbon sources for microalgae cultivation.The dominant microalgae strain Desmodesmus sp.XNY-2104 was isolated and purified,which could tolerate VFAs and had good nitrogen and phosphorus removal ability,and its cultivation condition was optimized.Then,the potentials of Desmodesmus sp.XNY-2104 and a heterotrophic microalgae Chlorella sorokiniana FACHB-275 which has been shown to be tolerant to VFAs in previous studies for utilizing acid-producing wastewater were explored.Finally,the effects of different nitrogen source types on microalgae growth,nitrogen and phosphorus removal and energy production were investigated,and the mechanism of efficient nitrogen removal by microalgae in heterotrophic condition was explored.The main conclusions of this study are as follows:（1）A microalgae strain with good nitrogen and phosphorus removal ability that could adapt to VFAs environment was obtained from the aerated biological filter of campus sewage treatment station through plate coating separation method and plate marking method.It was named Desmodesmus sp.XNY-2104 after morphological and molecular biological identification.The effects of different VFAs ratios（acetic acid:propionic acid:butyric acid）on the microalgae under mixotrophic and heterotrophic conditions were investigated.Considering the growth,nitrogen and phosphorus removal and productivity of microalgae,the optimal cultivation condition was VFAs ratio of8:1:1 under the heterotrophic condition.Under this condition,the biomass concentration was 0.27 g/L,the removal rates of total nitrogen and total phosphorus were 99.86%and99.28%,respectively,and the lipid content was 38.63%.The highest utilization rate of acetic acid was 56.37%,followed by butyric acid（54.35%）and propionic acid（33.65%）.After analysis,VFAs absorbed by microalgae were mainly used for biomass growth under the mixotrophic condition,and for nitrogen and phosphorus removal and lipid accumulation under the heterotrophic condition.（2）Chlorella sorokiniana and Desmodesmus sp.were cultured with actual acid-producing wastewater.Both strains of microalgae could effectively utilize acid-producing wastewater for growth.The highest dry weight（5.58 g/L）,specific growth rate(0.32 d^-1),dry weight yield（1.79 g/L/d）,total nitrogen removal efficiency（78.6%）,ammonia nitrogen removal efficiency（75.46%）,total phosphorus removal efficiency（81.73%）and lipid content（40.23%）were obtained for Chlorella sorokiniana under the heterotrophic condition.The utilization rates of COD and VFAs were 44.55%and 41.61%,respectively.For Desmodesmus sp.,higher dry weight（4.85 g/L）,specific growth rate(0.31 d^-1),and dry weight yield（1.50 g/L/d）were obtained under the mixotrophic condition,while higher total nitrogen removal efficiency（81.31%）,ammonia nitrogen removal efficiency（81.19%）,total phosphorus removal efficiency（82.28%）and lipid content（39.51%）were obtained under the heterotrophic condition.The utilization rates of COD and VFAs were 42.94%and 39.67%,respectively.According to the nitrogen and phosphorus removal of acid-producing wastewater by microalgae,both microalgae strains achieved higher nitrogen and phosphorus removal efficiency under the heterotrophic condition,and Desmodesmus sp.had better nitrogen and phosphorus removal effect.（3）The effects of different nitrogen source types on the growth,nitrogen and phosphorus removal and energy production of the two microalgae strains under the heterotrophic condition were investigated,and the mechanism of efficient nitrogen removal by microalgae was discussed.Overall,nitrate nitrogen was the optimal nitrogen source for the two microalgae strains.Under this condition,the biomass yields were0.90 g/L and 0.23 g/L,the total nitrogen removal efficiencies were 97.79%and 97.84%,the total phosphorus removal efficiencies were 95.96%and 96.54%,and the lipid content were 39.13%and 36.21%,respectively.In addition,the removal efficiency of total nitrogen could be effectively improved by increasing the proportion of nitrate nitrogen in the culture medium,while the removal efficiency of total nitrogen could be reduced by increasing the proportion of ammonia nitrogen.In this study,the two microalgae strains used nitrate nitrogen more efficiently than ammonia nitrogen,which might be due to the toxic effect of high concentration of ammonia nitrogen on microalgae cells and the existence of VFAs under dark conditions promoted denitrification,thus improving the removal efficiency of nitrate nitrogen.