Enhanced Cultivation Of Microalgae Biomass In Anaerobic Digestion Solution Of Swine Wastewater Based On Algae-bacteria Symbiosis System

The utilization of wastewater and the lack of plant protein are important issues in the field of environment.It is a potential technology to cultivate microalgae by anaerobic fermentation liquid.How to strengthen the production of microalgae is a hot topic in this field.Algal symbiosis constitutes the purification basis of natural water.The gas exchange between microalgae and bacteria,the utilization of nutrients and the output of high added biomass make the algal symbiosis system have great potential in wastewater resource.Therefore,it is necessary to carry out relevant research.In this paper,the key technological parameters of microalgae culture were optimized,and then the symbiosis system was constructed to treat anaerobic fermentation liquid of swine wastewater,and the effect of the system on the growth and nutrient removal of microalgae was studied.The operating mechanism,the path of nitrogen and phosphorus transfer,the microbial group structure and the biomass components of microalgae were analyzed.In this study,when using anaerobic fermentation liquid of swine wastewater to culture microalgae,the symbiosis system of algal bacteria was formed by adding activated sludge.The key technological parameters（algae proportion,concentration of microalgae inoculation,light cycle and light dark cycle）were optimized to analyze the suitable conditions and related mechanisms of algae culture and nitrogen and phosphorus recovery.The results showed that the symbiosis system of algae was helpful to improve the growth and nitrogen and phosphorus recovery efficiency of microalgae under the appropriate algal ratio（1:0.2）,initial inoculation concentration(0.2 g·L^-1)and light dark condition（12l:12d）.The final concentration of Chlorella was 0.6 g·L^-1,and the removal rates of COD_Cr,TN,TP and ammonia nitrogen were 65.5%,72.1%,33.2%and 46.1%respectively.Under the above optimized technological parameters,the algal symbiosis reactor and microalgae pure culture reactor were further operated.After 4 days of static culture,the semi continuous flow reaction was started.The effects of HRT on the growth of microalgae biomass,nitrogen and phosphorus recovery and microbial community in the symbiotic system and pure culture system were studied.After 40 days of continuous operation,it was found that the final concentration of Chlorella was 0.6 g·L^-1,and the removal rates of COD_Cr,TN,TP and ammonia nitrogen were 84.1%,66.3%,93.0%and66.5%respectively.The dissolved organic matter（DOM）in anaerobic fermentation liquid of swine wastewater is mainly humus like substances.The symbiosis system of algae has a good removal effect on DOM in wastewater,and the degree of humification is high.In addition,the symbiosis system of algae has more advantages in microalgae culture and pollutant removal than that of pure culture system.Within the HRT range,the microalgae in the pure culture system lost with the outflow of water after 18 days of reaction.Considering the amount of microalgae,the amount of wastewater and the consumption of nutrients,the HRT system with 8 days is the best.The concentration of microalgae was 0.05 g·L^-1·d^-1 under the condition;1.15 L·d^-1 of wastewater treatment;The removal rates of COD_Cr,TN,TP and ammonia nitrogen were 75.0%,91.4%,61.0%and 73.4%respectively.In addition,the microbial diversity,the transfer and transformation of N and P in a cycle（24 hours）and the quality of the final harvest of Chlorella were analyzed.The results showed that Proteobacteria and Bacteroides can degrade the organic matter in wastewater effectively and promote the removal of nitrogen and phosphorus;Microalgae and Xanthobacteraceae are symbiotic;Comamonadaceae and Chitinophagaceae can inhibit the growth and reproduction of microalgae by decomposing the cell walls of plants.From the removal mechanism of nitrogen and phosphorus,the consumption of N and P in the system was mainly 48 hours ago,which was affected by the mechanism of excessive absorption of N and P by microalgae.The transfer and transformation of N in the water mainly includes nitrification and assimilation.After stable operation,the consumption of N in the symbiotic system of algae was 64.1 mg·L^-1·d^-1,and the assimilation N was 17.9 mg·L^-1·d^-1.The transfer and transformation of P in the water mainly includes microalgae absorption and precipitation removal.The consumption of P in the system is 1.2 mg·L^-1·d^-1.Compared with the pure culture of microalgae,the volatilization rate of ammonia in the symbiotic system of algae is 21.1%,although it has doubled,the content of N and P elements assimilated by microalgae is not very different.The results showed that the symbiosis system could cultivate high protein（56.8%）and low fat（4.4%）algae biomass compared with the pure culture system.It can be seen that the symbiosis system of algae can be used to culture microalgae in swine wastewater fermentation liquid,which can enhance the production of microalgae and improve the protein content of microalgae.The results of this study provide support for the application of algal symbiosis system in water treatment and resource-based.