Bacteria Enhanced Nutrient Recovery From Piggery Biogas Slurry By Microalgae

With the rapid development of China’s pig industry,a large number of piggery biogas slurry has been produced,which will pose a serious threat to the surrounding ecological environment.Piggery biogas slurry is rich in nitrogen and phosphorus.How to recover these nutrient is an inevitable choice for the sustainable development of the pig industry.Microalgae has shown great potential in assimilating nutrients(such as nitrogen and phosphorus)from wastewater,but it is mainly applied in the wastewater with low nutrient concentration.Moreover,carbon source and dissolved oxygen inhibition are other two frequently overlooked challenges for microalgae.In this study,the relationship among microalgal growth,pH and ammonium concentration were firstly analyzed to find the pH regulation strategy for reducing the ammonia inhibition on microalgal growth.Organic pollutant degradation bacterial inoculum were used as bacteria-augmentation(BA)for providing endogenous carbon source and reducing dissolved oxygen inhibition for microalgae.The mechanisms and conditions of augmentation were investigated.A novel Flat-Plate Continuous Open Photobioreactor(FPCO-PBR)was designed based on the results above.The operating parameters,stability and resource utilization potential of FPCO-PBR were analyzed,the main results are as follows.(1)The relationship among microalgal growth,pH and initial ammonium concentration(IAC)were firstly analyzed to find the pH regulation strategy for reducing the ammonia inhibition on microalgal growth.Results showed that the growth of Desmodesmus sp.CHX1 was inhibited under high ICA conditions,with the free ammonia inhibitory level of 63.37 mg L-1.Regulating pH can reduce the inhibition of ammonia,with the optimal pH of 7.5 for Desmodesmus sp.CHX1 growth when IAC was not higher than 1200 mg L-1,and the optimal pH was 6.5 when IAC was higher than 1200 mg L-1.(2)Commercially available bacterial inoculum were used as BA for improving the performance of microalgae-based wastewater treatment,mechanisms and conditions of BA were investigated.It was proved that BA could not only improve the growth and accumulation of carbohydrate and pigment of Desmodesmus sp.CHX1,but also increase nutrient assimilation by microalgae,with the increase of ammonium nitrogen(NH4-N)and total phosphorus(TP)assimilation rate of 6.37,mg L-1 d-1 and 0.59 mg L-1 d-1 respectively,BA increased the microbial diversity of the microalgae,and mainly increased the frequency of organic pollutant degrading bacteria,such as Bacillus,and Paenibacillus,which can provide inorganic carbon source and eliminate dissolved oxygen inhibition for microalgae growth,and thus promote microalgal gro.wth and nutrient removal.The open system was beneficial to the growth of microalgae and bacteria and appeared more efficient in nutrient removal than closed system.This was because that microalgae and bacteria were in good contact with the air in the open system,and oxygen can be released into the air when the concentration of dissolved oxygen was too high,on the contrary,bacteria can take oxygen from the air when the dissolved oxygen concentration was too low.The optimal temperature for nutrient removal by microalgae-bacteria system was 30 ℃,at which the carbon source and oxygen supply were sufficient,which was more conducive to the growth of microalgae and bacteria,thereby having better nitrogen and phosphorus removal efficiency.The optimal light intensity for nutrient removal by microalgae-bacteria system was 400 ìmol photons m-2 s-1,at which the growth of microalgae was the best,providing sufficient oxygen for the growth of bacteria,thus achieving the highest nutrient removal efficiency.The optimal initial bacteria-algae inoculation ratio was 1:1.(3)A novel FPCO-PBR was designed,and its operation parameters,microbial stability and feasibility of resource recovery were explored.Results showed that the appropriate influent mode for FPCO-PBR was multi-point or spraying mode.The optimal operation parameters hydraulic retention time was 2 d.The time interval for biomass harvesting was 2 d.Microorganism remained relatively stable in FPCO-PBR.RE of NH4-N and TP were 95%and 75%respectively.Nitrogen and phosphorus recovery rate were 20%(30 mg L-1 d-1)and 80%(7 mg L-1 d-1)respectively.Biomass production rate of FPCO-PBR was 0.47 g L-1 d-1.The revenue generated from cultivation Desmodesmus sp.CHX1 and organic pollutant degradation bacteria in piggery biogas slurry was estimated to be 104.26 ￥ kg-1,which means that treating one ton of wastewater can generate ￥ 49.00 in revenue.The biomass from FPCO-PBR showed great potential in the production of organic fertilizer and aquatic animas feed and extraction of pigment,which:indicated that it was feasible to recover resource from piggery biogas slurry by FPCO-PBR.