Nitrogen And Phosphorus Removal And Microalgae Harvesting Of Forward Osmosis Membrane Photobioreactor Based On Marine Microalgaes

With the acceleration of urbanization and industrialization,the amount of wastewater from municipal,industry and agriculture has been gradually increasing.If the nitrogen and phosphorus nutrients contained in the wastewater are not properly treated and directly discharged into water,it will lead to eutrophication in water system and destruction in the balance of the water ecosystem.Microalgae can recover nitrogen and phosphorus from wastewater by assimilation and adsorption,however,due to its small size and similar density to water,it is difficult to separate them by sedimentation.Therefore,the present study constructed a forward osmosis membrane photobioreactor（OMPBR）to solve this problem.OMPBR uses the forward osmosis membrane to replace the ultrafiltration and microfiltration membrane in traditional membrane photobioreactor.It is because forward osmosis（FO）is merely driven by osmotic pressure,making water molecules spontaneously permeate through the semi permeable membrane from the feed solution with high hydrochemical potential to the draw solution with low hydrochemical potential that there is no need to use external pressure with a high energy consumption.Moreover,the forward osmosis membrane can not only intercept microalgae and reduce the risk of microalgae biological washing,but also intercept and concentrate nitrogen and phosphorus nutrients,which is conducive to the growth of microalgae and the improvement of effluent quality.However,the reverse diffusion of the draw solution to the feed solution may result in the increase of the salinity in the feed solution.Therefore,in this study,marine microalgae were used to remove nitrogen and phosphorus from simulated wastewater and resist the impact of reverse salt diffusion at the same time.In addition,the microalgae in OMPBR were harvested by flocculation combined with forward osmosis.The flocculation before the forward osmosis filtration enabled the fouling of forward osmosis membrane to reduce to some degree.This paper mainly carries out the following two aspects of experimental research.（1）The nitrogen and phosphorus removal performance of OMPBR based on marine microalgae was investigated,and the microalgae biomass,biomass composition and membrane fouling in the reactor were analyzed and monitored.The results demonstrated that among the three marine microalgae,only Nannochloropsis oculata could adapt to the simulated wastewater.OMPBR based on marine microalgae could achieve excellent treatment effect in the treatment of simulated wastewater and bear the nutrient load of 9.2 mg NH₄⁺-N/（L·d）and 2.2 mg PO₄^3--P/（L·d）.Due to the excellent interception performance of FO membrane and the assimilation of nutrients by microalgae,more than 90%removal rate of NH₄⁺-N and PO₄^3--P could be obtained during the operation.At the end of the operation,the conductivity of the feed solution reached 14 ms/cm,and the marine microalgae could survive well at this salinity because of its salt tolerance,reaching 6 g/L in the microalgae biomass at the end of the operation.（2）The effects of different concentrations of chitosan on flocculation efficiency,organic matter content in microalgae suspension and membrane fouling in forward osmosis filtration were investigated.As the chitosan concentration increased,it displayed a tendency of initial increase followed by a decrease in flocculation efficiency.The microalgae suspension containing 10 mg/L chitosan achieved the highest flocculation efficiency,which was more than 80%.At the same time,the maximum flocculation efficiency could be obtained in the shortest time at this concentration.Through the DOC,the content of organic matter in microalgae suspension containing 10 mg/L chitosan decreased the most after flocculation,which was about 46%.The decrease of water flux could be slowed down,and the fouling of the membrane could be reduced by adding appropriate amount of chitosan before the filtration.