A Novel Sequence Batch Membrane Carbonation Photobioreactor Developed For Microalgae Cultivation

Recently,microalgae have attracted much attention as a new generation of energy feedstock,which were widely applied in environmental treatment,medical care and other aspects.How to efficiently cultivate microalgae has become a current research hotspot.Optimizing the reactor form can effectively increase the yield of microalgae,which is of great significance for the efficient cultivation of microalgae.Traditional membrane bioreactors played a great role in the cultivation of microalgae,which are generally divided into two application forms:membrane carbonation and membrane retention.Membrane carbonation can reduce the bubble size,which making the microalgae absorb CO₂ easilier and accelerating the accumulation of biomass.However,the subsequent harvesting process is still required,and the process is cumbersome with higher economic cost.Membrane retention:Membranes are used to harvest and retain biomass and the solid-liquid separation of microalgae was realized.But membrane fouling is serious,and replacement of membrane modules will increase production costs.In view of this situation,a novel sequence batch membrane carbonation photobioreactor was constructed in this study,which aimed to combine membrane carbonation and membrane retention in the same reactor.Based on the intermittent operation mode of SBR reactor,membrane carbonation was performed in the reaction stage,accelerating the growth of microalgae.The membrane module at the effluent stage was used as a separation unit,retaining all microalgae in the reactor.Meanwhile,the fouling membrane module was gas-backwashed to alleviate the membrane pollution problem in the next reaction cycle?membrane carbonation?.The life span of the membrane module was extended,so that the novel reactor can stably cultivate microalgae for a long time.This study is divided into two parts:exploring the influence of a single variable?membrane aperture,photosynthetic active radiation and CO₂ concentration?on the growth of microalgae and the performance of microalgae in sequence batch membrane carbonation photobioreactor.The influence of a single variable on the growth of microalgae was evaluated by measuring the growth index of microalgae?TSS and pigment content?,and the optimal conditions were selected for the reactor experiment.In the reactor experiment,the growth index of microalgae(OD₆₉₀,TSS and pigment content),water quality index?the removal rate of NH₄⁺-N and P,the fluctuation of pH?and membrane pollution index?TMP,permeability flux,SEM images?were used to investigate the long-term operation feasibility and stability of the reactor,and the mitigation degree of membrane fouling.The main conclusions are as following:?1?The short-term experiment found that the smaller the membrane pore size?30/100/300 nm?,the higher photosynthetic active radiation degree?12/24/36 W/m²?,and the lower the CO₂ concentration?10/20/40%?leaded to the better the growth of microalgae.Therefore,the optimal experimental conditions for this experiment were obtained for the reactor experiment:the membrane pore size was 30 nm,the photosynthetic active radiation degree was 36 W/m²,and the CO₂ concentration was10%?v/v,N₂ balanced?.?2?Constructing the novel sequence batch membrane carbonation photobioreactor and operating three cycles of long-term culture experiments.Each cycle of experiments resulted in relatively stable and rapid growth of microalgae,with a maximum daily productivity of 150 mg/?L?d?and a maximum culture concentration of 1355 mg/L.And the maximum accumulation amount of chlorophyll a was 25.0mg/L,chlorophyll b was 14.9 mg/L,carotenoids was 6.6 mg/L,and the pigments were also greatly accumulated.The novel photobioreactor has a good cultivation performance.During the 48 h hydraulic retention time,the removal rate of NH₄⁺and P in the reactor?three cycles?were stable at about 80%.While,under carbonation conditions:10%CO₂?v/v,N₂ balanced?,the gas flow rate was 2 ml/min,the pH can be maintained above 5?suitable for microalgae survival?,and the pH are automatically controlled and adjusted in the novel reactor.For membrane fouling,the average daily increment of TMP in the three stages of operation is 9.05×10^-33 bar/d,7.27×10^-33 bar/d,and 7.26×10^-33 bar/d,respectively and the water flux recovered to 80%,75%,and 68%of the initial flux?104.8 L/m²?h?bar?after hydraulic cleaning.SEM images also showed that this type of reactor can greatly alleviate the problem of membrane fouling,thus,the reactor can run stably for a long time.