CO₂, Water Intake, Light-accelerated Algae Precipitation And Oil Production

With the"water crisis"and the"energy crisis"approaching,microalgae have gained more and more attention because it can achieve biodiesel production while purifying sewage.However,in conventional culture systems,the slow growth of microalgae,the high cost of harvesting,and the low oil content limit its industrial application.This study aims to enrich settleable microalgae in real secondary effluent,by applying“washout”pressure.Based on this,control strategies including carbon dioxide supply,light conditions and wastewater feeding mode were explored to increase the energy storage of settleable microalgae,with the purpose of determining the optimal conditions for sustainable microalgae biofuels production.Firstly,the effects of CO₂ on biomass productivity,sedimentation efficiency,purification performance and celluar components of the settlebale microalgae were investigated.The study found that CO2 supply had little effect on sedimentation performance,but CO2 affected dominant algae sinificantly.CO2 supply stimulated the enrichment of starch-producing filamentous green algae and oil-producing diatom?Navicula sp.?in raw?SiO2 8 mg/L?and silica amended?20mg/L?wastewaters,respectively.Without CO2 supply,filamentous cyanobacteria dominated regardless of silica concentration.CO2 supply increased biomass productivity by 51.3%,chlorophyll a content by 9.4 times,nitrogen and phosphorus removal rate by 67.21%and 4.2%,respectively.What's more important is that CO₂ can increase the productivity of oils,polysaccharides,and proteins by 22.8 mg/L/d,39.4 mg/L/d,and 14 mg/L/d,respectively.Then the impacts of light intensity and photoperiod on the settelable microalgae cultivation were invetigated.Low light intensity of 40?mol/m²/s facilitated the enrichment of filamentous diatom,while high ligh intensity of 400?mol/m²/s promoted the dominace of Navicula sp.Without CO2 supply,high light increase biomass concentration by 28.5%,as compared with low light.When high light was coupled with CO2 supply,biomass concentration was increased by 5.5 times,while nitrogen removal rate increased by 53%and phosphorus removal rate increased by 6%.High light intensity can increase the micro algae oil content to some extent.The increase in fat content is accompanied by a decrease in polysaccharide content,suggesting that high light may change the direction of photosynthetic carbon fixation and make it more fat-to-lipid.In the case of insufficient supply of inorganic carbon,high light will reduce the protein content,and vice versa will increase the protein content;again,the coupling effect of light and CO2 has a significant impact on the growth of microalgae.In continuous light mode,the both biomass productivity?20 mg/L/d?39 mg/L/d?and chlorophyll a increased?18 mg/L?28 mg/L?compared with the light/dark cycle mode,but the oil produtivity decreased slightly?7.7 mg/L/d?5.3 mg/L/d?.Finally,this study explored the impact of wastewater feeding mode?light-feed vs.dark-feed?under light/dark cycle.The dominant species in dark-fedd and light-fedd systems were Chlorella?rich in starch,up to 53.24%?and diatoms?rich in oil,up to 49.2%?repectively.This suggests that dark-feed mode would promote polysaccharides production,while light-feed mode could stimulate oil production.