Research On Light Strategy Optimization And Wastewater Treatment Of Microalgae Immobilized Culture

With the rapid development of the world economy,the problem of energy shortage and environmental pollution has become increasingly severe.As an excellent source of renewable energy raw materials,microalgae have attracted more and more attention of researchers.Compared with traditional suspension culture,microalgae immobilization culture can increase the biomass culture density and reduce the biomass harvesting cost,and is easy to realize large-scale culture,which has the prospect of industrialization.Microalgae require nitrogen and phosphorus as nutrients in the cultivation process.Combining microalgal culture with wastewater treatment can not only reduce the cost of microalgae nutrition sources,but also reduce the cost of wastewater treatment.However,in the process of immobilized culture,the absorption and utilization mechanism of light and nutrients by microalgae is significantly different from that of suspension culture.Therefore,in this study,the following aspects were studied to improve the biomass,lipid/carbohydrate production,removal of nitrogen and phosphorus in wastewater from microalgae immobilization culture,and reduced cost investment as an indicator.(1)Research on optimization of illumination strategy.Based on the capillary action-based immobilized culture reactor developed in the previous period,optimization of illumination strategy for immobilization of microalgae based on LED light source was carried out.The effects of different light intensity,light and dark periods,and variable LED lighting strategies on the biomass production and bio-oil accumulation of microalgae were studied.The results showed that the light intensity and light/dark cycle has significant effects on the biomass production and oil content of microalgae in different cultivation stages.At the light/dark cycle of 16:8 h,the light increment strategy was used to obtain the maximum oil yield of 66.65 gm-2.The light increment strategy provided suitable light intensity for immobilized microalgal cells in different culture stages and was beneficial to the production of oils and fats.(2)Optimization of cultivation conditions and harvesting strategy.Chlamydomonas sp.JSC4 was selected as the best biofuel production and wastewater treatment algae strain among five energy algal species.The effects of different total nitrogen concentrations,nitrogen inhibition strategies,and semi-continuous culture strategies on the growth and biofluel production of microalgae biofilm were investigated.The results showed that the biofilm production/yield and carbohydrate yield of microalgae was 268.67 gm-2(38.38 gm-2d-1)and 16.67 gm-2d-1 respectively in undiluted piggery wastewater with a total nitrogen concentration of 600 mgL-1.Meanwhile,the removal rate of total nitrogen and total phosphorus reached 96.58%(5.52 gm-2d-1)and 99.44%(0.44 gm-2d-1)respectively;Under the nitrogen inhibition strategy,The maximum content of carbohydrate was 47.89%(18.42 gm-2d-1)after two days.This strategy was beneficial to increase the yield of biofuel ethanol;semi-continuous cultivation strategy was applied in the immobilized culture,and the highest biofilm productivity was obtained at the 25%harvest rate.The biofilm yield was 37.39 gm-2d-1 and the carbohydrate yield was 15.48 gm-2d-1.Under this condition,the glucose content accounted for 79.7%to 88.9%by analyzing the carbohydrate composition,indicating that this condition was favorable to the production of fermented biofuels(such as bioethanol and biobutanol).(3)Amplified system design and outdoor cultivation optimization study.A new type of enclosed rotating disk photobioreactor was designed,and microalgae immobilization cultivation and wastewater removal of nitrogen and phosphorus were studied to obtain the technical parameters of the amplification process,and to assess its industrial application prospects.The effects of the medium volume and the time of supplemental light under outdoor culture on the biofilm yield,carbon sequestration,biofuel yield,decontamination rate,and photosynthetic efficiency were investigated.The results showed that the maximum biofilm yield was 69.77 gm-2d-1 and the carbon sequestration rate was 130.12 gm-2d-1 and the carbohydrate yield was 27.48 gm-2d-1 under a culture condition of 1.5 L and a light supply time of 7 hours.At the same time,the removal rates of total nitrogen and total phosphorus in wastewater reached 98.85%(11.86 gm-2d-1)and 98.35%(2.17 gm-2d-1)respectively.The photosynthetic efficiency of this immobilized culture system was 14.13%,which initially reached the goal of green production of energy microalgae.