Agar-immobilized Chlorella Sacchrarophila For Long-term Preservation At Room Temperature And Its Study For Nitrogen And Phosphorus Removals

Microalga has been widely used in many traditional fields such as human health food,aquaculture,environmental protection,and bioenergy.Nitrogen and phosphorus are indispensable elements in living systems.With the expansion of population and the development of industry and agriculture,nitrogen and phosphorus are overused in many industrial and agricultural production systems,which makes the eutrophication increasingly serious.Microalga absorb a large amount of N and P in the growth,which shows great application potential in the treatment of eutrophication.Recently,the combination of microalga culture and wastewater treatment has received widespread attention from more and more researchers,ant it has been considered a sustainable and cost-effective strategy with high application value.However,how to effectively preserve a large number of active microalga is a challenge in the industrial application of microalga.At present,the research on microalga preservation methods is mostly for conservation,such as traditional subculture,ultra-low temperature preservation,dry preservation,concentrated solution preservation,etc.There are relatively few studies on the industrialization of a large number of microalga preservation technologies.Therefore,seeking a simple,cost-effective method for long-term storage of a large number of active microalga has important practical significance for the industrial application of microalga.In this study,the microalgae Chlorella sacchrarophila was immobilized in agar,sealed in a transparent bag,and stored at room temperature.The number of viable cells in the agar was periodically measured,the morphological changes of the microalgae were observed using a scanning electron microscope.Microalgae are regularly inoculated into fresh medium to reculture.In addition,agar-immobilized microalga was used for nutrient removal in wastewater treatment,and test its reuse ability and long-term storage and treatment effect.Finally,agar-immobilized microalga was combined with a photoreactor to test the real wastewater treatment.The results of agar-immobilized microalgae stored at room temperature showed that after200 days of storage,the survival rate of microalgae was 47-52%,which was significantly higher than that of liquid storage.Initial algae and agar concentrations have little effect on preservation effect.Carbon and carbon-rich?Na₂CO₃?,nitrogen and phosphorus deficient and nitrogen and phosphorus?Na NO₃,K₂HPO₄?restrictions are not conducive to the preservation of microalgae.After 6 months of storage,the highest survival rate under low light(8?mol s^-1m^-2)was 63.3%.The scanning electron microscopy observation showed that the agar-immobilized algal cells remained spherical,and were no significant difference in the cell morphology before storage.Moreover,The re-culture of the algal after storage has the same growth tendency as the initial storage,and can restore activity in a short-term culture.Agar-immobilized microalgae was used for nitrogen and phosphorus removals in wastewater treatment.The removal efficiencies of NH₄⁺-N and PO₄^3--P are 97%-98%and 92%-94%,respectively.High ammonia nitrogen concentration may cause toxicity to C.sacchrarophila.During eight reuse treatments,the removal rates of NH₄⁺-N and PO₄^3--P were94%-99%and 66%-91%,respectively.After treatment for 432 h,the dry cell weight of agar-immobilized algae reached 3.2 g/L.After 60 days of storage,the immobilized algae still maintained a removal efficiencies of more than 80%of NH₄⁺-N and PO₄^3--P.The NH₄⁺-N and PO₄^3--P removal efficiencies of the improved columnar photoreactors were 97.3%and 96.1%,respectively,compared with the initial operation of the reactor,the removal efficiencies were greatly improved.Proper HRT is essential for the operation of the reactor system to remove nutrients?nitrogen and phosphorus?.After the reactor runs stably,the wastewater from the secondary treatment unit of WWTP is used as the test wastewater.After treatment,the concentrations of NH₄⁺-N,PO₄^3--P and COD were 0.5 mg/L,0.2 mg/L,and 74 mg/L,with removal efficiencies of 98%,95%,and 71%respectively.Ammonia nitrogen and phosphate reach Level A emission standards,and COD reaches Level 2 emission standards.