| Wastewater effluents containing nutrients would lead to eutrophication in natural water bodies,whereas microalgae-based treatment could effectively allow their safe disposal.Heterotrophic cultivation of microalgae provides the possibility to achieve fast growth and efficient removal of nutrients from wastewaters.It also facilitates high biomass productivities making the large-scale process quite feasible economically.However,there is a scarce of experimental data to make the sound evaluation and model the process for practical use of this approach.On the other hand,though biological assimilation has been widely recognized as the dominant mechanism in algal nutrients removal,the metabolic process of microalgae during nutrients removal is yet to be understood.Therefore,this study aimed to improve the understanding of cultivation of microalgae for nutrients removal under heterotrophic conditions and biological mechanisms of nutrients removal by microalgae,with Chlorella sorokiniana as a model organism.Gene expression analysis of nrt2,nr,and ppk(genes in nutrients assimilation mechanism)indicated that their expressions were directly influenced by the external concentration of nitrogen and phosphorus,which indicates the correlation between the nutrients assimilation process by C.sorokiniana and its metabolic activity rather than the change of biomass growth.With the initial nutrient content of 123.6 mg N L-1 and 26.8 mg P L-1,the highest removal efficiency up to 99%could be achieved.The dependency of nutrients removal rate and efficiency on the initial concentration of nutrients was revealed through mathematical modeling.It was also shown that the accumulation trends of biomass,carbohydrate and lipid storage and of removal efficiencies of carbon,nitrogen,and phosphorus are functions of carbon,nitrogen,and phosphorus content and ratio in wastewater;and the proposed model for this process was validated.To treat low-strength wastewater,a coupled system of heterotrophic cultivation of microalgae with the membrane was applied to improve both the loading of nutrients and separation of biomass form effluent,which achieved 96.31-98.08%and 93.97-99.4%removal of total nitrogen and total phosphorus respectively and 32.3 g L-1 biomass productivity after 62 days of operation.It enhanced considerably both the wastewater treatability and the accumulation of algal biomass.In all,compared to previous reports,this study validates that heterotrophic cultivation of microalgae is a promising technology for advanced wastewater treatment,with reasonable treatment performance and profound mechanism analysis.Nonetheless,further studies are required to optimize cultivation parameters,to perform a molecular track on metabolic pathway and to reconstruct a dynamic metabolic model to appropriately correlate extracellular and intracellular phenomena within algal nutrients removal. |
