Technology And Mechanism Of Efficient Oil Production By Serial Cultivation Of Microalgae With Landfill Leachate And Biochemical Wastewate

The gradual increase in the global population and the consequent mountain of waste has added a great deal of pressure to waste disposal.The waste disposal process produces a complex,highly toxic and turbid waste leachate,which poses a significant environmental hazard to water resources and must be treated strictly before it can be discharged.The usual waste leachate treatment methods are generally too costly and require some pretreatment before conventional treatment can be carried out.Microalgae are low-level,autotrophic aerobic plants,and they are a diverse and widely distributed group of low-level plants.Numerous studies have demonstrated that microalgae can efficiently remove pollutants from wastewater;at the same time,they can assimilate the pollutants from wastewater to promote their own growth and development,and produce beneficial biomass(starch,protein,lipids,etc.)for humans.Previous studies on microalgae treatment of waste leachate have only been conducted in single batch stage and were usually abandoned due to unsatisfactory results of wastewater treatment or Chlorella growth.In this study,three experimental groups were established:basal medium(BG11),waste leachate biochemical wastewater(BWLL)medium and waste leachate biochemical wastewater+phosphorus(BWLL+P)medium.Four successive cycles of culture were conducted to investigate the removal of pollutants and the growth of Chlorella in the BWLL medium,and finally to analyze the influencing factors and economic benefits.The specific results are as follows:(1)BWLL+P medium due to the purification of waste leachate biochemical wastewater environment by microalgae in the first cycle.It started to demonstrate good growth performance in the second cycle,obtaining a dry weight of 1.06 g/L and a growth rate of 87.06mg/L·d.Significant changes in cell division and cell elongation were observed in the BGLL+P group medium due to the stress of the growth environment summary metal ions,turbidity and other factors.The second and third cycle cultures promoted cell proliferation,which increased by approximately 190%and 140%to 6.5×10~9cells/L and 5.2×10~9cells/L,respectively.the values of Chl-a/Chl-b declined and the ratio of Caro/Chl elevated in the waste leachate wastewater medium.It indicates that the microalgae were stressed in the environment and the photosynthetic system received damage,but the defense mechanism could be initiated through its carotenoid production.Microalgae in the BWLL+P group in several subsequent incubations showed better lipid productivity(36.33,26.99 and 18.11 mg/L·d).The lipid productivity was 1.12-1.27 and 1.95-2.36 times higher than those in the BG11 and BWLL groups,respectively.In terms of fatty acid fraction,Chlorella vulgaris cultured in different batches performed better in BWLL+P medium than in BG11 medium,exhibiting a higher proportion of saturated fatty acids(SFA).As for the biodiesel properties,the microalgal biodiesel in the recovered BG11,BWLL and BWLL+P media met the European(EN)and American Society for Testing and Materials(ASTM)quality standards.(2)At the end of the fourth incubation,99%of the nitrogen and phosphorus were consumed,meeting the"Class A"standard of China’s urban wastewater treatment plant pollutant discharge standards.The abundant carbon source in the wastewater compensated for the poor growth of microalgae due to environmental stress and allowed a satisfactory biomass production of Chlorella in BWLL+P.The variation of TOC content in leachate wastewater always showed a trend of decreasing and then increasing,and the TOC content in wastewater also showed a gradual decrease in the end.In contrast to the trend of gradually increasing turbidity in BG11 medium,the turbidity in BWLL+P medium gradually decreased with the increase of cyclic batches.And the turbidity was reduced by nearly half(40NTU)in the first cycling culture,which laid a solid foundation for the healthy growth of subsequent microalgae in the cycling culture.The high concentration of metal ions in the environment can seriously affect the growth of microalgae and cause metal stress.Metal ions were effectively removed from the BWLL+P medium after the first cyclic culture only(Ca,Mg,K and Na consumption reached 110 mg/L,47 mg/L,447 mg/L and 1197 mg/L,respectively).And heavy metals were almost completely purified and consumed with the increase of culture batches.(3)The way in which stressors such as extracellular polymeric substances(EPS)inhibition,metal ions,high turbidity and photoinhibition affect the growth of microalgae is analysed in terms of their impact on growth and cost effectiveness.The economic benefits of the’whole chain’culture model were compared with the conventional pretreatment model.The new treatment mode can significantly reduce the cost of cultivating microalgae.Compared to other methods,the cost of cultivating microalgae is significantly lower,with a cost saving of99%compared to the BG11 culture model of the same group.In summary,the new treatment method of"microalgae total chain"provides a new method for microalgae cultivation and removal of pollutants from leachate biochemical wastewater,while reducing costs.