Performance And Mechanism Of Algal-bacteial Symbiosis System Based On Sequencing Bactch Biofilm-sludge Reactor For Nitrogen And Phosphorus Removal

Algal-bacterial symbiosis(ABS)system is a kind of wastewater treatment technology by combining the functions of bacteria and microalgae.Nowadays,in order to meet the technical requirements for sustainable development and energy-saving and emission-reduction,developing wastewater treatment technologies based on ABS system has aroused much attention.In this study,facing low nutrients removal efficiency,low growth rate of microalgae and unstable operation process of traditional ABS system,a novel ABS system,algae-assisted sequencing batch biofilm reactor(A-SBBR),was developed by introducing suspended carriers into ABS system to research and remit the problems above.In A-SBBR,microalgae biomass was adsorbed with activated sludge to form ABS system to improve flocs' settleability,and suspended carriers could provide a suitable environment for algae growth and improve light energy utilization efficiency of algae.Moreover,the performance of nutrients removal from wastewater was enhanced.The long-term operation stability and the removal efficiency of nutrients in A-SBBR system were studied.Results showed that: the enrichment of microalgae could be achieved in A-SBBR system with chlorophyll a(Chl-a)content of 4.80 mg/g MLVSS in biofilm phase and 0.66 mg/g MLVSS in sludge phase,which was 7.09-fold higher than that of algae-assisted sequencing batch reactor,indicating that the suspended carriers could improve both accumulation and activity of algae.Besides,the promotion of algae activity enhanced bacteria activity with the biofilm mass of 5.63 g,which was 1.94-fold higher than that of control SBBR(C-SBBR).TN and TP removal efficiencies in A-SBBR system were 69.91% and 94.78%,which increased 22.35% and 46.39% compared to that of C-SBBR system,respectively.According to TN removal pathways analysis,the fractions of assimilation by sludge,assimilation by biofilm and denitrification increased 3.67%,3.97% and 12.7% compared to that of C-SBBR system.According to TP removal pathways analysis,the fractions of assimilation by sludge,luxry uptake by biofilm increased 3.91% and 46.22% compared to that in C-SBBR system.The effects of sludge and biofilm characteristics on nutrients removal in A-SBBR system were investigated.Results showed that: microalgae cells mainly adhered to the outside of sludge flocs and biofilm,and the coverage of microalgae in biofilm was significantly higher than that of sludge.Compared with C-SBBR,the phosphorus storage capacity of EPS in A-SBBR was enhanced,and the TP contents of EPS in sludge and biofilm phases increased by 11.18% and 28.97%,respectively,at the end of aeration.Meanwhile,the introduction of algae to SBBR could modify EPS characteristics,and concentrations of LB-EPS in sludge,TB-EPS in sluge,LB-EPS in biofilm and TB-EPS in biofilm decreased by 15.20%,-16.73%,28.59% and 10.10% compared to C-SBBR.Values of SVI,zeta potential and partical size of sludge in A-SBBR were lower than that in C-SBBR,proving that the sludge settleability in A-SBBR was improved.The specific surface area of sludge flocs in A-SBBR was higher than that in C-SBBR,indicating that the contact area and adsorption rate of sludge for contaminants was improved.By analyzing the influence of microorganism community structure and activity change and key environmental factors on operation efficiency and stability in A-SBBR system,the change rule of functional microorganism of bacteria and algae community in was revealed,and the enhancement mechanism of nitrogen and phosphorus removal was illuminated.By analyzing microbial community structure,it was found that compared with C-SBBR system,the relative abundances of bacteria relating to nitrogen removal,such as Nitrospira,and bacteria relating to phosphorus removal,such as Flavobacterium and Micropruina,were all enhanced in A-SBBR system.In addition,the microalgae community also exhibited significant changes,which mainly happened in biofilm phase.The relative abundance of Desmodesmus,Scenedesmus,Chlorella and Prorocentrum and Akashiwo sanguinea,which had good nitrogen and phosphorus absorption characteristics,were significantly increased.It is worth noting that the changes and interactions of microbial community in ABS system were significantly affected by the external key environmental factors.Aeration for oxygen supply could promote bacterial respiration and then enhance bacterial activity,but the photosynthesis and activity of microalgae could be inhibited by "product inhibition" of bacteria,and then mutualism between bacteria and algae were weakened,which was not conducive to the stable operation of ABS system.The introduction of suspended carriers could promote the growth and accumulation of microalgae by accepting more optical energy and providing attachment environment to achieved microalgae enrichment,and then the proportion of microalgae in biofilm phase was improved,and finally the content of microalgae in ABS system was increase,which was conducive to the nutrients removal.In conclusion,as the A-SBBR system proposed in this study could provide a more suitable environment for the growth of both bacteria and algae,the enhanced nutrients removal performance and stable operation of ABS system were realized in a processing unit,which could not only provide some theoretical basis and technical support for the development of biological nitrogen and phosphorus removal technology,but also be of great significance for the popularization and application of ABS system.