Effect Of Carbon Source On Microalgae-based Reject Water Treatment Performance And The Inherent Self-flocculation Mechanism Investigation

Reject water originates from the anaerobic digestion process of sludge.Due to the characteristics of high ammonia nitrogen and low carbon to nitrogen ratio,it is difficult to rely on traditional sewage treatment technology for purification,which has become an urgent water problem.At the same time,microalgae,as“the third generation biofuel”,usually need to grow in a commercial medium rich in nitrogen and phosphorus,but the waste of freshwater resources and the consumption of nutrient elements have become the bottleneck of the large-scale microalgae resources.In view of the above two problems,coupling the treatment of reject water with the cultivation of microalgae is considered.However,only relying on reject water as a growth medium for microalgae will cause problems such as low biomass concentration and unsatisfactory pollutant removal efficiency.In this paper,based on this coupling technology,carbon sources was added into reject water,so as to achieve the improvement of microalgae biomass and lipid yield as well as the improvement of removal efficiency of pollutants in reject water.At the same time,the exact mechanism of microalgae self-flocculation difference induced by carbon source environmental changes was explored,which is aimed at harvesting microalgae in a low-energy and low-cost way,providing a theoretical basis for solid-liquid separation and subsequent harvesting of microalgae suspension.First,the effect of carbon source on the performance of microalgae-based reject water treatment sludge and the growth characteristics of microalgae were studied.In terms of microalgae culture,the result found that compared with the culture group supplemented with sucrose and sodium bicarbonate,the biomass of glucose group and sodium acetate group increased significantly,and the dry weight of algae cells reached 369.65±1.37 mg/L and228.74±13.23 mg/L,respectively,which were 4.7 times and 2.5 times higher than in BG11medium.Meanwhile,the lipid yield of these two groups of microalgae had also been greatly improved,reaching 3.5 times and 2.3 times that of original reject water,and the oxidation stability of biodiesel was optimized.In terms of purification of reject water,the glucose group and the sodium acetate group achieved all the removal of orthophosphate,and the ammonia nitrogen removal efficiency reached 35.97%and 35.92%,respectively,while the removal efficiency of ammonia nitrogen and orthophosphate in original reject water group were only14.32%and 54.07%.Therefore,it is believed that glucose and sodium acetate are suitable to be supplemented into reject water to assist the growth of C.sorokiniana.Based on the above result,the mechanism of self-flocculation considering microalgae cultured with glucose and sodium acetate was explored.After 6-days culture period,the 12 h flocculation efficiency of C.sorokiniana in sodium acetate group and glucose group were 66.42%and 42.87%,respectively.The results showed that this difference was mainly due to different composition and characteristics of EPS on microalgae.Compared with glucose group,the total amount of EPS in the sodium acetate group was relatively high(1.5 times)and more hydrophobic sites and looser protein secondary structure were found on the EPS.Proteomics showed that the high abundance of algal cell-adhesion molecules and metal-ion-binding protein in sodium acetate group EPS,which possessed v WF domain,FAS1 domain,Ig-like fold,LRR domain,Nod B,etc.,had positive significance for increasing cell viscosity and binding to metal ions,strengthening the bridging and flocculation effect between microalgae cells.In addition,the eDLVO theoretical model verified the difference in total interaction energy between microalgae cells caused by the difference in Lewis acid-base interactions.The glucose group had a high energy barrier value of 10.98×10~3 K_BT,which made flocculation difficult,while the sodium acetate group had no energy barrier,so it had good flocculation and sedimentation ability.The study of this mechanism provides path analysis for the high-efficiency separation of microalgae suspensions.