Study On Wastewater Treatment And Flocculation Mechanism Of Induced Algae-bacteria Symbiotic Syste

Algae-bacterial symbiosis system can realize deep treatment of wastewater,low operating cost,especially microalgae can capture CO₂ in the air,absorb nitrogen and phosphorus in wastewater to synthesize their own cells,and provide oxygen for aerobic microorganisms,which is a promising wastewater treatment technology.However,the microalgae cells are tiny（20-50um）and their specific gravity is close to that of water,making it difficult to separate the algae cells from the water body and losing the microalgae,resulting in the unstable effect of algae-bacteria symbiosis system and low wastewater treatment capacity.Therefore,obtaining microalgae with strong self-flocculation ability becomes the key to solve this problem.Atmospheric pressure room temperature plasma technology（ARTP）has been proved to have the characteristics of fast mutagenesis,high mutation efficiency and genetic stability in the process of microalgae mutagenesis.Therefore,in this study,the ARTP technology was applied to the mutagenesis of microalgae in order to obtain algal strains with high self-flocculation ability and to solve the problem of microalgal loss in algae-bacterial symbiosis.In this study,four mutant algal strains with high self-flocculation ability and high nitrogen and phosphorus removal efficiency were obtained from wild Chlorella species taken from local water bodies in Jinan after isolation and purification and（ARTP）mutagenesis.The mutant algae strains were used as the main body to build an algae-bacteria symbiosis system with activated sludge to treat wastewater,and the flocculation and wastewater treatment performance of the mutagenic algae-bacteria symbiosis system were investigated.The main research results are as follows:（1）Four highly efficient self-flocculating mutant algal strains were obtained by mutagenesis and named SDJZM-1,SDJZM-2,SDJZM-3 and SDJZM-4.Among them,SDJZM-1 showed the highest self-flocculation efficiency of 92.34%and SDJZM-2 showed the lowest self-flocculation efficiency,which was still improved by 78.85%compared with the original type.In addition,the four mutants showed a high ability to absorb nutrients from the wastewater.Compared with the wild-type strain,the mutant strain SDJZM-4 increased the removal of total nitrogen,total phosphorus and ammonia nitrogen by 29.39%,22.72%and 14.82%,respectively.（2）Based on this,the mutagenic algae-bacterial symbiosis system was constructed,and the Chlorella vulgaris mutagenized by ARTP had stronger flocculation and sedimentation ability and better synergistic effect when combined with activated sludge,with 99.7%spontaneous flocculation and sedimentation efficiency at the end of growth.In addition,an in-depth study of algal-bacterial symbiosis and its extracellular polymers in algae-bacterial systems revealed that algal-bacterial communities were more stable than single algal systems for the formation of extracellular polymers.The main mechanisms of flocculation and sedimentation of algal-bacterial aggregates include charge neutralization,adsorption and bridging,and netting and sweeping.Among the extracellular polymers（EPS）subfractions,the extracellular polysaccharide（PS）of soluble extracellular polymers（S-EPS）can enhance the adhesion properties of microbial cell surfaces to a certain extent,but excessive secretion can lead to loosening of floc structure and difficult sedimentation.High levels of loosely bound extracellular polymers（LB-EPS）can deteriorate the floc structure and affect flocculation and sedimentation.Tightly bound extracellular polymers（TB-EPS）plays a major role in the flocculation process of algal-bacterial aggregates,and tryptophan and tyrosine proteins are the key extracellular aggregates of microalgae and activated sludge.The higher PN/PS ratio in TB-EPS is also more conducive to the enhancement of algal-bacterial aggregate particle settling ability.（3）The algae-bacterial symbiotic system constructed with the mutant strain of Chlorella vulgaris and activated sludge was named group B,the algae-bacterial symbiotic system constructed with the wild strain of Chlorella vulgaris and activated sludge was named group C,and the pure algae system（group A）with the mutant strain of Chlorella vulgaris was used as the control group for wastewater treatment.Among all groups of systems,the mutagenic algae-bacterial symbiosis system showed the strongest wastewater treatment performance.After 24 h of system operation,the concentration of all pollutants in the system of group B had reached the national urban wastewater treatment plant pollutant discharge level A treatment standard,at which time the removal rates of COD and TN in the system were 35.48%and 14.06%higher than those in the pure algae system of group A.The removal rates of COD,TN and TP were 5.09%,2.05%and 28.77%higher than those in group C,respectively.It showed that the mutant strain of Chlorella was much better than the wild strain in removing phosphorus from wastewater,with stronger biological activity,faster cell metabolism and higher biomass production capacity.In contrast,for COD and nitrogen,the removal relied more on the synergistic effect of the algae-bacterial system.