Construction Of A Fouling-resistant Microalgae-bacteria Biofilm System Based On EVA-based Hot-melt Adhesive Biofilm Carrier

The anaerobic digestate from agricultural product processing by-products has become an environmental issue that cannot be ignored in the agricultural and ecological systems.The microalgae-bacteria symbiotic wastewater treatment technology has comprehensive advantages such as high yield,efficient use of light energy,environmental friendliness,resource conservation,and sustainability.However,traditional microalgae-bacteria symbiotic systems are mostly suspended culture,which has high energy consumption,low efficiency,and difficulty in biomass harvesting.Microalgae-bacteria biofilm technology has been developed based on the microalgae-bacteria symbiotic technology,with more concentrated and easily harvested biomass,low space requirements,and high wastewater treatment efficiency,which can effectively overcome the bottleneck of traditional microalgae-bacteria symbiotic systems.However,this technology still faces some challenges in engineering applications for the treatment of full-strength anaerobic digestate.On the one hand,conventional microalgae and bacteria cannot tolerate high concentrations of pollutants in full-strength anaerobic digestate,and thus,they usually need to be diluted to reduce their inhibitory effect on microorganisms.However,such operations increase water consumption and the cost of wastewater treatment.Additionally,the carrier material is one of the key aspects of the biofilm system,which supports cell adhesion and film formation.However,traditional biofilm carrier materials,such as metallic materials,fabrics,and membrane materials,have poor resistance to mechanical damage,are not durable,and are expensive,making large-scale application difficult.This study provides theoretical basis and technical support for the application of microalgae-bacteria biofilm technology in practical wastewater treatment engineering.The research content of this paper mainly includes the following aspects:（1）In order to enhance the tolerance of microorganisms to full-strength anaerobic digestate,indigenous microorganisms were artificially gradient-cultivated to obtain three indigenous microalgal strains and five bacterial strains that can tolerate high concentrations of pollutants(1400 mg·L^-1NH₄⁺-N,40 mg·L^-1Cu²⁺,40 mg·L^-1Zn²⁺,and 30 mg·L^-1tetracycline).（2）Four microalgal strains and six bacterial strains were artificially constructed into microalgal-bacterial symbiotic systems,including three indigenous microalgal strains and five bacterial strains that can tolerate high concentrations of pollutants vobtained through gradient cultivation,as well as one strain of microalgae and one strain of bacteria provided by the laboratory.By examining the biomass yield,photosynthetic efficiency of microalgae,and purification effect of anaerobic digestate in each microalgal-bacterial symbiotic system,three advantageous indigenous microalgal-bacterial co-cultivation combinations that synergistically enhance the purification of anaerobic digestate were selected（Chlorella sp.GZQ001-Lysinibacillus sp.SJX05,Chlamydomonas sp.LRM021-Shinella sp.YHB03,Chlorella vulgaris FACHB-8-Shinella sp.YHB03）.Results showed that compared with the microalgae-bacteria symbiotic systems constructed from commercially available microalgae and bacteria（Chlorella vulgaris FACHB-8-Bacillus subtilis1.821）,the indigenous microalgae-bacteria symbiotic systems obtained by gradient domestication were more favorable for improving biomass productivity and the recovery of nitrogen and phosphorus resources.The biomass productivity of the Chlorella sp.-Lysinibacillus sp.symbiotic system reached 113.25 mg·L^-1·d^-1,and the removal rates of total nitrogen,ammonia nitrogen,and total phosphorus reached82.07%,83.17%,and 76.60%,respectively.（3）The two selected indigenous microalgae-bacteria symbiotic systems were used to optimize symbiotic conditions and study their synergistic mechanisms in purifying anaerobic digestate.The enhancement rate was innovatively introduced to quantify the synergistic strengthening intensity of microalgae and bacteria co-cultivation relative to single cultivation,and the correlation between parameters related to microbial growth and parameters related to wastewater removal efficiency was studied using principal component analysis.The symbiotic system of microalgae and bacteria enhances the tolerance of pollutants in wastewater and reduces the rate of cell apoptosis.While microorganisms absorb and utilize nutrients in wastewater,they can also reduce the turbidity of wastewater.This process can promote microalgae to capture light energy for photosynthesis,thereby directly or indirectly promoting the exchange of metabolites between microalgae and bacteria,and further enhancing the purification of anaerobic digestate.The symbiotic system of Chlorella sp.GZQ001-Lysinibacillus sp.SJX05,with a 50:1 inoculation ratio,increased the biomass productivity of Chlorella sp.by 37.10%compared with single cultivation,promoted the photosynthetic activity of microalgae（Fv/Fm value of 0.72）,and increased the removal rates of TOC and ammonia nitrogen by 17.12%and 35.72%,respectively,with corresponding enhancement rates of 53.62%and 59.47%.For the Chlamydomonas sp.-Shinella sp.symbiotic system,the biomass concentration was increased by 118.75%compared with single cultivation,and the highest Fv/Fm value of microalgae reached 0.802.The symbiotic system effectively removed 97.72%of ammonia nitrogen from anaerobic digestate,with a corresponding enhancement rate of 70.56%.Moreover,the chemical composition of biomass was analyzed,and it was found that the yield of high value-added products was promoted.The highest FAME productivity in the two symbiotic systems was achieved with a 50:1 inoculation ratio,reaching 2-3 times the biomass productivity of each microalga cultivated alone.（4）In order to achieve a low-cost and durable biological membrane carrier,an innovative EVA-based hot melt adhesive was developed to immobilize a natural wood fiber microalgae-bacteria biofilm carrier.The construction conditions of the EVA-based hot melt adhesive carrier（amount and pressure of the hot melt adhesive）and its performance in wastewater treatment were investigated.The study found that with an increase in the amount of hot-melt adhesive,the adhesion quality of wood fiber particles on the glass substrate gradually increased and tended to stabilize,while the scratch rate showed an opposite trend.The optimal amount of hot-melt adhesive was 310.23 g·m^-2,and the hot-melt adhesive pressure was 150 N.At this point,the adhesion amount of 30-40 mesh bamboo scraps was 162.41 g·m^-2,with a scratch rate of 8.31%,In addition,the natural roughness of the wood fiber surface can increase the yield and adhesion strength of the biofilm.The biomass productivity was 44.35g·m^-2,the adhesion amount of 30-40 mesh pine sawdust was 94.19 g·m^-2,with a scratch rate of 3.30%.The biomass on the 30-40 mesh bamboo scraps and pine sawdust carriers was 44.35 g·m^-2and 28.03 g·m^-2,respectively,significantly higher than the control group PMMA(20.85 g·m^-2).The removal efficiency（30-40 mesh bamboo scraps）of TN,TC,and TP by the bacterial-algal biofilm compared to the control group PMMA was increased by 109.50%,23.75%,and 38.38%,respectively.