Study On The Formation Mechanism Of Microalgae Biofilm And Treatment Of Soy Sauce Wastewater

Microalgae wastewater treatment technology is a biological water purification technology developed in recent decades.At present,there are many problems in the practical application of traditional suspended microalgae wastewater treatment technology.The new wastewater treatment technology represented by microalgae biofilm technology realizes the efficient collection of algal biomass by scraping,which solves the problems of microalgae cell separation from water and the difficulty of pollutants treatment.However,at present,the domestic research on biofilm culture and wastewater treatment are still in the infancy,and the film-forming mechanism of algal biofilm is not clear.This study was carried out from the attachment culture of microalgae,through screening the best algal strains for biofilm forming,optimizing culture conditions,developing rapid biofilm forming technology and exploring its mechanism,exploring the biofilm formation of algae-bacteria based on soy sauce wastewater,and the treatment of soy sauce wastewater by small microalgae biofilm device.The main conclusions are as follows:（1）The growth trend of 10 selected algal strains was basically the same,and there was little difference in maximum biomass;The protein content of NCU-7 and NCU-12was relatively high,while the lipid content was relatively low,while the overall difference of lipid content was not obvious;NCU-12 had the best adhesion effect among the selected algal strains,and its adhesion biomass accounts for 87%;In conclusion,NCU-12 is the best algal strain for biofilm forming,which was identified as Chlorella pyrenoidosa.NCU-12 had the best adhesion effect when the initial p H was acidic,and the adhesion biomass and adhesion ratio reached 53 mg and 94%respectively;When the rotating speed was 90 rpm,the adhesion biomass and adhesion ratio of NCU-12 were the highest,reaching 54 mg and 91.3%respectively;NCU-12can grow normally at 25～35℃,and the temperature has little effect on its adhesion effect in this range.（2）When the culture p H of NCU-12 was 3,the biofilm forming effect was the best,the 24 h adhesion ratio reached 74%,and its adhesion biomass was as high as 16.7g/m²;Light intensity has little effect on the early stage of rapid biofilm forming at low p H,but has a greater effect on the later stage of biofilm thickening;There was no significant difference in the accumulation of adhesion biomass of NCU-12 at suitable p H（7,8 and 9）,which were all at the same level,about 48 mg,and the biomass per unit area was 49 g/m²,which was significantly higher than that of p H 3 group and the control group;There was no significant difference in the content of extracellular polysaccharide of NCU-12 at different p H,but the content of extracellular protein was the highest at p H 3 with the best adhesion effect,which was 4.85 mg/g.Extracellular protein played a decisive role in the rapid biofilm formation of NCU-12,and the Zeta potential of NCU-12 increased with the decrease of p H;A total of 17 up-regulated proteins were found by label-free quantitative proteomic analysis.The overexpression of the ribosomal metabolic pathway in algal cells was induced at p H 3,resulting in the production of a large number of ribosomal proteins,which synthesized a large number of different kinds of enzymes through the translation process.Then,these excess ribosomal proteins and enzymes are secreted out of cells,resulting in higher PNs content and rapid formation of biofilm;The results of the rapid biofilm forming experiment of small microalgae biofilm device showed that the adhesion biomass of p H 3 group was significantly higher than that of the control group（without adjusting p H）,and the 24 h adhesion biomass was 27.8 g/m² and 1.2 g/m² respectively.At 144hours,the adhesion biomass of p H 3 group was as high as 97.2 g/m²,while that of the control group was just 33.6 g/m².（3）The biomass of biofilm in RSW is the highest,which is 100 g/m²,followed by SSW,which is about 69 g/m²;The total amount of EPS in RSW group was the highest,with the total amount of EPS of 1.4 mg,followed by SSW,blank and BG11,with the contents of 1.07,1.06 and 0.7 mg respectively;The removal rates of N and P in RSW and SSW groups were almost the same,while the blank group was lower,but the removal rate of COD was similar to that in RSW group and significantly higher than that in SSW group;Compared with soy sauce wastewater,in,the relative abundance of Proteobacteria of algal biofilm cultured in RSW increased to 77.7%,while the relative abundance of other bacteria decreased;At the genus level,the dominant bacteria changed before and after biofilm forming.（4）The highest biomass productivity and specific growth rate of biofilm were get when the harvest time was 5 days,which were 2 g/m²/d and 0.57 d^-1 respectively;When HRT was 2 days,the lowest COD concentration of algal biofilm system was 220 mg/L after operating 3 HRT;The removal efficiency of nitrogen by microalgae biofilm was the best under all HRT conditions.The maximum removal rate reached 91.5%and 93%when HRT was 8 days,and the removal rates of other systems were less than 80%.The removal of phosphorus in wastewater is similar to that of nitrogen.Microalgae biofilm is absolutely dominant in the whole experimental process.When HRT was 8 days,the removal rate was as high as 98%;The highest C content in the initial adhesion biomass was 47.5%,but there was no significant difference in biomass C content under different HRT conditions;When HRT was 5 days and 2 days,the N content in biomass was almost the same,about 8.7%,the protein content was as high as 57%,the total lipid content in the initial adhesion biomass was the highest of 24.3%,and the ash content reached the highest of 9.5%when HRT was 2 days.The trend of carbohydrates increased first and then decreased with the shortening of HRT.Under different HRT,the content of C16-C18 in FAME reached 80.88%,76.01%,79.64%and 80.45%respectively.The high content of short carbon chain fatty acids in FAME indicates that the harvested algal biomass is an ideal raw material for biodiesel production.As the most promising wastewater treatment method,microalgae wastewater recycling still has some defects.On this basis,the emerging concept of microalgae biofilm wastewater treatment has more economic and environmental benefits.