Research On The Treatment Of Domestic Sewage, Secondary Effluent And Resource Utilization Of Newly Isolated Algae Strain

With the rapid development of social economy and the continuous increase of various pollutant emissions,the pollution carrying capacity of the water environment is facing serious challenges.Therefore,effective treatment of wastewater is crucial for improving the water body and ecological environment.Microalgae has the advantages of fast growth rate,high Photosynthetic efficiency,energy saving,etc.It can remove various pollutants in wastewater and make use of them as resources.Using microalgae to treat wastewater is a dual benefit method that can achieve sustainable development of resources.In this study,algae species were isolated,screened,and identified from the water and mud of Yingxue Lake,Shandong Jianzhu University,for treatment of domestic sewage and secondary effluent.Culture conditions were optimized by changing the initial p H value of domestic sewage and secondary effluent,and the initial OD of microalgae.Then,using summer domestic sewage as an experimental water sample,further optimize the carbon and nitrogen conditions of microalgae for domestic treatment,and analyze the fatty acid content of microalgae oil to explore the possibility of using microalgae to produce biodiesel.The main research results are as follows:（1）The microalgae in lake water and lake mud samples of Shandong Jianzhu University were isolated and purified by using dilute reverse dish and plate separation technology,and four kinds of microalgae were preliminarily isolated by morphological and molecular biology methods.The four microalgae isolated after gene sequence identification were isolated with Desmodesmus sp.XFLZ928.6 and Coelastrella rubescens var.oocystiformis isolate FACHB-2297),Scotiellopsis reticulata strain CCALA 474,and Chlorella sorokiniana strain ACSSI 364 are close relatives.They were named SNN1,SNN2,SNN4 and SNS1.After 12 days of culture,it was found that SNN1 had the highest removal rates of COD,NH₄⁺-N and TN,which were 41.04%,99.79%and 95.61%in domestic sewage,respectively.In secondary effluent,51.13%,98.62%and 82.26%,respectively.There was no significant difference in TP removal ability among the four algal strains.In terms of biomass,the light absorption values of SNN1 in domestic sewage and secondary effluent were 2.514 and 1.803,respectively,which was the best among all the algal strains.Therefore,algal strain SNN1 was selected as the subsequent experimental algae species.（2）The effects of initial p H,initial OD,and other conditions on the accumulation of biomass and oil in the treatment of domestic wastewater,secondary effluent,and biomass by algae strain SNN1 were studied.The results showed that the optimal conditions for treating domestic sewage with algae strain SNN1 were an initial p H of9.0 and an initial inoculation amount of microalgae OD₆₈₀ of 0.6.Under these conditions,the removal rates of COD,NH₄⁺-N,TN,and TP from domestic sewage by the algae strain SNN1 were 47.98%,99.58%,96.88%,and 97.13%,respectively,with a biomass of 1.34 g/L and a chlorophyll a content of 9.22 mg/L.The optimal culture conditions for microalgae oil production are:initial p H 9.0,initial inoculation amount OD₆₈₀ 0.1,oil content and oil yield 29.54%and 291.33 mg/L,respectively.The optimal p H value of algae strain SNN1 for treating domestic secondary effluent is 9.0,and the optimal initial inoculation amount(OD₆₈₀)of algae strain is 0.4.After 12 days of treatment,the removal rates of pollutants from secondary effluent by SNN1 are52.69%（COD）,99.99%（NH₄⁺-N）,89.09%（TN）,and 94.64%（TP）,respectively.In addition,it also produces 0.71 g/L dry weight,5.30 mg/L chlorophyll a,and 168.33mg/L lipids.Through comprehensive analysis and comparison,the biomass and oil production of the algae strain SNN1 are the highest in domestic sewage,making it the target algae strain for subsequent experiments.（3）Due to seasonal changes,the four microalgae strains obtained from the initial screening were cultured in summer domestic sewage,and the biomass,oil content,and oil yield of the algae strain SNN1 were the highest.Add sodium nitrate to summer domestic sewage and set different initial concentrations（0 g/L,0.05 g/L,0.1 g/L,0.2g/L,and 0.5 g/L,respectively）for research.The results showed that under the initial concentration of 0.2 g/L sodium nitrate,the growth status of the algae strain SNN1 in domestic sewage was relatively good,with a maximum biomass of 1.22 g/L.The removal rates of COD,NH₄⁺-N,TN,and TP in domestic sewage were 64.18%,94.89%,95.54%,and 97.85%,respectively.The oil production reached 275.17 mg/L.As the nitrogen concentration increased,the oil content decreased.（4）Add glucose to summer domestic sewage with an initial Na NO3concentration of 0.5 g/L and set different initial concentrations（0 g/L,0.25 g/L,0.5g/L,1 g/L,and 2 g/L）for experimental research.The experimental results indicate that the growth status of the algae strain SNN1 in domestic sewage is relatively good under the initial concentration of 2 g/L glucose.After 12 days of cultivation,the biomass was 2.40 g/L,and the removal rates of COD,NH4+-N,TN,and TP in domestic sewage were 95.72%,98.30%,93.60%,and 87.64%,respectively.Under these conditions,the oil production reached 281.00 mg/L,and with the increase of glucose concentration,the oil content decreased.The fatty acid analysis of the algae strain SNN1 indicates that it may be a natural microalgae species suitable for producing biodiesel.In summary,this study lays the foundation for the treatment and resource utilization of domestic sewage and secondary effluent,and provides pilot scale conditions for the large-scale production of microalgae oil.