Study On The Design Of Shallow Algal Bed Reactor And Effectiveness For Nitrogen And Phosphorus Removal In Tail Water Of Municipal Trement Plants

As water eutrophication,the energy crisis and global warming are the biggest challenges nowadays,microalgae have become important research objects in the field of advanced wastewater treatment due to their high effluent purification efficiency,resourcefulness of oil,and high photosynthetic capacity.Microalgae are able to use nutrients such as nitrogen,phosphorus and organic carbon in wastewater,and at the same time accumulate their own biomass and transform it into an economically valuable biomass energy source.In addition,microalgae absorb CO2 during photosynthesis,which has a positive impact on the mitigation of the greenhouse effect and the promotion of carbon neutrality goals.Thus,microalgae advanced treatment of wastewater is a sustainable green treatment technology.Algae species,environmental conditions and reaction devices are extremely important for the efficient treatment of wastewater in microalgae advanced treatment technology.In view of the above issues,microalgae species with good water treatment effects was selected to compare and analyze the decontamination efficiency;further the culture conditions in depth to promote the removal of nitrogen and phosphorus from wastewater was optimized;a shallow algal bed reactor for the advanced treatment of municipal domestic wastewater was developed,which aimed to provide a basis for the application of advanced microalgae wastewater treatment technology.The main research of the thesis included the following four aspects:In this study,Chlorella vulgaris,Chlorella proteinacea,Scenedesmus obliquus and Anabenaflos aquae were chosed to investigate the denitrification and phosphorus removal effect and growth conditions in the simulated secondary effluent.The results showed that Chlorella vulgaris had the highest organic carbon,phosphorus and ammonia nitrogen,and removal efficiency.The optimal effluent of COD,TP and NH3N concentration were 11 mg/L,0.34 mg/L and 4.1 mg/L respectively,which all met the first grade A standards in "Municipal wastewater treatment plants pollutant discharge standards "(GB18918-2002).The biomass of all algae species increased,which indicated that the removal of pollutants was by absorption of algae species rather than adsorption.On this basis,the microalgae bioreactor operating conditions were optimized.The results showed that the growth of Chlorella vulgaris was optimized at 26～30℃,and the best treatment effect was at 30℃;when the light intensity was 4000～8000 lux,growth of Chlorella vulgaris was optimized,and the best treatment effect was at 6000 lux;nitrogen and phosphorus removal efficiency was positively correlated with the light time,while COD was on the contrary,and the cell biomass accumulated the most when the light-dark ratio was 14 h:10 h;The best effect of nitrogen and phosphorus removal was achieved when the CO2 volume fraction was 1%,and excessive CO2 concentration would inhibit algal cells growth;the initial inoculum density was 1 × 106～1 × 107 cells/mL.A growth kinetic model of Chlorella vulgaris was constructed to describe the relationship between temperature,light intensity and CO2 concentration and the growth rate of Chlorella vulgaris,and the model fitted well.In this study,a shallow algal bed reactor was designed and developed,mainly to explore the influence of water depth and algal cell density on light attenuation,and the light attenuation model I=I0 exp[-(0.89610D680+0.19025)·L]was obtained,it was determined that the critical water depth of the reactor is about 25 cm when the incident light intensity is 100000 lux.At the same time,combining the advantages of open and closed microalgae bioreactor,the circulation system,light system,temperature control system and algae water separation system were designed,so that the reactor operating conditions can be controlled and suitable process conditions can be achieved.Among them,the circulation system allowed CO2 gas to be introduced from the bottom of the reactor.On the one hand,it increased the inorganic carbon source for microalgae photosynthesis,and on the other hand,it could play the role of mixing the algae solution in vertical direction,so that the algae cells can have more uniform contact with light and nutrients,and the energy consumption of the additional mixing devices was reduced.The reactor was operated outdoor,mainly using natural light,supplemented by artificial light system.Thus,the energy consumption of shallow algal bed reactor was lower than that of closed bioreactor.In this study,a small-scale dynamic test was carried out to explore the hydraulic retention time on nitrogen and phosphorus removal efficiency.The results showed that Chlorella vulgaris was most effective in treating the secondary effluent of the actual wastewater plant at a hydraulic retention time of 1～1.5 d.When the reactor was operated to a steady state,the concentrations of TP,NH3-N and TN in the effluent were maintained at 0.1～0.2 mg/L,3.5～4.2 mg/L and 5.1～6.5 mg/L,respectively,and the concentration of COD were maintained at 10～20 mg/L.The concentrations of TP and COD in the effluent have met Ⅳ and Ⅱ grade standards in the "Environmental Quality Standard for Surface Water"(GB 3838-2002);The concentrations of NH3-N and TN have met the first grade A standards.At the same time,the oil content of algae cells in the stabilization period was examined and it was found that the oil content of algae cells in the effluent was 17.8～19.2%,which was higher than that of algal cells in BG11 medium.The results show that the algal cells harvested from the deep treatment of the shallow algal bed reactor have a great potential for resource utilization.