Research On Phosphorus Removal Effect Of Constructed Wetland Based On Biochar Substrate

With the advancement of urban construction in our country,the number of sewage treatment plants has been increasing,and the tail water of sewage treatment plants has become the main new input channel of nutrients in the Taihu Lake Basin.Phosphorus is a key nutrient pollutant in the water body,and it is one of the limiting elements that cause lake eutrophication.It is necessary to carry out advanced treatment of the tail water of the sewage treatment plant to further reduce the phosphorus in the tail water.Constructed wetland is a green sewage treatment technology with landscape value.It achieves the removal of nutrients in sewage by simulating the structure of natural wetland.However,traditional constructed wetlands have limited phosphorus removal rate and difficult to recycle.However,the wetland matrix type and phosphorus removal effect Closely related.In this thesis,the modified biochar material prepared from the plant calamus harvested in the wetland is used as the wetland substrate,which not only strengthens the absorption and removal of phosphorus,but also realizes the utilization of wetland plants as a resource.The main research contents are:biochar material preparation,characterization and analysis;biochar adsorbent to phosphorus adsorption characteristics experiment;biochar matrix constructed wetland to phosphorus removal effect research.The main research results are as follows:（1）FeSO₄·7H₂O is used as modifier to prepare iron-modified calamus biochar（MBC）after NaOH pretreatment.The MBC is prepared at a pyrolysis temperature of400℃and a modifier concentration of 1.0mol/L.Biochar has the best phosphorus removal effect by adsorption,with a removal rate of 98.48%and an adsorption capacity of 24.62mg/g.Through element composition（C、H、O、N、S）,BET,scanning electron microscopy SEM/EDS,Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）and other characterization methods,analyze the modification of biochar materials The results show that the hydrophilicity and polarity of the biochar material are improved after modification,and the total specific surface area,micropore specific surface area and micropore volume are greatly increased;the material surface is successfully loaded Iron oxide compounds mainly exist in the form of Fe3O4 and-Fe2O3 crystals,and there are a large number of functional groups such as hydroxyl groups on the surface and between layers,which change the physical and chemical properties of biochar.The electrostatic adsorption,ion exchange and coordination reactions between hydroxyl and phosphate ions are The main mechanism of phosphorus removal is that the iron and phosphate on the surface of the biochar compound to form a precipitate,which is also an important reason for the rapid removal of phosphate.（2）Through the experiment of the dosage of biochar adsorbent,the optimal dosage of MBC is determined to be 0.2g/L,and follow-up experimental research is carried out with this dosage.The fitting results of the adsorption kinetic model and the intra-particle diffusion model show that the adsorption behavior of MBC on phosphate is more in line with the quasi-second-order kinetic model,and the fitted equilibrium adsorption value of phosphorus is closer to the experimental data.The fitting results of the internal diffusion model show that the intra-particle diffusion is not the only adsorption mechanism,but may also involve boundary layer and film diffusion;the fitting results of Langmuir and Freundlich isotherm adsorption models show that the Langmuir isotherm adsorption model can more accurately describe MBC to phosphate The adsorption characteristics of MBC to phosphate are mainly a single-layer adsorption mechanism.The maximum adsorption capacity at 288K,298K,and 308K are 51.82mg/g,56.69mg/g,76.34mg/g,respectively.MBC has a wide range of adaptation to the p H value of sewage water and strong selective adsorption to phosphate ions.（3）Use biochar,sodium alginate and water as raw materials to preferably prepare the immobilized biochar pellet wetland matrix filler,and determine the filling volume of the biochar pellet according to the dissolution experiment.Zeolite-gravel mixed filler（control 1）,zeolite-gravel mixed filler with blank sodium alginate/calcium chloride pellets（control 2）,and zeolite-gravel mixed filler with biochar pellets as filler（Experimental group）,respectively build simulated vertical flow constructed wetland reactors（CW1,CW2,CW3）.The inlet water of the simulated wetland reactor is the tail water of the simulated sewage treatment plant,and the hydraulic load is 0.25 m³/（m²·d）.In the 15-30 day operation period,the biochar-based constructed wetland（CW3）is compared with CW1 and CW2.The removal effect of TN and TP is excellent,and the average removal rate is 78.12%and64.93%,respectively.The results of the operation impact study show that when the hydraulic load is gradually reduced from 0.5 to 0.125 m³/（m²·d）,the phosphorus removal rate of each simulated wetland reactor gradually increases,and the phosphorus removal rate of CW1,CW2,and CW3 is increased by 12.73%,10.43%,46.4%increased to 21.16%,19.36%,and 81.72%.Among them,CW1 and CW2 are more significantly affected by hydraulic load.CW3 has a certain impact resistance to hydraulic load changes,and the removal rate fluctuates slightly.The removal effect of wetland system on TP in different months decreases with the decrease of seasonal temperature.The average removal rate of TP in October is 21.1%,19.7%and 64.7%,and the average removal rate of TP in December At 13.25%,12.67%and 46.79%,CW3 is less affected by temperature changes than CW1 and CW2 wetland systems.（4）Through the analysis of the influence of biochar based on plant and microbial community structure,it is found that the biomass of CW3 is less than CW1,and the removal rate of phosphorus by CW3 is greater than that of CW1 and CW2,indicating that CW3 wetland system plants contribute to the phosphorus removal rate.Small,but the addition of biochar can promote the growth and development of plant roots;CW3has a higher relative abundance of microbial communities than CW1 and CW2,with Proteobacteria,Firmicutes and Planctomycetes.The relative abundance of denitrifying bacteria such as Bacteroidetes and Bacteroidetes increased,while the relative abundance of Chloroflexus decreased,and the number of phosphate phagocytic bacteria decreased,which inhibited the degradation of phosphorus by microorganisms.Improve the removal effect of TN and COD.To sum up,the modified biochar material prepared by the wetland plant Acorus calamus has a good adsorption and removal effect on phosphorus,and the biochar pellets prepared with the modified biochar material as raw materials are used as the wetland matrix filler for sewage phosphorus.Stable removal effect.This research is funded by the 13th Five-Year Plan"Water Pollution Control and Treatment Science and Technology Major Special Project"project,and provides certain theoretical support for the resource utilization of wetland plants,the enhanced removal of phosphorus in the constructed wetland system and the recovery of low-phosphorus resources in sewage.