Preparation Of Magnetic PVA-SA-Diatomite Composite Carrier And Its Application On Treatmennt Of Ammonia-nitrogen Wastewater By Immobilized Microorganisms

Ammonia nitrogen（NH₄⁺-N）is the main reason causing the eutrophication of water.The rapid development of industry and agriculture in China have increased NH₄⁺-N discharge year by year,and excessive discharge of NH₄⁺-N brought serious harm to the earth’s ecological environment.Among various treatment methods,biological methods are cheaper,and less harmful,has been widely used for NH₄⁺-N treatment.In the traditional biological method,the microorganisms cannot be reused and are susceptible to external environment,making it difficult to be stably operated in the actual treatment process.Immobilization of microorganisms is a technique to embed microorganisms into the prosous carriers,which brings several advantages over free microorganisms:provide a larger biomass,protection of microorganisms from toxic action of harmful compounds in environment,and convenience for recycling for microorganisms.In this paper,the magnetic PVA-SA-diatomite composite immobilized microbial carrier was synthesized and applicated on NH₄⁺-N removal in wasterwater.Firstly,magnetic Fe₃O₄particles were prepared by chemical co-precipitation method.Considering the ease of pellet formation,mass transfer efficiency,NH₄⁺-N removal efficiency and chemical stability of carriers,the optimal composition of raw materials for preparation of carriers was:10 wt%polyvinyl alcohol（PVA）,1%sodium alginate（SA）,1%diatomite 2%Fe₃O₄ and 86%distilled water.The carrier was characterized by Fourier transform infrared spectroscopy（FT-IR）,vibrating sample magnetometer（VSM）,mercury pressure method（MIP）and scanning electron microscope（SEM）,and the results showed that saturation magnetization of carrier was 8.29emu/g,the average pore size was 704 nm,and the pore on surface of the carrier was more compact than the interior.When the immobilized microbial carrier was used for 15 days repeatedly with renewing wastewaster every day,the NH₄⁺-N removal efficiency maintained above 95%and the maximum wet weight loss was less than 2%,indicating that the immobilized microbial carrier has good NH₄⁺-N removal efficiency and chemical stability.The kinetic study showed that the nitrification reaction fits the first-order reaction kinetic model within 5～25mg/L initial NH₄⁺-N concentrations,and the zero-order reaction kinetic characteristics within50～300 mg/L NH₄⁺-N concentrations.The microorganisms’growth,respiratory activity and biocenosis distribution of carrier were investigated in domestication experiments to clariy the NH₄⁺-N biotransformation mechanism.Turbidimetric analysis showed that both magnetic and non-magnetic carriers were favorable for microbial growth,and microorganisms grown better in magnetic carriers.The respiratory activity of microorganisms in magnetic carriers after different cycles of domestication were higher than that in non-magnetic carriers,indicating higher oxidation activity of magnetic carrier.NH₄⁺-N removal efficiency,total nitrogen（TN）removal efficiency and nitrogen nitrite（NO₂^–-N）accumulation are 93.4%,33.3%and 33.7 mg/L for magnetic carrier and 76.3%,37.7%and 13.8 mg/L for non-magnetic carrier after domestication for 3weeks,indicating that the magnetic carrier plays an important role on facilitating the microbial nitrification process.Biome analysis showed that magnetic carriers increased microbial species diversity.The relative abundance of Proteobacteria,Gammaproteobacteria,Betaproteobacteria and Nitrosomonas in the domesticated magnetic carriers was 94.3%,43.5%,35.0%and 2.0%,respectively,which was higher than that in non-magnetic carriers（80.3%,24.5%,18.2,and 0.5%）.These bacteria could play a key role on the oxidation of NH₄⁺-N,and therefore,the differences in the abundance of these bacteria could be responsible for the differences in NH₄⁺-N and TN removal efficiency and NO₂^–-N concentrations by different carriers during biological denitrification.The study on the process conditions of NH₄⁺-N conversion reaction showed that the optimum reaction condition for NH₄⁺-N and TN removal were:the carrier dosage at 10%,C/N at 4,temperature at 30-40°C and p H at about 8.A high initial NH₄⁺-N concentration is helpful for maintaining high NH₄⁺-N removal load.The experiment of treatment of NH₄⁺-N in landfill leachate for 20 days（renewing waterwater every 24 h）showed that the NH₄⁺-N removal efficiency,TN removal efficiency,COD removal efficiency,NO₂^–-N and nitrogen nitrate（NO₃^–-N）accumulations were respectively 71.9%,26.3%,14.0%,92.2 mg/L and 15.8 mg/L for magnetic immobilized microbial carrier,and 55.2%,33.1%,9.6%,30.7 mg/L and 10.4 mg/L for non-magnetic immobilized microbial carriers,and 60.3%,52.4%,19.6%,18.2 mg/L and 4.9 mg/L for the free microorganisms.Magnetic carrier achieved the highest NH₄⁺-N removal efficiency and free microorganism obtained the highest TN removal efficiency.