| In order to further control water eutrophication,the emission standards of ammonia nitrogen and total nitrogen in wastewater treatment plants and the requirements of water environmental protection have become increasingly stringent in our country.Further removal of nitrogen from water has become one of the hottest issues in water pollution prevention and control field.At present,total nitrogen removal efficiency of many urban sewage treatment plants is not stable,and the difficulty of upgrade is widespread.The reduction of nitrogen from balck-and-malodorous water body in urban area is also crucial in pollution control.It is the urgent task of reducing the nitrogen load from water environment to further strengthen the stable removal of ammonia nitrogen and total nitrogen from the polluted water and the effluent of sewage treatment facilities.In recent years,microbial embedding immobilization technology has been one of the research directions in the field of biological wastewater treatment.It has some advantages,including enhancing the efficiency and tolerance of microorganisms,facilitating the maintenance of high microorganism concentrations in reactors and improving the removal efficiency of pollutants in wastewater.Based on improving the solubility and mechanical strength of embedding beads,poly(vinyl alcohol)-sodium alginate(PVA-SA)gel was modified with alumina nanoparticles(ALNPs)using ultra-sonication and used for activated sludge with a high nitrification rate immobilization.Batch experiments were conducted to measure nitrogen removal in synthetic wastewater with initial ammonium concentrations of 5~40 mg/L.The nitrification and denitrification characteristics,physical and chemical properties and microbial characteristics of modified embedding beads were investigated.The major conclusions were drawn as follows:(1)Preparation and optimization of modified beadsIn order to enhance the mechanical strength of the embedding beads and ensure good mass transfer ability,the modified beads were prepared by adding ALNPs with ultra-sonication.The single factor experiments were conducted to compare the apparent characteristics and the effect of ammonia nitrogen degradation of embedding beads which were added ALNPs with different volume fraction.The results showed that the removal efficiency of ammonia nitrogen was 99% in stable phase when the volume fraction of ALNPs was 0.5%,and the shape and elasticity of embedding beads remained well in the stable phase.Therefore,the best ratio of ALNPs was 0.5%.The modified beads were light grey,with good elasticity and easy flow ability.(2)Physical and chemical properties analysis of modified beadsPhysical and chemical properties analysis including functional groups,specific surface area and pore size of embedding beads before and after modification was investigated.The results showed that the functional groups of the embedding beads before and after modification changed greatly and their mechanical strength and solubility improved.The specific surface area and average pore size of embedding beads before and after modification were 11.7631m2/g,9.9997m2/g and 110.309?,75.092 ?,respectively.Compared with the unmodified beads,the pore size of modified beads were smaller,which were conducive to interception of microorganisms and the protection of microorganisms from the hydraulic disturbance,making the maintenance ability of microbial activity improved.(3)Nitrogen removal processes of modified beadsBatch experiments were conducted in a series of 0.5 L reactors to measure nitrogen removal in synthetic wastewater with the initial ammonia nitrogen,total nitrogen and COD concentrations were 5~40mg/L,10~45mg/L,80~100mg/L and C/N ratio of 1.78~10.When the packing ratio of modified beads was 10%,and the temperature of wastewater,dissolved oxygen concertration were maintained at 10~15?,2~4 mg/L,respectively,the results showed that the maximum ammonia removal loads were 9.63,25.02,41.90,59.58 mg/(L·h),respectively in reactors with different initial ammonia nitrogen concentrations.The ammonia nitrogen and total nitrogen removal efficiency were 89~100% and 22.25~44.93%,respectively.When the initial ammonium concentration was 5 mg/L,the reaction followed first-order kinetics.The reaction coincided with zero order kinetics under initial ammonium concentrations ranging from 15 to 40 mg/L.SEM observations showed the modified beads were highly porous and microorganism adhered to the interior and external surface of embedding beads,demonstrating that embedding beads acted as an ideal support material.Compared with unmodified beads,modified beads had better operational stability,and solubility and mechanical strength of beads were improved.During the reaction period,the shape of modified beads kept well.(4)Study on microbial community diversities of seed activated sludge and modified beadsThe microbial characteristics of seed activated sludge and modified beads were investigated using high-throughput sequencing analysis.The results showed that the structure of microbial communities in the embedding beads changed significantly under different classification after acclimation to the reactor environments and the reaction process compared with seed activated sludge.The advantage of denitrifying bacteria in embedding beads was obvious and the microbial diversity was good.Some microorganism which can conduct both heterotrophic nitrification and aerobic denitrification,were identified.These processes may facilitate pathways for untraditional biological denitrification in the modified beads.The results of PCA analysis showed that the new microbial community structure was formed in the embedding beads after acclimation and the reaction process.Although embedding beads shared similar bacterial phyla,the communities had different bacterial genus due to the different ratio of C/N in reactor and a DO concentration gradient existing on the beads' exterior and interior,indicating that the embedding beads had an important effect on screening advantage bacterium.The stable operation of reactor could be related to the good micro ecological features of microbial community in embedding beads.In this paper,PVA-SA gel was modified with ALNPs using ultra-sonication and used for activated sludge with a high nitrification rate immobilization to remove the nitrogen from secondary effluent or polluted water.The nitrification and denitrification performance,physical and chemical properties and microbial community diversity were investigated under different initial ammonia nitrogen concentrations.The results can provide theoretical support for the enhanced biological nitrogen removal in wastewater and offer a microbiological basis for enhanced denitrification technology of advanced treatment. |
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