Study On Treatment Of Nitrogen In Secondary Effluent By Flyash-Based Hydrotalcite Coupled Iron-Carbon Microelectrolysis Constructed Wetland

The TN content（15 mg/L）in the secondary discharge water of the town sewage treatment plant is much higher than the V water standard（2.0 mg/L）in the Surface Water Environmental Quality Standard（GB 3838-2002）.In order to achieve the same level of discharge,In this thesis,a flyash-based hydrotalcite（HT）coupled iron-carbon microelectrolysis constructed wetland system was constructed for deep nitrogen treatment with low concentration,which not only utilized the high absorption and buffer capacity of HT,but also provided electron donors for the wetland system,enabling it to efficiently remove nitrogen.In addition,the efficiency of nitrogen removal in constructed wetland was low at low temperature（<15℃）.In this thesis,electrochemical regeneration technology was used to enhance the continuous nitrogen removal of substrate.The main conclusions are as follows:（1）The hydrothermal synthesis of fly ash（FA）was verified as HT by XRD,FTIR and SEM analysis.At different initial pH values,the NO₃^--N removal rate of HT remained at a high level,and the buffer capacity was relatively high（the final pH value remained between 6-7）.The kinetic data were described by a pseudo-second-order kinetic model,which showed that the rate limiting step was chemisorption.The experimental data fit well with Langmuir model,and its maximum adsorption capacity at 25℃is 11.13 mg/g.（2）When the outdoor temperature was higher than 25℃,the results of pollutant removal in each wetland system showed that the average TN removal rate（68.98%and 66.46%）of the hydrotalc-like coupled iron-carbon microelectrolytic system（HT-Fe-C1）in the constructed wetland system without reed planting was significantly higher than that of other systems（P<0.05）when the HRT was 5 and 7 days.In the reed constructed wetland system,the average TN removal rate（96.21%and 99.04%）of the hydrotalcite coupled iron-carbon microelectrolytic system（HT-Fe-C2）was better than that of the hydrotalcite coupled iron-carbon microelectrolytic system（HT2）（96.04%and 97.45%）when the HRT was 1 and 3 days.It was significantly higher than that of Fe-C2（80.88%and 95.41%）（P<0.05）.The average TP removal rates（90.75%,83.02%,80.44%and 46.61%）of HT-Fe-C2 system under different HRTS were significantly higher than those of other systems（P<0.05）.Combining COD,ammonia nitrogen,total nitrogen and total phosphorus,HT-Fe-C2 wetland system can reach the surface water ClassⅣstandard under different HRTS.（3）When the outdoor temperature drops to 10-15℃,an electrochemical system is added to the hydrotalcite coupled iron-carbon micro-electrolysis constructed wetland system,and the electrochemical reaction time is 24 hours.When the HRT was 1 day,the average NH₄⁺-N removal rate of HT-Fe-C1 system and HT-Fe-C2 system was increased to 57.15%and 97.69%,and TN was increased to 33.61%and 84.07%,respectively.When the HRT was 7 days,the average TN effluent concentration of HT-Fe-C2 wetland system decreased to 1.43 mg/L,which reached the level of ClassⅣfor surface water.（4）The microbial composition analysis of HT-Fe-C1 and HT-Fe-C2 systems showed that,at the phylum level,the dominant bacteria in the functional filler layer of constructed wetlands were Proteobacteria and Bacteroides,with relative abundance of more than 99%,while the dominant bacteria in the quartz sand layer were Proteobacteria and greenbendomyces.Their relative abundance was 58.79%and 68.90%in the two groups of systems,respectively.At the genus level,the dominant microorganisms in the functional packing layer were chitinophage,Solanacearum and Phenybacterium,and their relative abundances were 75.23%and 79.79%respectively in the two groups.The dominant microorganisms in the quartz sand layer were Flavobacteria,Acinetobacter,decloromonas,Soxella and nitrocillum.Its relative abundance was 21.28%and 22.50%in the two groups,respectively.Therefore,the functional filler layer mainly removes nitrogen through degradation of organic matter and denitrification,and the quartz sand layer mainly removes nitrogen through nitrification and denitrification.