Study On Effect Of Denitrification Biofilter Based On Organic Slow-release Carbon Source On Advanced Denitrification Of Sewage

In recent years,many provinces and municipalities have introduced stricter sewage discharge standards（quasi-four water standard）in response to the problem of excessive nitrogen and phosphorus in natural waters.Under the general trend of strict environmental protection,many sewage treatment plants are facing the need for upgrading.Due to the low concentration of common sewage carbon sources in the southern region,the denitrification process cannot be thoroughly completed,which brings pressure and challenges to the advanced denitrification of sewage treatment plants.This paper compares deep denitrification processes commonly used in sewage treatment plants:MBR membrane bioreactor and denitrification biofilter.It is considered that there is insufficient adaptation for small and medium-sized sewage treatment plants although the MBR membrane bioreactor has better effluent effect.For small and medium-sized sewage treatment plants,the denitrification biofilter has the advantages of low operating cost and convenient operation and maintenance under the premise of ensuring better water discharge conditions,so it is greatly suitable for practical application.The previous research of the research group found that the biochar filter has better denitrification effect than the ordinary sand filter.Based on the biochar filter,this paper discusses the effect of organic slow-release carbon source on the advanced denitrification of the reactor.By comparing the denitrification rate of different organic slow-release carbon sources,the organic slow-release carbon sources with faster reaction rates are screened and then organic slow-release carbon source biofilters with different proportions are established for natural biofilm formation.The effects of surface hydraulic load,C/N ratio and reflux ratio on the effluent effect of the reactor are successively studied by the control variable method.Under the optimization of process parameters,the stability of the reactor for the removal of different pollutants and the effect of the standard treatment of simulated sewage are discussed.Besides,microorganisms on the surface of filter media in different heights in the reactor are preliminarily analyzed to explore the denitrification mechanism.Main conclusions obtained are as follows:（1）Comparative study on denitrification and denitrification of PLA（Polylactic acid）,PHBV（Amyl hydroxybutyrate）,PCL（Polycaprolactone）biofilter and sand filter is carried out.There is no significant difference between the biofilter of PLA as the organic slow release carbon source and the ordinary sand filter.The NO₃^--N removal rates are 24%and 26%respectively.From the 63th to 80th days,the NO₃^--N concentration in the PCL biofilter is stable below 1.8 mg/L and the NO₃^--N removal rate is over 93%.The order of denitrification rate for four biofilters is:PCL biofilter>PHBV biofilter>PLA biofilter=ordinary sand filter.（2）PCL is used as a better organic slow-release carbon source and PCL in different ratios and biochar are combined into four reactors for a 30-day biofilm formation.In the late filming period（21³0 days）,the average removal rate of NH₄⁺-N in the four reactors is above 75%.The removal rate of NO₃^--N in PCL-3 and PCL-4 reactors is above 90%,the removal rate of TN is above 90%and the COD concentrations of PCL-2,PCL-3 and PCL-4 reactors are increased in varying degrees;（3）In the PCL biochar filter,with the increase of surface hydraulic load,the COD concentration decreases and the NH₄⁺-N,NO₃^--N and TN effluent concentrations increase to different extents.The PCL-4 reactor has strong anti-shock load and has good denitrification effect under different surface hydraulic loads;（4）Different C/N ratios have little effect on the removal rate of NH₄⁺-N in the reactor and the removal rates of the four reactors are maintained for above 70%.The C/N ratio has a great influence on the NO₃^--N,TN and COD concentrations in the effluent of the reactor.The denitrification effect of the PCL-3 and PCL-4 reactors is better than that of PCL-1 and PCL-2 reactors.The C/N ratio increases from 3:2.3 to3:3.8 and the TN effluent concentration of the PCL-3 reactor increases from below10.00 mg/L to above 20.00 mg/L,while the TN effluent concentration of the PCL-4reactor is always maintained for 10.00 mg/L or less,but the COD concentration of the effluent in the PCL-4 reactor is higher;（5）Increasing the reflux ratio has a greater effect on reducing the COD concentration in the reactor effluent and enhances the reactor’s ability to remove other pollutants to some extent.PCL-3 and PCL-4 reactors have higher TN removal rates than those of PCL-1 and PCL-2 reactors.TN removal rate is above 80%,and COD effluent concentrations in PCL-3 and PCL-4 reactors are respectively reduced to 20mg/L and 45 mg/L;（6）Under the conditions of optimized process parameters,the surface hydraulic load is 0.19 cm³/（cm²·min）,C:N=3:2.8 and the reflux ratio is 200%.Under this process condition,the average removal rate of NH₄⁺-N in PCL-1,PCL-2 and PCL-3reactors is above 80%,and there is no obvious accumulation of NO₂^--N for four reactors and the effluent concentration of NO₂^--N is less than 0.017 mg/L.TN effluent concentrations of PCL-3 and PCL-4 reactors are less than 10.00 mg/L.The COD effluent concentrations for PCL-1,PCL-2 and PCL-3 reactors are below 30 mg/L and the COD effluent concentration of the PCL-4 reactor is about 40 mg/L;（7）Under optimized process parameters,the effluent of PCL-3 reactor meets the requirements of standard renovation in the standard renovation treatment of simulated first-level B standard sewage by four groups of reactors.In the standard renovation treatment of simulated first-level A standard sewage,the effluent for PCL-2 and PCL-3 reactors meets the requirements of standard renovation and the effluent quality meets the requirements of pollutant limits on quasi-surface Level IV water standard.（8）In the microbiological analysis for the fillers in different heights in the reactor,it is found that the microbial functions of the upper,middle and lower parts of the PCL-1 reactor are similar and there are some differences between the microorganisms in the PCL-3 and PCL-4 reactors.The dominant bacteria include Protebacteria（48.81%-75.42%）,Bacteroidetes（5.36%-19.72%）and Acidobacteria（0.51%-21.64%）.The dominant bacteria include Unclassified_fComamonadaceae（1.72%-34.59%）and Simlicrispira（0.12%-23.75%）and Terrimonas（0-16.66%）;In the genus level analysis,the PCL-3 and PCL-4 reactors in three reactors show more significant performance in the genus Zoogloea,Desulfovibrio and Nitrospira.