Study On Denitrification Of Tail Water From Municipal Wastewater Treatment Plant By Slow-release Carbon Source Fillers

In recent years,with the development of urbanization,the problem of insufficient water protection for ecological environment such as urban ecological water replenishment,greening water,and urban miscellaneous water has become increasingly prominent.China mainly uses the advanced treatment of urban sewage plant tail water to reuse it for ecological environment water.However,the high content of TN（more than 80%of NO₃^--N）in the tail water is the main reason that restricts its reuse.In sewage plants that dominate biological denitrification,the removal of TN is limited by insufficient carbon sources in the water.The denitrification effect is often improved by adding carbon sources.Plant carbon sources have attracted the attention of many scholars because of their wide sources,huge quantities,and low costs.However,the carbon release rates of different plant carbon sources are very different,and simple addition exists,The carbon release is not long enough and the effect is unstable.Therefore,this study screened several common plant carbon source materials in rural areas,and used different methanol-water-Na OH modification systems to prepare slow-release carbon source fillers,which were used in denitrification biological filters,To carry out deep denitrification on the tail water of the municipal sewage plant.The main research conclusions are as follows:（1）The Chinese wolfberry branches are a better plant solid carbon source.The carbon（C）content of five plant carbon sources such as flax stalks,Chinese wolfberry branches,soybean stalks,oil sunflower stalks and oil sunflower disks is between 36.23%and 68.64%,The C content of Chinese wolfberry branches was 67.09%,which had a great potential of carbon release.The five plant carbon source materials all have two stages:Phase I（1～3 d）was the rapid carbon release stage,and Phase II（4～21 d）was the stable release stage.The carbon release rate of Chinese wolfberry branches is the highest in the stable carbon release stage,which can reach 3.35 mg·（g-Straw·d）^-1.The carbon release process of the five plant carbon sources fitted Ritgre-Peppas equation and the second-order kinetic equation quite well.The c_m value of the Chinese wolfberry branches is relatively moderate and the t_1/2value is relatively large,which is beneficial to improve the denitrification performance and shorten the start-up period of the reactor.Compared with the other four kinds of plant carbon sources,the DOM in the extract of Chinese wolfberry branches contains more alcohols,aldehydes and small molecular organic acids.Therefore,it is preferred to use Chinese wolfberry branches as a plant carbon source for subsequent experiments.（2）The 20%methanol-water-Na OH（1%concentration）system is the optimal condition for the modification of Chinese wolfberry branches.The Chinese wolfberry branches were modified by the mixture of methanol（0%,20%,50%and 100%volumetric ratio respectively,hereinafter referred as 0%,20%,50%and 100%group）,water and Na OH（1%）.The cellulose and hemicellulose content of the modified branches were 8.7%～35.2%higher than those of the raw branches.The average denitrification rates are in the sequence of 20%group>50%group>0%group>100%group>the control group.The denitrification rate of the 20%group was the highest(0.76 mg·（g-Chinese wolfberry branches·d）^-1),which indicated that 20%methanol-water-Na OH（1%）was the optimum condition for the modification of the Chinese wolfberry branches.The surface of the modified branches was rough,irregular and had many voids,which was suitable for the growth of denitrifying microorganisms.The denitrification process of the modified branches followed the Monod kinetic equation（R²=0.96）.The Chinese wolfberry branches modified with 20%methanol-water-Na OH（1%concentration）is the prepard slow-release carbon source,and subsequent experiments will use it as a filling of denitrification filter for deep denitrification research.（3）HRT 3.5h and HRT 6.5h are the best HRTs for slow-release carbon source fillers to enhance the denitrification of tail water with low nitrate nitrogen concentration and high nitrate nitrogen concentration.When the slow-release carbon source is used in the deep denitrification of the municipal sewage plant tail water,under the condition of low nitrate nitrogen concentration(11.70±0.64 mg·L^-1),the effluent COD is between10.82～19.71 mg·L^-1 when HRT is 1.5h,2.5h,3.5h and 4.5h.The removal rate of NO₃^--N and TN was the highest at HRT 4.5h,79.58±6.75%and 70.23±5.64%,respectively,but the average removal rate was only increased by about 3%compared with HRT 3.5h.The denitrification filter basically does not remove TP.When HRT is 3.5h and 4.5h,the microbial film on the surface of the slow-release carbon source filler is denser and thicker than other HRTs,and the microbial community diversity is more abundant.The main bacterial genera detected are Saccharimonadales,OLB14,C10-SB1A and Microtrichales.Under the condition of high nitrate nitrogen concentration(21.66±0.51mg·L^-1),compared with other HRTs（4.5h,5.5h and 7.5h）,HRT 6.5h has the highest removal rate of NO₃^--N and TN,69.22±9.53%and 60.56±8.57%,respectively.At the same time,the comparison found that the removal rate of NO₃^--N and TN under the condition of low nitrate nitrogen concentration at HRT 4.5h was significantly higher than that of high nitrate nitrogen concentration,indicating that the denitrification effect was negatively correlated with the influent nitrate nitrogen concentration.（4）The slow-release carbon source enhances the long-term operation efficiency of tail water denitrification.When studying the long-term operation effect of the slow-release carbon source filler under typical conditions(the influent NO₃^--N concentration is 23.04±1.51 mg·L^-1,HRT=4h),the effluent COD concentration in the stable operation stage is 15.09±6.18 mg·L^-1,the average removal rate of NO₃^--N and TN in the slow-release carbon source group was increased by 15.31%and 14.05%,respectively,compared with the control group（unmodified Chinese wolfberry branches）,and the NO₃^--N removal rate was still more than 20%at 120 days.Compared with the control group,the surface biofilm of the slow-release carbon source group is more uniform and compact,and the microbial community diversity is more abundant.The proportion of Proteobacteria（21.05%）is higher.The main bacterial genera are Microtrichales,Saccharimonadales and Ilumatobacteraceae.The abundance of denitrification functional genes is also higher.It showed that the Chinese wolfberry branches were modified by methanol（20%by volume）-water-Na OH,which significantly enhanced the denitrification capacity of the denitrification filter.