Study On Denitrification And Nitrogen Removal Efficiency Of Different External Carbon Sources In Sewage With Low Carbon To Nitrogen Ratio

With the rapid development of the social economy,China is facing severe challenges such as water resource shortage and increased requirements for water pollution control.There is a common phenomenon in various regions of China where there is an excess of nitrogen containing substances in sewage and a low carbon to nitrogen ratio.The country is increasingly attaching importance to ecological environment protection,and the efforts to control water pollution are constantly increasing.The effluent discharge standards of many sewage treatment plants have been raised from Class A to Class IV water quality standards.COD has decreased from 50 mg/L to 30 mg/L,ammonia nitrogen has decreased from 5 mg/L to 1.5-3 mg/L,and total nitrogen has decreased from 15 mg/L to 10 mg/L.Faced with strict effluent standards and low influent carbon nitrogen ratio,ensuring the removal and up-to-standard discharge of COD,total nitrogen,and ammonia nitrogen is currently a challenge for sewage treatment plants.The removal of total nitrogen in water is mainly through the role of microorganisms.Carbon source provides electrons for denitrification,which is an important factor in the denitrification process.The lack of carbon source will restrict the denitrification process to a large extent,and the lack of carbon source will cause the accumulation of nitrite.Therefore,in view of the problem of insufficient carbon sources,additional carbon sources are selected in practical engineering applications to improve the carbon nitrogen ratio of sewage and promote the efficiency of denitrification.This study selected corn cob as the experimental material to prepare sustained-release carbon sources.The corn cob was subjected to three treatment methods:low concentration Na OH solution soaking at room temperature,low concentration Na OH solution water bath heating,and untreated.The pre treated corn cob was crushed to prepare sustained-release carbon sources with different pre-treatments.At the same time,sodium acetate was used as a single carbon source,and sodium acetate plus glucose was used as a composite carbon source as a control,Explore the economical and efficient removal of total nitrogen from low carbon to nitrogen ratio wastewater using additional carbon sources.All carbon source materials were subjected to 7-day static carbon release test.The control group was treated with different pretreatment corncob,polycaprolactone,polyvinyl alcohol sodium alginate,and the experimental group was treated with different slow-release carbon sources to study the carbon release capacity of different carbon source materials.Exploring the chemical composition and functional group changes of all carbon source materials before and after carbon release through Fourier transform infrared spectroscopy analysis;Observing the surface and internal structure of carbon source materials before and after carbon release through scanning electron microscopy,whether it is conducive to the release of organic matter and the attachment and growth of microorganisms;Through three-dimensional fluorescence spectrum analysis,explore the dissolved organic matter released by different carbon source materials,and reflect the organic matter such as humus and proteinoid in the leaching solution.Select the slow-release carbon source that is most suitable for the external carbon source,participate in the denitrification test study of different external carbon sources,and discuss the denitrification capacity and operation cost of three external carbon sources,namely,sodium acetate,sodium acetate plus glucose,and slow-release carbon source,for the denitrification test.The main research findings of this paper are as follows:（1）In the static carbon release experiment,alkaline foam CPSP released more COD than other carbon source materials,reaching 191.50 mg/g,with the lowest TN release,only 0.31mg/g,and TP release only 0.042 mg/g.The COD release of all carbon source materials shows a characteristic of first fast and then slow.Within 48 hours of the experiment,the COD release of all materials is relatively high,and after reaching 48 hours,the COD release begins to slowly decrease.The carbon release of carbon source materials follows a quasi second order kinetics.All carbon source materials have different levels of TN release,and slow-release carbon sources can reduce TN release due to their internal pore structure,and the TN content in the preparation of slow-release carbon source materials is low,resulting in less TN release in carbon release tests.The release of TP from all carbon source materials is relatively low,while the release of TP from alkaline foam CPSP is the lowest,only 0.042 mg/g.The daily release of TP can be ignored and will not affect the effluent quality.（2）The carbon source materials before and after carbon release were characterized and analyzed by Fourier transform infrared spectroscopy（FT-IR）and scanning electron microscopy（SEM）,and the leaching solution of the carbon source materials was analyzed by three-dimensional fluorescence spectroscopy（3D-EEM）.Fourier transform infrared spectroscopy shows that there are no new absorption peaks generated before and after the carbon release of the carbon source material,and there is no significant chemical reaction during the carbon release process.Scanning electron microscopy showed that the surface of all slow-release carbon sources in the experimental group was rough,with uniform pore distribution and easy attachment of microorganisms;In the control group,after alkali soaking and alkali heat treatment,the surface structure of corn cobs was damaged,with more pores and larger pore sizes,making them susceptible to microbial attachment and reproduction.After the carbon release test,the surface wrinkles of all corn cob carbon source materials in the control group deepened and the number of pores increased.All slow-release carbon source organic substances in the experimental group are dissolved and released,with an increase and deepening of pores and an increase in pore size.However,their skeleton structure is stable,providing more space for microbial attachment and growth.Three dimensional fluorescence spectroscopy analysis shows that a large amount of protein like substances that are easily degraded and utilized by microorganisms are present in the leaching solution of various carbon source materials.（3）In the denitrification experiment with sodium acetate,sodium acetate plus glucose,and alkaline bubble CPSP as three external carbon sources,sodium acetate is the most suitable external carbon source for sewage treatment with low carbon nitrogen ratio.The results of the denitrification test with different external carbon sources are as follows:the denitrification test with sodium acetate as the external carbon source has the best COD removal effect,the COD concentration in the effluent is 10.00 mg/L,and the removal rate of NO₃^--N is the highest,90.7%.The concentration of NO₃^--N in the effluent is 2.08 mg/L,the concentration of NH₄⁺-N in the effluent is 0.03 mg/L,the removal rate of NH₄⁺-N is 99.3%,the concentration of TN in the effluent is 2.23 mg/L,and the removal rate of TN is 92.6%.In the denitrification test with sodium acetate and glucose as the external carbon source,the effluent COD concentration was maintained at 15.00 mg/L,the effluent NO₃^--N concentration was 3.46 mg/L,the removal rate of NO₃^--N was 85.8%,the effluent NH₄⁺-N concentration was 0.14 mg/L,the removal rate of NH₄⁺-N was 97.0%,the effluent TN concentration was3.75 mg/L,and the removal rate of TN was 87.5%.In the denitrification test with alkaline bubble CPSP as the external carbon source,the effluent COD concentration is 23.00 mg/L,the effluent NO₃^--N concentration is 9.01 mg/L,the removal rate of NO₃^--N is 63.3%,the effluent NH₄⁺-N concentration is 0.74 mg/L,the removal rate of NH₄⁺-N is 85.1%,the effluent TN concentration is 9.87 mg/L,and the removal rate of TN is 67.1%.To sum up,the concentration of COD,TN and ammonia nitrogen in the effluent of all denitrification tests has reached the quasi Class IV water quality standard,of which the concentration of COD in the effluent of sodium acetate denitrification test is the lowest and the denitrification effect is the best.（4）The results of microbial high-throughput sequencing showed that the microbial diversity of the sludge sample（A1）from the alkali bubble CPSP denitrification test was the highest,and that of the sludge sample（A3）from the sodium acetate plus glucose denitrification test was the lowest.In terms of community composition,the highest bacteria content at the sample phyla level is Chloroflexi,Proteobacteria,Actinobacteriota and Bacteroidota.From the sample level clustering results,all samples can form three sample sets.The hierarchical clustering of inoculated sludge sample（A0）and sodium acetate denitrification test sample（A2）is closer,and the hierarchical clustering of alkaline bubble CPSP denitrification test sample（A1）is the farthest.（5）After cost calculation,it was found that adding sodium acetate had the highest operating cost,with a removal cost of 35.32 yuan/kg NO₃^--N per unit.Adding alkaline foam CPSP had the lowest operating cost,and a removal cost of 21.46 yuan/kg NO₃^--N.From the perspective of adding cost,alkali bubble CPSP is more suitable as an external carbon source for denitrification.