The Nitrogen Removal Pathways And Performance Of Nitrite Enhanced Simultaneous Nitrification And Denitrification In SBBR Wastewater Treatment System

Simultaneous nitrification and denitrification（SND）owns outstanding strongpoints like energy saving,land conservation,external carbon source addition saving.But the uncertainty of stable operation system construction and denitrification mechanism restrict its further development and engineering application.In this study,the sequencing biofilm batch reactor（SBBR）was choosen considering its advantages such as good diversity of microbial system,large biomass,stable operation and so on.By controlling the operation conditions and adding nitrite,an enhanced SND denitrification system with stable treatment performance was successfully constructed in the SBBR with intermittent aeration.By adjusting the influent nitrogen source composition and tracking the pollutant change in the whole cycle,the denitrification performance of the system was determined.The effect of biofilm in-situ and ectopic state,influent C/N ratio and nitrogen composition on the nitrogen removal performance of the system was studied.The main nitrogen removal pathways in the system were studied by using nitrification inhibitors.The main microbial community diversity,abundance,amount of nitrogen removal related functional gene in this SND system and the distinctions from common SND system were studied via 16S r RNA gene diversity analysis and real-time fluorogenic quantitative PCR method.The main research work and conclusions were as follows:（1）Construction of nitrite enhanced SND system in SBBRThe long-term addition of NO₂^--N has little impact on COD removal,yet can affect nitrogen removal greatly,and can significantly accelerate the formation of a stable and efficient SND nitrogen removal system.After stabilization,the ammonia nitrogen removal efficiency was over 99%,and the rate is relatively faster.The effluent NO₂^--N and NO₃^--N had been stable at low levels,0.36 mg/L and 0.13 mg/L,respectively.The SND rate was around 98%,which indicates an excellent SND performance.The COD removal was fast,with a removal efficiency of more than 85%within 3h.（2）Study on the nitrogen removal performance of nitrite enhanced SND system1)The nitrification and denitrification performance of the system were excellent.The nitrification performance keeps well under instantaneous exposure of ammonia nitrogen within 200 mg/L.If under long-term exposure of ammonia nitrogen,the nitrification performance may be further improved.The maximum denitrification capacity was approximately 186.17 mg/L with NO₃^--N as the only influent nitrogen source when the COD concentration is fixed at 400 mg/L.When NO₂^--N was the only influent nitrogen source,the fast denitrification rate and the good denitrification performance were presented,and the denitrification ability was at least 50 mg/L.The NO₂^--N removal is mainly attributed to direct denitrification.In addition,the dissimilatory nitrate reduction to ammonium is less essential in SND system.2)The biomembrane structure,that is,the existence of DO gradient in biomembrane,has a certain influence on the denitrification performance of SND system,but has a small effect on COD removal.The oxygen concentration gradient existence promotes ammonia and total nitrogen（TN）removal.NO₃^--N did not accumulate during the processing process.NO₂^--N can be removed well in both in-situ and ectopic biofilm states.3)The presence or absence of NO₂^--N in the influent has very limited influence on the overall nitrogen removal performance of the reactor.Excellent removal performance was achieved in both cases.The instantaneous exposure of NO₂^--N is not the key to the better treatment performance,but the influence of its long-term existence on the dominant flora of the reactor.4)Influent C/N ratio has very limited influence on ammonia nitrogen removal in SND system,but has a great impact on NO₂^--N,NO₃^--N and TN removal.When influent C/N ratio was low,the effluent NO₂^--N,NO₃^--N and TN concentrations were high.And the removal rates were all increased with the increase of the influent C/N.However,when C/N exceeded 5,the increase rate decreased.The SND systems for simultaneous removal of ammonia nitrogen and NO₂^--N,and simultaneous removal of ammonia nitrogen and NO₃^--N,both achieved a good denitrification performance when the influent C/N ratio was 5.（3）Study on the nitrogen removal pathways of nitrite enhanced SND system1)The study of adding inhibitors and changing the influent water quality found that the main paths of removing ammonia nitrogen in the system include autotrophic nitrification and heterotrophic nitrification by AMO,and other pathways that do not rely on AMO,such as microbial assimilation removal and possible initial adsorption of biofilms similar to COD removal.Tthe nitrification via AMO pathways contributing to the total ammonia nitrogen removal of about 91.73%.The NO₂^--N and NO₃^--N removal is due to aerobic denitrification and anoxic denitrification,and their contributions were close.When the Nar K of the denitrifying bacteria is inhibited to affect Nar activity,the denitrification can still be carried out smoothly catalyzed by Nap,but the removal efficiency is affected,resulting in 0.064 mg/L NO₃^--N residue in the effluent.2)Based on the study of microbial community structure in the system,it was found that the Proteobacteria phylum is predominant in both SND system（B1）constructed with long-term addition of NO₂^--N to influent and the SND system（B2）built without the addition of NO₂^--N to influent.And the abundance of Chloroflexi,Bacteroides and Acidobacteria were also high in the two systems.The abundance of Nitrospirae phylum associated with the nitrification process in B1 is greater than in seed mud and significantly greater than that in B2.At the genus level,the relative abundance of functional bacteria in B1 is relatively higher than B2.The bacteria includes nitrification bacteria associated with denitrification,hypoxic densitrification and aerobic antinitis bacteria.Quantitative PCR results of the target gene presented the copy number of nitrogen removal related function genes in B1 was higher than that in B2.This was consistent was the greater nitrogen removal performance in B1.The results of nitrogen removal pathways and performance of nitrite enhanced SND in SBBR wastewater treatment system will enrich the theory of SND processes,and provide technical support for its efficiency improvement in practical engineering application.