Performance And Mechanism Of Utilizing Soluble Microbial Products As Electron Donors For Denitrification In Sequencing Batch Reactor

Multiple complicated issues about environment pollution are gradually appearing along with the fast developing of society and technology.For realizing the sustainable development of environment,especially in control and remediation of water pollution,the increasing strict discharge standard of the wastewater treatment plant has been published.Druing biological nitrogen removal process,the efficiency of total nitrogen(TN)removal is usually limited due to the lack of organic carbon source(electron donor).And the commonly used method,that adding external carbon source,could effciently promote the TN removal rate,but also would highly increase the cost.A number of researchers have never stopt exploring the effective and economic carbon sources.On the other hand,soluble microbial products(SMP)are the by-products during biological metablisms.They usually have a large molecular weight and the low biodegradability,consequently could easily remain in the effluent of biological treatment plants,and contain up to 75%-95%of the total soluble COD.Another sound claims that SMP have the potential to be degraded if providing a long enough hydraulic retention time(HRT).However,there are few reports about if the SMP could be utilized in wastewater treatment systems as the electron donors for denitrification.Therefore,this research mainly focused on the feasibility of such process.And the efficiency and mechanism of in-situ utilizating SMP in SBR to enhance the TN removal performance were further investigated.Firstly,under denitrification condition(anoxic,organic substrates and nitrate initially fed),the fate of SMP under different carbon source feedings was investigated,and the occurring condition of utilizing SMP as the electron donors for denitrification process by the microorganisms was figured out.It was found that the influent organic matters(OMinf)determined the maximum accumulation of SMP.And the existence of the ISO also inhibited the utilization of SMP.Under feast-famine condition,unitizing the OMinf and SMP respectively,as the carbon sources and electron donors,contributed 43.6%and 56.4%for TN removal,61.0%and 39.0%for nitrogen reduction,and 57.1%and 42.9%for electron provision.Although nearly half of the removed TN attributed to the SMP,the electron release rate from SMP was only 1.3 M equivalent electrons/h,which was much slower than 42.4 M equivalent electrons/h from OMinf.Under famine condition,more SMP were released from cells by the stress of insufficient nutrients,which contributed more for denitrification process afterwards.However,due to the over endogenous consumption,the performance of the reactor,that running under famine condition,started to decrease from the 3rd operation cycle,and gradually lost the nitrogen removal ability.Secondly,the method of how to achieve the in-situ utilization of SMP in sequencing batch reactor(SBR)was explored,and the contribution from in-situ utilizing SMP on TN removal was discussed.It was found that the efficiency of SMP utilization could be effectively enhanced by providing a period of anoxic reaction following the aeration phase.The utilization-associated products(UAP),which were produced along with the process of OMinf metabolism by microorganisms,were degraded in aerobic phase.By contrast,the biomass-associated products(BAP),which were released in famine-carbon condition,could not be hydrolyzed and utilized until the reactor operated into anoxic phase.From analyzing the distribution of molecular weight(MW),the transformation from large MW fractions to small ones was clearly observed,and those small MW fractions were finally discomposed or assimilated.From analyzing the compositions of SMP,the available utilization compositions were targeted as aromatic proteins,polysaccharides,tryptophan-proteins,and fulvic acids.In single-cycle reaction,totally 21.1 mg/L of TN was removed.Herein,the OMinf,UAP,and BAP contributed 13.2 mg/L,3.9 mg/L,and 4.0 mg/L,respectively.The contribution from the SMP was as high as 37.2%.Additionally,extending the anoxic reaction time and solid retention time(SRT)could increase the TN removal performance partly from utilizing the SMP as electron donors.Nevertheless,extending SRT would make the ammonium release from the cells as well.Finally,the interaction among influent C/N,SMP in-situ utilization,and TN removal performed was investigated in SMP utilization enhanced reactor.It was found that the influent C/N not only influenced the direct contribution from the OMinf,but also affected the indirect contribution from the SMP as the intermediate electron media,on TN removal.Superficially,the TN removal performance of the enhanced reactor was in direct proportional to the influent C/N value,the amount of removed TN significantly increased from 16.1 mg/L(C/N=2)to 23.5 mg/L(C/N=8).Underneath,due to the decrease of the initial nitrate concentration by increasing C/N from 6 to 8,the direct contribution from OMinf adversely dropped from 40.3%to 36.6%.In comparison,more and more SMP were generated and accumulated by increasing the C/N from 2 to 8.Consequently,it supported more efficient denitrification process,which performed as the absolute contribution(removed nitrogen/initial nitrogen)of SMP obviously raised from 6.5%(C/N=2)to 43.0%(C/N=8),including 20.3%from UAP and 22.7%from BAP.In addition,TN removal performance could be effectively promoted by enhancing SMP in-situ utilization for treating high C/N wastewater.The increasing parts of TN removal at C/N=5,6,and 8 were 4.1 mg/L,5.6 mg/L,and 6.7 mg/L.However,the enhanced parts of treating low C/N wastewater were not obvious,which were 2.2 mg/L at C/N=3 and only 0.7 mg/L at C/N=2.