Formation And Characteristics Of Soluble Microbial Products In Wastewater Biological Treatment

Biological methods are the most widely used wastewater treatment.However,microorganisms produce a large number of soluble microbial products(SMP)as well as degrade pollutants.SMP are released into the solution during substrate metabolism,biomass growth,as well as biomass decay.Its composition is complex including humus,polysaccharides,proteins,nucleic acids,organic acids,antibiotics,thiol and so on.Since some components are not readily biodegradable or even difficult to biodegradable,SMP constitute a major part of effluent organic matter.The presence of SMP adversely influences the effluent quality and decreases the treatment efficiency.In addition,with the increasing urgent for wastewater reuse and reclamation,SMP have significant impacts on subsequent advanced treatment.For example,SMP can cause severe membrane fouling and affect the efficiency and stability of resin.Additionally,SMP are readily transformed to undesirable disinfection by-products during disinfection when chlorine is applied as the primary disinfectant.Therefore,SMP is an important constraint in wastewater reclamation process.The study on the formation and characteristics of SMP is the prerequisite and basis for the regulation and control.Based on the above background,this thesis investigated in-deep the generation and property of the SMP.The production mechanism and sources of SMP under stress conditions including chemical uncoupler and nanomaterials was studied.Fluorescence spectroscopy and quantitative analysis was applied to SMP studies under stress condition.In addition,interaction of SMP and metal was investigated to explore the chelation and environmental impact of SMP.The main contents and conclusions are as follows:(1)The effects of 3,3',4',5-Tetrachlorosalicylanilide(TCS),a typical chemical uncoupler,on SMP production and the related mechanism was studied.The results showed that TCS was an effective metabolic uncoupler for microbial yield reduction.However,the addition of TCS led to an increased production of SMP.The increase in SMP was related with the concentration of TCS.In addition,the SMP were produced in proportion to the substrate utilization at a low TCS concentration,while more non-substrate-associated SMP were released at a high TCS concentration.Both sludge reduction and SMP contents were related to TCS dosing.In order to balance these two aspects,optimal TCS concentration of 2.5 mg/L was chosen.TCS stimulated the production of extracellular polymeric substances(EPS)and enhanced cell lysis,which both contributed to SMP production.Specifically,EPS release was likely predominate at a low TCS concentration(<5 mg/l),while cell decay might be the main source at a high TCS content(>7.5 mg/1).FTIR and EEM analyses showed that the SMP,EPS,and cell lysis products had similar functional groups and fluorescence properties,indicating a similar origin of these substances.In addition,a dose of TCS increased the release of high molecular weight compounds due to cell lysis.(2)Ag nanoparticles(AgNPs),typical nanomaterial,were synthesized through aqueous chemical reduction and the influence of AgNPs on activated sludge system in terms of SMP formation and cytotoxicity was investigated.The results showed that AgNPs inhibited substrate degradation and microbial activity,while increased the formation of SMP,especially proteins and humic-like substances.In addition,AgNPs led to the changes in the structure of SMP components.Although the increased SMP was not from EPS release,the carboxyl on EPS of activated sludge was likely to be the action site of AgNPs.Lactate dehydrogenase activity analysis,live/dead staining and the ultra-thin section imaging showed the cytotoxicity of microorganisms caused by AgNPs which led to the damage of cell membrane,the increase in dead bacteria as well as the release of cellular constituents.Thus,the substrate utilization,microbial activity and SMP production were eventually influenced.AgNPs dosage had insignificant influences on other physicochemical properties of activated sludge,such as surface morphology,zeta potential and floc size.The transformation of AgNPs was also studied and the results suggested that AgNPs were stable and Ag(0)was the dominant form of Ag in activated sludge.(3)The impact of salinity on SMP production was investigated using fluorescence excitation-emission matrix(EEM)spectra with parallel factor analysis(PARAFAC)and fluorescence regional integration(FRI)analysis.The results showed that protein content was more substantially influenced by salinity compared with polysaccharide.EEM spectra of SMP with different salinities were analyzed by PARAFAC with core consistency to determine the appropriate number of PARAFAC components.One protein-like and two humic-acid-like components were identified which exhibited different trends.In addition,the scores of protein-like component were closely correlated with the protein concentrations by chemical analysis.EEM spectrum was divided into six excitation-emission regions.FRI analysis revealed that majority of protein fluorescence in SMP was attributed to tryptophan and tryptophan-like proteins.The normalized volume percentage of protein region increased,while those of humic-and fulvic acid-like region decreased significantly with the increasing NaCl concentration.(4)The interaction of SMP and metal was studied by fluorescence spectroscopy such as EEM and synchronous fluorescence spectroscopy.The influence of interaction on the acute toxicity of metal was also evaluated by luminescent bacteria.The results showed that the presence of metal ions could cause fluorescence quenching of the SMP.Compared with other metals,the interaction between Cu2+ and SMP was stronger.At pH=6,Cu2+ caused fluorescence quenching of a protein and two humic acid fluorescence peaks,which was static quenching.The number of binding sites between Cu and SMP was 2.Temperature and pH both affected the interaction of the SMP and Cu2+.Stronger interaction occurred under conditions of low temperature and high pH.Synchronous fluorescence spectra showed that protein conformation was not changed under experiment conditions.The interaction between SMP and Cu reduced the concentration of ionic Cu,resulting in the decrease in the acute toxicity of the metal.