Formation Of Aerobic Granular Sludge Under The Application Of Iron Electrolysis And Its Nutrient Removal Efficiency And Mechanism

Compared to activated sludge（AS）,aerobic granular sludge（AGS）is characterized of good settling ability,high biomass retention and strong resistance to influent shocking,and thus AGS technology is regarded as a novel biological wastewater treatment technology with the low footprint and high development prospect.However,when it is applied to treat the real municipal sewage with a low organic loading rate（OLR）,there are some application bottlenecks,such as the long start-up period,unstable performance during the long-term operation,and low removal efficiencies of nutrient.To address the above questiones,this study proposed to couple an iron electrolysis unit into AGS systems,and the main reasons were to 1)untilize the positive influences of electrical stimulation on the surface properties,migration behavior,and biochemical activties of microbes,2)improve the bioavailability for the element Fe by slowly releasing of Fe from an iron anode.Considering these two aspects,this study was proposed to provide a feasible method for overcoming the above-mentioned application bottlenecks of AGS technology.In addition,this study explored the varations on all kinds of compositions in sludge during the granulation process,and the response of fucntional microbes and genes in mature granules,with the purpose of revealing the interactive relationships among application of iron electrolysis,internal response of sludge,and ecological function of system,which advanced both engineering applications and theoretical development of AGS technology treating the municipal wastewater.A new strategy for enhancing granulation was developed through the couping of iron electrolysis in this paper,and then cultivated a novel kind of AGS which formed rapidly,maintained stably,and degraded pollutants effectively,denoted as“the in-situ iron minerals-deposited AGS”.A pair of reactive iron anode and inert titanium cathode was installed into a sequencing batch reactor,and then the AGS system couped with iron electrolysis was successfully constructed.In terms of the evaluation of both granulation rate and decontamination efficiency,the application mode and voltage intensity of iron electrolysis unit in this coupling system was majorly optimized.The results showed that a constant voltage（1.0～1.5 V）was only applied in the anoxic phase of the AN/O＿SBR operation mode,this coupling system achieved the start-up within 20 days,and compleleted the sludge granulation after 60 days.Besides,the cultivated AGS had an excellent settling ability,large specific gravity,and high bioactivity.The accumulated mineral composition in this novel AGS not only could enhance the granular stability,also created the interconnected channels for mass transfer of substrates and metabolites.As a result,this kind of novel AGS got rid of the inherent weaknesses of unstable structure the conventional AGS.After the sludge was completely granulated under the application of 1.5 V iron electrolysis,this coupling system was used to treat the real municipal sewage.It was observed that an excellent decontamination performance was achieved in this coupling system,in which the removal efficienices of COD,TN,and TP were respectively increased by 4.0%,27.3%,and 39.9%compared to the AGS system without the coupling of iron electrolysis.Moreover,the treated water met the Class A emission standard of Pollutant Discharge Strandard for Urban Sewage Treatment Plants（GB 18918-2002）.The synergistcal evolution of the three kinds of components within sludge,namely minerals,microbes and extracellular polymeric substance（EPS）,was explored during the sludge granulation,and then the formation mechanism of the in-situ iron minerals-deposited AGS based on mineral core theory was elaborated.The results revealed three aspects of variations during granulation process,1)the sludge mineral evolved from amorphous iron oxides to crystalline iron phosphates;2)the form of C-（C/H）in the EPS matrix was partially oxidized into the oxygen-containing forms of C 1s（C-OH,C=O,and O-C=O）,and then enabled EPS the binding properties;3)the microbial community was siginifcantly evolved,and gradually colconized the functional microorganisms related to EPS secretion and nutrient removal.Accordingly,the dominating component of sludge responsible for granulation process was identified,and its influencing mechanism was deduced and verified.To be specific,EPS played an important role in the initiation phase of granulation process,which not only retained small flocs by adhesion property,also binded the metallic cations(Fe²⁺,Fe³⁺,and Ca²⁺)for subsequent precipiation;the amorphous iron oxides present in sludge matrix obviously improved settling ability of sludge,and then retained more biomass for futher growth and granulation;when sludge was completely granulated,the dominating species was crystalline iron phosphates,which cooperated with microbes and EPS to maintain the structure stability of AGS.Underlying machanims of nutrient removal by multiple routes of the in-situ iron minerals-deposited AGS was revealved.The results showed that for the cultivated AGS under the application of 1.5 V iron electrolysis,both diameter of sludge（1.7 mm）and content of the element Fe（30 mg/g SS）provided suitable oxygen and iron amounts demanded for the Fe-metabolism microbes and various oxygen-demand bacteria.Compared to the conventional AGS,this novel kind of AGS enriched a variety of denitrifiers.,and heterotrophic,autotrophic,and mixotrophic denitrifiers respectively accounted for 74.5%,10.3%,and 15.2%in all of denitriers,which totally reached to 42%of microbal population.Through the batch tests of granular sludge,it was further verified that autotrophic iron-dependent denitrifcation was present in the in-situ iron minerals-deposited AGS,which cooperated with other denitrification routes to enable this coupling system a good TN remoal efficiency.In addition,it was indicated that the Fe-metabolizing bacteria strictly controlled accumulation of intracellular iron by up-or down-regulation of fucntional genes,and the reaction between Fe²⁺and NO₃^--N was restricted in the periplasm.Based on the phosphorus speciation of granular sludge,it was inferred and then verified that the superior TP removal efficiency was attributed to the synergistically biochemcial phosphorus removal,which included three routes,namely the biological phosphorus removal completed by PAOs,co-precipitation with ferrous or ferric ions,and adsorption of PO₄^3-by iron oxides.