In-depth Exploration Of The Mechanism Of Humic Acid Inhibition And De-inhibition Anaerobic Digestion

Nowadays,operating wastewater treatment plants?WWTPs?toward carbon neutrality is globally emphasized,and thus capturing organic energy from excess sludge has made anaerobic digestion?AD?regain an opportunity to be applied.However,the practical chemical energy from AD depends on the influent organic?COD?concentration,while the conversion efficiency of which transferred to excess sludge is not very considerable due to stable structure of bacterial cells and stubborn organics such as lignocellulose substances and humic substances?HSs?.Espacially,the HSs is not only difficult to be biodegraded but also inhibits AD process.Therefore,it is meaningful to study the mechanisms of HSs inhibiting AD of excess sludge and provide the basis for mitigating inhibition by metal ions.In present study,“clean”excess sludge was cultivated in laboratory to avoid HSs involving in.The cultivated sludge was applied in a series of batch experiments,with different concentration of humic acid?HA?.A single-phase and three sub-phases?HP:hydrolytic phase;AP:acidogenic phase;MP:methanogenic phase?of AD were respectively tested with and without HA dosed and then analyzed the changes of associated eneymes and microorganisms.Finaly,the tests foucus on the mechanism of mitigation the inhibition by Ca²⁺and Al³⁺metal ions.In the single-phase AD system,HA indeed inhibited the methanogenic efficiency by 35.1%at HA:VSS=15%.However,the effects of HA on the three sub-phases behaved totally differentdue to different mechanisms.In the HP process,it could lower the activities of the associated key enzymes by the interactions of electrostatic force,covalent bond and sweep flocculation,then inhibit the hydrolytic process.The hydrolytic efficiency in the test without HA dosed was at 10.2%,while the test with HA dosed?HA:VSS=15%?decreased to 6.3%,accounting for an inhibition efficiency up to 38.2%.In the AP process,HA could act as TEA or ECT to promote the acidogenic process,and the acidogenic efficiency increased to 124.1%?HA:VSS=15%?from 61.6%?HA:VSS=0?,elucidating a promotion efficiency up to101.5%.In the MP process,however,HA decreased the methanogenic efficiency by52.2%?250%at HA:VSS=0 was lowered to 130.5%at HA:VSS=15%?due to inhibiting the activities of the involved key enzymes.The FISH results demonstrate that HA had almost no effect on the involved microorganisms in both AP and MP processes with HA dosed.Overall,the combined efficiency of the three sub-phases without HA dosed is calculated at 15.7%,and that of the three sub-phases at HA:VSS=15%is at 10.2%.Finally,the whole inhibition efficiency of the three sub-phases is equal to 35.0%,which is almost identical?35.1%?to the result observed in the single-phase AD process.The effects of HA on hydrolase is various due to a variety of hydrolysis substrate.A small amount of HA has a strong inhibition effect on?-amylase and protease while increasing the hydrolysis efficiency of cellulose and lipase.However,the excessive HA eventually behaved irreversible inhibition.The HA-enhanced acidification experiment showed that HA could increase the yield of VFAs by 35.7%,nearly 17%more than the two-phase acidification yield?19.0%?,which was mainly limited by the inhibition of HA on HP process.The interactions of HA with metal ions mainly include electrostatic force,bridging adsorption,and sweep flocculation.Among them,electrostatic force happens between the functional groups of carboxyl,hydroxyl and metal ions,and belongs to the strong-bonded type.The capacity is directly relation to the amount of charge in the surface and little to do with the type of metal ion.However,bridging adsorption,and sweep flocculation are weakly bound types and are associated to metal ion species.Al³⁺has more charge and smaller ionic radius than Ca²⁺,and its hydrolysates have strong bound ability,therefore,Al³⁺has a stronger binding capacity to HA than that of Ca²⁺.In the mitigating experiment,the mechanism of Ca²⁺and Al³⁺is basically similar,but performed differently in the three sub-phases,resulting in the most effecitive mitigation at different concentrations in each phase.Achieved the same mitigation efficiency?65%?,the required concentration of Ca²⁺was at 100 mg/L,while Al³⁺was only at 70 mg/L.In the combined metal ion mitigating experiment,the combination of lower concentration(50 mg Ca²⁺/L+10 mg Al³⁺/L)may have a synergistic mitigation phenomenon,i.e.,“1+1>2”.As a result,the mitigation efficiency increased from 41.8%(Ca²⁺)and 16.5%(Al³⁺)alonely to 80.1%.,while the higher concentration combination could behave further inhibition than any one kind of metal ions.