Simulation And Reaction Kinetics Analysis Of Anaerobic Digestion Process In ABR

The efficient degradation of organic pollutant in anaerobic baffled reactors(ABRs)depends remarkably on the biophase separation characteristics.Employment of ABR in treating organic wastewater has gained much attention in these recent years.However,treatment efficiency and a steady reactor operation require further improvement.As it has been postulated that,simulation and reaction kinetics analysis of anaerobic digestion process are important means to optimize and control the process,a 4-compartment ABR was employed to treat wastewater and kinetic models were developed to simulate and analyse biochemical process,physico chemical reaction and microbial growth process of anaerobic digestion.The reactor was started up with the mixed liquor volatile suspended solid(MLVSS)of 5.66 g L-1,influent chemical oxygen demand(COD)of 2 g L-1,hydraulic retention time(HRT)of 2.0 d and temperature of 35oC.The addition of sodium bicarbonate(Na HCO3,3.36 g L-1)along with shortened HRT(form 2.0 d to 1.0 d)resulted in achieving steady state,remarkable biophase separation characteristics,as well as COD removal of 94% after 30 days of operation.The results in regulation period further indicated that volatile fatty acids(VFAs),pH,COD and biogas yield were influenced by HRT and influent COD concentration.Whiles HRT was stepwisely curtailed from 2.0 d to 0.3 d,and influent COD concentration maintained at 4 g L-1,it was observed that,VFAs had accumulated in the system,p H decreased from 6.8~7.4 to 5.2~6.4,and COD removal had decreased remarkably.When HRT was stabilized thus 1.7 d was implemented,and influent COD concentration had ranged from 2 g L-1 to 8 g L-1 by stages,p H in first and second compartments decreased while no obvious change of p H was observed in last compartments.MLVSS was significantly enhanced,resulted in CH4 yield and COD removal increased from 4.2 L d-1 and 88.0% to 18.0 L d-1 and 96.4%,respectively.VFAs model was developed by introducing thermodynamic inhibition coefficient of VFAs degradation into Anaerobic Digestion Model No.1(ADM1).It could simulate kinetic process of VFAs production and degradation and influence of impact load on anaerobic digestion.Confidence interval of fbu,su,fpro,su,fac,su and fh2,su were ±0.028,±0.070,±0.050 and ±0.004,respectively.The cellular automaton model,integrated with VFAs model was developed based on the growth probability of functional population.It could be used to simulate two-dimensional growth and spread of acidogens,H2-producing acetogens and methanogens and profiles of VFAs and biomass under varying p H.H2-producing acetogens and methanogens were inhibited by p H of 4.5~4.6,while acidogens were enriched.Noticeably,p H range of 6.7~7.3 had enriched H2-producing acetogens and methanogens in the core area of granules,while acidogens grew on the outside layer.Granules formed a multilayered structure comprising of functional population with different hydrophobicity.By introducing liquid-gas mass transfer and biomass retention coefficients into ADM1,a performance characteristic model was developed to simulate VFAs,COD,biogas yield and biomass in ABR with HRTs.The results suggested that when HRT was at the range of 1.0~2.0 d,high biomass retention capacity was achieved,and biomass retention coefficient was 0.55.Stoichiometry parameters and liquid-gas mass transfer coefficients remained constant.High biomass retention capacity and mass transfer efficiency ensured high degradation conversion of organics in waste water,resulting in COD removal above 90%.Substrate degradation-methane fermentation model was developed with integrated stoichiometry from methane fermentation by substrate degradation kinetic equation,which could be used to simulate effluent COD concentration and methane yield.Reaction kinetics analysis suggested that VFAs inhibition factor Ki was key kinetic parameter.When influent COD concentration was at the range of 2.0~8.0 g L-1,influent COD concentration increased,resulted in increase in Ki and enhancement of endurance capacity.Acidogenic fermentation model was developed by integrating hydrogen partial pressure(PH2)into acidogenic fermentation kinetic equations.The acidogenic stoichiometry parameters(fac,su,fpro,su,fbu,su,fet,su and fh2,su)estimated by PH2 could provide criterion for acidogenic fermentations.Ethanol-type,butyrate-type and propionate-type fermentation was identified by(fac,su + fet,su)/(fbu,su + fpro,su + fac,su + fet,su + fh2,su)?66.0%,(fac,su + fbu,su)/(fbu,su + fpro,su + fac,su + fet,su + fh2,su)?63.0% and(fac,su + fpro,su)/(fbu,su + fpro,su + fac,su + fet,su + fh2,su)?74.0%,respectively.Based on simulation and reaction kinetics analysis,strategy of ABR startup and regulation was discussed.The results suggested key regulatory factors for startup,regulation and acidogenic fermentation type were pH,OLR and PH2,respectively.Optimal p H was at the range of 6.7~7.3,OLR 2.0~4.0 kg COD m-3 d-1(constant influent COD,varied HRT)and OLR 1.2~4.8 kgCOD m-3 d-1(constant HRT,varied influent COD).Mixed acid-type,propionate-type and butyrate-type type fermentation was achieved at PH2 of 0~0.07,0.07~0.31,0.31~0.50 atm,respectively.Ethanol-type fermentation was achieved when PH2 was larger than 0.50 atm.