Treatment Of Sulfate-rich Organic Wastewater By Internal Circulation(IC)Anaerobic Reactor

Sulfate-rich organic wastewater is commonly generated from the operation of pharmaceutical,petrochemical and papermaking industries,which would lead to serious environemntal problem if discharged without effective treatment.Anaerobic biological treatment,as a key method for treatment of wastewater rich in organics and sulfate,has many advantages such as low cost and energy recovery,and has been widely applied to the treatment of industrial wastewater.However,the presence of sulfate would exert influence on the anaerobic digestion even lead to collapse of anaerobic system,which pose challenge to the effective removal of organics and sulfate by anaerobic process.In this study,internal circulation(IC)anaerobic reactor was applied to the treatment of sulfate-rich organic wastewater under varied operation parameters including organic loading rate(OLR),upflow velocity(Vtup),ratio of COD(Chemical oxygen demand)to sulfate(C/S),sulfate loading rate(SLR)and ratio of nitrate to sulfate(NO3-/SO42-,N/S).Besides,the influence mechanism of these parameters on the IC reactor was revealed from the perspective of dynamic behavior and microbial community.The main conclusions of this study are following:(1)Over 80%of COD could be removed by IC reactor at the OLR ranging from 2.50 to 18.94 kgCOD/m3/d with the Vup of 1.0 m/h.Under high OLR,decrease of Vup to 0.06 m/h after withdrawal of external hydraulic circulation led to deterioration of IC reactor,as the COD removal and effluent pH dropped to below 60%and 6.0 respectively,as well as 37.0%loss of specific methanogenic activities(SMA)of anaerobi sludge.At the same time,obviously changed microbial community and the shift of substrate metabolic pathway from propionate-type fermentation to hybrid fermentation were observed after decreasing Vup,as well as the replacement of Streptococcusby Raoultetilas the predominant bacteria genus.Dominance of hydrogenotrophic methanogens including Methanobacterium and Methanocorpusculum with a total abundance over 93%indicated a important role of H2 in the formation of methane.Three mathematical model,Logistic,Modified Gompertz and Transference,could be applied to simulate the behavior of substrate degradation and methane production by anaerobic sludge under high OLR.(2)Worsen performance of IC reactor was observed after decline of C/S from 2.5 to 1.67.Further reduction of C/S to 0.83 led to decreased removal of COD and sulfate to the level close to 50%due to the inhibition of sulfide,as well as a reduced percentage of electron flow utilized by methane producing archaea(MPA)to only 2.1%.Decreased SMA value of anaerobic sludge and strengthened role of propionate and ethanol in the utilization of subsrate were observed with decreased value of C/S.Lower C/S contributed to the enrichment of the phylum Proteobacteria,and two sulfate reduction bacteria(SRB)genera,i.e.Desulfovibrio and Desulforhabdus,as well as acetoclastic methanogen Methanothrix,but do harm to the hydrogenotrophic methanogens.Obvious response of mcrA and dsrA genes to C/S was observed.With decreased C/S,mcrA gene abundance declined remarkably while dsrA gene increased generally,in accordance with the performance of IC reactor at varied C/S.(3)Worsen performance of IC reactor was observed after the simultaneous increase of influent content of COD and sulfate to 7500 mg/L and 3000 mg/L respectively.However,the two-phase IC reactor,a modified versions of IC reactor with the combination of IC reactor and two-phase anaerobic digestion,could removed over 84%of COD and 90%of sulfate when the influent COD and sulfate reached 10000 mg/L and 4000 mg/L respectively,and over 40%of influent COD and 60%of sulfate could be removed in acidification phase.Under high loading of organics and sulfate,ethanol-type fermentation predominated in the acidfication phase of two-phase IC reactor with reduction of sulfate,while further oxidization of volatile fatty acids(VFA)and ethanol,methanation of acetate and H2/CO2,as well as reduction of sulfate took place in the methanogenic phase.Lactobacillus and Streptococcus were the predominant bacteria genera in the acidfication and methanogenic phase respectively.Acetoclastic methanogen Methanothrix and hydrogenotrophic methanogens Methanobacterium dominated in the archaea domain of the two-phase IC reactor.qPCR revealed higher abundance of mcrA and dsrA genes content in methanogenic phase than it in acidfication phase.(4)Obvious inhibition of nitrate on the sulfate reduction in the IC reactor was observed,as the average removal of sulfate decreased from 78.4%(N/S=0)to 41.4%(N/S=1.03).Sulfate reduction with acetate as electron donor might be affected by the competition of acetate between denitrifying bacteria(DB)and SRB and/or the inhibitory intermediates of denitrification on the sulfate reduction.Besides,nitrate could be reduced by SRB with the oxidization of propionate preferentially thus leading to a limited sulfate reduction based on the propionate.The SRB genus Desulfovibrio was inhibited by nitrate as its abundance decreased remarkably from 11.4%(N/S=0)to 2.2%(N/S=1.03).mcrA gene was found to be more sensitive to nitrate than dsrA gene,as the mcrA gene concentration decreased significantly with elevated influent nitrate content.