Effects Of Ni And Ni Complexes On Methane Fermentation And Their Mechanism

Biogas project employing anaerobic digestion and aiming at waste resource utilization is now becoming the most effective and promising technique used for the treatment of urban waste,industrial wastewater,poultry excreta,and excess sludge from wastewater plant.Trace elements play an important role in methane fermentation and lack of even a single element could lead to the activity decrease of microorganisms.Therefore,the treatment efficiency and process stablity of the anaerobic reactor should be influenced greatly.Sulfides,carbonate,and phosphate are always present in industrial wastewater,urban sludge,or their treatment processes.Because the trace metals can easily precipitate with sulfides and other anion,they also will not exist in adequate amount and effective biological-form in the actual anaerobic digesters.The maximal substrate conversion rate may not be attained.Focusing on nickel as research object,the bioavailability of trace metals was increased by the addition of metal ion chelators to the anaerobic reactors in this study.The nutrient condition for methanogens was optimized and methane fermentation was enhanced greatly.The main research contents are as follows:(1) The importance of trace metal nickel in methane fermentation was investigated. Nickel showed "Hormesis" effect on methane fermentation.That was,nickel at trace dosage could stimulate anaerobic digestion,while high dosage may cause inhibition.Methane production increased with increasing nickel concentration in definite dosage range.The presence of sulfides made a lot of nickel dosages.Nickel speciation in reactors changed because of the complexation between nickel and chelators.The formation of soluble complexes between nickel and chelators favored the dissolution of nickel from their sulfides, and nickel would not be the limiting factor in methane fermentation.Critric acid(CA), nitrilotriacetic acid(NTA),and ethylene diamine tetraacetic acid(EDTA) were also preferable stimulants for anaerobic digestion.At a sodium acetate concentration of 7 g/L,temperature of 35℃,and Ni²⁺ concentration of 20μM,methane production was enhanced 34.1%,49.2%, and 38.6%,by the addition of 10μM CA,NTA and EDTA,respectively.(2) The single factor experiments were conducted to study the effects of different substrate,substrate concentrations,initial pH and ammonia nitrogen concentrations on methane fermentation.The results showed that NTA stimulated methane production when acetate,propionate,butyrate was used as substrate respectively.The stimulant effect was related to the substrate concentration.Substrate concentration was more high,the stimulant effect was more obvious.When sodium acetate concentration was 6,12 g/L,methane production was increased by 30.0%,95.9%,respectively,compared with the control system.The kinetic constant V_max and the half saturation constant K_s in Michaelis-Mente equation were improved from 84.8 ml CH₄·gVSS^-1·d^-1 and 2.95 gNaAc·L^-1 to 147.1 ml CH₄·gVSS^{- 1}·d^-1 and 7.57g NaAc·L^-1.The application of NTA was related to the initial pH.When the initial pH was 5.5,6.0,NTA did not show any stimulant effect on methane fermentation.But the methane production was increased significantly at pH over 6.5.Ammonia nitrogen at concentration of 0.4 g/L was advantageous for methane fermentation and methane production was enhanced 20%.Ammonia nitrogen concentration at 2.0 g/L began to show inhibition effect on methane fermentation.The inhibition effect was mainly present in the latter period of anaerobic digestion.When ammonia nitrogen concentration was above 2 g/L,the methanogenic activity of the biomass decreased greatly and the inhibition prevailed in the entire anaerobic digestion process.At ammonia nitrogen concentration of 5 g/L,the methanogenic activity of the biomass decreased by 50%.Nickel had antagonistic effect on the ammonia nitrogen toxicity.The inhibition effect caused by the ammonia nitrogen may be alleviated or eliminated by the addition of nickel.This indicated the acquisition of trace metal was very important for MPB to increase their ability for resistance the deleterious effect of toxic substance.But NTA did not show any effect on methane fermentation of ammonia nitrogen containing wastewaters.(3) During batch methane fermentation of sulfate-containing wastewaters,the effect of NTA on sulfate reduction and methane production was studied in details.When sulfate concentration was below 5000 mg/L,methane yield was enhanced by 11.9%～44.1%and sulfate reduction was decreased by 33%～76%in the presence of 100μM NTA.The bioavailability of trace metal for methane production bacteria(MPB) was increased and MPB may therefore become more resistant to the toxicity produced by sulfides.MPB then should predominate over sulfate reduction bacteria(SRB) in the anaerobic reactors.When sulfate concentration attained 5000 mg/L,methane fermentation was inhibited severely.The presence of NTA did not increase methane production and the sulfate reduction was also inhibited.(4) In order to investigate the operation status after the applicaition of NTA to the continuous operation reactors,the treatment efficiency,the trace metal nickel speciation changes in anaerobic sludge with time,and the relation between cofactors F₄₃₀,CoM concentration and methanogenic activity were studied in a continuous stirred reactor(CSTR). The methanogen population changes were also analysed by use of fluorescence in situ hybridization(FISH).The results showed COD removal efficiency was enhanced by 20%and the methane production rate was increased by 60%by the application of 10μM NTA to the influent at an organic load rate of 3 g/L·d and influent COD concentration of 4500 mg/L. Nickel speciation distribution in anaerobic sludge changed with NTA addition.The sulfide fraction of nickel decreased from 62 to 29μg/gTSS and the residual fraction increased 1.89 folds.NTA boosted the internalization of Ni for MPB,hence the bioavailability of nickel was greatly improved.FISH results indicated NTA could stimulate the growth of Methanosarcinaceae which utilize substrate faster than Methanosaetaceae.There was a positive correlation between F₄₃₀,coenzyme M(CoM) concentration and methanogenic activity of the biomass.F₄₃₀ and CoM content increased 4.5 and 2.8 folds in the NTA amended reactor,compared with the control system.(5) The uptake of trace metal is related to the bioaffinity constant a and bioconversion capacity constant b.NTA stimulated the uptake of nickel for MPB.The uptake process did not conform to free ion activity model(FIAM).The uptake of trace metal was not simply determined by the free ion concentration in reactor.The formation of metal complexes may play an important role for the uptake of metals by MPB.