Enhancing Anaerobic Wastewater Treatment By Adding Iron And Activated Carbon

Anaerobic biological technology has been considered as an important strategy to convertthe organic wastes to energy,due to its potential advantages,such as the high oragnic loading rates(OLRs),energy saving and methane production.Slow metabolism of methanogens and its sensitive characteristics to environmental perturbation are liable to cause the unbalance between acidogenesis and methanogenesis.Therefore,it is necessary to develop the high-efficiency and practical anaerobic biological technology which meets the discharge standard.Zero-valent iron(ZVI),as cheap and environmental friendly reducing agent,has been widely used in preprocessing of refractory pollutants,repairing shallow groundwater and strengthening acidification process of hydrolysis.Due to these considerations,in this study,ZVI and rusty ZVI(mainly ZVI and Fe(III)oxides)were respectively used to enhance the anaerobic biological technology with the low-concentration wastewater as substrate.The results showed that,under the high-OLR(corresponding to the decrease of hydraulic retention time [HRT] from 12 to 4 h)and low-temperature conditions,as compared with control group,both the ZVI-added and rusty ZVI-added reactors could improve the chemical organic demand(COD)removal rate by 12%-37%.The diameter of granular sludge in the ZVI-added and rusty ZVI-added reactors was as 5 times as that in the control reactor.Moreover,FISH analysis showed that,compared with the control group,the abundance of hydrogenotrophic methanogens in the ZVI-added reactor increased about 40%,suggesting that ZVI might enrich the hydrogen-utilizing methanogens and promote the syntrophic metabolism via interspecies hydrogen transfer(IHT).In the rusty ZVI-added reactor,the abundance of Geobacter species increased about 13%,suggesting that the rusty ZVI might enrich the Fe(III)-reducing microorganisms(Geobacter species)and enhance the dissimilatory iron reduction.Besides,considering the low cost of the rusty ZVI,it is reasonable to be applied to enhance the low-concentration wastewater treatment.Granular activated carbon(GAC)can be served as the electron conduit to promote direct interspecies electron transfer(DIET)and enhance the syntrophic methanogenic metabolism.Based on the above conclusion that Fe(III)oxides could accelerate the decomposition of complex organic wastes via dissimilatory iron reduction,Fe(III)oxides and GAC were respectively supplemented to the acidogenic and methanogenic phase of a two-phase anaerobic digestion system to enahcne the anaerobic biological technology with landfill leachate as substrate in the second part,.The results showed that,under the influent OLR of8.8 kg COD/(m3·d),as compared with the control group,the two-phase anaerobic reactorwith Fe(III)oxides and GAC improved methane production rate,COD removal rate and methane conversion efficiency by 74.4%,12.2% and 13.4%,respectively.Adding Fe(III)oxides to the acidogenic phase could enrich the Fe(III)-reducing microorganisms that might participate into the decomposition of complex organic wastes via dissimilatory iron reduction,enhance the conversion of complex organics into simple organic acids and alcohols and and improve the acidification efficiency.Meanwhile,adding GAC to the methanogenic phase could establish DIET between syntrophs and methanogens to accelerate the conversion of organic acids and alcohols to methane,maintaining the acidity balance and stability of syntrophic metabolism.The study presented in this part proposed a strategy to solve the major problems of two-phase anaerobic digestion involved in the low-efficiency hydrolysis acidification as well as the slow syntrophic conversion of fermentative intermediates(organic acids and alcohols)to methane,which kept the two-phase anaerobic digestion resist the high organic load impact well.