Enhancement Of Anaerobic Digestion And Biogas Upgrading From Wastewater By Nano Zero-valent Iron Addition

The wastewater generation was growing with the development of economy.For the harmlessness treatment of wastewater,the energy recovery from wastewater also received wide spread attention.Anaerobic digestion technology was to degrade organic matter in wastewater and generate biogas by the metabolism of anaerobic microbial.However,anaerobic digestion technology of wastewater had some problems,such as being susceptible to environmental factors,poor system stability,low degradation efficiency of organic matter,low calorific value of biogas,etc.In recent years,the application of nano zero valent iron（n ZVI）in anaerobic digestion system had attracted attention.On the one hand,n ZVI as a reducing agent could enhance the treatment efficiency of anaerobic digestion,on the other hand,corrosion hydrogen（H₂）evolution from n ZVI in the anaerobic system could provide H₂ for biogas biological upgrading.Based on this,this paper first studied the effects of p H,properties of aqueous solution and n ZVI concentration on the H₂ evolution efficiency from n ZVI corrosion,and obtained the optimal H₂ evolution conditions.n ZVI was added to a trickle bed reactor（TBR）to study the effect of H₂ supply ways on the anaerobic digestion efficiency and biogas upgrading.In a continuous stirred tank reactor（CSTR）,the efficiency was improved and methane generation was stabilized by optimizing dosing ways and dosages.The main research conclusions were as follows:（1）The optimization condition study of anaerobic corrosion H₂ evolution reaction from n ZVI.The experimental results showed that a large amount of H₂ was produced in the early stage of reaction,but it was decreased after that.The H₂ evolution from n ZVI was affected by p H,aqueous solution and n ZVI concentration,H₂ yield from organic wastewater solution system was lower than that of deionized water system.Acid condition could promote H₂evolution from n ZVI corrosion while alkaline condition could be inhibited,excessive concentration would cause agglomeration and decrease the efficiency of H₂ evolution.The highest cumulative H₂ yield was 628.50 m L in the deionized water system with p H 3.After the reaction,the higher the corrosion H₂ evolution efficiency,the higher the p H rise,and the more significant decline of ORP.The concentration of Fe was higher in the lower p H and glucose synthetic wastewater system.The characterization data of corrosion products showed that a passivation layer was formed on the surface of n ZVI under alkaline condition,which resulted in the inhibition of H₂ evolution;n ZVI agglomeration was occurred in high concentration condition;there was reduction reaction of n ZVI competing with organic matter in wastewater system.（2）Four n ZVI corrosion H₂ supply ways were set through endogenous and exogenous H₂supply to improve the anaerobic digestion efficiency and biomethanation of the TBR.The results showed that n ZVI corrosion H₂ evolution promoted the degradation of organic matter in wastewater,and the degradation rate of COD increased from 59.43%in the stable stage to 67.86%in stage 4.In addition,the CH₄,biogas yield and CH₄ content of TBR were also increased.Compared with the stable stage,the average daily CH₄ in stage 1-4 was increased by 23.99%,35.76%,49.40%and 56.13%,respectively,the average daily CH₄ content reached the maximum in stage 3,which was 90.72%.The enzyme activity analysis of the reactor showed that n ZVI increased the activities of AK and DHA.However,the activity of enzyme F₄₂₀ was low,which might be because n ZVI enhanced the homoacetogenic acetic acid pathway,and more acetic acid was produced to participate in the acetoclastic methanogenesis pathway.The microbial community abundance analysis showed that the addition of n ZVI changed the structure of microbial community and increased the abundance of homoacetogenic bacteria（Clostridium）and hydrogenotrophic methanogens（Methanobacterium）.（3）The enhancement of anaerobic digestion efficiency of CSTR by n ZVI corrosion H₂evolution.The results showed that the wastewater treatment of CSTR was severely inhibited in stage 1.This was mainly due to the high concentration of n ZVI in the reactor,which resulted in a rapid increase in p H and inhibited the activity of anaerobic microorganisms.The performance of CSTR operation and the biogas upgrading were the best in stage 4.The effluent TCOD decreased from 722.24 mg/L of the stable stage to 445.87 mg/L,the average daily CH₄yield increases by 45.25%compared to the stable stage,and the CH₄ content reached 95%.The results of EPS,TEM and enzyme activities showed that in stage 4,EPS content was the highest,microbial cells could maintain normal morphology and structure,and the enzyme activity was high.In stage 4,the relative abundance of acid generation microorganism（Chloroflexi and Firmicutes）and methanogens（Methanothrix）were increased.