Effects Of Zero Valent Iron On Anaerobic Digestion Of Organic Fraction Of Municipal Solid Waste At High Organic Loading

Organic fraction of municipal solid waste（OFMSW）is the main component in Chinese MSW,and its effective separation and treatment is becoming increasly important in the optimization of MSW management system.The high-pressure extrusion pretreatment could separate the organic fraction with high biodegradability（wet fraction）from combustible waste（dry fraction）,then they would be anaerobic digested and incinerated respectively.However,with the pretreatment pressure increasing,the hydrolysis rate of wet fraction would be faster and the phenomenon of‘excessive acidification’easily appeared in anaerobic reactor.The study firstly discussed the upgrading and modification effect of high-pressure extrusion pretreatment on OFMSW.In laboratory scale experiment,after the pretreatment of 40 MPa,the low heat value of the dry fraction of simulated MSW reached to 18.7MJ/kg,which was increased by 243-293%compared with the raw MSW.The dissolution amount and rate of soluble chemical oxygen demand（SCOD）of separated wet fraction were higher than those of simulated OFMSW,and the biochemical methane potential（BMP）of wet fraction could reached to 674 mLCH4/gVS.High H2 partial pressure in the single-phase reactor was the main reason of‘excessive acidification’production.In the batch reactor operation,after zero valent iron（ZVI）added,the hydrogen partial pressure would decrease from 16%to 0 in 6 days,meanwhile,volume percentage of CH4 continuously increased.By contrast,in the control reactor at high organic loading rate（OLR）without ZVI addition,the hydrogen partial pressure maintained at 0.15-0.25 in a long term.Afterwards,under the effects of ZVI,the butyric acid converted to acetic acid rapidly and further utilized by methanogens,which increased the CH4 percentage to higher than 60%and achieved the goal of excessive acidification elimination ultimately.Furthermore,the CH4 yiled of the substrate also enhanced by 41.7%through ZVI dosing.In the operation of continuous anaerobic reactor with an effective volume of 40 L,the phenomena of‘foam accumulation’and‘mixture expansion’,which caused by VFAs accumulation,often occurred in two specific working conditions of‘high organic loading shocking’and‘restarting up’.These phenomena affected reactor’s stable operation.Comparing with the control reactors without ZVI addition,the sustainable organic loading shock could raise from 7.5 kgVS/m³/d to 10kgVS/m³/d through addition of 500 g ZVI in the reactor once.Furthermore,after the reactor stop running at a certain period,it could be restart up at the OLR of 5 kgVS/m³/d.High throughput sequencing and qPCR were also used for analyzing the alteration of bacterial and archaeal communities at vaious stages.The results showed that in the early stages,ZVI addition could increase the abundance of acidogenic bacteria and the amount of Methanobacteriales,which enhanced the methanogenesis pathway of interspecies hydrogen transfer（IHT）.During the middle and last stages of ZVI reactor,amounts of Methanosaetaceae and methyl coenzyme-M reductase（mcrA）gene were much higher than control groups.More importantly,the significant increasing of abundance of C.butyricum in ZVI reactor suggested it could be a conducting medium and construct a methanogenic mechnisim of direct interspecies electron transfer（DIET）with bacteria of Syntrophomonas and Methanosaetaceae.This mechanism enhanced the conversion from acetic to CH₄ and achieve‘excessive acidification’elimination and CH₄yiled improving.Life cycle inventory analysis was adopted to further evaluate the integraed environmental effects of various scenarios of MSW management process.The results indicated that the scenario of OFMSW separation,treatment through using anaerobic digestion and land utilization of digestate is an ideal method for MSW management in China.