Study On The Mechanisms Of Anaerobic Digestion Of Excess Sludge For Methane Production Enhanced By Waste Iron Scrap

The Paris Climate Treaty implies that the operational era of wastewater treatment plants?WWTPs?towards carbon neutrality is coming,which contributes to the reconsideration of anaerobic digestion?AD?of excess sludge.However,the low energy conversion efficiency of excess sludge severely limits the extension and popularization of this technique,so some measures must be taken to enhance it.Some trials have verified that adding waste iron scrap?WIS?to AD system can enhance methane?CH₄?production effectively,but the study on the mechanisms of this technique has not been reported yet.Therefore the experimental research and mechanisms of enhancing anaerobic digestion of waste activated sludge for methane production by waste iron scrap was deeply studied in this paper.Firstly,the author reviewed the research actuality of enhancing anaerobic digestion for methane production by WIS.It developed from hydrogen?H₂?evolution from iron corrosion;then the roles of iron in reducing ORP,affecting biochemical/physiological characteristics of microorganisms and the enzyme activities were fully analyzed;finally,the environmental and economic issues of WIS-based anaerobic digestion were evaluated using the method of life cycle assessment?LCA?.Overall,it fully demonstrated the the feasibility of enhancing anaerobic digestion for methane production by WIS from three points of view: technology,environment and economics.In the previous experimental work,WIS and Na₂S were added to some bottles with excess sludge in them respectively?without biological effects?to demonstrate WIS and Na₂S have the same effects in lowering ORP,and the results showed that the optimal dosages of WIS and Na₂S were10 g/L and 500 mg/L respectively,and the ORP could decreased to-350 m V.When Na₂S was used as the replacement of WIS and added to the AD system,results showed that the change of ORP caused by WIS was not obvious enough to exert significant influence on methane production,thus the possibility of decreasing ORP to enhance CH₄ production by adding WIS was excluded.In the main body of the research,3 two-phase AD reactors were adopted,one as the control system and the other two with 10 g/L of WIS in their acidogenic phase and methanogenic phase respectively.The main monitoring items contained the VSS removal ratio,ORP,VFAs,biogas production,the concentration of iron,microbial community structure and the activities of enzymes in the presence of WIS,and the mechanism of enhancing anaerobic digestion of waste activated sludge for methane production by WIS was discussed based on the above parameters.The study found that WIS can improve the VSS removal ratio by 5.0% when it was added to the acidogenic phase,while another 13.2% improvement was achieved when added to the methanogenic phase,and the biogas production increased 10.0% and 16.5%,while the methane production increased 10.1% and 21.4% respectively.Besides,WIS failed to change the ORP level to a large extent and the CH₄ increment was not that obvious;the contribution of evolutional hydrogen corrosion to methane production was only 3.7%,though it did have slight promotion to methane production.As a matter of fact,other factors,such as the activation of microorganisms,the improvement of biodegradability of refractory organics by WIS,and participating in biochemical reactions as electron donor?hypothesis?are the main forces of WIS-enhanced AD system,and their contribution accounted for about 96.3%.