Research Of Enhanced Anaerobic Digestion Of Waste Activated Sludge By Addition Of Zero Valent Iron

Anaerobic digestion is a promising technology to recover energy from waste activated sludge. However, the sludge digestion is limited by its low efficiency of hydrolysis-acidification. As a result, anaerobic digestion of waste activated sludge usually requires pretreatment procedure to improve the bioavailability of sludge, which involves considerable energy and high expenditures. Zero valent iron (ZVI) as a reducing material is expected to enhance anaerobic process including the hydrolysis-acidification process. The following conclusions were achieved from the experiments:(1) ZVI powder was added into an anaerobic sludge digestion system to accelerate the sludge digestion. The results indicated that ZVI effectively enhanced the hydrolysis-acidification of sludge. Compared to the control test (without ZVI), the degradation of protein was increased by21.9%and the volatile fatty acids production increased by37.3%with adding ZVI. More acetate and less propionate are found during the hydrolysis-acidification with ZVI. The activities of several key enzymes in the hydrolysis-acidification increased by60～100%. ZVI made the methane production raise43.5%and sludge reduction ratio increase by12.2%. Fluorescence in situ hybridization analysis showed that the abundances of hydrogen-consuming microorganisms including homoacetogens and hydrogenotrophic methanogens with ZVI were higher than the control, which reduced the H2accumulation to create a beneficial condition for the sludge digestion in thermodynamics.(2) This study proposes a cost-effective method for enhanced high-solid anaerobic digestion of sludge without a pretreatment by directly adding iron scrap and rusty scrap into the digester. The results showed that the rusty scrap iron could increase by29.5%of methane yield and27.3%of VSS reduction, which were higher than the powder iron and the clean scrap iron. The improved performance were ascribed to the Fe(III) oxides on the surface of the scrap enriched iron-reducing bacteria to decompose complex matters of the sludge, which accelerated the anaerobic hydrolysis-acidification to benefit methanogenesis and organic mineralization. This study provided a cost-effective method for simultaneous usage of waste scrap and enhancement of sludge digestion.(3) A pair of iron-graphite electrodes installed into an anaerobic sludge digester to form a MEC-anaerobic reactor could significantly enhance the production of SCFA and hydrogen from waste activated sludge. After the26-day fermentation, the cumulative SCFA yield in the MEC-anaerobic reactor was331mg-COD/g-VSS,3.5folds of the control anaerobic reactor. The hydrogen produced in the MEC-anaerobic reactor reached90.6mL/g-VSS, while it was almost undetectable in the control reactor. The increased pH in the reactor resulted from the excessive utilization of H+by the cathodic hydrogen production was an important reason for prohibiting methanogenesis. Increase of voltage supplied might obtain a higher coulombic efficiency to enhance the anodic oxidation of complex matters, thus producing more SCFA and hydrogen. The Fe-C electrode at the proper voltage was beneficial for enrichment of iron-reducing bacteria to biologically catalyze electron transformation between the two electrodes.