| Anaerobic digestion (AD) of organic matters is a widely used technology in the efficient treatment of organic waste and the simultaneous generation of renewable energy source. With respect to Upflow anaerobic sludge blanket (UASB), the presence of granular sludge is a major characteristic of anaerobic reactor, which leads to high biodegradation efficiency. However, sludge granulation is a long-term process that generally takes three to eight months. Efforts need to enhance this process. Also, anaerobic reduction of azo linkages (-N=N-) present economical and environmentally friendly ways to remove dyes from wastewaters, but the performance gets worse under high azo dye concentrations. In addition, anaerobic pretreatment, i.e., acidification often presents low performance in practical process and needs to be enhanced. Therefore, zero valent iron (ZVI), a reducing agent, is used to enhancing these processes at the aim of helping create an enhanced anaerobic environment that may improve the performance of anaerobic digsetion due to its reductive property. According to the experimental study and theoretical analysis, we can draw the following conclusions:1. A zero valent iron (ZVI) bed with a pair of electrodes was installed in an upflow anaerobic sludge blanket (UASB) reactor to create an enhanced condition to increase the rate of anaerobic granulation. The effects of an electric field and ZVI on granulation were investigated in three UASB reactors operated in parallel:an electric field enhanced ZVI-UASB reactor (reactor R1), a ZVI-UASB reactor (reactor R2) and a common UASB reactor (reactor R3). When a voltage of 1.4 V was supplied to reactor R1, COD removal dramatically increased from 60.3% to 90.7% over the following four days, while the mean granule size rapidly grew from 151.4μm to 695.1μm over the following 38 days. Comparatively, COD removal was lower and the increase in granule size was slower in the other two reactors (in the order:R1>R2>R3). The electric field caused the ZVI to more effectively buffer acidity and maintain a relatively low oxidation-reduction potential in the reactor. In addition, the electric field resulted in a significant increase in ferrous ion leaching and extracellular polymeric substances (EPS) production. These changes benefited methanogenesis and granulation. Scanning electron microscopy (SEM) images showed that different microorganisms were dominant in the external and internal layers of the reactor R1 granules. Additionally, fluorescence in situ hybridization (FISH) analysis indicated that the relative abundance of methanogens in reactor R1 was significantly greater than in the other two reactors. Taken together, these results suggested that the use of ZVI combined with an electric field in an UASB reactor could effectively enhance the sludge granulation.2. A zero valent iron (ZVI) bed with a pair of electrodes was packed in an anaerobic reactor aiming at enhancing treatment of azo dye wastewater. The experiments were carried out in three reactors operated in parallel:an electric field enhanced ZVI-anaerobic reactor (R1), a ZVI-anaerobic reactor (R2) and a common anaerobic reactor (R3). R1 presented the highest performance in removal of COD and color. Raising voltage in R1 further improved its performance. Scanning electron microscopy images displayed that the structure of granular sludge from R1 was intact after being fed with the high dye concentration, while that of R3 was broken. Fluorescence in situ hybridization analysis indicated that the abundance of methanogens in R1 was significantly greater than that in the other two reactors. Denaturing gradient gel electrophoresis showed that the coupling of electric field and ZVI increased the diversity of microbial community and especially enhanced bacterial strains responsible for decolorization.3. A novel strategy for enhancing anaerobic wastewater treatment via dosing Fe0 powder in acidification reactor was reported in this study. Experimental results showed that Fe0 powder dosed efficiently improved the acidification performance. Acidification reactor with Fe0 dosed (Al) presented higher and more stable performances in COD removal (ranging from 45% to 56%) and the degree of acidification (ranging from 84% to 91%) when lowering the HRT from 6 h to 2 h; however, those of the reference reactor without Fe0 (A2) significantly declined from 45% to 25% and 65% to 30%, respectively. Fe0 dosed could help optimize fermentation type, especially decreasing propionate which was lower in acetification as compared with butyrate. Reactor A1 presented 14%-20% less of propionate composition, but 8%-11% and 6%-9% more of acetate and butyrate composition respectively than reactor A2. Fe0 dosed improved 17-fold of the activity of pyruvate-ferredoxin oxidoreductase which is a crucial enzyme in the hydrolysis/fermentation, leading to an enhanced acidogenesis. Fuorescence in situ hybridization analysis indicated that Fe0 dosed increased the abundance of acidogens, and especially acetogens, coincident with higher acetate production. These effects facilitated the following methanogenic performance. Therefore, the Fe0 enhanced acidification is helpful to form the favorable fermentation type and low the subsequent treatment loading, which is expected to broadly apply in wastewater pretreatment, two-stage anaerobic digestion and even waste sludge fermentation.
|
PDF Full Text Request