Iron Enhanced Microbial-electrocatalysis Of Anaerobic Wastewater Treatment

Anaerobic digestion (AD) is a desirable and widely used technology for high-strength organic wastewater treatment, simultaneously with the generation of bio-energy resources. Slow metabolism of methanogens and its sensitive characteristics to environmental perturbation are liable to cause the unbalance between acidogenesis and methanogenesis, thereby resulting in the accumulation of volatile fatty acids (VFAs), inhibition of methanogenesis and even failure of anaerobic process. Therefore, it is necessary to develop a new approach to enhance the effective degradation of VFAs and improve the ability of methanogenesis. This study is meaningful for efficient treatment of lugh concentration organic wastewater, bio-energy recovery and the application of AD in a wider field.To solve the above issues, we explored the performance and potential mechanisms of zero valent iron (ZⅥ) and Fe(Ⅲ) on the enhancement of AD, and we mainly focused on the study of microbial electrocatalysis of anaerobic methanogenesis system based on the iron electrode and Fe(Ⅲ) oxides. Furthermore, we also investigated the coupling effects among electrochemical technology, iron (0, Ⅲ) enhanced technology and anaerobic digestion, and clarified the synergistic relationships and mechanisms among Fe (0, Ⅲ), electricity and anaerobic microbial community. Main contents and results are as follows:(1) A novel method via dosing zero-valent iron (ZⅥ) into an anaerobic reactor significantly enhanced anaerobic methanogenesis in sulfate-containing wastewater treatment. The results showed that ZⅥ as a reducing agent could buffer acidity, maintain neutral pH(7-8) and reduce the inhibition of H2S generated from sulfate reduction on methanogenesis, resulting in high COD removal and methane production. Molecular biology analysis showed that the fractions of methanogens and SRB presented highest in the upper portion and the bottom of the ZⅥ-anaerobic reactor, respectively. This method realized the functionalized partitioning of microbial activities that was similar to a two-stage anaerobic reactor for sulfate-containing wastewater treatment, but in a single anaerobic reactor. The addition of ZⅥ in an anaerobic reactor helped to enhance anaerobic methanogenesis.(2) Based on the characteristics of microbial Fe(Ⅲ) reduction for the degradation of organics, a novel approach via dosing Fe2O3into an acidogenic sulfate-reducing reactor was developed to enhance the degradation of organic acids in the sulfate-containing wastewater treatment. The results showed that the addition of Fe2O3resulted in microbial reduction of Fe (Ⅲ), which enhanced the degradation of organic acids through assisting sulfate reduction process. In this way, high COD removal (27.3%) and sulfate reduction (57.9%) were realized. The qualitative and quantitative analysis of molecular biology revealed that this method realized the enrichment and symbiosis of iron reducing bacteria, sulfate reducing bacteria and acidogenic bacteria. Thereinto, the abundances of sulfate reducing bacteria Desulfovibrio marrakechensis and Iron-reducing bacteria HN54were significantly higher than that in the control reactor.(3) Using the characteristics of ZVI on the enhancement of anaerobic methanogenesis, a Fe anode-based microbial electrocatalysis cell (MEC)-upflow anaerobic sludge blanket reactor (UASB) was developed i.e. a pair of Fe-graphite electrodes was inserted into an UASB reactor with a proper voltage input. The addition of ZVI could strengthen the anaerobic process including the enhancement of anaerobic reducing atmosphere and the growth of anaerobic bacteria, further realizing the effective treatment of high salinity wastewater, dye wastewater and nitrogen-containing wastewater. In addition, this coupled reactor also enhanced acidogenic process of organics. For the high salinity wastewater treatment, this coupled reactor presented high salt-adapted ability and high degradation ability. Under the high salt condition, the COD removal of this coupled reactor reached93%while the COD removal of the control reactor was only53%and the accumulation of organic acids occurred. The molecular biology analysis indicated that a large amount of salt-adapted bacteria and archaea were enriched in this coupled reactor and the relative abundance of propionate-utilizing bacteria was higher. For the high azo dye wastewater treatment (1200mg/L reactive brilliant red X-3B), this coupled reactor showed high decolorization (83.4%) and COD removal (84.7%). The coupling effect of Fe2+generated from anode and electric field enhanced the production of extracellular polymer substances (EPS). The dynamic analysis of microbial community indicated that the coupling of Fe electrode and electric field significantly accelerated the enrichment of anaerobic bacteria, which was meaningful for shortening the start-up time and increasing organic loading. The coupling of Fe electrode and electric field significantly improved the efficiency of hydrolysis and optimized the composition of organic acids, realizing the high COD removal and the dominant acetate production. Additionally, the yields of methane of this coupled reactor was about two times higher than that of the control reactor. The qualitative and quantitative analysis of molecular biology indicated that a large amount of archaea was retained in this coupled reactor, in which the acetate-utilizing methanogens was dominant. From comparsion, the abundance of archaea in the control reactor was low and the hydrogen-utilizing methanogens was dominant. The Fe anode-based MEC-UASB reactor could enhance the enrichment of ANAMMOX bacteria, which further shortened the start-up time (about24%). Through investigating the effects of different voltage on the nitrogen removal of ANAMMOX, we discovered that raising the voltage applied for the electrode in a given extent (≤0.6V) enhanced the performance of the reactor, while a voltage more than0.8V reduced the anammox performance.(4) Using the characteristics of microbial Fe(III) reduction on the degradation of organics and the enrichment of iron reducing bacteria (IRB), a novel method via dosing Fe(OH)3into a MEC-UASB reactor enhanced both anaerobic digestion and anodic oxidation of organics, realizing the high treatment performance of organic wastewater. Molecular biology analysis indicated that the addition of Fe(OH)3increased the abundance and biodiversity of bacteria and archaea communities. Meanwhile, the effects of electric field changed the microbial community structure on the surface of electrodes, which created a vertical distribution of different functional microbial community in a single reactor. On the other hand, this method also presented high performance for color removal. Enzyme activity analysis showed that the coupling of Fe(III) and electric field could improve the activity of azoreductase that was important for the decolorization. Subsequently, a novel iron reducing bacteria (IRB) Aeromonas hydrophila. XB (KC507819) with electrochemical activity was isolated from the anodic biofilm of MEC-UASB reactor. The strain XB had the electrochemical activity, Fe(III) reducing activity and decolorization capacity.