08.08.2015   BIOENERGY. RESEARCH:. ADVANCES AND. APPLICATIONS

MFCS FOR WASTEWATER TREATMENT. WITH CONCOMITANT ELECTRICITY. PRODUCTION

MFC Reactor Designs

There are many different types of MFC bioreactors. They include single-chamber, dual-chamber, multi­chamber, membrane-less, multianode, multicathode and so on. Many MFC reactors were discussed by Du

et al. (2007). More recently, Zhou et al. (2013) reviewed some new MFC reactors and their combinations, including MFCs operated as microbial electrolysis cells (MECs) to produce bio-products such as hydrogen and methane. It should be pointed out that improvement in MFC reactor design must consider cost and mainte­nance. Complicated designs are not only costly but also prone to biofouling, causing maintenance and sus­tainability problems. A simplistic tubular MFC reactor

with convective axial flow was proposed (Zhou et al.,

2013) . To reduce cost and fouling, no membrane was used. To prevent oxygen back-diffusion into the anodic region, a substantial flow rate from the anode to the cathode is required. This means that the biofilm has to be highly efficient in the digestion of organic matter in wastewater streams. This type of design will become attractive only when robust "super-bug" biofilms are successfully engineered.

FIGURE 9.11 Microcentrifuge tubes containing chloroform — extractable pyocyanin (blue bottom layer and the "merlot" colored) and water-soluble pyorubrin layer (top). The tube on the left is derived from a lasI rhlI mutant that is incapable of quorum sensing and, as such, is incapable of producing pigments, while that on the right is from rpoS mutant bacteria that overproduce both pyocyanin and pyorubrin. Source: Suh et al. (1999). (For interpretation of the references to color in this figure legend, the reader is referred to the online version of this book.)

Substrates Used in MFCs

In MFCs, the substrates greatly impact their perfor­mances such as power density and Coulombic efficiency (Pant et al., 2010). The substrates range from the simple volatile fatty acids (VFAs) to complex compounds such as lignocellulosic biomass. Anaerobes evolved when the earth’s atmosphere was still anaer­obic long before aerobes evolved. Many of them lack
the metabolic pathways to utilize high-grade organic carbons such as cellulose, hemicellulose, various hex — oses and phenylpropane moieties (components of lignin). Most of the electrogenic microbes capable of DET feed only on low-grade organic carbons such as VFAs and alcohols. Only a few organisms such as R. ferrireducens (Chaudhuri and Lovley, 2003; Schroder, 2007) utilize glucose, while Geobacter and Shewanella strains cannot (Lovley, 2006a). This limits MFC power output because high-grade organic carbons are unutilized.

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