Acetate and glucose are two common substrates in laboratory studies. Compared to the recalcitrant substrates, they are far more readily utilized by microbes for energy generation. Thus, they are usually used as the carbon source for microbes used in MFCs. Acetate has an advantage that at normal temper­atures, it is not a good nutrient for fermentation and methanogenesis. In contrast, glucose is a fermentable sugar that can be consumed by the processes of fermen­tation and methanogenesis (Pant et al., 2010). Thus, the Coulombic efficiency of acetate is usually higher than glucose. However, glucose can be used to promote the microbial diversity of a biofilm consortium. When glucose was used as the substrate, a maximum power density of 216 mW m~2 was achieved (Rabaey et al.,

2003) , while it reached 506 mW m~2 for acetate (Liu et al., 2005b). Some other simple substrates such as buty­rate have also been used as the substrate in MFCs.

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Wastewater Types

Various wastewaters have been tested as substrates for MFCs because they contain many different kinds of organic carbon molecules. They are attractive for use in MFCs because the organic carbons are otherwise wasted. As shown in Table 9.3, the output power density is dependent on the wastewater quality (high COD values) and the MFC reactor structure. For example, a maximum power density of 528 mW m~2 for brewery wastewater was obtained (Feng et al., 2008), while an average power density of 72 mW m~2 was achieved for domestic waste­water (Sharma and Kundu, 2010). Some biorefractory wastewaters such as dye, leachates and pharmaceutical wastewater have also been tested for MFC power gener­ation. A landfill leachate containing heavy metals, dis­solved organic matters and other matters achieved a maximum power density of 1.38 mW m-2 (Greenman et al., 2009). A maximum power density of 9.1 W m~3 was achieved when using phenol as the sole carbon source. While glucose was added as a supplement, the maximum power density increased to 28.3 W m~3 (Luo et al., 2009a). In addition, some refractory compounds such as pyridine, quinoline and indole were also used as substrates for MFCs (Hu et al., 2011).