Lignocellulosic biomass includes corn stover, straw, wheat stover, algae and others. The primary components in lignocellulosic biomass are cellulose, hemicellulose and lignin. Compositions differ for different types of biomass. Lignocellulosic biomass is considered unfer­mentable because most microbes cannot degrade it

without pretreatment and lignin is optimally degraded under aerobic conditions via several dioxygenase — type enzymes, although some anaerobic bacteria can degrade it, albeit slowly. Pretreatment methods include mechanical, hydrothermal, biological, chemical, ammo­nia or supercritical CO2 explosion and ionic liquid extraction (Gu, 2013). An MFC using corn stover after steam-explosion pretreatment as the substrate achieved a maximum power density of 861 mW m~2 (Zuo et al.,

2006) . MFCs fed with Chlorella vulgaris and Ulva lactuca powders achieved maximum power densities of 0.98 Wm-2 (277 W m~3) and 0.76 Wm-2 (215 Wm-3), respectively (Velasquez-Orta et al., 2009).

Cellulose is relative easy to utilize by MFCs compared with lignocellulosic biomass. A maximum power den­sity of 272 mW m~2 was achieved using carboxymethyl cellulose as substrate in an MFC (Rezaei et al., 2009). This means that it is possible to utilize the tissue paper (cellulose) in municipal wastewater as substrate. Table 9.3 shows the list of substrates used for MFCs studied until 2013.

SUMMARY AND PERSPECTIVES

This chapter discusses the operating principles of MFCs and various aspects in bioelectrochemistry in MFC research. Although tremendous advances have been made around 2013 in academic MFC research including a much better understanding of biofilm electro­chemistry and better reactor designs, major technological hurdles remain for practical MFC applications beyond powering sensor devices. It is unreasonable to expect MFCs to reach power densities on par with those from chemical fuel cells because MFCs are powered by low- energy-density fuels such as dilute organic matter in wastewaters. However, it is still necessary to increase MFC power generation to what would be considered a useful level (e. g. to offset part of the energy input in wastewater treatment), much higher than what has been achieved.

Various approaches have been attempted to increase MFC performance including improved reactor designs, electrode and membrane materials, feedstock selection and modification, introduction of exogenous mediators, and utilization of secreted endogenous mediators. Unfortunately, many of the improvements come with inherent cost increases with little hope for practical applications. Some MFC researchers have come to realize that a breakthrough in biofilm engineering should be explored. Recent discoveries such as interspe­cies electron transfer, conductive cell aggregates and long-distance conductive filaments provide new hope for means to engineer robust "super-bug" biofilms with greatly enhanced electron transfer capacity and a voracious appetite for complex organic matter digestion. The dawn of a new era for MFC research might be in sight and the synergistic involvement of biochemical and environmental engineers, microbiologists and mo­lecular biologists may soon bear fruit in this exciting field of practical research.