Environmental Engineering Reference
In-Depth Information
the insoluble constituents. One such modification of the anode was reported by
Niessen et al. [41]. A platinized electrode was combined with a fermentative organ-
ism (
Clostridium beijerinckii or C. butyricum
) to demonstrate utilization of starch
and molasses as the carbon source for electricity production at high current densities
(between 1 and 1.3 mA/cm
2
). Another study investigating utilization of cellulose in
MFCs reported use of enzymatic hydrolysis to solubilize the cellulose, with effective
electricity production with cellulase as the substrate [41]. The process of conversion
of complex organic matter into electricity or hydrogen can be broken down into three
steps. The first step is the depolymerization or breakdown of the complex organic
matter into its monomeric constituents. The second step, (using carbohydrates as
an example), is fermentative degradation of the sugars into volatile fatty acids and
hydrogen. The third step is the conversion of the VFAs and hydrogen to electricity.
Use of MECs can result in conversion of the VFAs to hydrogen. Direct conversion of
sugars to electricity is also possible [45]. However, when using microbial consortia
a recent study demonstrated that the primary route for electricity production from
glucose was via VFA and hydrogen formation [46]. This study used an inoculum
enriched on acetate. A similar study using an microbial consortia enriched on glu-
cose or sugars is needed to determine the path of conversion of sugars to electricity.
In MFCs using consortia enriched on acetate, potential for conversion of either the
VFAs or hydrogen to other products such as methane (via methanogenesis), has been
demonstrated [46]. Methane is unsuitable as a substrate for exoelectrogens, which
leads to lower coulombic efficiencies. The role of fermentative and methanogenic
bacteria is therefore quite important while considering electricity or hydrogen pro-
duction from complex organic matter, as in the case of food processing wastewaters.
Use of methods to minimize methanogens in MFC anodic communities, such as
intermittent aeration, have shown some success [34] but long term studies have not
been conducted.
The overall COD removal from most wastewaters in MFCs is usually very high
(approaching 90% or higher), which indicates that degradation of the organic car-
bon is not a problem. The problem is related to the electron acceptor used for
the bioconversion process. In addition to the diversion of the electrons towards
methanogenesis, other paths to electron oxidation also exist via use of nitrate, sul-
fate and oxygen as electron acceptors. Nitrate may be present at high concentrations
in wastewaters originating from fermentation operations such as brewery wastew-
ater. Sulfate may be an issue in food operations using groundwater as the source
of washwater. Oxygen leakage into the anode chamber also impacts coulombic
efficiencies (CEs), and becomes significant in membrane-free MFCs. In a study
investigating electricity production from starch processing wastewater, the COD
removal efficiencies of 96-98% were reported, while the CE was only 7% [39].
The huge inefficiency was essentially attributed to oxygen diffusion, although it
also included contribution of other electron acceptors in the wastewater. CEs for
MFCs fed with glucose have ranged from 28 to 59% [37, 46, 47]. Studies using
complex organic matter or wastewaters as the substrate report CEs in the range of
5-40 [25, 37-39]. The relatively lower CEs for the latter may also be due to the
