Environmental Engineering Reference
In-Depth Information
as ethanol, acetate, butyrate, and propionate, have been found to accumulate in
the anode [21, 22].
One study is particularly illustrative in highlighting the positive syntrophic
interactions between ARB and fermenters. Ren et al. [22] demonstrated colla-
boration between ARB and fermenters to transfer electrons from cellulose -
polysaccharide derived from beta-glucose linkage - to the anode. A pure culture
of Geobacter sulfurreducens, a well-known ARB, was incapable of cellulose
hydrolysis and anode respiration. However, in the presence of Clostridia cellulo-
lyticum, which hydrolyzes and ferments cellulose to acetate, ethanol, and hydro-
gen, G. sulfurreducens used some of these fermentation byproducts to respire the
anode to support the generation of electricity by an MFC. This collaboration
between ARB and fermenters is analogous to the processes that occur in anaero-
bic digesters: McCarty describes anaerobic digestion as an assembly line in which
several microbial species cooperate to break down organic materials to methane
[23]. In the case of the MFC anode, several different microbial species appear to
collaboratively funnel the electrons from the waste ultimately to the anode.
In the biomass-to-energy field, the important niche of an MFC is that it
directly generates electrical energy, which relates measurable outputs from an
MFC: current and voltage. Current describes the amount of charge (or elec-
trons) passing through the circuit per unit time. Voltage describes the amount of
energy that is carried by a unit charge (e.g., an electron equivalent) that passes
through the circuit. Multiplied together, current and voltage define power,
which is the rate at which energy is generated. Current also relates to three
wastewater treatment objectives of an MFC:
The rate and amount of the organic waste that is removed
The generation of organic byproduct generated by fermentative bacteria
The substrate flux and organic loading rate
When comparing MFCs to other biomass-to-energy processes, researchers
consider two types of calculations to asses the energy efficiency of the process.
First is the energy capture efficiency (ECE), in which an energy balance is made
to determine how much energy is converted into electrical energy from the
original amount of energy fed into the reactor as substrate. Second is the
process energy efficiency (PEE), which is the ratio between the energy needed
to run the biomass-to-energy process and the energy collected as electrical (or
chemical) energy. Additionally, if the system is also used as a wastewater
treatment process, then it is important to determine the treatment efficiency
(TE), in which the chemical oxygen demand (COD) removal is calculated.
We use mathematical models that help us understand the interrelationships
among current, voltage, ECE, and TE for an MFC process. We present the
model in three parts:
1. Voltage in an MFC
2. Current density in the MFC's anode
3. Coulombic efficiency, ECE, and COD balance in an MFC
