The idea of a microbial fuel cell that runs of sewage has been around for a long time and many prototypes have been built, including the school prototype shown above. The concept definitely works and produces electricity while purifies sewage. From the treatment angle, it reduces the cost of treating sevage in some 50% (because it generates instead of consuming electricity), from the electricity angle, sewage contains oxidizable organic compounds (600 mg/liter) that produce electricity directly, without passing through the methane phase and the need to burn methane. I do not foresee this technology competing on a commercial basis with other forms of generation, but under the current hysteria small prototypes could be sold including for schools.
Bacteria in a fuel cell work as they do in any sewage treatment system: They oxidize the organic matter, and they get energy from it and make new cells, but instead of sending electrons to oxygen, which is expensive in terms of energy to dissolve in water, they send those electrons to an electrode. In one model, bacteria are placed in the anode chamber of a fuel cell separated from oxygen. As the bacteria began to digest, they transfer electrons to an enzyme, which then transfers those electrons to an electrode. Because they do not have oxygen, they must transfer the electrons that they obtain from consumption (oxidation) of their food somewhere else other than oxygen; therefore, they transfer them to the electrode. In the microbial fuel cell, these electrons are transferred to the anode, while the counter-electrode (the cathode) is exposed to oxygen. At the cathode the electrons, oxygen, and protons combine to form only water.
Most microbial fuel cells use a two-chamber design (see above). A single-chamber microbial fuel cell (as shown here) consists of an acrylic cylinder with eight graphite anodes (or negative electrodes) inside, to which the bacteria attach, and a hollow central cathode (or positive electrode). Electrons flow along a circuit wired from the anode to the cathode. A better design is an upflow microbial fuel cell fed continually that works with chambers atop each other rather than beside each other. The cell is based on a carbon-based foam with a large pore size on which biofilm grows, making it possible to connect two electrodes in the anode and cathode chambers with a conductive wire.
Prof. Bruce Logan has the latest model (see pic). He writes that "While current generation using bacteria has been known to be possible for over a decade, only recently has it been shown that chemical mediators (toxic chemicals added to a reactor) are not needed. This finding that chemicals do not need to be added to wastewater could drive development of a completely new wastewater treatment technology based on MFCs. What is needed is a method to increase power flow by optimizing a fuel cell for use with bacteria (as opposed to hydrogen and inorganic catalysts). In a MFC, the potential created between electron carriers in the bacterial respiratory chain and oxygen is harvested by allowing bacteria to transfer electrons from respiratory enzymes to an electrode (anode) while still in an anaerobic environment. A second electrode (cathode) is kept in an aerobic environment, so that a potential is created. The flow of electrons from respiratory enzymes located on the outer membrane of the bacteria across this potential creates current which can be captured. The purpose of this SGER proposal is to demonstrate the feasibility of this MFC approach for wastewater treatment, and to show for bacteria we must optimize current flow at the cathode. In this project we will demonstrate power generation from wastewater is optimal for current flow in an MFC when substrate is first fermented, and that power generation can be much higher than previously believed. We will construct three types of MFCs in the laboratory: a batch system with a salt bridge that mimics a seawater type of fuel cell; an otherwise identical system that uses a proton exchange membrane (PEM); a fully optimized cathode that uses a direct air system."
The only patent I found on the subject is the vertical (non-batch) cell: Title:Upflow microbial fuel cell (UMFC)
Document Type and Number:United States Patent 20060147763
Kind Code: A1 Link to this page:http://www.freepatentsonline.com/20060147763.html
Abstract:An upflow microbial fuel cell in one embodiment is comprised of a generally cylindrical cathode chamber containing a cathode sitting atop a generally cylindrical anode chamber containing an anode, with a proton exchange membrane separating the two chambers, so that as influent is passed upwardly through the anode chamber electricity is created in a continuous process not requiring mixing such as with a mechanical mixer or the like. Electrodes are connected to each of the anode and the cathode for harvesting the electricity so created. Effluent may be recirculated through the anode chamber by a second inlet and outlet therein. A multiphase fuel cell includes a plurality of electrode couples arranged in a single chamber with an influent inlet near its bottom and an effluent outlet near its top, with the electrode couples connected in series to generate electricity at higher voltages. In another embodiment, the cathode chamber--preferably U-shaped--is positioned inside the anode chamber.
My comment: It is terrible vague. The direction of the flow is not material to the process, it could be downflow or sideflow. I am no patent lawyer but this is undefensible.
My idea is a "generally more or less double celled fuel cell with graphite anode and platinum cathode embodied as per the Secret Engineer's Secret Design" (the design is so obvious that I will not publish it here). The cell produces electricity that can be measured only by a sensible measuring instrument, and there is a need of several cells to light a lamp, so I cannot propose its use as a bonafide sewage treatment system nor as a economically competitive or significant energy source. It could find a market as (1) scientific parafernalia to be sold to school labs interested in environmental science (2) to "green" buildings as a green object of interest or conversation object, (3) idem to wastewater treatment plants (4) green sewage related sensors (to power instruments in places where there is no electricity, but I am not sure if it is reliable and powerful enough).