I’ve written beforeabout methods and processes to treat wastewater  (here and here, for starters- you can find more by searching the index to the right for wastewater)- including methods that may be able to provide power to attenuate the costs of such treatment. Now, another concept to employ microbes to generate electricity has been published in the Proceedings of the National Academy of Sciences (right now, it’s ahead of print) describing work being done by Dr. Craig Criddle’s group at Stanford.

Drs. Criddle, Xiea,Yea, Hsub, Liuc, and Cuib described their prototype microbial battery. The microbes degrade the waste (by oxidizing them) at the anode, and the released electrons are collected at a solid-state cathode. The cathode can be repeatedly reused (even though electrons and other materials accumulate on it).

To reverse the changes (electron capture and material accumulations) from the process, the cathode is removed from the cell, oxidized by the oxygen in our atmosphere [the researchers have tried various temperatures to optimize the process], and then reinstalled in the battery. No oxygen is released in the process- nor does this process require the use of ion-exchange membranes, which can become fouled, thereby reducing the process efficiency.

Criddle et.al.’s process uses a single chamber (as shown above), with silver oxide as the cathode. Or course, this process has only been tested on a prototype stage (currently the size of a D cell battery), but the authors claim its efficiency approaches 49% (based upon the combustion enthalpy of consumed materials) or 44% (if the results are based upon the amount of organic matter added).

These high efficiencies, however, only consider the wastewater treatment portion of the process.   There are energy costs associated with regenerating the capabilities of the cathode.  (Otherwise, one would have to continually purchase new cathodes, which would make the process very expensive.)  When the energy expenditure for the cathode regeneration is included, the overall efficiency of the process is reduced  to some 30%. The authors claim this overall energy yield is equivalent to that of anaerobic digestion (where methane is produced in the process of treating the waste; the methane is then used for power).

Criddle et. al. are not proposing this to treat domestic wastewater; instead, they hope this process could be used in dead zones (where high organic levels exist in lakes, rivers, and oceans).   The dead zones cause the surrounding water to become depleted of oxygen, rendering life in said zones impossible. The microbes they have employed on the anode are anoxic (do not require the presence of oxygen; they are also called exoelectrogenic), so they would certainly be able to grow and convert the waste in the dead zones to energy.

Now, let’s see how the process works as it’s scaled up.

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