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[SANET-MG] Bug power
http://www.i-sis.org.uk/BugPower.phpISIS Press Release 03/06/05
Waste-gobbling bacteria may be our dream ticket to clean renewable
energy. Dr. Mae-Wan Ho
A fully referenced version of this paper is posted on ISIS members’
website. Details here
Resources and energy from wastes
Bacteria that gobble wastes are a godsend. They prevent the build up of
wastes in our environment and play an indispensable role in making
wastewater safe for domestic animals, wild life, and human beings. In
many Third World countries, these same bacteria are working miracles
turning manure and other wastes into valuable resources to support
highly productive farms that require no input and generate little or no
waste ("Dream farm", this series). When these bacteria are confined in
anaerobic digesters with limited or no access to oxygen, they ferment
the wastes, release and conserve nutrients for livestock and crops, and
produce ‘biogas’ as by-product, which typically consists of about 60%
methane (CH4) and a small amount of hydrogen (H2), both of which can be
burnt as smokeless fuel.
Within the past two years, these same bacteria are showing even more
remarkable potential for producing clean and renewable energy while
reducing greenhouse gas emissions.
Hydrogen economy on potato waste
The "hydrogen economy" is on everyone’s lips as the answer to the
ultimate clean energy. Burning hydrogen produces pure water instead of
green house gases, and it is by far the most energetic fuel on earth,
weight for weight. But in order to really reduce green house gas
emissions, hydrogen must be produced sustainably with renewable sources
such as sun, wind and biomass. About half of all hydrogen produced
currently is from natural gas, the rest is produced primarily using
other fossil fuels. Only 4% is generated by splitting water using
electricity derived from a variety of sources.
At BIOCAP Canada’s First National Conference in February 2005, a
research team at the Wastewater Technology Centre and the University of
Waterloo in Ontario, Canada, presented a poster describing a prototype
process for producing substantial amounts of hydrogen as well as methane
from potato waste .
The team used a two-stage anaerobic digestion to get first hydrogen and
then methane. In this way, it was possible to optimize the first stage
for producing hydrogen. The key appears to be an acidic pH of 5.5 in the
hydrogen reactor, instead of pH 7 in the methane reactor. Both reactors
were run at 35C.
They pulped the potatoes bought from a store and treated the slurry with
peptone (an enzyme that breaks down protein), then seeded the two
reactors – one for hydrogen the other for methane - with digested sludge
from the local wastewater treatment plant to get the bacteria in place.
For the hydrogen reactor, the seed sludge was pre-cultivated in a
sucrose medium for a few days before switching to potato waste when high
hydrogen production was confirmed. For the methane reaction, no
precultivation of the sludge was required.
From the 4th day, the potato pulp replaced sucrose and hydrogen biogas
was produced continuously for a further 90 days. The maximum production
rate from the one litre reactor was 270ml/h on the 17th day, and the
average rate over the entire 90-day period was 112.2ml/h. The hydrogen
fraction fluctuated between 39 and 51 percent of the biogas (v/v). The
average chemical oxygen demand (COD) concentration (a measure of the
amount of waste present) of the fluid coming out of the hydrogen reactor
was 7 220mg/L, at an input concentration of 12 800mg/L. So more than 40
percent of the waste was removed.
Once hydrogen production became stable after the 20th day, the outflow
from the hydrogen reactor was transferred to the second, bigger
(methane) reactor, 5 litres in volume. During the 70 days of operation,
methane biogas was produced continuously; the maximum rate was 410ml/h,
and the average rate, 213 ml/h. The concentration of methane in the
biogas was between 69 and 79 percent. The average COD concentration in
the methane bioreactor outflow was 4 130 mg/L. Again, the process
removed more than 40% of the wastes. Together, the two reactors removed
68% of the waste.
Based on the hydrogen and methane production rates, the average energy
yield from each kilogram dry weight of potato waste was 4.96 MJ (1.4kWh)
and the maximum energy yield, 9.58 MJ (2.7kWh). For comparison, burning
1 kg wood yields about 20MJ . But because the energy is generated
from waste, it is essentially free, and does not require chopping down
Potato is the third largest food crop in the world, and Canada is one of
the leading producers (4.7million tonnes annually). Large amounts of
potato waste come from food and potato processing plants. This is
potentially a huge source of renewable, clean energy.
Dual purpose microbial fuel cell
A research team in Pennsylvania State University has also discovered how
to coax the same bugs to make plenty of hydrogen while they are gobbling
When the bacteria ferment glucose, they generate a maximum of 4
molecules of hydrogen per molecule of glucose and end up at best with
two molecules of acetic acid that they cannot convert further to
hydrogen due to an electrochemical barrier. But, given a little
electrical boost, the bacteria can jump over the barrier to generate
The research team, led by Dr. Bruce Logan, already made news in 2004
, when they succeeded in getting the bacteria to produce electricity
while removing wastes.
The bacteria were put into a microbial fuel cell that generated 26mW m2
of electricity while removing up to 80% of the wastes that flowed through.
These waste treatment bacteria, numerous species belonging to many
genera including Geobacter, Shewanella, and Pseudomonas, have the
ability to transfer electrons obtained by fermenting wastes to external
metals . When the bacteria are attached to electrodes, the electrons
are transferred to the electrodes (the anode), to flow through an
external circuit to the cathode where they combine with oxygen from the
air and protons (hydrogen ions) to form water.
The reactor then used was a single cylindrical plexiglass chamber the
size of a soda water bottle in which the anode, consisting of eight
graphite rods, was placed in a concentric arrangement surrounding a
central cathode that was exposed to air. The air-porous cathode
consisted of a carbon/platinum catalyst/proton exchange membrane layer
fused to a plastic support tube.
The efficiency of the system, based on waste removal and current
generation was less than 12%, indicating that a substantial fraction of
the organic matter was lost without generating current; perhaps in
producing more bacteria. But as the bacteria were doing their intended
job, which was to remove waste, any electricity generated at the same
time was an extra bonus.
Excluding air and boosting electric potential
Now, the team has discovered that by excluding air from the cathode, and
by giving the bugs a boost of about 250mV, they can make the bugs
produce hydrogen at high efficiency. They refer to this process as
electrochemically assisted microbial production of hydrogen.
Normal fermentation converts glucose to dead end products such as acetic
and butyric acid:
In the first case, four molecules of hydrogen are generated, and in the
second, only two molecules. The greatest theoretical yield possible is
four molecules of hydrogen per molecule of glucose.
The microbial fuel cell, however, offers a new solution to the problem.
By augmenting the electric potential in the microbial fuel cell circuit,
it gave just the little help needed for the bacteria to make hydrogen
out of acetic acid.
In a typical fuel cell, the open circuit potential of the anode is about
–300mV. If hydrogen is produced at the cathode, the half reactions
occurring at the anode and the cathode with acetic acid oxidized at the
anode, are as follows:
In order for the bugs to donate electrons to the anode from acetic acid,
however, the anode potential has to be made less electronegative.
To improve the efficiency of the intended process, the researchers also
created a two chamber microbial fuel cell instead of the one-chamber
version they had previously constructed. One chamber contained the
anode, the other the cathode, separated by a proton exchange membrane. A
major advantage of housing anode and cathode in separate chambers is
that the hydrogen produced at the cathode is separated from the carbon
dioxide at the anode at source. Instead of being exposed to air, the
cathode chamber was sealed. A voltage of 250mV or greater was applied to
the circuit by connecting the positive pole of a power supply to the
anode, and the negative pole to the cathode.
The external power supply increased the anode potential from –300mV to
-291mV with a boost of 250mV and to –275mV with a boost of 850mV,
producing hydrogen and degrading more than 95% of the acetate in the
process. The recovery of electrons as hydrogen was over 90%. The
Coulombic efficiency - defined as the recovery of total electrons in
acetate as current - ranged from 60 to 78% depending on the applied
voltage. Thus 2.9 of the theoretical maximum 4 molecules of hydrogen are
obtained from the acetic acid reaction with water by an injection of
250mV of electricity (see equation 3). This compares favourably with the
costly1800-2000 mV needed for getting hydrogen from splitting water .
A combined fermentation and bioelectrochemically assisted anaerobic
microbial fuel cell has the potential to produce as much as 8 to 9
molecules of hydrogen starting from a molecule of glucose (The
theoretical maximum is 12, see equations 1, 3 and 4.)
With this bioelectrochemically-assisted reactor, hydrogen can be
produced from any type of biodegradable organic matter. Combined
hydrogen production and wastewater treatment will offset the substantial
costs of wastewater treatment as well as provide a contribution to the
hydrogen economy. As the technology is rather simple, it can be adapted
for use at different scales, in third world countries as well as
At the BIOCAP Canada conference referred to earlier, another poster
pointed out that 45 of 56 wastewater treatment plants in large urban
areas of Ontario, Canada incorporate an anaerobic digestion process to
reduce the volume of disposable sludge; but the methane produced is
mostly wasted by being flared off to the atmosphere. A conservative
estimate suggests that if all the wastewater sites were to use anaerobic
digesters and simply recover the methane to generate electricity, this
would produce 1.51 GWh/day . It was a small percentage of the total
of 317GWh consumed each day in Ontario. But on average, 0.3kg of CO2 is
emitted per kWh energy produced from Ontario Power Generation, so simply
recovering the biogas energy from the current sites using anaerobic
digesters represents a saving of 432 tonnes of CO2 per day.
Imagine what can be achieved if waste treatment were optimised for
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