ISIS Report 03/06/05
Bug Power
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 [1].
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 [2]. But because the energy is generated from waste, it is
essentially free, and does not require chopping down trees.
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 wastes
[3].
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 more hydrogen.
The research team, led by Dr. Bruce Logan, already made news in 2004
[4], 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 [5]. 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 [6].
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 industrialised countries.
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 [7]. 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
hydrogen production.
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