Bug PowerThe Institute of Science in Society
Science Society Sustainability
http://www.i-sis.org.uk
Waste-gobbling bacteria may be our dream ticket to clean renewable energy. Dr. Mae-Wan Ho
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 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.
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