Page 24 - Plastics News September 2017

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The same microbes What is Anaerobic digestion
will treat plastics and
Anaerobic digestion is a series of biological processes
water simultaneously.
in which microorganisms break down biodegradable
The microbes then
material in the absence of oxygen. One of the end
die and leave behind
products is biogas, which is combusted to generate
liquid and solid
electricity and heat, or can be processed into renewable
biomass, which can
natural gas and transportation fuels.
be used as fertilizer,
and biogas, which Anaerobic digestion is the simple, natural breakdown
can be separated into of organic matter into carbon dioxide, methane and
methane and carbon water, by two groups of microorganisms, bacteria and
dioxide. archaea. Since many of these are intolerant to oxygen,
this process is known as anaerobicAnaerobic digestion
Moeover the methane could be used to create heat and
occurs naturally in some soils and in lake and oceanic
energy, possibly to power the wastewater plant, and there
basin sediments, where it is usually referred to as
was potential for the carbon dioxide to be captured and
"anaerobic activity".[This is the source of marsh gas
reused. "Basically, plastics go into an anaerobic situation
methane as discovered by Alessandro Volta in 1776
in wastewater where the microbes digest the plastic and
create energy," he said. The digestion process
Thompson said Australia has more than 550 wastewater It begins with bacterial hydrolysis of the input materials.
treatment plants: "At least half would have anaerobic Insoluble organic polymers, such as carbohydrates,
digestion facilities attached so it's a good opportunity are broken down to soluble derivatives that become
to go down that path. "I have already got inquiries from available for other bacteria. Acidogenic bacteria then
overseas, including a large consortium in South America convert the sugars and amino acids into carbon dioxide,
that is quite interested to get involved and take the hydrogen, ammonia, and organic acids. These bacteria
technology over there," he said.Thompson said the poet convert these resulting organic acids into acetic acid,
system did not impact existing recycling practices because along with additional ammonia, hydrogen, and carbon
it targets plastics destined for the landfi ll and will add dioxide. Finally, methanogens convert these products
a new revenue stream for waste industry companies. He to methane and carbon dioxide. The methanogenic
said more than 1 million metric tons of plastics unfi t for archaea populations play an indispensable role in
recycling goes to landfi ll in Australia annually. anaerobic wastewater treatments.Anaerobic digestion
Thompson expects to have the fi rst two machines, each is used as part of the process to treat biodegradable
capable of processing 20 metric tons of plastic a week, waste and sewage sludge. As part of an integrated waste
operating commercially in mid-2018. management system, anaerobic digestion reduces the
emission of landfi ll gas into the atmosphere. Anaerobic
The machines will be built at wastewater treatment plants
digesters can also be fed with purpose-grown energy
in the state of Victoria. According to reports more than
crops, such as maize.
one million tonnes of contaminated plastic deemed unfi t
for recycling is sent to landfi ll in Australia each year. In Anaerobic digestion is widely used as a source of
the United States the fi gure is almost 10 million tonnes. renewable energy. The process produces a biogas,
consisting of methane, carbon dioxide and traces of
David Thompson said his plastic to energy technology
other ‘contaminant’ gases. This biogas can be used
had so far successfully been applied to polyethylene,
directly as fuel, in combined heat and power gas engines
polypropylene, polystyrene and expanded polystyrene.
or upgraded to natural gas-quality biomethane. The
He said the anaerobic digestion process varied upon
nutrient-rich digestate also produced can be used as
disposal feedstock and depended also upon temperature
fertilizer. With the re-use of waste as a resource and
and system set up. Poet is a semifi nalist in the 2017
new technological approaches that have lowered capital
Australian Technologies Competition, which promotes
costs, anaerobic digestion has in recent years received
startup technologies with global potential. Winners will
increased attention among governments in a number
be unveiled in October and November.
of countries.
Plastics News | September 2017 24