'Digestion chamber' creates energy
Mar 22 - McClatchy-Tribune Regional News - Hiroko Sato The Sun, Lowell,
Mass.
It's a giant green dorm that's 92 feet tall on Sawmill Road by the
Merrimack River. And it's home to countless invisible bugs that are
hungry for anything smelly and slimy.
From mechanical bar screens to aerated grit chambers, the Nashua
Wastewater Treatment Facility uses all kinds of mechanisms to remove
toxins and germs from sewage water. When cleaning is complete, the thick
and heavy sludge left over gushes into the steamy-hot chamber inside the
dorm, where millions of micro-organisms wait for the sediment to be
devoured. The metabolic processes reduce the volume of the sediment
while releasing methane gas -- a fuel used to generate electric power
for the plant.
"This is like the human stomach," says plant Superintendent Mario
Leclerc, looking at the egg shell-shaped "digestion chamber."
Since 2000, the Nashua Wastewater Treatment Plant has been using
"anaerobic digestion" -- a biological process that converts organic
waste into energy. Anaerobic digestion is not a new way to treat
wastewater. Rather, it's an add-on feature that allows treatment plants
to drastically reduce the amount of sludge, which saves on hauling fees,
while harvesting electricity.
The technology has been used throughout Europe for many years
to help reduce waste and is a requirement in Germany, according to
Leclerc. It's increasing in popularity in the U.S., as well, with Boston
using 12 digestion chambers that are much larger than the one in Nashua
is,
Leclerc said.
Motivating the trend is cost savings. The Nashua plant, which receives
12 million gallons of wastewater from across Nashua and Hudson, has so
far saved $1 million a year in sludge-hauling fees and $120,000 on
electric bills.
So how does the system work?
When sewage water comes in, treatment plants typically remove food
scraps and other large solids and add micro-organisms that break down
toxins, Leclerc says. After that process, treatment plants disinfect the
wastewater, and further separate water from smaller solids. Purified
liquid is then discharged into local streams, leaving large amounts of
sediment behind.
Many treatment plants have this sludge trucked to landfills. But at the
Nashua plant, the sludge goes directly into the anaerobic digestion
chamber.
The chamber has two pipes running through it -- one that sends the
sludge upward and another that sends it downward. That prompts a
constant and thorough mixing of sludge, Leclerc said.
"There is no dead spot in this tank," he said.
The micro-organisms used for anaerobic digestion are different from
those used for removing toxins from wastewater, according to Leclerc.
The digestion chamber has no air inside, so the bugs take oxygen from
sludge components. The key is to keep the chamber inside at 92 degrees
Fahrenheit to promote the biological process, Leclerc said.
In the end, the sediments are reduced in half, and the methane gas --
the byproduct of the process -- is sucked into a purification
compartment, where workers add wood chips that are impregnated with iron
oxide to the gas to remove hydrogen sulfate from it. The wastewater
treatment center then uses the gas to run a generator, and uses the
electricity to run the anaerobic digestion complex and parts of the
plant.
The electricity has so far reduced the plant's $60,000-a-month electric
bills to $50,000, about a 17 percent savings. The sludge that comes out
of the digestion chamber after the anaerobic process is applied to
cornfields in central New Hampshire to help enrich the soil, although
the plant pays a contractor for shipment.
The city spent $10 million to construct the anaerobic digestion complex.
Updating the system is also costly; a new generator cost the plant
$460,000.
But, at the savings rate of $1.1 million a year, the investment more
than pays off, Leclerc said. More importantly, he added, it helps reduce
waste and save the environment.
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McClatchy-Tribune Information Services
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