'Digestion chamber' creates energy

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.