Waste from thousands of toilets will soon be transformed into the renewable energy biogas, the first of its kind in the Ocean State.
EAST PROVIDENCE — As a porous conveyor belt rolls forward, the murky soup it carries drains of water and leaves behind a thick sludge the consistency of wet cement.
There’s no question about it. This is a foul mix of the contents of tens of thousands of toilets from East Providence and the Blackstone Valley that has made its way through the Bucklin Point Wastewater Treatment Facility. It’s dirty, unsightly and only a little less smelly than you’d expect.
But don’t dismiss it as merely waste. It’s actually fuel.
All the sludge collected here is used to create biogas, a renewable energy source that will soon be harnessed to produce a quarter of the power for this sprawling sewage treatment plant on the Seekonk River.
When the Narragansett Bay Commission finishes testing this spring and brings it into operation by the summer, the 677-kilowatt biogas engine will be the first of its kind in Rhode Island.
Similar types of power generators have been built near the Central Landfill in Johnston — one that burns gas given off by the landfill itself and another that gets it from food waste — but none of the 18 other major wastewater treatment plants in the state has anything like it.
The bay commission, which also runs a larger plant at Fields Point on the Providence waterfront, installed the new system as part of a years-long plan to remake itself as an energy-neutral operation and reduce its carbon footprint.
With a trio of wind turbines in Providence, another three in Coventry, two proposed solar fields also in Coventry and the biogas facility, the largest wastewater system in Rhode Island, which serves a third of the state’s population in an area stretching from Providence to Cumberland, could soon produce as much energy as it uses.
The nameplate capacity of the biogas piece of the portfolio is relatively small, but unlike weather-dependent wind and solar, it will run around the clock. After all, as long as people keep flushing, there’s a guaranteed supply of fuel.
“When we’re generating biogas, which is all the time, it will be producing electricity,” said James C. McCaughey, environmental manager with the bay commission.
Biogas is produced when microorganisms break down organic material in the absence of oxygen. The process is known as anaerobic digestion.
The gas is naturally emitted by landfills as material trapped below the surface slowly biodegrades, but the process can be accelerated and intensified inside airtight tanks, called digesters, when heat is applied.
The main components of biogas are methane and carbon dioxide. Just like in natural gas that’s formed over millions of years deep underground, it’s the methane that makes biogas a combustible fuel. Biogas from landfills typically has a methane content of about 50 percent, but when it’s produced in digesters the level can be much higher.
Recent attention has focused on companies that are developing facilities, like the Blue Sphere Corp. plant in Johnston, that produce biogas from rotting food remnants, a relatively new technology on this side of the Atlantic but one used widely in Europe.
Biogas from cow manure is also being tested in parts of the country — Vermont has dubbed its program “Cow Power” — and studies have found that manure has the potential to meet the energy needs of millions of homes in the United States.
In the wastewater industry, digesters are nothing new, but they’re not common. Because of the cost of building them, only about a tenth of treatment plants across the country — generally the larger ones — have them, according to the U.S. Environmental Protection Agency. And less than a tenth of those actually produce electricity.
They include the Newtown Creek facility in New York City with its striking stainless-steel digester “eggs,” as well as plants in Boston, Chicago and Philadelphia.
It takes a staggering amount of power to run the aerators and pumps to clean up wastewater in the 16,000 or so treatment facilities across the country, consuming nearly one percent of the nation’s total electric usage.
But the potential energy that can be harvested at the plants is huge. It’s estimated that there’s five times as much energy in wastewater as it takes to process it, according to the American Biogas Council.
The Bucklin Point plant has been producing biogas since it went into operation in 1954. Unlike the other treatment plants in Rhode Island, its original design included a “pancake”-style digester — a large, flat-roofed cylinder — and two more were added in subsequent years.
For decades, the plant has been using the digesters, but the aim wasn’t to generate power. It was to break down the sludge produced by the plant. The digestion process is able to cut the volume in half, reducing the cost of trucking away the sludge, which in the past went to the landfill and more recently has gone to an incinerator in Woonsocket.
As for the resulting biogas, half is burned to heat the digesters while the rest is flared off.
The methane concentration at the Bucklin Point plant is about 60 percent, due in part to the large concentration of households that use the system, says McCaughey. With a higher ratio of human users than plants that have more industrial customers, the concentration of organics in its sludge is also higher.
It’s collected in two stages. First, after the wastewater that enters the plant passes through debris and grit screens, it makes its way into a settling tank. Most of the solids fall to the bottom, where they get pumped out, while some float to the top, where they get skimmed off, and then it’s all transferred underground to the digesters.
Then, the water gets filtered again inside the building marked by a sign reading “Gravity Belt Thickener.” What passes through the thickener is a mixture of organic matter, bacteria and a polymer additive used to hold it all together. It goes into a basement storage tank and then “Sludge Master” sewage pumps send it too to the digesters.
They heat the material up to 95 degrees, slashing the time it takes to produce biogas.
“What takes years at a landfill takes 15 days in the digester,” McCaughey said, as he stood beside one of them on a recent morning.
“We’re trying to speed up nature,” added Jamie Samons, public affairs manager for the bay commission.
The new cogeneration system will produce the equivalent amount of power used by 767 typical Rhode Island households and the equivalent amount of heat used by 233 households. All that energy will be used by the treatment plant.
Although it’s not expected to reduce customer rates, it could ease the financial impacts of a series of massive infrastructure projects the bay commission has undertaken to store and treat contaminated stormwater before it enters Narragansett Bay.
“It is a way of mitigating the impacts of these huge federally-mandated projects,” Samons said.
The payback on the $8.2-million project is projected to be 14 years. With regular maintenance, the biogas engine is expected to last about 15 years, but replacing it won’t cost nearly as much, because all the associated infrastructure will already be in place.
It’s hard to say whether it would be cost-effective to duplicate the system at other treatment plants, such as the one at Fields Point, where digesters would have to be built as part of any project. But the Bay commission isn’t ruling anything out.
“We’re on a much tighter footprint there,” Samons said.
“But you never know,” McCaughey said.
As they led a tour of the Bucklin Point plant, the flare next to the digesters steadily burned despite a steady drizzle, using up biogas with no real benefit.
When the new generator fires up sometime in the coming months, the flare will be extinguished, a tell-tale sign that the newest source of renewable energy in Rhode Island is in use.