“This is just a larger, industrial-size scale,” he says.
The remaining solid matter used to go to a landfill. Now, it’s sent to a plant near Philadelphia International Airport, to be converted into pellets to fertilize golf courses and farms, or fuel cement kilns.
But back to the gas.
It is piped to the new biogas “cogeneration” facility on the site of the Northeast plant.
In essence, cogeneration is the simultaneous production of electricity and heat from a single fuel source, according to the U.S. Environmental Protection Agency, which endorsed the technology.
After treatment to remove moisture, hydrogen sulfide, and “siloxanes” — residue from the decomposition of soap, shampoo, and other personal-care products — the gas is burned in four massive reciprocating engines.
The result is 43 million kilowatt hours of electricity a year, enough to power more than 4,000 typical homes — or, more pertinent to the project, enough to meet 85 percent of the power needs of the sewage treatment plant.
The combustion also produces heat. Much of it is captured and returned to the digesters, where it helps keep the sludge mix at 95 degrees, the optimal temperature for microbes to do their job.
“Very simple,” says Paul M. Kohl, Water Department energy program manager. What’s not simple, he says, is getting there — the miles of piping, the heat exchangers, the monitors, computers and other instrumentation, all in communication.
All told, the process captures more than 80 percent of the available energy for heat and electricity, according to the Water Department. In contrast, a coal-fired electricity generating station is about 35 percent efficient.
The plant’s carbon emissions have been reduced by about 22,000 tons a year — the equivalent of taking 4,833 cars off I-95, or planting 5,390 acres of pine forest, by Water Department calculations.
Department officials had mulled a biogas cogeneration plant for a while. But, ironically, electricity rates were low enough that the project would not be economically feasible.
Recently, however, rates increased to a point where it made financial sense.
The city partnered with Ameresco Inc., a Massachusetts-based company that specializes in energy efficiency and infrastructure upgrades for facilities across North America.
Financing came from Bank of America, which technically owns the facility; the city is leasing it for 16 years, with an early buyout option.
