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A NovaThermal unit extracts thermal energy from wastewater in a mixed liquor channel to fulfill the heating needs of the plant’s compressor building and gallery.

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The City of Philadelphia is using new geothermal technology to harness energy from wastewater to meet energy conservation objectives and save a projected $15,000 annually.

Mayor Michael Nutter launched the Greenworks Philadelphia initiative in 2008, aimed at making his community the No. 1 green city in America. In alignment with Greenworks Philadelphia, the Philadelphia Water Department (PWD) developed a strategic plan to meet energy conservation and generation objectives and formed a four-person energy team to identify and evaluate green energy opportunities.

Looking at sewage

The PWD provides water, wastewater and stormwater services for the Greater Philadelphia Region. The utility serves the city's population of 1.5 million along with portions of Montgomery, Bucks and Delaware counties. Its 3,000 miles of sanitary sewers convey wastewater to three treatment plants that process a combined average of 491 mgd.

In 2009, the PWD began discussions with Philadelphia-based NovaThermal Energy about the possibility of installing a wastewater geothermal unit. At the time, wastewater geothermal technology was commonly used in Asia but had not achieved widespread use in the United States.

Traditional geothermal technology is a well-established renewable building climate control method but is not a viable option for densely populated urban centers such as Philadelphia. The drilling required for geothermal well installation can be challenging and cost-prohibitive in a heavily urbanized area.

"It became clear that a traditional geothermal application wasn't the way to go," says Paul Kohl, energy program manager at PWD. "But when we looked at sewage, we realized that instead of going vertically down, we could go horizontally. We wouldn't have to drill."

The PWD chose the Southeast Water Pollution Control Plant, its smallest and newest, as the location for the geothermal system. NovaThermal Energy agreed to own and maintain the equipment for five years. The company also provided $90,000 of the $240,000 project cost; the balance was covered by an Energy Efficiency Community Block Grant via the Philadelphia Industrial Development Corporation.

Power from wastewater

In 2011, contractor Limbach Company designed the piping and pumping portions of the system and subsequently installed the sewage geothermal system, which is comprised of piping, a heat pump, and a proprietary filtration device. The system went online in April 2012.

The system taps a small sidestream of graywater, which has already passed through preliminary and biological treatment. The sidestream enters a highly efficient heat pump that extracts 978,000 Btu per hour from the graywater. The heat pump generates four units of heat for every unit of electric power supplied, significantly more efficient than the natural-gas boiler that handles all additional heating needs at the plant.

"A good gas-fired boiler takes 85 percent of the energy you put in to heat the water," Kohl says. "In contrast, using the heat source of the sewage, we can extract a lot more energy at a lower-cost rate. The geothermal unit is basically an amplification machine."

The sewage geothermal system fulfills about 20 percent of the facility's heat load, which translates to a projected $15,000 in annual energy savings. Although the system can both heat and cool, it is currently used only for heating. Use of its cooling capacity would significantly increase its return on investment.

In order to make the best use of the existing piping system, the sewage unit is tied into the plant heat loop ahead of the boilers, so the boiler controls are still in operation. When the heat pump produces enough water at 130 degrees F, the boilers remain off. If the load exceeds the capacity of the heat pump, then the boilers respond.

"Operationally, it's easier to be tied into the existing heating control loops so the operators don't have to switch from one heating system to another," says Doug Cowley, assistant plant manager at the Southeast plant. "The standard heating system just comes on automatically so the operators don't have to worry about it. It takes care of itself."

Public education

To increase public interest in the use of sewage as a renewable energy source, the PWD hopes to add a sewage geothermal system display to the Fairmount Water Works Interpretive Center, a department-owned and -operated educational facility that teaches school groups and the general public about aquatic ecology, stormwater control, pollution, and water and wastewater treatment.

That facility is housed in the Fairmount Water Works facility, which was the first successful water pumping station, using water wheels and turbines powered by the river to pump water to a reservoir that provided gravity-fed water to the city as early as 1822.

"If we get funding, we would like to install sewage geothermal technology in the center so we can let the next generation know that we can extract heat energy from sewage," Kohl says.

Partnering with consultant Black & Veatch, Kohl and his energy team focus on ways to conserve and generate energy throughout the PWD. Several large energy projects have been implemented over the past few years. A 248 kWp photovoltaic solar array installed at the Southeast plant in April 2011 generates 350,000 kWh annually, supplying 5 percent of the facility's electrical load and saving about $30,000 annually.

Funding for that $1.6 million project was split evenly between the PWD and a U.S. Department of Energy (DOE) Energy Efficiency and Conservation Block Grant. The DOE Solar America Cities program provided technical assistance.

The PWD also partnered with the Philadelphia International Airport in 2009 to co-digest aircraft deicer runoff. Annually, the airport sends 2.8 million gallons of deicer runoff to the Southwest plant to be added to the anaerobic digesters. "This is a sustainability partnership that meets everyone's needs," Kohl says. "We can provide a cheap alternative method of disposal for the airport, and, in return, we produce more gas."

The digester gas at the Southwest plant fuels a biosolids recycling facility that pelletizes the material from the city's treatment plants. These biosolids pellets are used for farmland application and as fuel at a cement kiln.

A biogas-fueled cogeneration facility at the Northeast Water Pollution Control Plant, designed to generate 40 million kWh annually, is expected to be online in summer 2013. It will be the city's biggest alternative energy system project to date. "The PWD has a rich history of energy management and energy control," Kohl says. "What the department has done well for years behind the scenes is now being showcased to the public."