Converting Biogas at the Back River WWTP into Renewable Energy
Built almost a century ago, the Back River Wastewater Treatment plant stands on roughly 466 acres on the west bank of the Back River, a tributary that winds its way into nearby Chesapeake Bay.
The plant serves an estimated 1.3 million people who live in the 140 square mile Baltimore area. The Back River Plant operates around the clock with a staff of nearly 350 employees and can treat some 180 million gallons of wastewater every day.
The Back River plant has two Egg Shaped digesters and six in-ground digesters which has a total capacity of almost 14 million gallons. The biosolids are heated to 98 degrees F, using steam injection, and have a minimum 15 day detention time in the digesters. Biogas generated by the process has always been used to heat the campus and digestion process, but more gas was produced than could be used; so the excess was flared off to the atmosphere. This wasted excess biogas would become the potential fuel source for consideration in a co-generation project.
The City of Baltimore is known for its commitment to energy efficiency and sustainability. In 2006, the city entered into an energy savings performance contract for improvements at the Back River Wastewater Treatment Plant with Johnson Controls, a global provider of energy efficient systems.
In the preliminary analysis phase of the project, Johnson Controls determined that the Back River facility produced approximately 1.7 million cubic feet of methane gas per day. Of that, approximately 511,000 cubic feet were used to provide heat for the digestion process, and another 163,000 cubic feet were used to cool and heat buildings. The remaining roughly one-million cubic feet of excess methane gas was being burned off in flares.
The company determined the excess gas could be used to generate electricity that would reduce the amount of energy purchased to power the plant. In addition, waste heat from the electrical generation process could be used to further reduce the amount of energy used to produce heat in the plant. Total reductions in electricity demand were estimated at 19.4 million kilowatt hours.
To achieve these savings, Johnson Controls undertook the following energy efficient improvement measures and featured the development of a co-generation facility at Back River WWTP:
- Designed and installed gas conditioning equipment to remove impurities from the methane gas produced in the wastewater treatment process.
- Installed three 12-cylinder engine/generators, fueled by the excess methane gas. Each engine/generator is capable of producing 1,050 kW of electricity using a “lean burn” technology that results in reduced air emissions.
- Installed equipment to recover heat from the engine/generators and use it to produce hot water and steam for other plant operations.
- Upgraded digester recirculation pumps to improve their performance and efficiency.
- Made energy-efficient lighting improvements throughout the plant.
- Improved heating, ventilation and air conditioning systems in four major buildings.
- Upgraded plumbing fixtures to promote water conservation in five major buildings.
Johnson Controls provides ongoing day-to-day maintenance of the gas conditioning, engine/generators, heat recovery and other equipment installed under the contract. Curtis Engine supplied the Waukesha engines, heat recovery equipment, and electrical switchgear and provided operations and maintenance services for the co-generation system.
The facility upgrades at Back River WWTP offer a variety of additional annual benefits, including:
- $1.8 million minimum in reliable energy and operational savings.
- Reduction of 19.4 million kWh of electricity.
- Effective utilization of methane gas, reducing the amount of purchased power and the need for 20-foot methane flares which were visible across town.
- Reduction of 12.9 million pounds of carbon monoxide and,
- $14 million in energy savings and plant improvement projects over the next 10 years.
Supplying 20 percent of the plant’s electricity needs with gas that was previously burned off acts as an excellent hedge against fluctuating energy prices. And because it reduces the amount of electricity utilities must generate, lower greenhouse gases emissions are the result.