Pittsfield, Mass., wastewater treatment facility saves money and shrinks environmental footprint.

Nestled within the Berkshire Hills in Western Massachusetts, Pittsfield is noted for its blend of New England tranquility and charm and its embrace of modern progress. Once frequented by writers such as Herman Melville, Henry Wadsworth Longfellow and Nathaniel Hawthorne, the city of roughly 46,000 people is also known as the plastics technology center of the nation because of its many locally-based plastics companies.

Now, thanks to a project that will implement energy and environmental reform, Pittsfield is about to earn another notable distinction: a city at the vanguard of the green movement. Pittsfield is in the process of constructing an upgrade to its wastewater treatment facility, which will feature a combined heat and power (CHP) system, using three 65-kilowatt rated microturbines for a total rating of 195 kilowatts. The microturbines will be fueled by the treatment facility's digester gas, which is a methane-based byproduct derived from its wastewater sludge treatment process. The digester gas is a biofuel and a renewable energy source.

The Pittsfield CHP project is not only good for Mother Earth, but for the wallet as well. Along with a reduced environmental impact, the city will benefit from a reduced utility bill because the digester gas will be used to generate both electricity and heat for on-site use. In addition, the plant will generate approximately $45,000 per year in revenue through the Renewable Energy Credit market.

Pittsfield is more than just a center of culture and industry. It is also a worthy example of how individual municipalities can be both economically savvy and environmental stewards at the same time.

The Design

The design for the upgrade to Pittsfield's wastewater treatment facility has a number of features, including a brick-and-block building to house the entire CHP system, space provisions to allow for a fourth microturbine should the city decide to expand and the furnishing and installing of electrical systems for interconnection of the CHP generated power for use within the entire treatment facility.

Furthermore, there are plans to replace the existing shell and tube sludge heat exchanger with a single 1.2 MMBTU per hour spiral heat exchanger and furnish and install three proposed double-disc digested sludge transfer pumps to replace the existing pumps. Also, the design calls for:

Installing instrumentation to monitor sludge and hot water temperatures

Installing instrumentation to automate sludge mixing through use of programmable timers and interlocking mixing operations with sludge pumping

How the CHP System Works

The sludge, or biosolids, processing infrastructure at the wastewater treatment facility includes primary and waste-activated sludge pumping, gravity belt thickeners, thickened waste activated sludge pumps, secondary and primary anaerobic sludge digesters, belt filter presses and dewatered sludge pumps. The purpose of this infrastructure is to reduce the overall mass and volume of the sludge for disposal.

Anaerobic digestion at the facility is a two-stage process. The first stage uses active heating and mixing to facilitate the destruction of volatile solids. Mixing is accomplished by recirculating compressed digester gas from the headspace of the primary digester to the bottom of the vessel through lances. Heating of the primary digester is accomplished by recirculating sludge from the primary digester through a heat exchanger located within the digester building. The second stage allows for solids-liquid separation through quiescent settling of the sludge.

A byproduct of the anaerobic sludge digestion process is digester gas (primarily composed of methane—approximately 62 percent by volume—and carbon dioxide), which is then used as a fuel for boilers located in the plant's pump and power building. The boilers are used to heat a water loop, which then heats the sludge in the digesters, maintaining it at the optimal temperature for anaerobic digestion. Excess digester gas is used to heat the pump and power building in the winter or is flared through a waste gas burner located on the top of the digester building.

Under the proposed CHP system, the digester gas will be sent through a fuel gas conditioning system that removes contaminants from the digester gas and boosts the gas's pressure to the microturbines. The target contaminants of the fuel gas conditioning system include water vapor, hydrogen sulfide and siloxanes—silicone-based compounds contained in many health and beauty care products.

The conditioned digester gas will then be used to fuel the microturbines, which will generate heat and electricity to meet the plant's base-load demands. The waste heat in the exhaust of the CHP system will be used in a heat exchanger to produce hot water. This hot water's purpose is to heat the sludge in the primary digester and serve as building heat in the digester building.

It is anticipated that the heat generated will not be enough to meet the peak sludge heating requirements under design winter conditions—making use of the existing boilers in the pump and power building for sludge heating and building heat a possible necessity. However, the microturbine system is expected to reduce the operating time of these boilers and, therefore, the volume of required diesel fuel for the operation of the existing boilers, as well as minimize significantly the amount of digester gas diverted to the flare-gas burner.


Economically and Environmentally Conscious

Thanks to these proposed upgrades, the Pittsfield wastewater treatment facility is expected to reduce the flaring of the digester gas and consumption of diesel fuel, provide an estimated 30 percent reduction in the plant's electric bill—saving taxpayers over $200,000 per year.

So this small Western Massachusetts city, long known for its persons of letters and as a pioneer in plastics technology, is now leading the way in the green revolution, setting the example as an environmental steward with its wastewater treatment facility and combined heat and power project.

 

Pumps & Systems, October 2011