2009 E3 Design Honor

Back River Wastewater Treatment Plant

Baltimore, Maryland
ENTRANT: AECOM
ENGINEER IN CHARGE: Ralph B. "Rusty" Schroedel, P.E., BCEE and Michael J. Zapinski, P.E.

The site for construction of the new facility was placed in the location of an abandoned pad for a past attempt to use the excess digester gas. Some of the gas flares are visible in the foreground.	After demolition of the existing structures, the clean site is ready for construction of the new engine/generator facility. The treatment plant and the Back River can be seen in the background.	Near completion of construction, the new facility is placed close to the existing digesters, flares, and steam boilers to minimize piping runs.
Steam generators were installed before the roofing was placed to ease installation.	The new gas handling system could not interfere with existing operations, including those of the famous egg shaped anaerobic digesters at Back River. Specifically, the gas withdrawal and steam heating need to remain fully operational while connecting the gas piping and the engine's steam generators.	Construction of the new facilities (at left) needed to be carefully coordinated with existing operations including the existing conventional digesters (at top) and the steam boilers and associated equipment in the building at the right.

Entrant Profile

Entrant's role in the project

The Back River Wastewater Treatment Plant is the larger of the two treatment plants owned and operated by the City of Baltimore, Maryland. It is a secondary wastewater treatment facility incorporating the activated sludge process. The City's Energy Office was interested in selecting an energy services company (ESCO) capable of developing and delivering a performance contract to utilize the digester gas that was also experienced in wastewater treatment. The team of Johnson Controls with subcontractor AECOM (formerly Earth Tech) was selected because they had a history of service to the City of Baltimore, and of working together.

AECOM provided engineering services during the study, preliminary design and final design/construction document phases of the Back River WWTP cogeneration facility. Cogeneration refers to the process of using a single plant to generate electricity and useful heat. The final design consisted of an electrical cogeneration plant that uses internal combustion engines fueled by the digester gas that was previously flared.

Other Firms/Agencies of Assistance

Johnson Controls, AECOM's client, provided to the project owner, City of Baltimore, a total package of energy savings measures that reduced 19.4 million kWh of electrical use annually and $14 million in energy savings and plant improvements over 10 years. AECOM's design services helped Johnson Controls and ultimately the City of Baltimore to achieve these energy savings.

Project Description

Introduction

Back River WWTP generates approximately 1.7 million cubic feet of digester gas daily, a by-product of treating and degrading wastewater sludge. Of that, approximately 1/3 was used in the plant primarily to heat the sludge in the digesters, and the remaining 2/3 was burnt in large utility flares to destroy methane present in the gas. These flares represented a visible waste to the community of more than 675 million BTUs of recoverable energy per day. The City of Baltimore, under the leadership of the Mayor's office, the Department of Public Works and its Energy Office wanted to aggressively attack this energy waste while ensuring superior service to the community.

Demonstration of a comprehensive, integrated approach using original and innovative techniques

A project this large integrating performance contracting and unique design applications for sustainable purposes is rare in the U.S. Forward thinking to address energy waste and the comprehensive project approach allowed the City to apply energy and operating savings necessary for the plant's capital improvements. Keys to savings included several original and innovative techniques:

Recovered heat from plant engines is used to generate steam to heat sludge in the anaerobic digestion process - a nearly one of a kind design application. Because the plant requires low pressure steam for the process, the heat recovery steam generators produce approximately 50% of the required steam freeing up additional digester gas for burning in the engines
Closing down digester gas flares and using the gas to instead fuel internal combustion engines to power the cogeneration plant. While using digester gas is relatively common, generating 3MW of energy is virtually unheard of. This electricity represents approximately 30% of the base electrical load at Back River, enabling the plant to pull less power from the local power grid.
Use of subzero chilling to condense water and other compounds (including siloxanes) to condition all digester gas. This modern method of conditioning gas removes 90% of siloxanes and reduces all gas utilization costs, significantly improving operations.
Because of the nature of this comprehensive project, the performance contracting model and project design placed great emphasis on generating a cash flow and payback meeting the requirements of the ESCO. An integral part of the process was Johnson Controls guarantee of capital cost and energy savings to the City at the end of the preliminary design phase, similar to a design-build contract.

Quality evidenced by degree of user satisfaction and proven performance
Utilizing AECOM's design, Johnson Controls is implementing a total of $14 million in energy conservation and facility improvement measures, saving the city $1.8 million in energy and operational savings annually in the next 10 years.

In addition, the proven performance provides environmental benefits to the City. Baltimore's Mayor Sheila Dixon, who ceremoniously started the generator and shut off the flares in November 2008, was quoted in the Baltimore Sun saying, "The burden on Baltimore taxpayers has been diminished, and the air that we breathe will also be cleaner." In addition to features and savings already cited, plant design provided additional benefits:

Administration, Maintenance, and Activated Sludge Control Building upgrades reduce energy consumption and operational costs.
Energy efficient lighting improves lighting quality plant wide and reduces annual energy cost.
Water conservation projects reduce energy consumption and annual water costs.
Reduction of approximately 21 million of kWh of electricity previously generated elsewhere, results in a smaller carbon footprint, reducing carbon dioxide emissions by 25 million pounds.

Addressing Project Complexity

A number of technical and project management challenges were successfully addressed in designing and constructing the Back River Cogeneration Facility.

These include:

Ensuring cogeneration process did not interfere with the wastewater treatment. This was achieved by integration of process steam into the plant's existing digester heating system.
Digester gas system design could not interfere with the use of the digester gas in other plant applications. The cogeneration project actually improved the use of the gas, cleaning it of moisture and siloxanes, thus improving maintenance and operations.
Performance contracting model required a firm contract price prior to start of final design. Therefore, it was important the preliminary design provided enough information to supply firm capital costs. AECOM worked closely with Johnson Controls, their selected contractors and a major equipment supplier so pricing could be prepared to meet the financial metrics for the project. Overall design and efficiency of the plant was key to the overall financial plan because the project is funded through energy savings. This was as important, if not more important than, the initial capital cost budget.

Contribution to Social or Economic Advancement

This project combines societal and sustainable goals, providing beneficial use of a renewable energy source previously wasted. In doing so, greenhouse gas emissions are reduced. In offsetting the production of approximately 21 million kWh per year, 25 million pounds per year of carbon dioxide and 38,000 pounds of nitrogen oxide are offset. Clean water, a basic societal good, benefits from the cost savings provided by the operation of the cogeneration facility. By freeing up funds ordinarily used to purchase electrical power, the City is able to provide cost effective wastewater treatment to the Baltimore area. This is a particularly sensitive issue since the receiving body of water is Chesapeake Bay, the largest estuary in the United States, and has undergone substantial degradation over the past century.

Conclusion

The City of Baltimore is a distinguished leader of municipal efforts in energy efficiency, innovative application of energy technology, and is committed to sustained stewardship of community resources. Wastewater treatment facilities are often one of the largest energy consumers in a municipality, and as energy costs of all kinds increase, it's imperative to explore all energy saving options. The Back River Cogeneration Facility is a prime example of the ability to finance a wastewater energy savings project through the use of performance contracting and unique design applications. This successful project is well on its way to long term sustainability through the joint efforts of the City of Baltimore, Johnson Controls, and AECOM.

A roof opening is provided to allow for future maintenance or removal of the steam generators. By generating steam from the engine exhaust, the steam can be used directly in the existing digester heating system, reducing the demand for digester gas for the steam boilers, increasing the system efficiency.	The gas conditioning skid treats all of the digester gas for removal of moisture, hydrogen sulfide, and siloxanes. This not only protects the new engines, but significantly improves operations and reduces maintenance on other equipment throughout the site that uses digester gas.	The engine generators are each capable of just over 1 MW of electricity production. The installation provides adequate room for equipment operation and maintenance.
The electrical equipment is integral to the facility. Transformers increase the voltage to match the on-site distribution system voltage.	The engine/generator building houses three engine generators with space for a future fourth unit should gas production increase. At the far right, a doorway to a separate electrical and control room is provided.	The gas conditioning building (at center) and engine/generator building (at right) were constructed to be both functional and aesthetically pleasing. The renowned egg shaped digesters can be seen in the background.
The engine/generator package was provided by a local supplier. Under the Performance Contract, Johnson Controls will initially be responsible for operations and maintenance.