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Hanover Township: Digesters and Cogeneration Hanover Township: Digesters and Cogeneration

Powering a treatment plant with its own methane gas

The new design reduced sludge processing energy costs with an innovative gas-piston sludge mixing system. Excess methane gas, formerly flared off, is now being recovered through cogeneration.

Hanover Township: Digesters and Cogeneration


During the American Revolution, George Washington and his troops often marched and camped in Hanover Township. Ironically, the township was named years earlier to honor a very different George: the king of England, whose family came from Germany’s House of Hanover.

Created in 1956, the Hanover Sewerage Authority provides service to the township and adjoining residential areas of Morris Plains, Morris Township, Parsippany-Troy Hills, and to industries in East Hanover.

In 1961, the Authority completed construction on its wastewater treatment plant on Troy Road in Whippany. Between 1976 and 1979, the plant was expanded from a capacity of 1.5 million gallons per day (MGD) to 3 million MGD, and upgraded to meet new water quality criteria with a secondary treatment system. Additional upgrades between 1979 and 1982 brought capacity to 5 MGD and equipped the plant to meet tertiary discharge limits requiring a filtration process.

More recently, the plant’s digester system reached the end of its useful life. Primary Digester No. 1 and Secondary Digester No. 1 were built in the 1960s, and Primary Digester No. 2 was added in 1978. As of 2008, Primary Digester No. 1 had been out of service for many years, and Primary Digester No. 2 had been in continuous service for 30 years. Its performance was degrading, likely due to its failing Perthgas mixing system.


The Authority retained Mott MacDonald in 2008 to design improvements to Primary Digester No. 1, and again in 2011 to improve Primary Digester No. 2. We provided all necessary design services, including structural, architectural, electrical, mechanical process design, and instrumentation and controls. Mott MacDonald also has provided bid and construction phase services including permitting, funding assistance, and full-time onsite resident observation.

Primary Digester No. 1 and Secondary Digester No. 1

Upgrades to the No. 1 digesters required retrofitting new piping and valves in areas of limited space, and achieving a high degree of gas pressure containment by reconditioning the existing concrete tanks.

Polyurethane was applied to the upper interior of the tank walls to improve gas containment, and Mott MacDonald designed flexible membrane covers that would increase the digesters’ storage capacity from about 4,000 to 28,000 cubic feet (113 to 793 cubic meters). Air blowers and controls maintained air pressure between the cover membranes.

Gas generated in the digesters was used to fuel the plant’s boiler system. Excess gas was automatically transferred between the digesters for storage and use during periods of low production.

We designed a new sludge mixing system for Primary Digester No. 1, connected with the older sludge recirculation system to allow new mixing pumps to be used for both mixing and recirculation. Structural repairs were made to the digester’s walls, and a coating of foam insulation improved its heat capture and retention.

In addition, a motor control center was replaced in the sludge pumping station and an elevated steel access platform was erected.

Primary Digester No. 2

As with the other digesters, Primary Digester No. 2 was fitted with a two-layer membrane cover that increased gas storage capacity from about 8,400 to 39,500 cubic feet (238 to 1,119 cubic meters).

The digester was upgraded with a gas-piston-type sludge mixing system. Methane drawn from the digester is compressed and fed into the bottom of three 24-inch-diameter (0.6 meters) vertical tubes. The gas forces sludge up the tube while drawing in more sludge at the bottom. The result is a circulating flow that requires less horsepower than jet mixing systems. Hot-water jackets on the mixing tubes eliminate the need for heat exchangers and circulating pumps, warming the sludge to enable anaerobic digestion.

In addition, We designed new automatically cleaned bar screens to replace raw wastewater comminutors and minimize the accumulation of solids. A liquid sodium bisulfite feed system will replace the existing sulfur-dioxide gas dechlorination system, and electrical upgrades will allow the existing standby generator to power the entire plant during utility outages.


In addition to bringing one digester back into service and extending the useful life of two others, Mott MacDonald's design reduced sludge processing energy costs with an innovative gas-piston sludge mixing system.

Excess methane gas, formerly flared off, is now being recovered through cogeneration. Digester gas generates electricity and hot water, and helps heat the plant’s buildings. To make the gas suitable for use in an engine generator, Mott MacDonald specified pretreatment equipment to remove sulfur dioxide, siloxanes, and moisture.

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