SEWAGE POWER

A wastewater treatment plant runs its processing equipment on methane gas from its own sludge.

Ottawa-Carleton’s wastewater treatment plant used to burn the 27,000 cubic metres of methane gas it produced each day simply to heat digester sludge and provide space heat in the plant. However, since installing cogeneration equipment in 1997, the plant can use the methane gas to generate its own electricity as well.

CH2M Gore and Storrie were the consulting engineers on the $4.5 million cogeneration installation, which saves the municipality almost $650,000 a year (energy savings of $900,000, minus maintenance costs). Of course, the cogeneration project also saves damage to the environment — in two ways. It significantly reduces the greenhouse gas emissions from the anaerobic digestion process itself, and means the electricity board is not producing carbon dioxide in providing power to the plant. In total, the energy savings from this project amount to the power provided for 1,000 homes.

Three 810 kW Toromont combustion engines/generators were installed to draw from the methane gas produced when sewage sludge in the anaerobic digester tanks decomposes. The engines themselves are located in the nearby blower building (see photo).

About 48% of the energy goes to heat, and 37% to producing electricity. The electrical power from the cogenerator units is fed into a substation, which then distributes it to run the equipment in the plant, such as the massive aerator and centrifuge machinery used to clarify the wastewater.

Heat generated by the engines is transferred to the plant hot water heating system. The thermal energy is captured two ways. First, by circulating coolant round cavities in the engines themselves. The coolant passes to a heat exchanger, raising the temperature to 120C. Second, the exhaust gas leaving the engines at about 450C also passes through a heat exchanger and is cooled to about 150C. Cooled gas leaves through silencer chimneys on an outside platform.

Each cogeneration engine has a Woodward controller and regulator to slow down its output when necessary, as well as a PLC linked to a master control and SCADA system. — BL

Consulting engineer: CH2M Gore & Storrie, Ottawa/Toronto.

J. Shawn Gibbons, P.Eng., Peter Burrowes, P.Eng.,

K. Clare Humphrey

Supplier: Toromont Cat

General Contractor: J.C. Sulpher Construction

Mechanical subcontractor: Black & McDonald

Electrical Subcontractor: EdisonCCE