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As one of Europe’s largest production and research site, the Industriepark Höchst, (in Frankfurt Germany) is home to 90+ companies in pharmaceuticals, biotechnology, basic and specialty chemicals, crop protection, food additives and services.

The waste water these companies generate is treated in a double stage biological treatment plant. Part of the water passes a biological reactor, which is 20 meters high and has a volume of 20.000 m³. After the water has been purified to the desired quality, it exits at the top of the reactor. It enters a tube which leads it down to a canal that takes it back to the river.

"We investigated several ways to harness the potential energy of the falling water", reported Wolfgang Zang, a project manager at Infraserv Höchst, the operator and service provider of the site. "Ultimately, we decided to go with a simple yet cost-effective solution – running a motor-driven pump in reverse, which works like a turbine with a generator.”

The pump is attached to the downpipe of the reactor at ground level. The falling water hits the pump's impeller and drives the motor, which produces power like a generator.

A Pump-As-Turbine (PAT) system has a wide availability because its method of construction is nodifferent than that of a standard pump, which is produced and marketed in practically every place in the world. The investment cost for the pump is therefore considerably lower than that for a turbine. Generally a PAT can be designed for different volume flow levels, but the water source should have a reasonably stable volume flow in order to ensure the proper operation and efficiency of the pump.

At the waste water treatment plant of the site in Frankfurt, there are between 850 and 950 m³/h of clean water available to drive the system. The falling water is able to produce around 30 kilowatts of power before continuing its way to the canal. The pump is expected to produce around 250 megawatts hour, the equivalent to the energy demand of 60 single-family households.

The energy produced is fed to the public electrical net and is paid at a rate of 0.1267 euros per kilowatt hour according to the tariffs of the German Renewable Energy Sources Act.

The investment costs of the project have been around 150,000 euros, including the pump, pipe modification, valves and others. Infraserv Höchst expects a payback period of approximately seven years.

The PAT started working in November 2010 and since then has been feeding the net with emission free electrical energy.

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