Drives play key role in subterranean wastewater treatment

Paul Boughton

More than 50 Vacon AC variable speed drives, with ratings from 1.1kW to 355kW, are helping to meet the requirements for accurate, dependable and efficient control of the pumps and compressors used in a new wastewater treatment plant at Kakolanmäki in Turku, Finland. The plant, which cost more than €125 million to build, is jointly owned by ten municipalities and treats the wastewater from 280,000 people.

The plant removes more than 70 per cent of the nitrogen from the water, thereby significantly reducing the eutrophication (adverse accumulation of excess nutrients) in the Baltic Sea, to which the water is ultimately discharged. The plant also removes in excess of 95 per cent of the phosphorous and oxygen-consuming organic material from the water, together with 90% of the solid matter.

Essential requirements for the plant were accurate regulation of the water treatment process, together with energy efficiency. For this reason, almost all of the pumps are controlled by AC variable speed drives, rather than by the throttle valves that been commonly used in similar applications in the past.

The AC drives also improve plant availability, since pump and compressor output is accurately matched to current requirements. This means that, for much of the time, the pumps and compressors run at speeds well below their maximum, which not only saves energy, but also reduces strain on the plant, thereby cutting maintenance requirements and extending its operating life.

For the control of the incoming water pumps at the Kakolanmäki plant, Vacon 130 kW AC drives are used. There are six pumps, one of which is held in reserve, and each is capable of pumping up to 765 litres of water per second. A further five Vacon 335kW AC drives are used to control the compressors that feed the aeration discs used to add oxygen to the active sludge as part of the clarification process. Additional drives are used to control the sludge pumps, and in subsidiary applications around the plant.

All of the drives are linked to the plant’s main control system via a Profibus network. This arrangement significantly reduces the amount of cabling and the number of interfaces needed, both of which are important benefits in underground applications.

Pictures: Jouni Saaristo/Valokuvaamo Saaristo

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