Variable drives key to subterranean treatment

Energy efficiency was key at a new underground treatment plant in Finland. Stephen Takhar, managing director of Vacon UK, explains how variable speed drives helped the client take control and reduce pollution to the Baltic Sea

More than 50 Vacon AC variable speed drives, with ratings from 1.1 kW to 355 kW, 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 £106M to build, is jointly owned by ten municipalities and treats the wastewater from 280,000 people.

In order to minimise its impact on the visual environment and to eliminate the need for it to occupy valuable building land, the Kakolanmäki plant was built deep underground in an artificial cavern; the construction of which involved moving almost 500,000m3 of rock.

The plant removes more than 70% of the nitrogen from the water, thereby significantly reducing the eutrophication in the Baltic Sea, to which the water is ultimately discharged. The plant also removes in excess of 95% 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.

In order to meet the competitive tendering requirements of the client, Turun Seudun Puhdistamo (Turku Region Wastewater Treatment), drives from three different suppliers were considered for the project.

"Vacon was chosen to supply the AC drives after a comparison of technical and financial factors," says Eero Kaurio, project manager for main contractor, YIT Environment (since transferred to ECO Environment). "The factors in favour of Vacon were its quality and the maintenance service. Co-operation has been exemplary; they have kept to its delivery schedules and answered queries without delay."

For the control of the incoming water pumps at the Kakolanmäki plant, Vacon 130kW AC drives are used. There are six pumps, one of which is held in reserve, and each is capable of pumping up to 765l/s of water.

A further five Vacon 335 kW 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.

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