Articles & Case Studies

Vacon drives play a key role in subterranean wastewater treatment

Posted: Thursday 21st October 2010

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 €125 million 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 has been built deep underground in an artificial cavern; the construction of which involved moving almost half a million cubic metres of rock.

The plant removes more than 70% 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% 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 ultimate client, Turun seudun puhdistamo Oy (Turku Region Waste Water Treatment Limited), 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,” said Eero Kaurio, Project Manager for main contractor, YIT Environment Oy (which has since been transferred to ECO Environment LTD Oy). “In my opinion, the factors in favour of Vacon were its quality and the maintenance service it provides. Co-operation has been exemplary; they have kept to its delivery schedules and answered our queries without delay.”

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 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.

For water treatment projects like the Kakolanmäki plant, Vacon offers a wide range of variable speed AC drives with ratings up to 5 MW. The drives are easy to install and commission, and they offer extensive communications options making them simple to integrate with control and supervisory systems of all types. In addition, application software packages that have been specifically developed to meet the needs of the water treatment industry are readily available.




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December 2018

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