A new waste water treatment plant jointly owned by 10 municipalities has been operating at Kakolanmäki in Turku, Finland, since the beginning of 2009. The plant, which has been excavated inside the rock, is able to effectively remove the nitrogen from the water, reducing the eutrophication of the sea. The plant is packed with high technology. Vacon’s AC drives control the pumps and compressors, since it is essential that the processes of the plant, which has been designed to handle the waste water of 280,000 people living in the area, can be regulated.
Beneath the former Kakola regional prisonin Turku — declared a listed building by the National Board of Antiquities — space has been excavated for what has been called the finest waste water treatment plant in Finland and for the four kilometres of underground caverns that surround it. Altogether 490,000m3 of rock were blasted and then removed to make room for the plant. A neight-story building would fit in the space made.
The treatment plant will handle the wastewater from Turku, Kaarina, Lieto, Paimio, Rusko, Raisio, Naantali, Masku, Nousiainen and Mynämäki. The initial impulse for building a joint waste water treatment plant for the area came from the stricter denitrification requirements announced in 1994, which the existing treatment plants owned by the individual municipalities proved incapable of meeting. The company ‘Turun seudun puhdistamo Oy’ (Turku region waste water treatment plant Ltd) was established in 2002, and planning of the Kakolanmäki treatment plant began in 2003.
The plant, which cost EUR 125 million to build, started operations at the beginning of 2009. It is able to process the waste water from 280,000 people in a way that considerably reduces the nutrient load on the Baltic Sea. “A joint plant is effective. It has been estimated that the overall load from the area will decline by 35 %,” says Timo Anttila, managing director of Turun seudun puhdistamoOy.
The waste water is fed into the underground caverns along four pipes with a diameter of one thousand millimetres, either gravity fedor by pumping. The pumps for the incoming water, which are controlled by Vacon 130kW AC drives, are capable of pumping up to 765 litres a second. There are six pumps, one of which is a reserve pump.
The incoming waste water contains a verywide range of fractions that have to be removedfrom the treated water before it isdischarged into the Turku harbour basin.The waste water is treated in a process that includes physical, chemical and biological processing units. The permits issued for the plant state that it must remove at least 95 % of the phosphorus and oxygen-consuming organics in the incoming water, 70 % of the nitrogen and 90 % of the solids found in the water.
“It is fashionable today to build waste water treatment plants out of sight,” says Mr Anttila. The cities of Helsinki and Lahti already had plants built inside the rock before Turku obtained its plant, and Tampere and Espoo are considering doing the same. Since the plant is built underground, it is not a visible part of the cityscape, and makes building land available for other purposes. Premises built inside the rock have a more constant AC drives controlwaste water pumps Beneath the former Kakola regional prison in Turku, Finnland, an innovative water treatment plant has been built into the rock. AC drives control pumpsand aeration compressorswhich handle waste water of280,000 people. and higher temperature than those built in the open, which is an advantage in removingnitrogen. Closed premises within the rockalso give better control over the odours from the treatment process.
It is essential to be able to regulate the water treatment process. Pumps, for example, are increasingly being controlled by an AC drive, which has replaced the throttle valve.
About 50 AC drives, in the power range 1.1 kW– 355 kW, have been supplied to the Kakolanmäki waste water treatment plant, to control the pumps for the incoming water, flushing water and sludge, and for the aeration compressors. The AC drives improve availability, since the pump and compressor output can be adjusted to the actual needs. When the rotation speed of a pump is reduced by the ACdrive to half its speed, this not only adjusts the process as required but also reduces the strain on equipment, doubles the operating life ofthe pump and cuts energy consumption.
Being located inside the rock makes the Kakolanmäki waste water treatment plant a challenging project, so an industrial-grade automation system has been installed, like those used for example in the chemicals industry. Process management comprises control, supervision and monitoring. The AC drives are linked to the automation system by a Profibusfieldbus. The use of Profibus reduces theneed for cabling and the number of interfaces,and gives significant benefits when the AC drives are located underground.
Oxygen is added to the active sludge in the clarification process. Small bubbles of air are pumped into the sludge through aeration discs on the bottom of the basin by five compressors, which are controlled by 355kWAC drives. The surplus heat generated in the aeration compressors is recovered by the LTO system. The highest parts of the underground caverns are over 20 metres high. The deepest tunnels are 16 metres and the aeration basins 9 metres below sea level.
“The client Turun seudun puhdistamo Oy required the contract documents in the competitive tendering stage to contain prices for the main equipment from three different manufacturers. Vacon was chosen to supply the AC drives after a comparison of technicaland financial factors,” states Eero Kaurio,project manager from YIT Environment Oy*,which was responsible for supplying andinstalling the waste water treatment plant,the main plant and the ancillary plant. (* Thebusiness operations of YIT Environment Oywere transferred to ECO Environment LTD Oyon 1 April 2009.)
“In my opinion, the factors in favour of Vacon were that it is a Finnish company, its quality and the maintenance services it provides.Cooperation has been exemplary; it has kept to its delivery schedules and has answered our queries without delay,” he adds.