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September 10, 2015
The sewage treatment plant in Putzhagen, Germany, requires large quantities of high-pressure compressed air for its exceptionally deep sedimentation tanks.
Three ZB 130 VSD centrifugal compressors with magnetic bearings and variable rotation speed and two older single-stage screw compressors supply the compressed air for the biological process that occurs in the tanks.
Each year, the plant treats an average of between 6.5 and 7 million m³ of sewage water. "The biological process takes place in eight tanks that are sub-divided into four lines," explains Reinhard Schweinforth, Sewage Water Technician. The tanks are 10 m high with a water depth of 8.5 m. "These dimensions are unusual for sewage treatment plants," says Schweinforth. Tanks are normally 4 to 5 meters deep. The biological cleaning processes are carried out in the eight aeration tanks. The air required for this procedure is blown into the tanks from below via diaphragm tube aerators — an energy-intensive process that accounts for 40% of the plant's entire power consumption.
The compressed air required for the biological process was previously provided by five single-stage screw compressors. Because these aging compressors needed to be completely overhauled at great cost, the search for a new energy-efficient solution began. Centrifugal compressors with magnetic bearings were deemed to be the best option because a comparatively high pressure is required for the exceptional depth of the tanks.
“We are able to produce higher pressures with the centrifugal compressors than we were previously able to with the screw compressors," explains Schweinforth. "We have purchased the new compressors with the aim that the machines will now be more energy efficient and easy to maintain," says plant manager Karl-Heinz Schröder. "Now that we are using the centrifugal compressors from Atlas Copco, we are saving around EUR 15000 each year on maintenance, energy and resources compared with screw compressors.”
The three speed-regulated centrifugal compressors generate between 2500 and 7500 Nm³ of oil-free compressed air per hour depending on the amount of sewage water and the polluting load. The interaction is engineered by the ES 130 master control system from Atlas Copco. The process control system determines the air requirement on the basis of the oxygen content in the tanks.
“At the moment, we are still using a constant pressure regulator," says Schröder. "Our next goal is to optimize this regulatory process and integrate a variable pressure regulator. Up to now, we have wasted a lot of energy as we have been operating against closed gate valves at a constant pressure of 1020 millibar. With the variable pressure regulator in place, all eight gate valves are linked and the pressure can be adjusted in line with demand, thereby enabling us to reduce the pressure used.”
Written by Michael Gaar