Sewage works saves energy and improves reliability

Published:  04 January, 2019

Replacing unreliable belt drive systems with gear motors and variable speed drives has enhanced control and reduced breakdowns at a German sewage treatment plant. PWE reports.

The residents of Ensdorf in Western Germany probably give little thought to their local sewage treatment works. Many won’t even know where it is since the facility, opened in 1995, has been carefully designed to blend into the landscape. Yet Ensdorf sewage treatment plant plays a vital role in the social, economic and ecological fabric of the German municipality.

Situated on the right bank of the River Saar, close the borders of France and Luxemburg, the sewage treatment plant processes the waste water generated by the region’s 50,000 or so inhabitants, together with effluent generated by several local industries.

Like all modern sewage treatment facilities, the Ensdorf plant relies on a combination of, mechanical and biological processes to remove dissolved contaminants and solid materials from the incoming effluent. Its job is to ensure that water leaving the facility meets strict cleanliness rules, allowing it to be safely discharged into the Saar, and that the remaining solids are also transformed into a condition suitable for safe disposal, typically as agricultural soil enhancers.

That’s a delicate balancing act. The inflow rate to the plant can vary significantly, from 26,000 m3 per day in dry weather to 57,888 m3 per day during periods of heavy rain. The volumes of contaminants and solid wastes processed by the facility similarly vary over time. To keep the plant’s processes running properly, the plant’s staff must adjust the operating speeds of equipment across the facility as demand rises and falls.

Originally, speed control in some of the plant’s important equipment was achieved via mechanical means. As part of the sludge dewatering process, the pumps that supply sludge to the two centrifuges were powered by 7.5 kW asynchronous geared motors with belt drive adjustment. Additional pumps used a similar system with 4 kW motors to transport the thickened sludge away from the centrifuges and on to the next process.

While the belt drive approach did enable the necessary speed control, the system proved unreliable in operation. Adjusting the belts frequently was inconvenient for operators, and breakages were common, leading to downtime and disruption.

In search of a better approach, the plant’s managers contacted drive technology specialists, Klebs + Hartmann GmbH, who proposed a solution that not only resolved the reliability issues, but also promised useful energy savings.

Klebs + Hartmann is a Bauer Gear Motor Gear Centre which offers its customers engineered solutions for improved performance. In this case Matthias Klee, pProject eEngineer Drive Technology, and Peter Siepel, pProject eEngineer Inverter Solutions, worked together to integrate their specialisms into a bespoke solution. That solution was based on the introduction of variable speed drive (VSD) controls and high-efficiency permanent magnet gear motors supplied by Bauer Gear Motor, part of the Altra Industrial Motion Corporation.

The new approach solved Ensdorf’s reliability challenges at a stroke. The introduction of variable speed drives eliminated the need for belts and allowed the automation of speed control in the transport systems for the centrifuges. Furthermore, as an additional aid to reliability, the new Bauer motors selected for the project are IP66 rated, providing increased protection against corrosion and the risk of water damage from cleaning activities.

There was a significant secondary benefit from the change in technology, however. The greater efficiency of the modern motors enabled significant energy savings. In fact, the new IE3 motors are so much more efficient than the older, unrated units they replaced that significant downsizing was possible. The pumps that supply the centrifuges are now powered by 6.3 kW units rather than 7.5 kW and the downstream pumps are powered by 3 kW motors instead of 4 kW.

After successful testing with one of the centrifuges, Ensdorf has now installed the new Bauer motors in all four locations. While the change has made life easier for the plant’s operating and maintenance teams, and provided useful energy savings, it is unlikely that the people of the region, or the record numbers of fish that now thrive in the river Saar, have noticed any difference.

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