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Case study

Energy and equipment savings at the Svenljunga sewage treatment plant

When the sewage treatment plant in Svenljunga, a community of 10,000 inhabitants, was renovated in the summer of 2005, investments were made in order to increase operational efficiency and reliability. One measure taken was to install Emotron variable speed drives and softstarters to control pumps, blowers and mixers, and Emotron shaft power monitors to protect equipment. The result is more reliable control, lower energy consumption and reduced wear.

[15 November 2006]

The whole plant is now managed by one operator, instead of the two or three that used to be required for daily control and supervision. The increased efficiency means, among other things, that time has been freed up for preventive measures and maintenance.

“We used to regulate most of the equipment manually, but today everything is controlled by Emotron products,” says electrician Johan Arvidsson.

 
Photo: Johan Arvidsson, electrician at the Svenljunga sewage treatment plant, supervising an excentre pump pumping sludge to a thickener. The pump is controlled by an Emotron FDU variable speed drive. An Emotron M20 shaft power monitor ensures nothing is blocking the screw conveyor.

Constant flow despite varying demand

In a sewage treatment plant, the objective is to keep a constant flow despite the large variation in load during the day and night. In Svenljunga, Emotron products ensure efficient operation continuously adjusted to demand. Variable speed drives and softstarters regulate pumps, blowers and mixers, and shaft power monitors protect screw conveyors.

“One advantage of Emotron products is that they can withstand harsh environments and be located within the plant,” says Johan Arvidsson. “No costly cabling or cabinets are required. We also minimize the risk of damage and downtime, since we are warned in good time in case of process disturbances.”
 
 

Photo: The equipment at the Svenljunga sewage treatment plant used to be controlled manually to a large extent. Today everything is regulated by Emotron products, such as these inlet pumps.

The Emotron products react immediately to deviations and send a warning or stop the process if operation is not optimal, for example if a pipe is blocked or a valve is not fully opened. The operator can take action quickly in order to prevent damage and downtime. Common and costly problems such as cavitation and dry-running can be avoided.

The stop sequence is a critical moment in pump operation, often causing stress on pipes, valves and other equipment. The Emotron solution offers soft stops by gradually reducing the pump speed, thereby avoiding water hammer and other potential damage.
 
The sludge is dewatered in centrifuges, the screw conveyors for which are controlled by seven Emotron MSF softstarters. The sludge is then used as fertiliser or sent for final disposal.

Minimized energy consumption and equipment wear

A total of 17 Emotron FDU variable speed drives are installed in the plant. They control, among other things, three inlet pumps that pump in untreated sewage water.

Two of the pumps are normally used to handle overflow water, i.e. water that is allowed to pass the plant after only limited treatment, for example in the case of heavy rainfall.

The flow must not exceed 220 m³/hour in order for the particles that are to be removed to have time to settle to the bottom and not run out with the water. If the flow is too high, the system regulates this by starting the third pump to pump water past so that the particles can settle.

The operation of the blowers supplying oxygen to the sewage water was also made more efficient by installing Emotron FDU variable speed drives.

“Until now the blowers were either running at maximum speed or standing still,” says Johan Arvidsson. “That was wasting energy. The Emotron FDU now continuously adjusts to demand, ensuring that the water maintains the correct oxygen content. In this way we minimize our energy consumption and equipment wear.”

 
Photo: Regulating the blowers using Emotron variable speed drives has reduced energy consumption and equipment wear. On the right Johan Arvidsson and on the left Peder Wale, salesman at Emotron.

More efficient sludge treatment

Emotron FDU variable speed drives control the sludge pumps that pump out surplus sludge from the basins. The FDU units are located in cabinets out by the basins. An excentre screw pump that pumps sludge to a thickener is also controlled by an Emotron FDU. An Emotron M20 shaft power monitor monitors the screw conveyor and sends a warning or stops the process if a blockage occurs.

Mixers placed down in the basins are regulated by Emotron FDUs, as are the sludge pumps that pump sludge to centrifuges where it is dewatered. Seven Emotron MSF softstarters control the screw conveyors for the centrifuges. The sludge is used as fertiliser or sent for final disposal.

Photo: The sludge scrapers in the sedimentation basins are monitored by Emotron M20 shaft power monitors. The monitors send a warning or stop the process if operation is not optimal, for example if a scraper is blocked, a blade is worn or a chain is broken. Equipment wear and downtime are minimized. The monitors are connected to the motor cable. No sensors are required.

Local industry built own sewage treatment plant

As Svenljunga sewage treatment plant was being renovated, the largest local industry, Elmo Leather, built their own plant next to the municipal plant. The sewage water from the tannery had been putting high demands on its capacity.

“Now our plant is rated at 3000 pe*, compared to the 100,000 pe required to treat the sewage from Elmo Leather,” says Johan Arvidsson.

Elmo Leather’s new wastewater treatment plant is the first of its kind. A fully biological process is used that has never been tested before in an operation that uses such large quantities of chemicals.

The EU’s environmental fund contributed to the financing and the good results mean that the company will lead the way for other tanneries in Europe. The new technique reduces nitrogen emissions by 80%, compared to 30% in the old plant. Emotron variable speed drives control, amongst other things, the blowers that oxygenate the waste water.