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Controlled by Liquiline Control

Automated nitrogen removal in Stadtlohn wastewater treatment plant

The Stadtlohn WWTP regulates nitrogen reduction using the Liquiline Control. The control system regulates the air supply into the aeration basins depending on the current concentration of nutrients in the wastewater.

  • Liquiline Control controls the nitrification and denitrification phases in Stadtlohn WWTP. ©Endress+Hauser

    One of the aeration basins at Stadtlohn WWTP. The Liquiline Control system monitors the online measurements of oxygen and ammonium to control the nitrification and denitrification phases.

  • The ISEmax CAS40D sensor measures nitrate and ammonium in the Stadtlohn wastewater treatment plant. ©Endress+Hauser

    The ISEmax CAS40D sensor measures nitrate and ammonium. In Stadtlohn WWTP, a sensor has been installed in the outlet of each basin.

The results

  • Safe compliance with limit values.

  • Stable operation.

  • Optimized energy usage.

  • High level of process transparency combined with ease of use.

  • The switching of the basins from parallel to cascade operation during a plant upgrade posed no problem for the control system.

  • The Stadtlohn WWTP is designed to handle wastewater from a population equivalent (PE) of 30,500. At present, a PE of approximately 21,000 is connected to the plant, roughly 15% of which is industrial wastewater. The plant‘s two intermittent basins are activated sequentially (in cascade formation). Commissioned in 1991, the facility has undergone extensive modernization works over recent years

  • Ewald Rathmer
    With Liquiline Control, we reliably adhere to the limit values at the outlet of our wastewater treatment plant without burning energy through too much aeration.

    Ewald Rathmer

    Plant Manager, WWTP Stadtlohn (Germany)

  • Markus Pries
    Key advantages of Liquiline Control are its user-friendliness and transparency. It can be operated using only a few very explicit parameters. This makes things easy for us.

    Markus Pries

    Electrician, WWTP Stadtlohn

The challenge

The customer required a control system that automated nitrogen reduction while also ensuring compliance with the official nitrogen value in the plant outlet (TN ≤ 8 or 12 mg/l, depending on the time of year). It was also important that the control system would be able to function after the clarifiers were switched from parallel to sequential operation. The upstream basin is now used at the peak-load stage, while the second basin is subsequently used at the low-load stage.

The nitrogen concentration in the outlet had to be as constant as possible during both peak- and low-load phases. Furthermore, the solution had to be easy to operate, to ensure that staff could make settings quickly and easily when on standby duty during the night and at weekends.

Our solution

The WWTP operator opted for a complete solution comprised of several different components:

  • One oxygen sensor per basin as well as an ion-selective sensor for ammonium and nitrate.

  • Programmable logic controller (PLC) with Liquiline Control software, which controls both basins individually.

  • Touchscreen display used to operate the system from the control room.

  • Various services, such as consulting, design and configuration of the controller, process integration, system training.

Load-dependent aeration control in Stadtlohn

The customer wanted the two aeration basins in the plant to be aerated individually. Sensors in both basins measure the parameters ammonium, nitrate and oxygen. Using the measured values, the Liquiline Control control system configures a dynamic set point for oxygen: depending on the ammonium concentration, the exact amount of air required for complete removal of the ammonium (NH4-N) is injected. This results in process stability while also optimizing the energy efficiency of the blowers.

In addition, the controller dynamically adjusts the duration of the nitrification and denitrification phases. This improves the reduction of total nitrogen (TN), ensuring that it remains at a constant level in the plant outlet.

Remote access to Liquiline Control control system

For the Stadtlohn WWTP, the controller was fitted with a remote control unit. This means that staff on standby duty are able to monitor the process and intervene if necessary from the comfort of their own homes.


Endress+Hauser Instruments International AG/ Asia Pacific Support Center
Unit 3, 1st Floor, CentrePoint Building 106 Nguyen Van Troi Street Phu Nhuan District Ho Chi Minh City Vietnam
Tel.: +84 28 3842 0026
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