Reliable stormwater measurement

In the course of modernisation of one of its stormwater basins, the wastewater treatment plant Waiblingen-Hegnach decided to implement radar measurement technology from VEGA. The basin, which is 5 kilometres away from the treatment plant, is monitored reliably by three radar sensors.

The employees of the sewage treatment plant at Waiblingen-Hegnach, Germany, certainly cannot be accused of not being close to nature. Yet they do have mixed feelings about spider webs!

Spun beneath the ultrasonic sensors, spider webs used to cause measurement uncertainties again and again. But animals and insects were not the only culprits: foam and even sunlight also had adverse effects on the ultrasonic measurement. Indeed, the Waiblingen staff has really had enough of foam. Once, the wastewater discharge from a nearby shampoo manufacturer caused major problems for the ultrasonic measurement due to the huge amounts of foam generated.

The shaft of the sewage pumping station is located in the middle of a flood-endangered area, so the radar gauges were designed with an IP68 protection rating.

While planning the new instrumentation for the stormwater basin, plant manager Frank Sura knew exactly what he wanted. “As a matter of principle we use non-contact methods for wastewater wherever possible. Because of the oil, dirt and debris in the water, ultrasonic technology was often deployed, accompanied of course by the very well-known difficulties of using that method.” That is why, in the course of modernisation of the stormwater basin, Vegapuls WL 61 radar level sensors were used instead, at three different measuring points.

Since May 2011, the instruments from VEGA have been delivering reliable and stable readings. “It is certainly not a particularly difficult application, but it has to function reliably,” explains Sura. “Beside their reliability, what really convinced us about radar instruments was their price, which was hardly different from that of ultrasonic devices.”

All plant components are connected via a control system, so the team working at the sewage treatment plant has access to all functions at all times. This is important because such mid-size treatment plants tend to be minimally staffed and many different areas have to be monitored. A remote control system provides security, setting off an alarm in case of damage or contamination by toxins or other hazardous substances.

The stormwater basin, one of a total of 13, is 5 km away from the treatment plant and situated directly on the banks of the river Rems.

The measurement data is transmitted every half hour to the control system via a mobile phone network, and the radar sensors are deployed at three measuring points. All units have Ex certification, a necessary precaution since fuels or solvents can also be present in the wastewater. Location Number 1 is directly in the open stormwater basin. There, the sensor monitors the level in the basin and, if necessary, puts the discharge pumps into operation.

In the nearby sewage pumping station, another radar sensor is installed in the pump shaft. Previously, this measurement was achieved via a pressure measurement. The entire facility is located in a flood-prone area - the Rems has a small channel, but a large drainage area, and as a result, the water level can rise substantially within a few hours. That’s why the measuring instruments have an IP68 protection category, which allows them to be immersed in floodwaters.

The third instrument is located some distance away, hidden in the middle of a meadow. There, a vortex valve regulates discharge into the sewer line. Most of the water generally flows past the basin. Only when there is a heavy rain, for example, and the rainwater runoff increases to a multiple of the normal wastewater flow, do the stormwater basins come into play. They buffer the dirty water and take the load off the treatment plant. The tank contents stored during the heavy rain are forwarded to the sewage treatment plant little by little after the rain event. To ensure proper control of this process, the vortex valve has to work flawlessly.

“Very few people have an idea of what can be found in sewage. We’ve already had to remove boulders, two-by-fours and flashlights. A large tree branch can block the vortex valve very quickly,” continues the plant manager.

In the past, someone from his team always had to go and check if there was really a blockage or if it was just another industrious spider that had built its web under the ultrasonic transducer! This is no longer necessary, since a radar level gauge is keeping watch there now. Further upgrades, including additional stormwater basins within the catchment area of the wastewater treatment plant, are planned for the future.