Flow measurement of wastewater in an open channel

A French sewage treatment plant is fed via a mixed sewer system with wastewater loads from municipal wastewater and rainwater. In order to measure the flow rate in the open channel, the operator uses level measurement as a common method. This involves measuring the height of the liquid as it passes through a measuring channel (Venturi flume) or over a weir. These specially developed channel shapes characterize the flow of water so that the flow rate can be calculated from the water level.

The flow rate is measured at various points of the channel. This allows the operator to calculate the inlet and outlet of the treatment plant which serves as a basis for optimizing the system and setting operating costs and tariffs.

So far, the liquid level in these open channels was measured by ultrasonic level measurement. This technology was found to be unreliable in case of deposit, foam and agitated product surface. In addition, the frequent cleaning of the device made it expensive on the long-term.

The plant operator was looking for a more reliable and accurate level measuring instrument, easy to install, operate and maintenance-free.

KROHNE delivered an OPTIWAVE 1400 radar level transmitter with polypropylene (PP) Drop antenna and orientable (45°) bracket. Suspended above the open channel, this non-contact FMCW-radar continuously measures the liquid level.

With its small beam angle of just 8° the radar level transmitter is perfectly designed for wastewater applications with open channels. The device features a stainless steel housing and is flood-proof (IP68 / NEMA XP).

The DTM of the device offers a large selection of flow channel types, all matching international standards. Installation and setup are straightforward and secure. The operator just selected the suitable flow channel shape and entered the dimensions. Based on this information, the radar is able to convert level into flow.

The OPTIWAVE 1400 provides the customer with an undisturbed flow measurement at the plant inlet. Thanks to FMCW-technology and the small beam angle of the Drop antenna, the radar provides continuous, reliable and accurate measuring values despite the moving product surface. The values are sent to a PLC (programmable logic controller) via 4…20 output and further processed in the control room to calculate the in- and outlet of the treatment plant and optimize plant operation.

As compared to ultrasonic level technology, the operator benefits from a maintenance-free alternative that is less susceptible to faulty measurements and more cost-efficient over the total operating time. Installation was secure and uncomplicated. The customer could quickly access the DTM of the device via PACTware^™and select the corresponding channel type.