Flow measurement of wastewater in a Parshall flume

A food company in Central America specialises in the production and marketing of commercial broiler chicken breeds. Among other outlets, the poultry products are sold to well-known local retailers and supermarket chains across North America.

The company discharges wastewater from production to an adjacent wastewater treatment plant (WWTP) on a daily basis. When the volume of wastewater exceeds the typical average, the WWTP operator must be notified to adjust treatment capacities and prevent process interruption. The flow rate at the poultry plant outlet also serves as an indication of production efficiency and quality. Therefore, any discharge of excessive flow rates must be detected and its root cause identified.

Wastewater is discharged via a 6-inch Parshall flume (ISO 9826). This hydraulic structure features a constricted throat section, which accelerates the flow and lowers the water level. By measuring the water level at a specific point of the converging section upstream of the throat, the flow rate can be calculated using standard equations or flume charts.

Previously, the poultry producer relied on an ultrasonic level transmitter for many years. Following an audit, the company was required to implement more stable and accurate instrumentation to enable reliable calculation of sewage loads and consistent readings in cubic metres per hour (m³/h). As the level transmitter was exposed to environmental conditions, humidity, wind and temperature changes caused failed measurements and drift over time. A more robust measurement technology that operates independently of environmental influences was therefore required.

The ultrasonic device was replaced by the OPTIWAVE 1540 radar level transmitter. The cost-effective 80 GHz device is optimised for flow rate monitoring in open channels. Due to its radar technology, the level transmitter operates reliably regardless of condensation, rain, wind or temperature fluctuations. It features a robust PVDF design with front flush Lens antenna. For this application, the IP68 version of the OPTIWAVE 1540 was selected, offering enhanced ingress protection in the event of downpours or flooding.

Commissioning of the KROHNE radar was carried out quickly using a mobile device with Bluetooth^® connectivity and the intuitive installation wizard of the OPTICHECK Level Mobile App. As the template for Parshall flumes is already pre-configured in the app, parameterisation of the 80 GHz radar only required the input of the radar-to-bottom distance and the diameter of the flume throat. The KROHNE app enabled the customer’s personnel, who had limited experience with process instrumentation, to correctly and efficiently set up the measurement independently.

The 80 GHz radar was installed upstream the flume constriction. It accurately measures the distance to the water surface and calculates the flow rate. The KROHNE radar was also easily integrated into the plant infrastructure. To totalise wastewater volumes and support overall process control and signalling, the flow readings are transmitted to an existing control unit at the plant. Upon customer request, KROHNE could also have supplied control and operating units such as the SCU 200 or SHD 200 from a single source.

The OPTIWAVE 1540 has proven to be highly reliable, delivering significantly more stable and accurate measurements than the previous instrument. This enables the customer to respond swiftly in the event of unusual readings. Installed outdoors, the radar is far better suited to prevailing weather conditions, eliminating common issues associated with ultrasonic level measurement.

The readings from the KROHNE 80 GHz radar now serve as verified data for the WWTP operator, who can adjust treatment capacity as needed. With a maximum measurement error of just ±2 mm / ±0.08 in, the level transmitter ensures accurate level measurement for Parshall flume flow calculations, while remaining well within budgetary constraints. As with all OPTIWAVE 1540 models, the radar was supplied with a factory calibration certificate confirming its stated accuracy.