Extensive Reconstruction Using 5 Flow Sensors

“Abwasserzweckverband Breisgauer Bucht” is a municipal association for sewage treatment in the south of Germany and runs a central wastewater treatment plant for its members (29 communities and cities). The WWTP’s treatment capacity is 600,000 population equivalents and it covers a catchment area of 650 km². The plant is also treating wastewater from 375,000 residents from industrial and commercial enterprises within the catchment area.

Flow Measurement

The municipal association requires an accurate flow measurement in the intake of the pre-clarification stage, offering dependable data on approaching water volumes for billing purposes. The measurement spot is positioned between grit trap and pre-clarification and includes significant asymmetric flow conditions because of the structural and operational situation. An inductive mag meter was initially included during the planning stage, but such a measurement system would have needed major investment and constructional alteration of the measurement site.

NIVUS Measurement System

Rather than installing a mag meter, NIVUS suggested the use of three CS2 sensors as well as an external level measurement within the rectangular channel of the measurement site. The municipal association accepted this suggestion. However, the procurement of the measurement system after the trial period required adherence with certain provisions – the maximum permissible deviation between the day totals established in the intake using the NIVUS measurement system and the day totals defined in the outlet of the secondary clarification using a calibrated mag meter was typically ±3%.

Test measurements covering a period of three months exposed that the deviations between the day totals determined using the inductive mag meter and the NIVUS system were nearly exclusively within the preferred range of ±3%. Thus, the installed NIVUS measurement system was able to realize the anticipated quality even at this early stage.

NIVUS COSP Technology

Besides comparing the measurement outputs from the mag meter and the three CS2 sensors in the WWTP outlet, the NIVUS measurement system was calibrated using the Correlation Singularity Profile (COSP) system. The NIVUS COSP technology is a method that helps to calibrate and alter flow velocity measurements even under poor flow conditions. Based on the cross correlation technique, this calibration can be performed using the latest NivuFlow 750 M9 transmitter along with up to 9 flow velocity sensors. Five CS2 sensors were used during the COSP calibration of Forchheim WWTP.

With the help of these sensors, flow velocity profiles could be acquired from 16 individual velocities in each of five measurement paths with high temporal resolution. Then, these flow velocity profiles have been evaluated – stretched by velocity components detected in skew flow areas and dead zones – and lastly interpolated to acquire a 2D flow velocity array.

Additionally, the average flow velocity within the examined flow cross section was also established. Then, the deviations between the average flow velocities coming from the three-sensors-measurement and the average flow velocities coming from the COSP application were calculated. The final assessment revealed that the readings acquired from the solution using three permanent sensors within the intake section are only 0.7% higher than the average flow velocity determined by the reference measurement using COSP calibration. The applicable correction factor was then applied to regulate the measurement accordingly.

The result of additional comparative measurements over a period of one month was that the average deviation between the NIVUS intake measurement and the measurement in the WWTP outlet can be decreased to less than ±1% by applying the COSP calibration.

"Abwasserzweckverband Breisgauer Bucht" performed a comparative evaluation between the flow measurements in intake and outlet sections of the Forchheim WWTP. The NIVUS team expressed its appreciation to the association for supporting the COSP calibration and for delivering the comparison results.

The NIVUS GmbH Urban Drainage Monitoring department performed the COSP calibrations. Application is recommended for calibration and adjustment of flow measurements installed at the measurement sites featuring weak flow conditions (for example, asymmetric oncoming flow).

This information has been sourced, reviewed and adapted from materials provided by NIVUS GmbH.

For more information on this source, please visit NIVUS GmbH.