Using 5 Flow Sensors for Extensive Reconstruction

Abwasserzweckverband Breisgauer Bucht" is a municipal association for sewage treatment in the south of Germany and operates 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 commercial and industrial enterprises within the catchment area.

Flow Measurement

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

NIVUS Measurement System

Instead of installing a mag meter, NIVUS recommended 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 proposal. However, the procurement of the measurement system after the trial period required compliance with certain provisions – the maximum permissible deviation between the day totals determined in the intake using the NIVUS measurement system and the day totals determined in the outlet of the secondary clarification using a calibrated mag meter was typically ±3%.

Test measurements spanning a period of three months revealed that the deviations between the day totals determined using the inductive mag meter and the NIVUS system were almost exclusively within the desired range of ±3%. Therefore, the installed NIVUS measurement system was able to achieve the desired quality even at this early stage.

NIVUS COSP Technology

In addition to 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 COSP system. The term COSP stands for Correlation Singularity Profile. The NIVUS COSP technology is a technique that helps to calibrate and adjust flow velocity measurements even under poor flow conditions. Based on the cross correlation method, this calibration can be performed using the latest NivuFlow 750 M9 transmitter together with up to nine 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 obtained from 16 individual velocities in each of 5 measurement paths with high temporal resolution. Subsequently, these flow velocity profiles have been assessed – extended by velocity components detected in skew flow areas and dead zones – and finally interpolated to obtain a two-dimensional flow velocity array.

Furthermore, the average flow velocity within the investigated 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 evaluation revealed that the readings obtained from the solution using three permanent sensors within the intake section are just 0.7% higher than the average flow velocity established by the reference measurement using COSP calibration. The appropriate correction factor was then applied to adjust the measurement accordingly.

The result of further 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 reduced to less than ±1% by applying the COSP calibration.

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

The NIVUS GmbH Urban Drainage Monitoring department performed the COSP calibrations. Application is advised for calibration and adjustment of flow measurements deployed at the measurement sites featuring poor flow conditions (e.g. 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.