Leading by Innovation

Suppliers Of Water Quality
Instrumentation & Bespoke
Measurement Systems For
Waste-Water Since 1990

Welsh Water FDO evaluation performed in activated sludge at Cross Hands WwTW


Pollution & Process Monitoring was invited to install instrumentation for evaluation at Cross Hands WwTW. The trial started 2 March 2010 and concluded 29 April 2010.

The objective of the trial was to establish whether the FDO 700 IQ (the more recently developed optical DO sensor), would provide the same measurement performance as the site installed Trioxmatic 700 IQ sensors that have already demonstrated excellent performance and low maintenance. Furthermore, the Company had installed alternative optical dissolved oxygen technology on the premises that it would provide reduced maintenance and improved reliability which had not materialised.

Cross Hands WwTW was selected as the test site because the site instrumentation would allow a comparison against proven site instrumentation, the site was manned enabling the trial to be properly monitored and most importantly, the application was considered to be one of the most challenging. Significant bio-fouling and blinding from fats derived from a local trader would test the ability of the sensor technology to remain clean and report concentration correctly without the need for regular manual intervention.

WTW FDO Dissolved Oxygen Sensor Installation

One of the site's Trioxmatic DO sensors was disconnected to allow the FDO test sensor to be installed for comparative measurement. The two sensors were calibrated using the same barometric pressure adjustment to ensure that the best possible initial correlation could be provided. Both sensor were mounted using the site rigid pole installation immediately adjacent to each other at an approximately depth of 0.70m and the sensors digitally transmitted the data to the site DIQ/S 182 transmitter provided analogue signals to the site telemetry system.

Periodically the sensors were removed by the Area Scientist during the trial period so visual inspection could be made as to the performance with regards bio-fouling however no manual cleaning took place during the evaluation period.

The standard deviation between the FDO 700 IQ sensor and the reference instrument was better than 0.998 for the eight week evaluation period. The test data showed an excellent correlation between the FDO sensor and the site Trioxmatic 700 IQ sensor. This demonstrates that the calibration of both devices during the test period did not drift and that the membranes of the sensors were unaffected by bio-fouling. The Silicone membrane fitted to the FDO sensor performed equally well as the porous Teflon membrane that is fitted to the Trioxmatic 700 IQ sensor.

Comparative trace for last week of evaluation ending 29 April 2010

Comparative trace for last week of evaluation ending 29 April 2010Both sensors follow each other during the last week of the 8 week installation period when bio-fouling and calibration drift would have had an effect if the design was not suitable for the application.

Image of FDO 700 IQ sensor at end of trial period

Image of FDO 700 IQ sensor at end of trial periodThe image clearly demonstrates that the complete sensor including the end- cap which performs the measurement, is free of biological fouling and ragging that can otherwise prove detrimental in waste- water applications. The only biological fouling that occurs is on the installation armature at surface level well away from any measurement surfaces.


The FDO 700 IQ sensor has the advantage that the replacement membrane cap is supplied pre-calibrated and the internal reference technology automatically maintains the calibration during the 24 month guaranteed life. Whilst with the optical technology a Silicone membrane is fitted, the trial has demonstrated that the sensor would not foul and need manual cleaning. Consequently the sensor can be used in confidence as a reliable and low maintenance control instrument.

For further information on how PPM can support your instrumentation requirement please contact:

Steven Tuck
Pollution & Process Monitoring Ltd
Tel: 01732 882044
Fax: 01732 780190

Email: s.tuck@pollution-ppm.co.uk