Roundtable Highlights Need for Improved Maintenance of DO Sensors

A roundtable discussion with water industry and regulatory experts has agreed that there is a need for improved maintenance of dissolved oxygen sensors to increase the accuracy of dissolved oxygen measurements.

The roundtable, which was hosted by the specialist electrochemical-sensor manufacturer, Analytical Technology (ATI), was made up of leading industry and regulatory experts.

The panel was chaired by Michael Strahand, General Manager of ATI Europe and included Alan Henson from Yorkshire Water, Andy Morse, Richard Bragg and Khaled Gajam from United Utilities, Jorgen Jonsson from The Water Research Centre and Robin Lennox from South West Water.

The panel was invited together to share their views concerning dissolved oxygen sensors for reliable dissolved oxygen measurement.

Optical D.O. sensors developed in the last few years use a technique that essentially measures the rate at which oxygen absorbs an optical signal generated within a membrane impregnated with a fluorescent dye.

Galvanic oxygen sensors measure the current produced in an electrochemical reaction cell. A membrane serves as a barrier to allow molecular oxygen to diffuse into the reaction cell where it is reduced at the working electrode.

This reaction produces a small current which is proportional to oxygen concentration.

There are some fundamental differences between the technologies. Galvanic dissolved oxygen sensors are inherently more precise when measuring low levels of dissolved oxygen. At zero oxygen concentration there is no current.

At low concentrations there is little quenching of the fluorescence, the processor has to reliably measure the small difference between two large numbers.

Galvanic sensors consume oxygen and need a flowing or moving sample. Optical sensors work in stagnant water. One important thing that both types of sensor have in common is that both have a membrane.

The membrane on a galvanic sensor controls the rate of diffusion into the electrochemical reaction cell; on an optical sensor its primary role is to prevent ambient light affecting the measurement.

The cleanliness of the membrane is vital to both types of sensor.

Although most water companies use optical sensors due to the guidance of their frameworks, there is a highlighted concern from the industry as to which type of sensor, if any, is more accurate and efficient.

The panel started proceedings discussing why many water treatment plants are running their treatment plants at dissolved oxygen levels higher than strictly necessary to optimize the process.

The main factors that emerged were a historic lack of confidence in the ability of monitors to reliably measure low dissolved oxygen levels and also the need to comply with regulations mapped out by the Environment Agency.

This over aeration was blamed in the discussion as a reason for large energy costs built up by water treatment plants. Producing high dissolved oxygen concentrations requires large amounts of energy, a waste if dissolved oxygen levels become too high.

Feedback from the Analytical Technology’s roundtable was that water companies prefer to remain on the safe side by over-aerating sewage in order to over-treat it and thus incur large electricity bills rather than incur penalties for breaching consents with the Environmental agency.

There was a consensus among the panel that the most pressing problem with dissolved oxygen sensors is the difficulty cleaning both optical and galvanic sensors, resulting in the build up of sludge which affects the accuracy of both instrument types.

Problems with cleaning dissolved oxygen sensors also unearthed an issue of damaged trust between manufacturers of sensors and water companies, as previously some manufacturers had sold sensors as “maintenance free”.

Those present at the meeting claimed large amounts of time and resources were being spent each year on cleaning sensors. Panel members asked suppliers of dissolved oxygen sensors present, for more information and guidance on the cleaning of these instruments.

The overarching conclusion of the discussion was that instrument maintenance is more important to users than the technology; the instrument must be properly maintained and regularly cleaned in order to measure dissolved oxygen accurately.

It was accepted that electrochemical and optical sensor are both capable of delivering the accuracy and reliability required by water companies if they are kept clean and well maintained.

It was suggested that manufacturers include a maintenance warranty and a technological demand was raised for the auto-cleaning functions of dissolved oxygen sensors to include a sensor to detect when it needs cleaning and to sound an alarm.