Sensors create a stir in WWT sector

Issues around the maintenance of dissolved oxygen (DO) sensors and the accuracy of measurements provided by these devices are causing significant concern in the UK water and wastewater industries.

In this sector, galvanic DO sensors were the device of choice up to about 10 years ago, until the arrival of optical DO probes, which removed the need for frequent recalibration and maintenance.

Optical DO sensors employ a technique that 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.

Galvanic dissolved oxygen sensors are inherently more precise when measuring low levels of dissolved oxygen, experts say. At zero oxygen concentration there is no current. At low concentrations there is little quenching of fluorescence, the processor has to reliably measure the small difference between two large numbers.

Galvanic sensors, however, consume oxygen and need a flowing or moving sample, whereas optical sensors work in stagnant water.

Both types of sensor 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 often uncertainty about using these sensors and the measurements they provide. This, in turn, leads to costly over-treatment of wastewater to meet regulatory requirements.

The energy required for an activated sludge plant represents about two thirds of a wastewater treatment plant’s total costs, so controlling it is extremely important, notes Graham Meller, a specialist in this subject as environmental writer at Buttonwood Marketing.

“Water companies are also under pressure to reduce their carbon footprint, whilst complying with tighter EU discharge permits,” he said.

Over-aeration is often there to cope with any surge in influent load, but this results in wasted energy/cost.
Real-time controls are an obvious solution to the problem, Meller noted that many plants around Europe have been able to make energy savings of 20% or more with such systems.

A recent roundtable discussion on DO sensors brought together industry and regulatory experts, including: Alan Henson of Yorkshire Water; Andy Morse, Richard Bragg and Khaled Gajam, United Utilities; Jorgen Jonsson of The Water Research Centre and Robin Lennox, South West Water.

Their consensus was that many water companies are running their treatment plants at dissolved oxygen levels higher than strictly necessary to optimise the process, and so comply with environmental regulations.

Lack of confidence

A key factor behind these issues was an historic lack of confidence in the ability of monitors to reliably measure low dissolved oxygen levels, found the panel, which was chaired by Michael Strahand, general manager of Analytical Technology Europe – the manufacturer of electrochemical-sensor devices which hosted the discussion.

This overaeration was blamed for large energy costs built up by water treatment plants. Producing high dissolved oxygen concentrations requires large amounts of energy, particularly if dissolved oxygen levels become too high, the speakers noted.

Water companies, however, prefer to remain on the safe side by over-aerating sewage in order to over-treat it. Incurring large electricity bills, it seems, is a greater evil than being hit by penalties for breaching consents with the Environment Agency.

There was a consensus among the panel about the most pressing problem with dissolved oxygen sensors: the difficulty of cleaning both optical and galvanic sensors, due to the build up of sludge which affects the accuracy of both instrument types.

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

Experts at the meeting went on to complain that large amounts of time and resources were being spent each year on cleaning sensors. The panel members also asked suppliers of dissolved oxygen sensors – some present at the meeting – to provide more information and guidance on the cleaning of these instruments.

Indeed, one of the most important points to emerge from the discussion was that instrument maintenance was more important to users than the  actual technology: the instrument must be properly maintained and regularly cleaned in order to measure dissolved oxygen accurately.

Manufacturers were also urged to offer maintenance warranties, and to work to develop technologies such as auto-cleaning functions for dissolved oxygen sensors, as well as capabilities to detect and alert end users when a device needs cleaning.

The overall conclusion of the roundtable discussion, though, was 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.