UK Water Industry gives ldo™ the thumbs up - Nikki Mellor

Water Companies are constantly in search of technologies that can both improve processes and provide operational savings, and one such area of potential improvement lies with the measurement of dissolved oxygen. A new sensor known as the LDO™ was launched at IWEX 2003 amid a blaze of publicity in which the manufacturer claimed that most of the problems associated with dissolved oxygen measurement had been eliminated.

A part of the launch involved a number of UK Water Companies that put the LDO™ on trial in their wastewater treatment plants. Most of these trials have now been running for several months, so this article will summarise the experience to-date of these operators. However, before outlining the users’ responses, a description of the historical problems with dissolved oxygen (DO) sensors will be given, in addition to an overview of the LDO™’s method of operation.

Many water treatment technologies involve aeration in order to maximise the effectiveness of bacteriological degradation. However, in addition to the need for ideal bacteria growth, it is essential that wastewater is not aerated excessively because this would result in a significant loss of energy, and it has been estimated that 60 to 70% of a treatment plant’s energy costs come from the aeration of activated sludge. It is obvious therefore, that accurate DO measurement represents a vital component of successful plant management.

For more than fifty years galvanic and polarographic sensors have been used to measure dissolved oxygen. These sensors employ membranes, anodes, cathodes, and electrolyte solutions that generally require a high degree of maintenance. The sensors also suffer from drift, and as a result have to be recalibrated frequently.

Historically, there have been a number of problems associated with galvanic and polarographic sensors. The membranes are relatively delicate, and can become contaminated or damaged, in which case it would be necessary to replace the internal electrolyte. The sensor’s anode is consumed over a period of time and will require replacement, or it may need replacement if it, or the electrolyte, becomes poisoned by gases such as hydrogen sulphide.

There are other factors that can affect the accuracy of these traditional sensors, including variations in pH or the presence of chemicals that induce voltage, such as iron and aluminium salts, and polymers.

The manufacturer of the LDO™ claims to have solved these long-standing problems with the launch of a sensor that, in contrast to its predecessors, does not consume oxygen as part of the measurement process, and does not require frequent recalibration because it does not suffer from drift (gradual loss of accuracy). So, how does it work?

The sensor is coated with a luminescent material, called luminophore, which is excited by blue light from an internal LED. As the luminescent material relaxes it emits red light, and this luminescence is proportional to the dissolved oxygen present. The luminescence is measured both in terms of its maximum intensity and its decay time. An internal red LED provides a reference measurement before every reading to ensure that the sensor’s accuracy is maintained.