Trial of the two technologies, Dissolved oxygen monitoring put to the test

Analytical Technology with a head office in Philadelphia and a European sales and service centre in Delph ( UK), specialises in electrochemical sensors.

A trial was undertaken back in August 2009 to investigate the performance of optical and galvanic dissolved oxygen monitors as part of the "Galvanic versus Optical DO sensors" debate. Two dissolved oxygen monitors, one using a galvanic sensor and one using an optical sensor were installed on an aeration lane at a wastewater treatment site in the UK. Both were equipped with the ATi air blast AutoClean technology.

After two years of maintenance-free operation with no manual cleaning of the sensors, data was gathered which clearly shows that both monitors were still performing almost identically. See the graph of 1 week's comparative data. When the calibration values were checked the pre- calibration and post-calibration slope values for both sensors were less than 1% apart; further evidence of the effectiveness of the ATi AutoClean system.

While both sensors have been proven to exhibit the same levels of performance when equipped with air blast AutoClean technology, they do differ in terms of cost of ownership and life span. Galvanic sensor cost around 20 times less to own. This is explained below.

The long term costs of ownership of optical DO monitors are still relatively unknown as they have not been around very long. The membranes are impregnated with optically active materials and most optical sensors need a new membrane every 12-24 months. However, advanced membrane technology has been developed that is expected to last three to five years. A new optical membrane costs around £75 to £150 depending on the manufacturer. Optical sensors are estimated to have typical lifespan of ten years. Once an optical sensor has failed it is not economical to repair it and so it must be replaced. A best estimate of the annual consumable cost for optical sensors is around £100/€110/$150

In contrast, galvanic sensors contain maintainable elements, including electrolyte and electrodes. This means that they can work for over 15 years after installation. A galvanic membrane sensor will need a new membrane every 12 months and a new anode every two to three years. This gives an annual consumable cost of around £5/€6/$8.

In summary, newly developed auto-clean technology enables wastewater treatment plants to reduce energy, operation and maintenance costs by ensuring that sensors in both galvanic and optical DO systems are kept clean and in calibration. While previous comparisons between galvanic and optical DO sensors without auto-clean technology have revealed optical DO sensors as the most reliable and accurate option, auto-clean technology allows customers to choose between galvanic and optical DO systems that deliver equal levels of performance. If a comparison is made between efficiently auto-cleaned sensors, the cost of ownership for optical systems is around 20 times that of a galvanic system for no major performance improvement, meaning the end user can choose whichever sensor they feel best suits their application.

ATi supplies both optical and galvanic DO systems, leaving the choice down to the end user. The company's Model Q45D-ODO Optical Dissolved Oxygen monitor utilises a luminescence/fluorescence-based sensor to provide a reliable oxygen measurement. This sensor is designed to provide years of service and will not be degraded by exposure to direct sunlight. The optical DO sensor contains a fluorophore element that absorbs liqht at one wavelength, and then radiates light at a different wavelength (fluorescence). Oxygen molecules collide with the fluorophor which causes a non-radiative reaction, meaning the fluorophore element gives off less light. In short, molecular oxygen reduces fluorescence and the concentration of dissolved oxygen can be determined by this quenching process.

Analytical Technology's Model Q45D Dissolved Oxygen monitor with a galvanic sensor is in use on thousands of aeration lanes worldwide and provides reliable oxygen measurement and reduced maintenance costs. The galvanic DO sensor uses a rugged 5 mil Teflon membrane to resist mechanical abrasions and tears. The membrane serves as a barrier to allow molecular oxygen to diffuse into the reaction cell where it is reduced, producing a small current which is proportional to oxygen concentration.

In order to ensure reliable operation, both sensors feature an Auto- Clean function which generates a series of high pressure air blasts to remove foulants from the face of the sensor. Each cleaning cycle lasts approximately three minutes, during which the monitor outputs are placed in a HOLD condition to prevent false readings or alarms. The user can vary the cleaning cycle frequency from as often as once every two hours, to as little as once a day depending on process conditions.

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