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Following a successful 12-month trial, Wessex Water has selected Analytical Technology as its preferred provider of the SO2 dechlorination monitoring in one of its clean water treatment plants.

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Comparison data over the 12 month period demonstrated that the ATi monitors were 75% more cost efficient and accurate than the water company’s current monitoring system, which was inaccurate and very expensive to maintain.


Based on these results – their most accurate ever – Wessex Water has now made the installations permanent, which will see them save upwards of £2,000 a year in reagents and maintenance costs, with the monitors paying for themselves within two months,


A Gas-Phase Approach to Sulphite Measurement


Wessex SO2 monitorAs measuring the SO2 ion in solution is relatively difficult, ATi’s Q46S/66 Monitor takes a different approach to the measurement, employing a unique gas-phase method to continuously monitor sulphite, without contact between the sensor and the water sample.  In operation, a small amount of sample is pumped into the system and mixed with acid.  In acidic solution, the sulphite ion is converted to Sulphur dioxide gas.


The mixed sample flows into a special chamber where the Sulphur Dioxide is stripped from the sample.  A gas conditioning module prepares the gas sample for measurement prior to contact with a special SO2 gas sensor designed for the system.  Sensor signals are amplified and displayed on a large format, backlit LCD display in the Q46S electronics unit



Benefits of ATi’s Q46S/66 Monitor


Dechlorination of wastewater effluent is common practice in many wastewater treatment facilities. Strongly reducing sulphur compounds are used to eliminate chlorine residuals that might prove toxic to fish in the receiving stream. Because residual chlorine discharge limits are often very close to zero, getting water quality monitoring residual values to comply with regulations has become very difficult, and controlling residuals at values between zero and 10 or 20 parts-per-billion is often not achievable.


To meet these stringent discharge limits, the sulphite or bisulfite used for dechlorination is added in slight excess, providing a small sulphite residual to ensure complete dechlorination. ATI’s Model Q46S/66 provides operators with a reliable tool for maintaining a small sulphite residual, while reducing excess chemical consumption due to overfeed.



Accurate results and cost benefits


Wessex SO2 installationsGarry Tabor, ATi’s European Sales Manager, believes continuous measurement data is crucially important to confirm treatment facilities are operating efficiently: “ATi’s Sulphite Ion Monitor provides the solution to dechlorination control. The system allows continuous measurement of sulphite residuals over ranges of either 0-2 or 0-20 PPM.


“Sulphite measurement in dechlorinated effluent has frequently been plagued by fouling problems. An important feature of the Q46S/66 system is the fact that the sensor never comes in contact with the wastewater sample. The result is a system that will continue to function, regardless of the quality of the effluent, or the presence of sulphur, reducing bacteria that can proliferate in water containing excess sulphite.


“The Sulphite monitors offer a completely new and improved approach to continuous on-line monitoring, which is simpler, cheaper and more accurate than conventional monitoring equipment.”

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