Residual chlorine is normally removed by injection of either sulfur dioxide gas or a solution of sodium sulfite of sodium bisulfite. Because the resulting sulfite ion is a strong reducing agent, any residual chlorine in solution is destroyed. As long as there is an excess of sulfite ion in solution, residual chlorine is effectively zero. In practice, most plants subject to dechlorination requirements run relatively high sulfite residuals to ensure complete chlorine removed at all times. While this practice is effective from a chlorine removed standpoint, one result is excessive chemical consumption.

ATI's Model A15/66 Sulfite Ion Monitor provides the solution to dechlorination control. The system allows continuous measurement of sulfite residuals over ranges of either 0-2 or 0-20 ppm. An analog output from the monitor can be used for control of the chemical feed system to maintain a safe residual sulfite concentration while reducing chemical expense to a minimum.

The A15/66 Monitor takes a unique approach to the measurement of sulfite ion concentration. In operation, a small amount of sample is pumped into the system and mixed with acid. In acidic solution, the sulfite ion is converted to sulfur dioxide according to the following reaction: The mixed sample flows into a special chamber where the sulfur dioxide is stripped from the sample. A sensor located in the gas stream measures the released SO2 concentration and displays the results in terms of equivalent sulfite ion concentration.

Sulfite measurement is dechlorinated effluent has frequently been plagued by fouling problems. An important feature of the A15/66 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 sulfur reducing bacteria that can proliferate in water containing excess sulfite.

A15/66 Sulfite Monitors consist of three separate components, a chemistry module where the sample is pH adjusted for measurement, an inlet overflow assembly where raw sample is delivered to the system and an electronic readout containing the sulfite concentration display, analog output and alarm contacts. Readout modules are available in either wall mount NEMA 4X of general purpose panel mount versions. A 20 foot interconnecting cable is supplied to connect the monitor to the chemistry module, and this separation can be increased to a maximum of 100 feet if required. An optional stainless steel system panel is available for mounting all components and providing a convenient shelf for reagent bottles.

Sample is connected to the inlet overflow assembly using 1/4" id flexible tubing. Recommended sample flow rate is 3-30 gallons per hour (.2-2 LPM). While the monitor uses only a small fraction of this sample, higher flow keeps sample delivery times to a minimum. Excess sample simply overflows to a drain chamber. A 1/2" id hose barb is provided for connection of drain tubing.

Sulfite monitoring systems are extremely easy to operate and maintain, with acid usage limited to one gallon every 40 days at standard flow rates. Acid usage of pH adjustment in the chemistry module is inexpensive and consumption is limited to 1 gallon every 40 days. Peristaltic pumps are used for sample and acid using long life tubing that requires replacement every 6 months. Pump heads are designed for easy tube changes, requiring about 10 minutes to replace both tubes. The sulfite gas sensor requires no maintenance other than an occasional visual inspection to ensure that no deposits have collected on the sensing membrane.

Other methods of monitoring dechlorination, such as "biased" chlorine monitors or ORP monitors, cannot match the sensitivity, selectivity and overall accuracy of the A15/66 Residual Sulfite Monitor. If better dechlorination process control is your goal, continuous sulfite measurement will provide the key to insuring dechlorination while reducing chemical consumption to a minimum.