How Are Chlorine Residuals Removed from Treatment Facilities?
Dechlorination of wastewater effluent is common practice in many treatment facilities throughout the U.S. Strong reducing sulfur 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, monitoring residual values to comply with regulations has become very difficult, and controlling residuals at values between zero and a few hundred parts-per-billion is often not achievable.
Residual chlorine is normally removed by injection of either sulfur dioxide (SO2) gas or a solution of sodium sulfite (Na2SO3) or sodium bisulfite (NaHSO3). Because the resulting sulfite ion is a strong reducing agent, any residual chlorine in the solution is destroyed. As long as there is an excess of sulfite ion in the solution, residual chlorine is effectively zero. In practice, most treatment plants subject to dechlorination requirements run relatively high sulfite residuals to ensure complete chlorine removal at all times. While this practice is effective from a chlorine removal standpoint, one result is excessive chemical consumption.
A15/66 Residual Sulfite Monitor Description
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 Residual Sulfite 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. 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 in dechlorinated effluent has frequently been plagued by fouling problems. An important feature of the A15/66 residual sulfite 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.