Dechlorination of wastewater effluent is common practice in many treatment facilities throughout the US. Strongly 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 gas or a solution of sodium sulfite or 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 removal at all times. While this practice is effective from a chlorine removal 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.  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 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.

________ (Quantity) Residual Sulfite Monitors shall be supplied for continuous measurement of the residual sulfite at the ________ (Specify Location). The sulfite monitoring system shall consist of and electronic monitor, a chemistry module containing a gas phase sulfite sensor, pumps and associated accessories as listed below. The residual sulfite monitoring system shall be ATI Model A15/66 or equal.

Sulfite Monitor

The Sulfite Monitor shall be a compact ¼ DIN size instrument suitable for panel mounting. For outdoor applications, the monitor shall be supplied in a NEMA 4X enclosure with a clear hinged door to allow tool-less access to controls and for viewing the LCD display. The display shall be alphanumeric LCD and capable of indicating the residual sulfite in a range up to 20 PPM. The display shall also indicate alarm setpoints with all configuration information programmable through the front keypad. The keypad shall be protected with a software lock and all configuration programming to be protected by an access code.

The monitor shall provide two independently programmed alarm/control relays selectable over the entire range of the instrument as either low or high alarm. In addition, both relays shall be programmable for variable dead and variable time delay. A third and fourth independent relay shall be provided for the purpose of automatic cleaning of the chemistry module sample and chemical line.

An isolated 4-20 mA analog signal driving up to a 600 ohm load shall be provided and capable of being programmed throughout the entire range of the instrument.

Chemistry Module

The chemistry module shall be housed in a separate NEMA 4X enclosure with hinged door and contain a special SO₂ gas phase sensor with acrylic sample chamber, a sample pump, an acid pump, an air spurge pump with acrylic air stripping chamber and precision flow control rotometer.

The chemistry module shall be designed to receive the sample intended to be measured and then precisely mix with a diluted acid for the purpose of pH reduction. The mixed sample is then air stripped of the sulfur dioxide and measured by a special high humidity gas phase sulfur dioxide sensor.

A 3-way sample line hand valve shall be provided for the purpose of bypassing the process sample and providing the system with a zeroing sample.