Sulfides can be found naturally in well water and can build up in wastewater collection systems due to anaerobic conditions that frequently occur. They are also used in mercury removal processes and are frequently found in tanning wastes. In drinking water systems, sulfides cause taste and odor problems. In wastewater systems, they can cause damage to concrete structures in collection systems and contribute to odor problems in treatment facilities.

Measurement of dissolved sulfide concentrations has been done primarily by the use of analyzers employing selective ion electrodes (SIE) for sensing. While providing adequate sensitivity, SIE based systems require frequent zero and span adjustments to maintain measurement accuracy. Because of this, most SIE based monitoring systems are relatively expensive and require frequent service.

ATI’s Model A15/81 Dissolved Sulfide Monitor provides an improved method for measuring sulfides in solution. Rather than using an SIE sensor, the A15/81 employs a polarographic H2S gas sensor that is isolated from the sample. The result is a system that can operate continuously on many types of water and wastewater streams with minimal maintenance and adjustment.

The A15/81Monitor takes a unique approach to the measurement of sulfide in solution. In operation, a small amount of sample is pumped into the system and mixed with acid. In acidic solution, hydrosulfide and sulfide ions (HS- and S-2 ) are converted to hydrogen sulfide.  The mixed sample flows into a special chamber where the hydrogen sulfide is stripped from the sample. A sensor located in the gas stream measures the released H2S concentration and displays the results in terms of equivalent sulfide ion concentration in mg/l.

Because sulfide measurements are often made in samples of poor quality, fouling of the analytical system has been a major concern. An important feature of the A15/81 system is the fact that the sensor never comes in contact with the sample. Only the gas stream containing the stripped H2S reaches the sensor. The result is a system that will continue to function, regardless of the quality of the sample. The only requirement is that large particulate be strained from the sample. The analytical system will easily pass particulate as large as 100 microns, so only course screening is required.

________ (Quantity) Residual Sulfide Monitors shall be supplied for continuous measurement of the residual sulfide at the _______ (Specify Location). The sulfide monitoring system shall consist of and electronic monitor, a chemistry module containing the gas phase sulfide sensor, pumps and associated accessories listed below. The residual sulfide monitoring system shall be ATI Model A15/81 or equal.

Sulfide Monitor

The Sulfide 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 sulfide in a range up to 20 PPM. The display shall also indicate alarm setpoints, and all configuration information and be 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 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 H₂S gas 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 Hydrogen Sulfide 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  zeroing and calibration reference samples.