Since its construction in 1977, the Stockton East Water District’s Dr. Joe Waidhofer Water Treatment Plant in Stockton, Calif., has used two Fischer & Porter model 70C4500 series gas chlorinators with 71P1100A pressure reducing valves for water disinfection. For more than 29 years, the equipment provided reliable and efficient disinfection for the 290,000 plus residents of the Stockton area, located 80 miles due east of San Francisco.
In 1997, the district began investigating replacing its system with new, state-of-the-art gas chlorination equipment. However, budget constraints prevented the district from purchasing new equipment until 2005.
At that time, Richard Bermudez, operations supervisor, worked with Ward Technical Products, Inc. to investigate ways of meeting the Uniform Fire Code (UFC) while keeping gas chlorination equipment as their primary disinfection treatment. The UFC requires that a self-contained scrubbing system be available for emergency use when using gas chlorination.
Dry vs. wet scrubbers
The most common scrubber systems used by municipalities are wet scrubbers. Such systems are economical, simple to operate and can easily be fabricated of corrosion-resistant materials.
However, dry scrubbers are increasingly being placed in municipal water systems across the country. While these systems are typically more expensive than wet systems, the full long-term investment of dry versus wet scrubbers is not always understood. The cost of a dry system’s media should be compared to the practical cost of the alternative wet caustic soda solution.
Caustic soda scrubbing solution used in wet scrubbers will last about five years in an installation without a leak. The reaction caused by CO2 in the air – without a chlorine leak – changes the caustic soda into sodium carbonate solids that precipitate from the solution and affect the operation of the scrubber.
With 20 years of normal use, a wet emergency scrubber system will incur the costs of replacing caustic soda solution, hazardous liquid waste disposal, and labor and operator certifications for handling the spent caustic four times. In the event of an emergency, there would also be the additional costs of caustic solution replacement and disposal for each major leak.
With the EST Dry Emergency Scrubber Type DES  from Severn Trent Services, dry media beads are permanently bonded with sodium thiosulfate and are landfill disposable and non-hazardous in the fresh and spent forms. The spent dry media cannot be regenerated and thus should be discarded in a non-hazardous landfill or as required by local regulation. The solid reaction products are permanently locked in the alumina oxide substrate. When not exposed to chlorine, the dry sodium thiosulfate media has an indefinite life expectancy. When using dry media in an emergency chlorine gas scrubber over 20 years, there would be no replacement cost unless there was a major leak. Therefore, while the cost of most dry scrubbers is higher than that of wet scrubbers, dry media replacement and disposal costs over a 20-year operational period, assuming complete media exhaustion, could be half the cost of the caustic solution required for wet scrubbers.
The right scrubber for the job
Ward Technical Products recommended the EST Dry Emergency Scrubber Type DES  from Severn Trent Services. The system was engineered and tested for use in municipal and industrial applications where the potential exists for the accidental release of heavier-than-air hazardous gases, such as chlorine and sulfur dioxide. Dry scrubbers are safe, user-friendly, low-maintenance systems that consist of a fan and a vertical cylindrical FRP vessel containing a bed of chemically impregnated 1/8-in. diameter dry pellet media. The only moving part, the fan, produces a vacuum on the contaminated room and draws the gas-laden air from top to bottom through the media bed and out to the atmosphere. The media reacts with the gas and reduces the concentration at the scrubber discharge to within the guidelines as set forth by the prevailing codes. The media substrate permanently bonds the chemical impregnate and salt products from the gas reaction, allowing clean, non-toxic landfill disposal.
In addition, dry scrubbers operate at sub-zero temperatures without the use of heaters and do not require liquid chemical leak containment or double wall vessel construction.
The EST Type DES is designed to neutralize a leak from an overfilled one-ton portable tank in accordance with the UFC worst-case release of hazardous gas through a fusible plug. For one-ton applications, three model sizes are offered corresponding with the required room exhaust rate and type of dry media utilized. The EST Type DES 3000 has a room exhaust rate of 3,000 cubic feet per minute. Higher exhaust rates such as 5,000 or 7,000 cu ft per minute are normally considered when the gas capacity needs to be split between two or more rooms, or when a single room volume exceeds 50,000 cu ft. When sulfur dioxide is present and needs to be scrubbed either alone or in conjunction with chlorine, the model suffix will be “PHD” indicating the kind of chemical media to be used.
The “STS” media is considered when chlorine is the lone containment gas present.
In December 2005, the district issued an award to Ward Technical Products for the EST Type DES. The district also awarded Telstar Instruments for the installation of the scrubber and for new chlorination equipment, the Capital Controls Advance gas feeder  series WX4144 from Severn Trent Services.
The equipment was installed and started up by June 2006. Award, submittal, supply, installation and startup were completed in six months.