Take A Measure of Prevention

April 12, 2005
How to prevent toxic and combustible gas tragedies in clean water and wastewater treatment facilities

About the author: Alan Austin is the manager for product line management at General Monitors, Inc. He can be reached at 949/581-4464 or by e-mail at [email protected].

Every year, municipal and industrial workers are injured in accidents at clean water and wastewater treatment plants or processes. Chlorine (Cl2), methane, hydrogen sulfide (H2S), sulfur dioxide (SO2), nitric oxide (NO) and carbon monoxide (CO) are all part of clean water and wastewater treatment processes.

Typical areas where toxic and combustible gas monitoring are necessary include:

  • Cl2 Storage Room;
  • Sewage Collection (H2S, CH4, O2);
  • Sludge Dewatering (H2S and CH4);
  • Digester Tanks (H2S, CH4, O2); and
  • SO2 Storage Room.

Monitoring toxic/combustible gases

Fixed-point and/or portable gas detectors are essential to protect workers and equipment, as well as an OSHA and EPA requirement.

A fixed system typically consists of one or more electrochemical cells, metal oxide semiconductor, catalytic-bead, or infrared sensors placed in key locations. When toxic or combustible gas is present outside of acceptable limits, gas exposure data are communicated via hard wiring to a control station. Portable detectors are utilized in confined spaces or large areas where fixed systems are impractical or cost-prohibitive.

When planning a toxic or combustible gas safety monitoring system, there are many important factors to consider. Some of the most important factors are:

  • Identify the potential combustible and toxic gas hazards;
  • Review the plant layout, potential leak sources and placement of detectors;
  • Study the plant’s ambient operating environment (watch for temperature and humidity extremes);
  • Review the various sensing technologies for performance and reliability;
  • Look at ease of detector installation, calibration, training and operation;
  • Consider the detector’s maintenance and repair needs; and
  • Determine the detector’s expected life in your plant’s environment.

For example, protecting waste treatment plants against H2S is best achieved with MOS gas-sensing technology using fixed gas detectors placed throughout the facility.

You could give every employee a portable gas detector, but how would you guarantee they would always wear and maintain their monitors in good working order? On the other hand, in confined spaces with infrequent access, the technology of choice is generally electrochemical cell sensors packaged as a portable gas detector. Tanks need to be cleaned, pumps require maintenance and oxygen deficiency can be deadly in such environments. Placing fixed gas detectors inside tanks is both impractical and costly given the small amount of time people enter them. One solution may be a portable detector.

No single type of sensor or controller can provide the sensitivity and response time required for every combustible or toxic gas hazard in a clean water or wastewater treatment process or plant. The potential gas hazards must be first identified, then various gas detection sensing technologies must be analyzed, the plant operating environment must be studied, along with factors such as ease of installation, maintenance and total installed cost.

About the Author

Alan Austin

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