NFPA 820 Standards for Wastewater Treatment Plants: Part 2

As we explored last month, the National Fire Protection Association takes a special interest in wastewater treatment plants. The buildup of hazardous substances in and around processing facilities has been associated with highly dangerous and destructive workplace accidents. Understanding the nature of sewer and sludge gas is the first step in preventing these incidents.

What’s In These Gases?

According to the EPA, sludge and sewer gases have different components. Here’s a typical breakdown:

  • Sludge = 65% methane, 30% carbon dioxide, and 5% other gases
  • Sewer = 70% carbon dioxide, 25% other gases, 5% methane

Those “other gases” in sludge and sewer gas may include hydrogen sulfide, ammonia, carbon monoxide, sulfur dioxide, and nitrogen oxides. Some of these substances create a toxic or asphyxiating atmosphere. But for the purposes of the NFPA, it is the potential fire hazard that is of interest. Methane and hydrogen sulfide, in particular, are flammable and highly explosive. An ignition source such as an electrical switch that creates even a small spark has the potential to set off these combustible materials. The higher the concentration of gases, the bigger the explosion.

What Level of Risk Is Normal?

According to the National Electric Code (NEC), wastewater plant processes create a Class 1 hazard (defined as the presence of flammable gases or vapors in the air). The production of hazardous gases in the wastewater industry is unavoidable. But the dangers posed can be minimized. When considering appropriate controls and safety, it’s important to understand what is normal and what is exceptional in terms of hazards.

The NEC draws a dividing line between Division 1 conditions (expected during normal operations) and Division 2 conditions (which might occur due to an accident or malfunction). The types of equipment and wiring methods deemed appropriate depend on this classification. If an area might be subject to a higher risk condition in the event of a rupture or leak, additional precautions may be needed to safeguard against fire or explosion.

Is Downgrading Classifications Possible?

In some cases, a previously hazardous area or condition can be downgraded to a less hazardous rating by making changes to equipment or processes. When feasible, it is in the interest of safety to reduce the classification of an area of the facility to make it safer. This is often achieved by increasing ventilation in a space or taking other measures to reduce the likelihood of gas accumulation.

Next month, we’ll look at some example scenarios of preventive measures that a wastewater treatment plant might take to minimize hazards.