Hydrogen Sulfide In A Collection System: What We Should Know And Do To Prevent Human Injury And Sewer Asset Deterioration.

NASSCO Tech Tips
By NASSCO member Jim Harris, PE, TFE Resources | October 2013, Vol. 68 No. 10

Imagine this:
• A sewer worker enters a manhole for one more look late in the day, just to check one final thing. He does this without an entry permit, a harness and tri-pod or a safety person on the surface. His co-workers, not knowing what he is doing, miss him. They look in the manhole and he is not there. He is found an hour or so later, dead at the sewage treatment plant.
• A sewer inspection crew begins their day by placing their smoke machine on a manhole and pulls the cord without first taking gas readings in the manhole. The resulting explosion breaks most of the bones in the face of the inspector.
• The public works director of a moderate size southern city receives bad news upon arriving at his office. The trunk sewer carrying one third of the city’s wastewater has collapsed and is overflowing into sensitive wetlands.
• An odor is wafting over an outdoor amphitheater located next to a sewage pump station disturbing concert goers.
Each of these is a true story,of incidents that occurred from a single culprit –hydrogen sulfide gas in our sanitary sewer wastewater.
Why is this happening? It begins with sulfates. Wastewater is a perfect environment for sulfate formation. Sulfates are the natural result of the decomposition of sulfur in fecal material and nitrogen in urine. Under anaerobic conditions sulfates convert to sulfides. When sanitary sewer systems are stagnant, surcharged, full pipe or excessively laminar, the wastewater does not readily aerate with oxygen and sulfides rapidly build up. As this anaerobic, sulfide-laden flow encounters turbulence, perhaps at the intersection with a force main or a steeper slope gravity line, air is mixed with the flow, resulting in hydrogen sulfide gas production. This is the gas that smells like rotten eggs, causing troublesome complaints, but which also is explosive and can asphyxiate and kill those who are directly exposed. This hydrogen sulfide is also the gas that, when it condenses on the surface of iron and concrete sewer pipes, manholes and mortar between bricks, is converted by bacteria into sulfuric acid which is highly corrosive to many sewer collection system materials.

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