Gas Detection in Sewers – Is It Important?

Would you buy a house with ten rooms and only look at one room to base your decision if the house is in good shape with no problems? Is that one room representative of the whole house? Most likely the answer to both questions is NO, yet when it comes to pipeline condition assessment, many decisions are based on only looking at a few data points.

Such is the case with gas detection in the sewer industry. Typically, it’s only collected and monitored at the entry or exit manhole, with no information in between. We have found high gas concentrations in the middle of a pipeline run, which creates a real problem. Hydrogen Sulfide gas in the pipe is a major cause of corrosion when it turns to sulfuric acid. The emergence of new technology with multiple sensors enables a detailed gas detection analysis of not just the manholes but the entire pipeline. This information can now be used and correlated with other sensor data to provide a complete pipeline condition assessment. In turn, this assessment can validate and provide insight into potential critical areas of concern.

During low-flow, dry-weather periods, sanitary wastewater solids deposited in combined sewers become a major pollutant of our water system. They have significant adverse impacts on the integrity of the sewage system and receiving-water quality. Even sewers that are supposedly designed to be ‘self-cleansing’ will have transient sediment deposits, and part of the load in transport will move near the sewer invert.

These solids contain high concentrations of sulfates, and the conditions of a sewer are ideal for transforming them to hydrogen sulfide. The hydrogen sulfide is then oxidized to sulfuric acid, a highly toxic and corrosive gas, by biochemical transformation.

Gas monitoring and gas detection inspections are conducted prior to and during any task involving a sewer because of the potential for the presence of sewer gases, especially in large diameter pipes where flow rates tend to be less and the area above the flow level is greater than smaller diameter pipes. The potential for gas problems is also greater in lift stations and chambers than in small-diameter sewer pipes. This is because sewers under most design situations have natural ventilation that is pulled along by friction from the flowing water in the pipe (and we don’t tend to get that kind of ventilation in lift stations, chambers and in large diameter pipes.) Although there are many kinds of sewer gases that are created, I will focus on Hydrogen Sulfide (H2S) in this article.

The production of H2S can prove devastating to our sewer systems. H2S in sewer pipes is produced by a combination of bacteria. Some of the bacteria then convert the H2S gas into sulfuric acid. The acid corrodes interceptors and can cause the collapse of other concrete waste treatment structures. Collection systems vary widely in their vulnerability to hydrogen sulfide corrosion. For example, vitrified clay and plastic pipes are very resistant to hydrogen sulfide corrosion. On the other hand, increases in sulfuric acid rapidly accelerate the deterioration of concrete, steel and iron pipes.

H2S levels should be monitored over time to ensure that the concentration remains below the threshold required to produce sulfuric acid (less than 2.0 ppm). The collection system utility should have a program under which it monitors areas of the collection system that may be vulnerable to the adverse effects of hydrogen sulfide. It may be possible to perform visual inspections of these areas. The records should note such items as the condition of metal components, the presence of exposed rebar (metal reinforcement in concrete), copper sulfate coating on copper pipes and electrical components, and loss of concrete from the pipe crown or walls. “Coupons” may be installed in structures or pipelines believed to be potentially subject to corrosion. Coupons are small pieces of steel inserted into the area and measured periodically to determine whether corrosion is occurring.

The collection system utility should also be aware that a system in which wastewater inflow has been reduced may face an increased risk of corrosion, since the reduction of flow through the pipes allows un-submerged conditions to occur and acid to be deposited. A terrain that encourages the wastewater to move at a higher velocity will be freer of hydrogen sulfide than one where the wastewater may experience longer detention times in the pipes. Therefore, some systems may need a more comprehensive corrosion control program, while some might limit observations to vulnerable points.

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