Dangerous Gases Threaten Pumpers, Cause System Maintenance Woes

Methane and hydrogen sulfide are silent, sometimes odorless killers that should be top of mind every time you approach a septic tank inspection.

Dangerous Gases Threaten Pumpers, Cause System Maintenance Woes

Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at editor@pumper.com.

Recently a colleague shared research he and others have been doing, looking for causes of and a solution to corrosion in concrete sewage tanks. As I have indicated in past columns, the common answer has been corrosion — particularly at the outlet baffle in septic tanks — is due to the lack of adequate venting.

Over the years, this has resulted in a lot of discussion about where gases that cause the deterioration come from, where they are generated and what can be done to prevent the problems. This is a topic I will address in an additional column.

When I looked at some of the data presented, I was struck by some of the measured levels of hydrogen sulfide gas (H2S) during these studies and the implication for safety of workers and others who may come into contact with sewage tanks.

The two main gases released during anaerobic digestion processes in the septic tank are methane and hydrogen sulfide.


Methane is an odorless, colorless, flammable gas. It is used primarily as fuel to make heat and light. It is also used to manufacture organic chemicals. Methane is lighter than air. Methane can be formed by the decay of natural materials and is common in landfills, marshes, septic systems and sewers.

Methane can form an explosive mixture in air at levels as low as 5 percent. You can smell leaking methane only when commercial gas utility companies add a chemical scent to it or when it mixes naturally with hydrogen sulfide, causing a “rotten egg” odor. This combination of gases leads to the odor we would call sewer gas. If you can smell it, the level may be too high to be safe. In a well-ventilated situation, methane is dissipated into the atmosphere rapidly. However, in a confined space such as a sewage tank, gases can accumulate and be breathed in by the service provider or others who open the tank.

Exposure to high levels of methane gas depletes the oxygen level in the body, causing difficulty breathing and suffocation. If the oxygen level in the body depletes to anything less than 12 percent, the person can become unconscious, which can be fatal. As the levels of oxygen in the body deplete, the body tries to make it up by using the oxygen contained in the bodily fluids. This leads to dehydration. Nausea and vomiting are also methane gas poisoning symptoms. Another symptom is heart palpitations. It causes an uncomfortable sensation of the heart beating rapidly, abnormally and out of sequence. Due to the depletion of oxygen in the body, it gives rise to cognitive problems. The person is inattentive, has memory loss and poor judgment. These symptoms aggravate when the exposure to this gas is higher.

The person will collapse soon after exposure. I hear or read about service providers who die because they “fell” into the septic tank; it is likely they stopped breathing, passed out, and fell into the tank. Sometimes in these situations, rescuers are likely to pass out as well and suffer the same fate.


Hydrogen sulfide is also an extremely toxic and irritating gas. It is heavier than air, so it will collect in low areas such as at the outlet baffle of a septic tank. Free hydrogen sulfide in the blood reduces its oxygen-carrying capacity, thereby depressing the nervous system. Hydrogen sulfide is oxidized quite rapidly to sulfates in the body, therefore no permanent effects occur in cases of recovery from acute exposures unless oxygen deprivation of the nervous system is prolonged.

There is no evidence that repeated exposures to hydrogen sulfide result in accumulative or systemic poisoning. Effects such as eye irritation, respiratory tract irritation, slow pulse rate, lassitude, digestive disturbances, and cold sweats may occur, but these symptoms disappear in a relatively short time after removal from the exposure.

Response to exposure to hydrogen sulfide as taken from information provided by the American Standards Institute follows:

Odors become detectable in concentrations as low as 0.008 ppm, but the sense of smell is lost after 2-15 minutes at 100 ppm. At 100 ppm, eye and throat irritation, coughing, and drowsiness can occur after 15-30 minutes of exposure. After several hours of exposure without treatment, death may occur within 48 hours. At concentrations of 500-700 ppm, loss of consciousness may occur after 30 to 60 minutes. At concentrations above 700 ppm, loss of consciousness can happen immediately with death in a few minutes, even if those exposed are removed to fresh air at once.


From a safety perspective, tanks should never be entered without proper respiratory equipment and ventilation. Measurements of H2S taken in a pump station with corrosion problems was measured at 100 ppm. In a vented septic tank, studies showed concentrations as high as 700 ppm, which demonstrates concentrations in those tanks were high enough to be a significant safety concern to any service providers or others who might think about entering the tanks. If you as a service provider have tanks that show corrosion problems, this should be a red flag to you that the likely cause is hydrogen sulfide gas.

The focus of the study was the impact on venting on the microbially induced corrosion in concrete tanks; but it serves as a good safety reminder. Next month I will take a look at the impact of venting and tank construction on the corrosion problems.


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