Set the Record Straight About Septic System Ventilation

Fight off homeowner misconceptions by explaining how and why air moves through the components of their onsite systems.

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I often get questions from homeowners, and they’re probably the same questions you hear regularly from customers. Sometimes they’re not really questions, but opinions thrown out based on something they heard somewhere. They’re seeking validations for their opinions about septic systems.

Recently a homeowner with a septic tank and gravity distribution into a series of trenches asked about the need for system ventilation. But the person was essentially arguing that adding oxygen into the system increases heat to improve treatment and avoid freezing problems. 

It is obvious they have read and/or heard from various contractors about how systems are supposed to operate, but have become confused. Wrapped into their questions were some major misconceptions. I thought addressing these may help in some of your conversations with homeowners in the future.

A statement was made that by adding ventilation to the end of each trench there would be additional oxygen available to allow aerobic bacteria to break down waste. This shows a lack of understanding that the purpose for venting a septic tank or other components of the system is to remove and disperse to the atmosphere gases that otherwise accumulate at different points in our systems. Proper ventilation reduces odor and corrosion problems in and around the system.


In a conventional septic tank with gravity distribution, these gases are vented back through the house plumbing stack. Lack of proper venting can lead to corrosion in concrete tanks and distribution devices. Over the years there has been a lot of discussion about corrosion and how to prevent it. With the addition of pretreatment components, pumps and filters, venting the entire system back through the house plumbing stack becomes more problematic. Additional venting of the individual components is often necessary. The purpose of venting is not to introduce oxygen into the system.

In a conventional gravity fed trench with a developing biomat; the conditions in the trench will be primarily anaerobic or without oxygen. Do we want to have oxygen in and around the trench to help with treatment? The answer is yes. We want the soil outside the trench to be unsaturated and be permeable to water and air, so oxygen is in the soil around and under the trench. 

In a soil environment around the trench, if there is organic food (from the sewage), water and air all the elements are there to allow aerobic bacteria and others to thrive. Aerobic organisms are more efficient at breaking down the organic material brought with the wastewater. Biomat formation is controlled and limited, and treatment improved if the trench is not overloaded organically or hydraulically.  

How does oxygen get around the trench? It comes from the atmosphere through the soil. It is one of the major reasons good system installation starts with having the soil dispersal and treatment unit installed shallow. Upper soil horizons have the most bacteria, other organisms and is most permeable to air and water. Adding one or several 4-inch-diameter pipes into the trench will be good to facilitate inspecting the system but will not supply enough oxygen to the system.


To demonstrate, think about this: The area of a circle representing the 4-inch-diameter sewer pipe is 3.14 (2in x 2in) = 12.56 square inches. That is the area oxygen would move through. 

Now consider an operating sewage treatment trench, 3 feet wide by 100 feet long with an operating depth set at 1 foot. Surface area around the perimeter of the trench will be 3 feet x 100 feet + (1 foot x 100 feet) + 1 foot x 100 feet) = 300 square feet + 100 square feet + 100 square feet = 500 square feet. 

Depending on soil texture and structure, soil consists of 35 to 50% pore space. Sandy soils have about 35% and clay soils have 50%. Pore space around the trench (depending on soil type) varies from 175 square feet to 250 square feet! We are talking feet rather than a few inches! Much more oxygen will enter the trench through the soil. Venting a conventional gravity trench will not improve trench aeration. 

The homeowner suggested venting should possibly be suspended during the winter to avoid potential problems due to freezing. Having 4-inch-diameter inspection ports in the trenches will not cause freezing to occur. The system may encounter freezing problems but they will be due to other conditions.  


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