Question:
I am curious to know the function of a vent on an ATU. Homeowners hate to see them, hiding them costs time and money, and they can be a source of odors. I install Clearstream units, and they don’t have a vent. They seem to work just fine. If the ATU can vent through the house plumbing stack (which it should unless there is a serious dip in a pipe that creates a P trap) and the air pump is not drawing more amps than it should, why do most manufacturers install a vent?
Answers:
Not all ATUs are created equal. Some are not allowed to vent back towards the house, so there is no other option. It just goes to show you why some are more popular than others.
It depends on the airflow rate (cfm). Some systems use a low flow rate and are all right to vent through the plumbing stack. Possibly Clearstream is one of them. Bio Green is another. If you install a system that uses a blower rather than a compressor and has a very high airflow rate, it will try to vent through the plumbing stack and all that air will slow down. I have seen it actually stop the flow down the pipe to the septic system.
Also, many older homes only had small vent stacks (1.5 to 2 inches), which are not large enough to handle the ATU venting. Some even vented into the walls, which could cause more problems. When you flush the toilet watch the water in the bowl; if it fluctuates up and down, you know the vent is not big enough or is partially plugged.
When I do an install, any brand, I always use an underground biofilter and use drain rock to surround perforated pipe, filter cloth and soil to neutralize the odor problem. You can use chamber sections and soil or bark mulch — it will work just as well.
In fact, when I went back to one of my installs during a rainy day, you could see the bubbles coming up through the ground. Just be sure to use a breathable sandy loam, minimum 6 inches, maximum 9 inches, and make sure water cannot flow down the vent pipes back into the ATU, overloading it. Double-check the amperage of the blower when you are done to confirm. The nice part is that it’s all underground out of sight. There are no complaints of “a thing sticking out of the ground.”
I’ve been operating, maintaining and installing aerobic units for 35 years, from 1000-gallon household units to 6-million-gallons-per-day municipal plants.
I like the vents on aerobic units. Assume the air compressor puts out 2 to 20 cfm. Each cubic foot contains about 7.5 gallons. In an hour, that’s 900 to 9,000 gallons of sewage air that has to escape the ATU. If it goes up through the house vents and stinks, then what? Your customer is going to be unhappy.
Some ATUs have timers to adjust on/off cycles. If the homeowner has house guests contributing more sewage, and the run time is not increased, more unpleasant odors will be created. If there is not enough aeration, the bacteria will shift to the anaerobic mode, creating a rotten egg smell. The heavy, unpleasant odors will spill out over the roof, and unless there’s a stiff breeze, create a problem.
An aerobic unit that is working properly will smell like wet earth. If it stinks, the unit is undersized or overloaded, or the blower is too small or is not running enough.
