Question:
I have occasionally had problems with homeowners complaining about getting odors now and then caused by their septic system. I have read replacing the outlet tee of the septic tank with an elbow can eliminate the odor. What do you think of this idea?
Answer:
Odors from a septic system do present a problem now and then. There is usually a fairly simple solution. However, the solution is not to change the vented tees of the septic tank.
The bacteria active in a septic tank are anaerobic. Anaerobic means the bacteria operate without oxygen from the air. The result of their anaerobic activity generates odors, the most common of which is hydrogen sulfide gas. The odor from hydrogen sulfide is like the odor of rotten eggs. Not too pleasant!
There is not a great deal of gas generated in a septic tank, but the gas must be released so pressure does not build up in the tank. If the septic tank has inlet and outlet baffles, they must be vented. If the inlet and outlet are tees, they must have upward extensions and be vented.
GAS PRESSURES
I have heard of the inlet tee being replaced by an elbow. If this is done, then the gases from the anaerobic bacteria would need to move out into the drainfield in a gravity flow system. I don’t see much of a problem with this, unless the inlet elbow becomes plugged for some reason. It would seem an inlet elbow could not be cleaned out as easily as an inlet tee.
If the septic tank discharges into a pumping tank, then there can be some gas pressures building up in both the septic tank and pumping tank. Whether this will cause a problem depends on how tight the tanks are.
I have also heard of the outlet tee being replaced by an elbow, and this supposedly cures the odor problem. The gases are still being generated, and as the pressure builds up in the septic tank, the gases will move out through the inlet and into the plumbing vent system. The plumbing vent outlet is on the roof of the dwelling.
Why isn’t there still an odor? I suspect the reason is the soil absorption system is vented and air circulates through the septic tank and out the vent pipe on the roof, carrying the odor with it. Replacing the outlet tee with an elbow prevents this air circulation. If there were no air movement through the septic tank, there likely would be very little gas movement from the tank and no detectable odor.
Vent pipes are of no value in a soil absorption system. If all the installed vent pipes were capped or eliminated, this would have the same effect of restricting the air movement as replacing the outlet tee with an elbow.
DOWNDRAFTS THE CULPRIT
The theory for the installation of vents in a soil absorption system is to keep the system aerobic. However, the theory is wrong since the oxygen must be introduced on the soil side of the biomat. That is where the oxygen is needed. There is no way sufficient oxygen can be introduced into a trench by a surface vent to keep the septic tank effluent aerobic.
So, I definitely suggest the septic tank continue to have vented baffles or vented tees. If air circulation from the soil absorption system is prevented, then there will be only a small amount of gas flowing out of the vent pipe on the roof.
The vent pipe of the plumbing system must extend well above the roof line according to most plumbing codes. As I recall, some plumbing codes state the top of the vent pipe shall be one or two feet higher than a level line 10 feet long from the vent pipe location to the roof. The sketch shown here may help explain why the vent pipe should extend well above the roof line.
If the top of the vent pipe is not far enough above the roof line, wind currents may carry the odors along and down toward the ground.
Downdrafts along the side of the roof where the vent is located may be caused by wind blowing over the roof from the opposite side. Wind blowing over nearby trees, which are higher than the house, can also cause downdrafts. The downdrafts may occur only when the wind is from a certain direction and at a certain speed. This is why the odors may happen only every now and then.
QUICK FIXES
What are the solutions to the odor problem? One simple solution may be to lengthen the plumbing vent and extend it well above the roof line.
In some cases some pruning of nearby trees may be in order. This is a fairly drastic solution, however.
I think there may be a better and easier solution. Many have successfully used an activated charcoal filter, which fits on the top of the existing plumbing vent. The charcoal filter does not restrict the flow of air or gases, but takes the odor out of the escaping sewer gases.
I suggested this type of filter to one of my clients, and it was very successful in eliminating the outside septic odors. There are usually a few suppliers of charcoal filters for plumbing vents advertised in Pumper.
In conclusion, don’t replace either of the septic tank vented tees with an elbow. Look elsewhere for the solution to the odor problem.
