An issue that comes up repeatedly at our workshops is what to do about odors from an onsite system. As usual, the time to deal with odors is at the time of installation. If the system is installed properly, any odorous gases will vent out through the system and out the plumbing stack on the roof of the house.
Air must be able to flow from one part of the system to another for proper ventilation of the sewer gases through the plumbing stack. After installation of the septic tank, a smoke test can be used to verify that the tank is venting back through the stack. For dual-compartment tanks, the installer should verify that the air can pass from one compartment of the tank to another across the gap in the top of the baffle wall.
If there is no space for this air movement to occur, you should discuss that with the tank manufacturer or supplier and ask them to fix the problem. In some concrete tanks with a slot or center hole positioned over the wall, a thin layer of concrete left over from the pouring process may need to be removed. Often, a hammer can be used to knock the excess off the top.
Sometimes additional vents are added to the tanks themselves, but recognize that there will likely be odors without the placement of a carbon filter in this vent pipe.
Hydrogen sulfide issue
A side problem to improper venting in concrete tanks is the accumulation of hydrogen sulfide gas, which can be converted to sulfuric acid in the head space of the tank. When improperly vented, this gas typically settles around the outlet baffle — because the gas is heavier than air. Where it settles, it will cause corrosion or breakdown of the concrete. This causes baffle deterioration and ultimate failure.
Most often, this problem occurs because the inlet piping from the house to the tank is pushed in too far so that it is up against the sidewall of the inlet baffle. In other cases, during backfilling around the tank and piping, the pipe on the outside ends up lower, so that it enters the tank at an angle.
Either of these conditions will cut off the free flow of air back through the tank — and also provide a place for plugging and freezing to occur. This is one reason why you should visit the site about a month or six weeks after installation, open the tank, and check to see if everything is working properly.
If a pump station or another pretreatment unit tank is part of the system, this tank should also vent back through the system. Here again, you can check with a smoke test. A pump tank will effectively cut off the venting of the soil treatment part of the system. However, since unsaturated soil has anywhere from 40 to 50 percent pore space, the gases will naturally vent through the soil.
Conduit for odors?
One odor problem we have encountered with pump tanks is the venting of gases back through the conduit that carries the electric lines for the pump and the pump alarm system. Any opening for electrical wiring should be sealed at the tank. Even if the line only runs to an outside box or panel, odors can be noticeable around the pump tank and system.
Of course, another reason to seal this opening is to make sure that the tank is watertight. This same pathway under the right conditions can allow surface water to drain into the tank.
Another potential cause of odors around tanks is the lids. Common practice now is to bring risers and lids to the surface to facilitate system maintenance. If the risers are at all exposed, they need to be sealed. This also insures that water cannot enter.
The lids need to be installed so that the covers are tight-fitting. Just last summer, we were at the site of new construction before landscaping and finishing work had been done. There was a stiff breeze blowing, and downwind there was a strong septic system odor.
Upon inspection we found that only one screw had been placed in the lid, and there was a crack about a quarter-inch wide between the cover and riser. Sure enough, the air blowing across the manhole was causing the tank to vent through the cover.
There are two other reasons to make sure all the screws are used: To prevent children or others from entering the tank, and to keep surface water from entering the system. There is another reason to visit the system after installation.
Temporary issues
Odors may be apparent around newly installed tanks that have not yet developed normal biological processes, or just after a tank has been pumped. These should dissipate with time.
There also can be normal odor from the roof stack. This is most often noticeable at times of still air, partic-ularly during temperature inversions in the atmosphere that may occur in the early morning or late evening.
Here, possible remedies are to extend the roof stack higher to an elevation above the roof ridge, or to install an activated carbon filter. If you extend the stack, recognize that in cold climates this may result in freezing problems and blockage of the stack during winter. If you use filters, they will need to be replaced every six months. Filters also may cause vent-freezing problems in the winter.
If you are troubleshooting odors for older systems, everything mentioned above applies and should be checked. Additional items to look for include:
• Broken tank top or other tank deterioration.
• Submergence of the inlet due to blockage or system backup (this indicates potential drainfield failure).
Then of course there are the odors associated with sewage seeping to the surface in the drainfield area. We hope this gives you some idea how to deal with potential odor problems during installation.
