Why your aerobic septic system smells, and what aerator pressure has to do with it

An aerobic treatment unit that is working properly does not smell like sewage. At most there is a faint musty, earthy smell at the tank lid. Not at the spray heads, not in the yard. The whole point of the design is that the bacteria doing the work are the kind that need oxygen, and their byproducts are mostly odorless.

So when an aerobic system starts to smell, it is telling you something specific: the treatment chamber is short of air. Nearly every cause traces back to that.

What the smell is

Without enough oxygen, a different set of bacteria takes over, the kind that live in a conventional septic tank. Their byproducts include hydrogen sulfide, which is the rotten-egg smell, and a range of other compounds that read as sewage. If your system sprays treated water onto the lawn, that smell goes with it.

The smell can start within a few days of the air supply dropping. It can also start gradually, as air output falls over months, which is the case people miss.

One thing it is not: a chlorine problem. The chlorinator on a spray system disinfects the water going out; it does nothing for odor, and adding tablets will not fix a smell. Every septic company gets that call.

Where the air comes from, and where it goes

An aerator sits above ground, usually in a housing near the tank or at the control panel. It pushes air through a line to a diffuser near the bottom of the treatment chamber. The diffuser breaks the air into small bubbles so the oxygen dissolves. The pressure in that air line depends on how deep the diffuser sits, how clean it is, and how hard the aerator is pushing.

That pressure is a diagnostic in its own right. A typical residential aerator runs at roughly two to three and a half psi, and the panel’s alarm switch trips somewhere around one, so there is a wide band where the system is weak but nothing alarms. Read the pressure against the system’s own normal and it tells you where the problem is.

Pressure falling: the aerator or the line

If pressure has dropped and the smell has started, air is not reaching the diffuser. The three usual causes, roughly in order:

1. Worn diaphragms. Most residential aerators are diaphragm pumps. The diaphragms flex thousands of times an hour and go slack over three to six years, and the valves wear with them. Output falls gradually, long before anything tears, while the pump still runs and still draws power, so a panel that senses current will not alarm. A full tear usually trips the aerator’s own safety switch and stops it, which the panel does catch, but by then output has been poor for a while.
2. A leak in the air line. A cracked fitting at the aerator, a split in the line where it enters the tank, or a connection shaken loose. Easy to fix once found. Listen for hissing.
3. A clogged air filter on the aerator intake. The pump starves and output falls. Most aerators have a filter you can clean in five minutes; manufacturers suggest cleaning it every six months and replacing it yearly.

A panel with a pressure switch will eventually alarm, but the threshold is set well below normal, and the smell usually comes first.

Pressure rising: the diffuser

If pressure has crept up over months, the diffuser is fouling. Biofilm and mineral deposits narrow the openings, the aerator pushes harder, and less air gets through where it is needed. The aerator also runs hotter, which shortens its life. Membrane diffusers typically go three to five years; ceramic stones in hard water can clog in a year or two. A rising pressure trend is a reason to have the diffuser cleaned or replaced before it takes the aerator with it.

No change in pressure, but still a smell

If the air side looks normal, think about these:

• Overloaded system. A long stretch of heavy water use, a houseful of guests, or a lot of fats and grease can overwhelm the bacteria even with full air. Usually clears with normal use.
• Chemicals. Bleach in quantity, antibacterial cleaners, drain cleaners, automatic toilet-bowl cleaners, paint thinner, or medications can knock the bacteria back. Recovery from a single dose usually takes a day or two; continual dosing keeps the system down. A water softener that backwashes into the tank is a common continual one.
• Vent problems. A sewer smell inside the house is more often a plumbing vent or a dry trap than the septic system. A smell at a roof vent in still weather is normal.
• Sludge build-up. If the tank has not been pumped on schedule, every three to five years for most households, solids carry over into the treatment chamber and bury the diffuser.
• Lids and vents. An unsealed riser or lid lets the tank breathe at ground level, and a blocked tank vent pushes the smell out somewhere else. Both are cheap fixes.
• Coming back from vacation. Bacteria starve with no inflow and take a few days to recover when the house fills up again. Ease back into full water use.

What to do

If the aerator has stopped, or you can hear it running and no bubbles are reaching the chamber, that is a repair for the next day or two. The bacteria start dying off within a day or two without air, and if the system sprays, it is spraying poorly treated water.

If the system is simply weaker than it was, the fix is still the same parts, just on your schedule instead of after the smell.

Catching it before the smell

The thing to know about every cause above is that the pressure changed before the smell started. Sometimes by days, often by months. Septic Brain records aerator pressure continuously, so a slow drift down toward a torn diaphragm, or up toward a fouled diffuser, shows as a trend you can act on. It also records the aerator’s real power draw, so an aerator that has stopped entirely is obvious even on a panel that would not have noticed.