A septic tank separates solids from liquid. The solids accumulate, and when they accumulate far enough they start leaving with the effluent.
Why pumping matters
The tank does not fill up and stop working in an obvious way. It gradually loses the clear space in which settling happens.
Once the sludge and scum layers encroach on the outlet, solids carry through to the drainfield, where they clog the soil interface permanently.
Pumping is cheap. A drainfield replacement is not. That is the entire economic argument.
The usual interval
Every three to five years for a typical household, which is the guidance most health departments give.
It moves with:
Household size relative to the tank. Four people on a 1,000 gallon tank fill it faster than two.
A garbage disposal, which substantially increases the solids load — some guidance suggests halving the interval.
Water use, since high flow reduces retention time.
Whether the system has an effluent filter, which catches solids and needs cleaning at the same visit.
The measurement that gives the real answer
A pumper can measure the sludge depth at the bottom and the scum thickness at the top.
The common trigger: pump when the sludge and scum together occupy about one third of the tank's depth, or when either layer comes within a set distance of the outlet baffle.
Having it measured at the first inspection establishes the household's actual rate, and after that the interval can be set from evidence rather than a rule of thumb.
What happens at a pumping
Both compartments are pumped, through their own access lids. A tank pumped through one lid only is half pumped, and it happens.
The baffles or tees are inspected — a missing or broken outlet baffle lets scum straight into the drainfield.
The effluent filter is rinsed back into the tank.
The tank is checked for cracks and for evidence of the liquid level being wrong, which points at a field problem.
Ask for what was found, not just that it was done.
What not to do
Do not add "septic tank treatments." The bacteria a tank needs arrive continuously in what enters it. Additives are unnecessary, and some — particularly the ones that claim to dissolve sludge — can carry solids out to the field, which is exactly the failure they claim to prevent.
Do not skip pumping because the system seems fine. A system on its way to failing a drainfield gives no symptom until the field is already damaged.
Do not pump a tank to "fix" a saturated drainfield and expect it to last. Pumping empties the tank; if the field cannot accept effluent, the tank refills and the problem returns within days. It is a temporary measure while the real fault is diagnosed.
Access
Tanks buried with the lids a foot down get inspected less often than tanks with risers to grade.
Risers cost little, they can be fitted at any pumping, and they make the difference between maintenance that happens on schedule and maintenance that happens after a problem.
Keeping the record
Date pumped, sludge and scum measurements, condition of the baffles and filter, and anything noted.
Over two or three visits that record establishes the household's real interval, and it is exactly the documentation a buyer's inspection will ask for.
Before a sale
A tank pumped and inspected shortly before listing gives a buyer's inspector something to look at and the seller a document to produce.
Many jurisdictions require an inspection at transfer, and some require a pumping with it. Where they do not, doing it anyway removes one of the standard negotiating items on a rural property.