Why a Septic System Fails in February and Behaves in August

Half the job is done by soil

A septic tank does less than most owners assume. Solids settle to the bottom as sludge, fats and lighter material float to the top as scum, and the relatively clear liquid in the middle flows out to the drainfield. The tank is a separator, not a treatment plant.

The actual treatment happens in the ground. Effluent disperses through perforated pipe into gravel and then into soil, and it is the soil, and the biological layer that develops at the interface, that does the final work. That arrangement is elegant and cheap and it has one absolute requirement: the soil has to be able to accept liquid.

Everything about the seasonal pattern follows from that requirement, because soil in this region does not accept liquid at the same rate all year.

What the wet season does

From roughly October through May this is a wet region with a persistent rather than dramatic rainfall pattern, and by midwinter the ground has been taking water for months without much opportunity to shed it.

Saturated soil has less capacity available for effluent. The drainfield is competing with groundwater for the same pore space, and where the water table rises seasonally, which it does substantially on the valley floors south and east of us, the field may be sitting in ground that is effectively already full.

So a system with a comfortable margin performs identically all year. A system operating close to its limit performs perfectly in August, when the soil is dry and eager, and fails in February when it is not. The household has not changed anything. The ground has.

This is why the pattern is so consistently misread. Come spring the problem disappears, the system appears to have recovered, and the underlying situation, which is a field near or at the end of its capacity, goes unaddressed for another year.

Reading the signals

Multiple fixtures draining slowly together, particularly the lowest ones in the building, and particularly if it worsens after heavy rain rather than after heavy use.

Gurgling from drains when nothing else is running, which is air moving in a system that is not clearing properly.

Standing water or unusually vivid green grass over the drainfield area. Grass that is noticeably better fed than the rest of the lawn in one rectangle is being fertilised by something, and that something is effluent reaching the surface.

Smell outdoors near the tank or the field, especially in still damp weather.

And backups that arrive with weather rather than with household volume. That last one is the diagnostic. A blockage does not care whether it is raining.

Worth separating out one symptom that looks like septic failure and is not. If a single fixture is slow while everything else in the building behaves normally, that is a local clog within a few feet of it, and it is the same problem it would be on a sewered property. Septic trouble presents across multiple fixtures because it originates downstream of all of them. One slow basin is a drain cleaning job whatever system the house is on.

Why cabling is the wrong first move

If a drain backs up, the reflex is to call for a drain cleaning, and on a sewered property that is usually correct. On septic it can be actively counterproductive.

A tank at capacity or a saturated field is not a blockage. There is nothing in the line to clear, and running a cable through it achieves nothing except a bill. Worse, a cable pushed through into the tank can damage the baffles, which are what keep scum and sludge from escaping into the field, and damaging those turns a capacity problem into a fouled drainfield.

A fouled field is the expensive failure in septic ownership. Fields are replaced rather than cleaned, and replacement involves siting, permitting and excavation on a scale nothing else in this article approaches.

So the first question on any slow drain south or east of the city has to be which system the property is on. If nobody has asked you that, they are about to guess, and the septic page sets out how the two diverge.

Working out which you are on

Pull out a utility bill and hunt for a sewer charge on it. An absent sewer line usually settles the question: no charge, no public sewer, and the property is on its own system.

Inside Federal Way most addresses connect to a public sewer, so septic here is the exception rather than the rule. Move south and east across our working area and that inverts, particularly around the plateau fringes where individual septic systems are common rather than unusual.

Paperwork for a septic system is held by public health rather than by anyone who bills you for water, and that catches almost everybody out. North of the county line it is King County Public Health. South of it, the health department covering Tacoma and Pierce County. One of those two can usually confirm whether a system was ever permitted at your address, and may still have the drawing that came with it.

Get that drawing before anybody digs. Knowing where the tank lid, the baffles and the field lines actually sit turns an excavation into opening a lid.

What actually helps

Reduce hydraulic load, particularly in winter. Spread laundry through the week instead of doing eight loads on a Saturday, because a field that can handle a household's daily volume may not handle a day's worth arriving in four hours. Fix running toilets promptly, since a silently cycling toilet loads a tank continuously and it is one of the more common hidden causes of a system that seems to have shrunk.

Keep water away from the field from outside. Roof downpipes, driveway runoff and surface water directed anywhere near the drainfield are competing for the capacity you need, and redirecting them is cheap.

Keep vehicles, sheds and serious landscaping off the field. Compaction reduces the soil's ability to accept liquid, which is the one thing the whole system depends on.

And inspect rather than pumping on a fixed calendar. Measuring the sludge and scum layers tells you whether pumping is actually due, which stops you paying for one you did not need and stops you leaving it long enough to push solids into the field.

There is a large market in septic additives and very little evidence that they improve a system which is already working. The bacteria a tank needs are established in it already. What genuinely shifts the outcome is the unglamorous list above.

One last thing, aimed at anyone buying a property on a septic system. Treat the inspection with the same seriousness you would give a camera run on a sewered house. A standard home inspection does not evaluate a tank or a drainfield, the seller may honestly know very little about either, and a failed field sits among the larger unexpected bills in residential property. Ask for the permit record, ask when it was last pumped, and ask whether anyone has ever measured the sludge layer. Vague answers to all three are themselves informative.