How your septic system works, and how to keep it working
A septic system is a small wastewater treatment plant buried in your yard, and most owners were never handed a manual. This page is that manual: how the system treats sewage, what Washington requires of you as the owner, what failure looks like and the habits that add years to a drainfield.
The short version of how it works
Everything that goes down your drains flows to a buried tank. Inside, solids sink to the bottom as sludge, grease floats to the top as scum, and the liquid in the middle, called effluent, flows out to the drainfield. The drainfield spreads that effluent through perforated lines or chambers into the soil, and the soil does the real treatment: billions of soil microbes consume the waste and pathogens as the water percolates down toward groundwater.
That last part is worth sitting with. Your drainfield soil is the treatment plant. The tank just protects it by holding back the solids. Almost every septic rule, habit and repair bill traces back to keeping that soil alive and unclogged.
Why counties require ongoing inspections
Washington's onsite sewage rule requires owners to have their system inspected regularly: at least once every three years for a conventional gravity system, and at least once a year for every other type, including any system with a pump or advanced treatment. Whatcom, Skagit and Island counties each run programs to track this, and some transactions, like selling a home, trigger their own inspection requirements.
The reasoning is not bureaucratic. A failing system pollutes groundwater, streams and shellfish beds, and in our corner of the state those are close neighbors. Inspections catch cheap problems, a cracked lid, a tired pump float, a full tank, before they become drainfield replacements.
Signs your system is in trouble
Slow drains through the whole house, gurgling plumbing, sewage odors in the yard, unusually green or spongy grass over the drainfield, standing water near the tank or field, or an alarm from your pump panel. Any one of these deserves a phone call. Sewage surfacing in the yard or backing into the house means stop using water and call right away.
One pattern we see a lot: symptoms that show up during heavy rain or holiday visits, then fade. That is a system running at its limit. It is telling you something while the fix is still affordable.
Habits that protect your drainfield
Pump the tank on schedule and spread laundry through the week instead of running six loads on Saturday. Fix running toilets, they can double your daily flow. Keep grease, wipes of every kind, paint and harsh chemicals out of the drains. Nothing that hasn't been eaten or isn't toilet paper really belongs in the tank.
Above ground: keep vehicles, livestock and structures off the drainfield, and keep deep-rooted trees away from it. Compacted or root-invaded fields fail early. Rain gutters and footing drains should discharge away from the field so the soil is not treating your roof water too.
Common questions
How long does a septic system last?
A well built, well maintained system commonly runs for decades, and the tank itself often outlives the drainfield. Neglect flips that math fast. The single biggest factor in drainfield life is keeping solids out of it, which is what routine pumping does.
How often should my tank be pumped?
Every three to five years suits most households, but tank size, household size and habits move that number in both directions. A pump visit includes measuring the sludge and scum layers, which tells you the right interval for your tank rather than a rule of thumb.
What is the difference between a gravity and a pressure system?
Gravity systems let effluent flow to the drainfield on slope alone and load the field unevenly, which is fine in deep, well drained soil. Pressure systems use a pump to dose the whole field evenly through small diameter pipes, which lets shallower or tighter soils treat effluent safely. The soil evaluation and design determine which one your site needs.
What is an advanced treatment system?
A system that cleans effluent beyond what a standard tank can do before it reaches the soil, using components like aerobic treatment units, media filters or UV disinfection, including proprietary packages such as NuWater and OSCAR. Counties require them where soils or setbacks cannot carry the full treatment load.
Do I need a permit to repair my septic system?
Generally yes. In Whatcom, Skagit and Island counties, repairs to an onsite sewage system run through the county health department, and larger repairs need an approved repair design. A permitted repair also keeps your property records clean for a future sale or refinance.
Can I build a deck, shed or driveway over my drainfield?
No. Structures and traffic compact the soil, crush components and block the oxygen the soil microbes need. Keep the field in grass, keep it accessible and know where it is before you plan any project in the yard.
Are septic additives worth using?
Your tank already contains the bacteria it needs, delivered free with every flush. Additives do not replace pumping, and some can actually harm the system by stirring solids into the drainfield. Save the money for a pump-out.
What are those green lids in my yard?
Riser lids. Risers extend the tank openings up to finished grade so the tank can be pumped and inspected without excavation. If your lids are buried, adding risers is an inexpensive upgrade that pays for itself on the next service visit.
Who is allowed to install a septic system in Washington?
Installation must be done by an installer licensed with the county where the property sits, building to a design the county has approved. Whatcom and Skagit counties both publish rosters of their licensed installers, and Shuksan Septic Installers is on both lists.
My system was installed decades ago and I have no records. Where do I start?
Start with your county health department, which may have the original permit and as-built drawing on file. If no records exist, a locate and inspection visit can map what you have and assess its condition. Knowing what is in the ground is the first step to taking care of it.
Have a question this page did not answer?
Call us. Explaining septic systems is genuinely part of the job, and we would rather you ask before a small problem grows.
Or just call.
If it goes to voicemail we are in a trench. Leave a message and we call back.