Septic Systems

Most septic systems are a simple design with a two-compartment tank and drain field. The septic tank is a watertight box, usually made of concrete or fiberglass, with an inlet and outlet pipe. The septic tank treats the wastewater by holding it in the tank long enough for solids and liquids to separate. 

Sludge and scum

Solids lighter than water (such as fats, oils, and grease) float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. The layers of sludge and scum remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. The sludge and scum that can't be broken down are retained in the tank until it is pumped. 

Standard drain field 

A standard drain field is a series of trenches or a bed lined with gravel or course sand and buried 1' - 3' below the ground's surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater. The drain field treats the wastewater by allowing it to slowly trickle from the pipes out into the gravel and down through the soil.

Caring for your septic system

There are many ways to extend the life and functionality of your septic system. Read the "Caring for your system" section on this page.

Have your septic system inspected

Your septic system needs an annual inspection and needs to be pumped at least every three years, or more frequently depending on what type of system you have and how you use it. Search for a septic system professional in your area.

Be smart when you hire a septic contractor

Check out the Labor & Industries website to learn how to "hire smart" and protect your home in the process.

Get a copy of your septic system as-built

Health departments often have septic system as-builts stored in a public database. Check your county's health department to see if yours is available.

• King County Public Health
• Snohomish Health District

Looking for other septic-related records? Please submit a public records request.

Septic systems glossary

EPA’s Glossary of Septic System Terminology contains terms commonly used in the wastewater treatment field and their definitions.

Septic systems are underground wastewater treatment structures, commonly used in rural areas without centralized sewer systems. They use a combination of nature and proven technology to treat wastewater from household plumbing produced by bathrooms, kitchen drains, and laundry.

A typical septic system consists of a septic tank and a drainfield, or soil absorption field.

The septic tank digests organic matter and separates floatable matter (e.g., oils and grease) and solids from the wastewater. Soil-based systems discharge the liquid (known as effluent) from the septic tank into a series of perforated pipes buried in a leach field, leaching chambers, or other special units designed to slowly release the effluent into the soil or surface water.

Alternative systems use pumps or gravity to help septic tank effluent trickle through sand, organic matter (e.g., peat and sawdust), constructed wetlands, or other media to remove or neutralize pollutants like disease-causing pathogens, nitrogen, phosphorus, and other contaminants. Some alternative systems are designed to evaporate wastewater or disinfect it before it is discharged to the soil or surface waters.

How a typical septic system works

1. All water runs out of your house from one main drainage pipe into a septic tank. 
2. The septic tank is a buried, water-tight container usually made of concrete, fiberglass, or polyethylene. Its job is to hold the wastewater long enough to allow solids to settle down to the bottom forming sludge, while the oil and grease floats to the top as scum. Compartments and a T-shaped outlet prevent the sludge and scum from leaving the tank and traveling into the drainfield area. 
3. The liquid wastewater (effluent) then exits the tank into the drainfield. 
4. The drainfield is a shallow, covered, excavation made in unsaturated soil. Pretreated wastewater is discharged through piping onto porous surfaces that allow wastewater to filter though the soil. The soil accepts, treats, and disperses... wastewater as it percolates through the soil, ultimately discharging to groundwater. If the drainfield is overloaded with too much liquid, it will flood, causing sewage to flow to the ground surface or create backups in toilets and sinks. 
5. Finally, the wastewater percolates into the soil, naturally removing harmful coliform bacteria, viruses and nutrients. Coliform bacteria is a group of bacteria predominantly inhabiting the intestines of humans or other warm-blooded animals. It is an indicator of human fecal contamination.

Information from the United States Environmental Protection Agency website, www.epa.gov/septic

Ten essential tips about caring for your septic system

1. Conserve water. The more wastewater you create, the more your soil must treat and dispose. Here are some ways to cut back on your water use:

• Use water-saving devices
• Repair leaky faucets and plumbing fixtures
• Reduce toilet reservoir volume or flow
• Take shorter showers
• Take baths with a partially filled tub
• Wash only full loads of dishes and laundry

2. Keep accurate records. Know where your septic tank system is and keep a diagram of its location. Your local health department may be able to provide you with records of its size and location. You should also keep a record of the system's maintenance. These records are helpful if problems occur, and they will be valuable to your home's next owner.

3. Inspect your system once each year. Check the sludge and scum levels inside your septic tank to be sure the layers of solids are not within the "early warning" levels. Your tank should also be checked to see if the baffles or tees are still in good condition. Inspect the drainfield periodically for odors, wet spots, or surfacing sewage. If your drainfield has inspection pipes, check them to see if there is a liquid level continually over six inches. This could be an early indication of a problem.

4. Pump out your septic tank when needed. Don't wait until you have a problem. Routine pumping can prevent failures, such as clogging of the drainfield and sewage backing up into your home. Using a garbage disposal increases the amount of solids entering the septic tank and requires more frequent pumping.

5. Never flush harmful materials into the septic tank. Grease, cooking fats and oils, newspaper, "flushable wipes," paper towels, napkins, facial tissue, rags, coffee grounds, pads and tampons, condoms, and cigarettes cannot easily decompose in your tank. Chemicals like solvents, oils, paint, and pesticides are harmful to the system's proper operation and may pollute the groundwater. Septic tank additives don't improve the tank's performance, and they don't reduce the need for pumping. Learn how to  dispose of hazardous household waste properly at hazwastehelp.org. 

6. Keep all runoff away from your system. Water from surfaces like roofs, driveways, and patios should be diverted away from your septic tank and drainfield area. Soil over your system should be slightly mounded to help surface water runoff.

7. Protect your system from damage. Keep vehicles, heavy equipment, livestock, and other heavy items off your drainfield. The pressure can compact the soil or damage your pipes. Before you plant a garden, construct a shed, or install a pool, check on the location of your system so you don't build on top of or near it.

8. Landscape your system properly. Don't place impermeable materials over your drainfield. Materials like concrete and plastic reduce evaporation and the supply of oxygen your soil needs for proper effluent treatment. They can also make it difficult to get to your system for any pumping, inspection, or repair. Grass is the best cover for your system.

9. Never enter any septic tank. Poisonous gases or the lack of oxygen can be fatal. Any work to the tank should be done from the outside.

10. Check with your local health department for help with system problems. Although some malfunctions may require complete drainfield replacement, many problems can be corrected with a minimum amount of cost or effort.

• King County Public Health: 206-477-8050
• Snohomish Health District: 425-339-5250

Don't ignore warning signs

Here are some signs that your septic system may be failing:

• Odors, surfacing sewage, wet spots or lush vegetation growth in the drainfield area 
• Plumbing or septic tank backups 
• Slow draining fixtures 
• Gurgling sounds in the plumbing system

Is your system failing?

Learn what to do if your septic system fails.

Resources from Washington Sea Grant

Septic Sense (PDF) 

Pumping Your Septic Tank (PDF)

Landscaping Your Septic System (PDF)

Septic System Manuals 

Gravity  (PDF)

Pressure (PDF)

Sand Filter (PDF)

Mound (PDF)

Proprietary (PDF)

Maintaining pressure

Video: How to maintain pressure in your system 

Pest management

Mole management

Vole management 

Septic-friendly cleaners

Clean and Simple (PDF)

Hiring a professional

L&I's "Hire Smart" worksheet (PDF)

More resources

Bothell septic system maintenance brochure (PDF)