septic system drain field diagram

Septic Systems and Onsite wastewater treatment

Do you have a home septic system or are you buying a home with a septic system? If you have or are buying a home outside the city, there is a good chance you do use septic systems to treat their sewage. These systems discharge  millions of gallons of wastewater into Washington’s soils and waterways annually, and this figure grows each year. The Puget Sound Water Quality Authority estimates number of septic systems in the Puget Sound region at about a half million; the Environmental Protection Agency estimates that 16 million on-site systems treat roughly half the domestic sewage in the U.S.

Once thought of as temporary sewage treatment measures til the sewer pipe came along-or to serve vacation & weekend retreats in the short term-we now recognize these systems are here to stay. Individual, on-site sewage treatment systems have traditionally been regulated solely by requiring a permit for installation. Septic systems dispose of household sewage, or wastewater, generated from toilet use, bathing, laundry, and kitchen and cleaning activities. Because septic systems are underground and seldom require daily care, many homeowners rarely think about routine operations and maintenance. However, if a septic system is not properly designed, located, constructed, and maintained, groundwater may become contaminated.

As of September 15, 2005, the sellers must disclose and transfer all septic maintenance records to the buyer and the septic system must be inspected by a licensed/certified Monitoring and Maintenance Specialist. Our standard home inspection does not include this type of specialized, intrusive inspection. We can refer you to a licensed operator who is certified to inspect your system.  To properly inspect the system, the Monitoring and Maintenance Specialist may need to dig holes to access the underground parts of the system. This will include inspecting the tank, as well as the leach field.

It makes good sense to have the tank pumped at the time of this inspection. A professional septic contractor can perform both the inspection and pump the tank, killing two birds with one stone and assuring that you begin with an empty tank and a system that has been inspected. Often, you can negotiate with the seller to have them pay for the pumping.

Household Wastewater

Households that are not served by public sewers depend on septic tank systems to treat and dispose of wastewater. Household wastewater carries with it all wastes that go down the drains in our homes, including human waste, dirt, food, toilet paper, soap, detergents, and cleaning products. It contains dissolved nutrients, household chemicals, grease, oil, microorganisms (including some that cause disease), and solid particles. If not properly treated by your septic system, chemicals and microorganisms in wastewater can travel through the soil to groundwater and pose a health hazard. The average person uses between 50 and 75 gallons of water per day; mostly in the bathroom. Reducing your water use will help your septic system to work more efficiently.

Your Septic System

A conventional septic system has three working parts: a septic tank, a drain field, and surrounding soil.

Septic tanks can be made of concrete, fiberglass, or plastic and must be approved by the state. Minimum sizes of tanks have been established for residences based on the number of bedrooms in the dwelling. A 1,000-gallon septic tank is normally required for homes with three or four bedrooms. Larger tanks are required for larger homes. Local district health departments issue permits for septic systems and specify the minimum size tank. Some systems installed before the current rules and regulations may have smaller septic tanks.

Septic drain fields are used to remove contaminants and impurities from the liquid that emerges from the septic tank. This is typically done by burying perforated pipes in trenches and allowing the liquid to leach out and the surrounding soil absorbs the unwanted waste. The size of make up of the field is determined by the size of the system and the soils ability to absorb moisture.

The soil below the drain field provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. Soils are critical to the treatment of septic tank wastewater. A system that is not functioning properly will release nutrient-rich and bacterial-laden wastewater into the groundwater and/or surface water. These contaminated waters pose a significant public health threat to people that come into contact with them. Wastewater that moves with groundwater can transport bacteria considerable distances. This can result in a threat to public health and adversely affect the quality of ground and surface waters.

For a more complete explanation about the workings see Septic Systems Explained

Caring for Your Septic System

Installing Your System

In order to have a septic system installed on your property, you must first obtain a permit. Permit applications are available from your local district health department. Next, you must have a site evaluation performed. Make arrangements for this with your district health department and with a licensed septic system installer. Note that not all property is suitable for septic systems, so some permits may be denied. It is recommended that you have a site evaluation performed before you purchase property. Finally, have your system installed by a licensed installer and inspected by your local health district. Provide regular, preventative, maintenance to keep your system running smoothly.

Inspecting Your System

When too much sludge and scum are allowed to accumulate in your tank, the incoming sewage will not have enough time in the septic tank for solids to settle. Solids may flow to the drain field and clog the pipes, causing the sewage to overflow to the ground surface, where it exposes humans and animals to disease-causing organisms. To prevent this from happening, it is very important to inspect your tank regularly and have it serviced when needed. All tanks have accessible manholes for inspecting and pumping. Some excavation work may be needed to uncover the manhole. Properly designed tanks should have enough capacity for three to eight years of use before needing service. This is dependent upon the amount of wastewater generated. It is recommended that an average family of four have its septic tank pumped out every three to five years. Don’t wait for signs of system failure to have your tank pumped. Your tank should be checked annually to measure sludge and scum levels. A licensed septic tank installer can provide a septic tank inspection and recommend when the tank should be pumped. A tank inspection should include measuring the depth of scum and sludge and inspecting the tees in the septic tank. In Skagit County this person must be a Certified Monitoring and Maintenance Specialist. A list can be found here

Maintaining Your System

Pumping your septic tank every three years (or as determined by your inspections) will remove accumulations of solids, help keep the drain field from becoming clogged, and help prevent you from experiencing sewage backups or septic system failure. An accumulation of sludge exceeding 35% of the total water depth in the septic tank could cause solids to enter the drain field and clog the system. Hire a licensed septic tank pumper to pump your tank for you.

Mapping Your System

In order to take proper care of your septic system, you must know the location of the septic tank and drain field. The location of your septic tank can be determined from plot plans, septic system inspection records, architectural or landscape drawings, or from observations of the house plumbing. If you do not have access to drawings, find where the sewer pipe leaves your house. Some installers mark the location where the waste pipe comes out of the house with an “S” on the foundation. You may want to do this as well. Probe in the ground 10 to 15 feet directly out from the location where the pipe leaves your house to find your tank.

Once the septic tank has been located, make several plot plan diagrams (with measurements) that include a rough sketch of your house, septic tank cover, drain field area, well, and any other permanent reference points (such as trees or large rocks) and place them with your important papers.  You may also want to hang a diagram in your garage and file a copy with your local district health office. Maintain a permanent record of any septic system maintenance, repair, sludge and scum levels, pumping, drain field condition, household backups, and operations notes.

Warning Signs of System Failure

While proper use, inspections, and maintenance should prevent most septic tank problems, it is still important to be aware of changes in your septic system and to act immediately if you suspect a system failure. There are many signs of septic system failure:

  • surfacing sewage or wet spots in the drain field area;
  • plumbing or septic tank backups;
  • slow draining fixtures;
  • gurgling sounds in the plumbing system;
  • sewage odors in the house or yard (note that the house plumbing vent on the roof will emit sewage odors and this is normal)
  • tests showing the presence of bacteria in well water.

If you notice any of these signs, or if you suspect your septic tank system may be having problems, contact a licensed septic system professional or your local district health agency for assistance.

Septic System Dos and Don’ts

Proper operation of a septic system can prevent costly repairs or replacement. Observing the following guidelines will help to keep your system running efficiently.


  • practice water conservation. The more wastewater you produce, the more wastewater your system must treat and  dispose. By reducing and balancing your use, you can extend the life of your system and avoid costly repairs.
  • Use water saving devices such as low flow showerheads.
  • Repair leaky faucets and plumbing fixtures immediately.
  • Reduce toilet reservoir volume or flow.
  • Take short showers.
  • Take baths with a partially filled tub.
  • Wash only full loads of dishes and laundry.
  • Shut off the water while shaving or brushing your teeth.
  • Balance your water use (e.g., avoid washing several loads of laundry in one day).
  • Keep accurate records. Know where your septic tank is, keep a diagram of its location using the space provided in this booklet, and keep a record of system maintenance.
  • Inspect your system annually. Check the sludge and scum levels inside the tank and periodically check the drain field for odors, wet spots, or surfacing sewage.
  • Pump your system routinely. Pumping your septic tank is probably the single most important thing you can do to protect your system.
  • Keep all runoff away from your system. Water from roofs and driveways should be diverted away from the septic tank and drain field area. Soil over your system should be mounded slightly to encourage runoff.
  • Protect your system from damage. Keep vehicles and livestock off your drain field. The pressure can compact the soil or damage the pipes. Before you dig for any reason, check the location of your system and drain field area.
  • Landscape your system properly. Plant grass over the drain field area. Don’t plant trees or shrubs or place impermeable materials, such as concrete or plastic, over the drain field.
  • Use cleaning chemicals in moderation and only according to manufacturer’s directions.


  • flood irrigate over your system or drain field area. The best way to irrigate these areas is with sprinklers.
  • use caustic drain openers for clogged drains. Use boiling water or a drain snake to clean out clogs.
  • enter a septic tank. Poisonous gases or a lack of oxygen can be fatal.
  • use septic tank additives. They are not necessary for the proper functioning of your tank and they do not reduce the need for pumping. In fact, some additives can even harm your system.
  • lush harmful materials into your tank. Grease, cooking oil, coffee grounds, sanitary napkins, and cigarettes do not easily decompose in septic tanks. Chemicals, such as solvents, oils, paints, and pesticides, are harmful to your systems operation and may pollute groundwater.
  • use a garbage disposal. Using a garbage disposal will increase the amount of solids entering the septic tank and will result in the need for more frequent pumping.

The TP question?  (from Skagit County DOH)

Which toilet paper should you use in your septic system? Curiously, the battle line isn’t drawn in the sand between 1-ply or 2-ply, but on its integrity. Try this simple test: Put a square of your t.p. in a jar of water. Let sit. A good long while. Does the paper just sit there, floating around, like wet paper in water? That’s what it’s supposed to do in the tank-join the scum & stay put til pumped. But if it breaks up & disintegrates-like wispy, filmy ghosts hanging in suspension-it’s going to float out the tank with the wastewater & settle its wispy, filmy self in your drain field … and come back to haunt you. Get t.p. with integrity.

The Additive Question?
Septic system owners are always asking about miracle additives. Additives to septic systems are widely recommended by many pumping companies, but their benefits are doubtful. Controlled studies have not shown them to improve the performance of septic systems in any significant way. Added enzymes and bacteria cannot break down non-organic sediment such as dirt and plastics. And added bacteria must compete with the bacteria already in the tank. In most cases, the established bacteria simply eat the added ones.

Skagit County’s Sewage Code

A “Maintenance Specialist” is a private business-person who has the following qualifications: is a Washington State licensed septic system designer and/or a Skagit County certified installer, and has further training in the monitoring and maintenance of septic systems.

Skagit County Septic Search – see if your system is listed

Sewage related Links: 

Washington State Department of Health – Waste Water Management Program

Summary of Bill 1458The Septics Bill

Washington On-site Sewage Association:

Caring for your septic system (from the state of Washington)