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Clean, healthy lakes are ideal places for people to fish, swim and enjoy life in many ways. Healthy lakes also provide habitat for fish and wildlife to thrive. To preserve a lake's quality and beneficial uses, people living within the watershed have a special stewardship role. Proper maintenance of home septic systems is one way home-owners can help keep a lake healthy and clean.

How a septic system works

A conventional septic system consists of two main parts: the septic tank and the drain-field (which includes the surrounding soil). Raw sewage from the home flows into the septic tank, usually a water-tight tank made of concrete. Solids settle to the bottom of the tank while fats, oils and other floating waste collect at the top. Bacteria slowly begin to digest the wastes in the tank, converting them to sludge. The liquid portion, or effluent, flows into a second chamber then out of the tank to the drainfield. All newer septic tanks have one or two chambers and baffles to keep solids from moving out to the drainfield.

A drainfield contains a network of perforated pipes lying in gravel filled trenches underground. Unsaturated soil beneath the drainfield acts as a filter and purifies the effluent as it moves slowly downward. In the soil, microbes utilize the nutrients. The purified liquid is eventually taken up by vegetation or seeps into groundwater or surface water.

septic system

Alternative systems

To accommodate a home septic system, King County requires a minimum lot size of 12,500 square feet for new homes. In cases where the soil does not have the appropriate permeability or depth to effectively treat effluent, an alternative system may be proposed. In King County, the most common alternatives include mound, sand filter and pressure distribution systems. For more information about these alternatives, contact the Public Health - Seattle & King County at their district field offices: East King County (206/296-4932).

Caring for your septic system

With regular care and maintenance, home septic systems can function for many years. By following these steps, you can keep your system functioning properly-without causing harm to nearby lakes or ponds:

Public Health - Seattle & King County

  • Know where your septic tank and drain-field are located. If you are unsure of their location, contact one of the Public Health - Seattle & King County District Offices, in Eastgate (206-477-8050) which keep plans of most home systems on file.
  • Have the tank inspected annually and pumped as needed (every three to five years) to prevent the sludge and scum in your septic tank from overflowing. Hire a licensed septic system pumper to do this job. Approved contractor lists are available at Public Health's District Offices. Ask the contractor to check the tank for leaks and to inspect the tank's baffles.
  • Do not use septic tank additives. They are ineffective at best and can actually harm the environment. State law presently prohibits the addition of chemical additives to on-site wastewater disposal systems. All additives, both chemical and biological, will be banned for sale, use or distribution without approval by the Washington State Department of Health after January 1, 1996.
  • Don't flush non-degradable materials such as oil, grease, plastic products, disposable diapers, rags, paper towels or cigarette butts. They'll only cause problems by clogging your septic system.
  • Garbage disposals can significantly increase the accumulation of solids in septic tanks and contribute to drainfield failure. Use of disposals will require more frequent pumping and monitoring of the system.
  • Household hazardous wastes should never be flushed or put down the drain. This includes strong acids or bases, petroleum products, solvents, heavy metals and pesticides.
  • Consider disposal alternatives for non-hazardous solid waste, including wormbin composting for food waste.
  • Conserving water can also help septic systems function efficiently. Install flow restrictors on faucets and showerheads, repair leaky pipes and hose cocks and select water-saving toilets and other household appliances. Space laundry loads throughout the week, rather than doing all of your household's wash on a single day.
  • Protect your septic system by not parking, driving or building over the tank or drainfield. By compacting the soil, you harm its natural ability to treat and transport the effluent.
  • Don't plant trees and shrubs over septic tanks or drainfields. The water seeking roots of these plants can damage your home septic system. Grass or shallow-rooted plants tend to be the best cover for a drainfield.

Signs of system failure

Catch problems early and prevent costly repairs. If you notice any of the following signs of system failure appear, contact your local office of the Public Health - Seattle & King County (206-477-8050) for advice:

  • Offensive odors, surfacing sewage, wet spots or lush vegetation over the drainfield
  • Toilet back-ups into the tub or shower
  • Slow draining toilets or sinks (Before calling the County, check for clogged pipes).

For questions about lakes in King County, please contact or call the Water and Land Resources Division front desk at 206-477-4800.