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On-Site Sewage Systems (OSS)

Households that are not served by public sewers depend on an on-site sewage disposal system (septic tank and drain field) to treat and dispose of their wastewater.  A well designed, installed, and maintained septic system can provide years of reliable low-cost service.  When these systems fail to operate effectively, property damage, groundwater and surface water pollution, and disease outbreak can occur.  Therefore, proper design, installation, operation and maintenance are critical in any use of on-site sewage disposal systems.

The type of on-site system utilized is dependent on the soil type present at the site and the site conditions.   A minimum of 3 feet of good soil is required between the bottom of a drain field trench and a restrictive layer (solid rock of ground water) to assure that adequate treatment of wastewater can be achieved  prior to disposal.  When adequate soils and site conditions are present, a conventional gravity on-site sewage system can be designed and installed.

The OSS Application 2022 packet and Lincoln County licensed installers can be downloaded and printed off (see Forms/Installer lists below), picked up at the Lincoln County Health Department office located at 90 Nicholls, Davenport, WA  99122 or by calling (509) 725-1001.


The  typical septic tank is made of concrete, fiberglass, or polyethylene.  Wastewater from your toilet, bath, kitchen, laundry, etc., flows into the tank.  The tank is divided into two compartments to increase the amount of separation of solids from the wastewater.   Heavy solids settle to the bottom where bacterial action partially decomposes them into digested sludge and gases.  Most of the lighter solids, such as fats and grease, rise to the top and form a scum layer.  Tees or baffles are provided in the tank to reduce the disturbance caused by incoming water and to assist in the settling of solids.  The baffles also reduce the amount of solids that flow through the tank into the drain field.

Many products on the market claim to reduce the amount of scum and sludge and reduce the need for septic tank pumping.  However, there is no scientific evidence that these products provide any benefit over the natural bacterial environment which exists within the tank.

The wastewater leaving the septic tank is a liquid called effluent.  It has been partially treated but the separation and bacterial action taking place in the septic tank but still contains disease-causing bacteria and other pollutants.  Therefore, further treatment is required prior to disposal into the environment.

A list of Lincoln County Licensed pumpers can be downloaded (see the Forms/Installers Lists below).



The drain field receives septic tank effluent.  It has a network of perforated pipes laid in a gravel-filled trench (usually 3′ wide) in the soil.  Effluent trickles out the pipes, through the gravel, and infiltrates into the soil.  The size and type of drain field depends on the estimated daily wastewater flow and the soil type and condition.

Every new drain field is required to have a designated replacement drain field area reserved for future use in the event of a failure of the primary drain field area.


The soil below the drain field provides the final treatment and disposal of the septic tank effluent.  After the effluent has passed in to the soil, most of it percolates downward and eventually enters the groundwater.  A small percentage is taken up by plants through the roots or evaporates through the soil.  The soil filters effluent as it passes through the pore spaces.  Chemical and biological processes treat the effluent before it reaches groundwater, or a restrictive layer. These processes work best where the soil is somewhat dry, permeable, and contains plenty of oxygen for at least three feet below the drain field bottom.

A repair or an alteration to an existing system is made due to a change in the design or strength of the waste water.


Once an on-site system has been installed, a final inspection is conducted by the Health Department.  As part of the inspection, a final as-built diagram is prepared showing the exact location of all components of the system and included in the systems permanent record.  A copy of the as-built diagram can be obtained by contacting the Health Department.  The information needed to find the as-built is the installation permit number, the parcel number, the section, township and range, or the name of the individual who took out the installation permit.


Onsite Septic Systems (OSS) Guidelines

Lincoln County OSS Code 8.33.

WA State Department of Health On-Site Septic Systems