Conventional Gravity Septic System: ***PICTURES BELOW***
Generally, a conventional gravity septic system in Whatcom County consists of a two compartment septic tank (minimum 900 gallons). We always like to install a quality effluent filter in the outlet baffle of the septic tank. Its purpose is to keep most solids in the tank and help prevent solids from entering the drain field. You also get some extra biological treatment with the use of a filter.
The septic tank's outlet is a 4" transport pipe that delivers effluent (liquid) to the conventional gravity drain field. The drain field is generally a gravel type bed system consisting of a series of 4" drain pipes installed to ensure proper distribution within the gravel bed . Sometimes, a gravity trench system can be installed at the County Health Department's discretion. The gravel bed generally consists of 12" of clean, washed drain rock usually 1 1/2" - 2 1/2" in size. On top of the drain rock, "Typar 3201" filter fabric, or other approved material is used to keep the topsoil from mixing in with the drain rock. 4" monitoring ports are also a good idea to install in the bed to have a visual idea of the condition of the system. These monitoring ports allow a licensed inspector to determine the amount of ponding (standing liquid) within the drain field, if any. Usually, one monitoring port is installed at the natural soil interface up to grade and one installed from the top of the drain rock up to grade. If the drain field is functioning properly, you normally will not see any standing liquid in these monitoring ports.
The below generic picture shows how the conventional gravity septic system works and how the effluent (liquid) gets treated and returned back into nature.
In certain instances, approved gravel-less type chambers can be installed in lieu of drain rock. Generally, a reduction in drain field size can be achieved with these systems with County Health Department approval.
The following are some pictures of various conventional gravity systems including a common gravel-less type chamber system:
The following are some pictures of various conventional gravity systems with washed drain rock & piping:
The following picture is a typical "step-down" gravity trench system with a curtain drain installed up-slope of the drain field to catch and properly divert ground/surface water away from the drain field area.
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