Typically, construction of a new septic tank system starts at the house by installing a building sewer, then installing a septic tank and, finally, installing whatever wastewater distribution component (i.e. drain field) is necessary for the local codes, site and soil conditions.
A building sewer is part of a house drain system that extends beyond the home and carries waste to its proper disposal whether it is a private onsite system (septic system), public sewer, or other form of treatment. Before it exits the home or structure, it is referred to as the building drain.
The first step is locating where the building drain exits the house and becomes the building sewer. This is done utilizing house plans and visually finding its location within the house. Depending on the construction schedule for the home, a system can be installed prior to the building drain or waste lines. In this case, a consultation with the interior plumber is necessary to confirm the correct system location and that the depth will be beneath the foundation footing.
The pipe choice for waste lines in North America is PVC, or polyvinyl chloride. Usually 4 inch diameter PVC is used, although depending on code requirements, amount of waste water expected per day and flow velocities this can be smaller or larger. In addition to sizing the diameter of pipe, it is important to determine the correct pitch of the pipe for correct gravity flow of waste. Pressurized systems follow different construction guidelines.
Most municipalities and uniform plumbing codes require that a cleanout, or a way to access the building sewer or building drain be within several feet of the exterior wall. This cleanout must be brought above or at grade, whether it is inside or outside the home.
The depth of the building sewer below the surface, or below “grade”, is also dependent on several factors, such as depth of frost, type of soil, possibility of flooding and what the ground above the building sewer will be used for. Use of insulated pipes and surrounding the building sewer pipes with more permeable and less compactable material can help to keep this part of the septic system more functional if perfect depth and soil type are not available.
Soil type, local codes, presence of rock and stones, the possibility of water flood events and distance to set backs can affect methods of burying the pipe. Unless the soil type is such that it will evenly support the building sewer and prevent any settling or compaction of soil, the soil will have to be tamped down and/or surrounded by material that will not further compact such as one to two inch stone. In northern locations where frost depths extend potentially below the depth of pipe, and when the building sewer travels a distance that could increase the chances that the waste water could freeze, insulated pipe must be used or foam sheathing added.
This information is based on a typical new construction and represents general code requirements and site, soil and climate conditions which can greatly vary.