This post describes the process by which you can construct a septic system for a single-family residence located on soil with sufficient permeability. The system is based on the preservation of a constant gradual slope for all the piping from the house drain to the leach field. Because it uses gravity, it is not subject to failure from power outages or pump failures.
First, a bit about the law to keep us all out of trouble… You will need to permit your system with the local permitting authority, usually the county health department. Check to see what supporting information you’ll need for the permit application, but usually it’s just a drawing of the designed system, a soil analysis report, and a standard questionnaire. A licensed engineer can help you design and build a system, but in most areas, you can build a standard system like the one I’m proposing for yourself. You cannot build this type of system for someone else and charge them a fee without being properly licensed.
Before going over the design, let’s make sure your building site has adequate characteristics for this type of system. You want to verify two things: (1) that there is no shallow groundwater present; and (2) that the soil has sufficient permeability to attenuate the leachate from your septic system. If you have already dug a test pit at your site to verify your soil’s bearing pressure for the foundation design, you can use the observations from this test pit for your septic evaluation. If not, dig a test pit where you plan to install the septic tank, so that you don’t have to refill the pit later. The pit should be as low as the planned elevation of your leach field. You can verify Condition 1 by waiting overnight to see if your pit fills with any groundwater. You can verify Condition 2 by filling your pit with water and verifying that the water drains from the pit within about 24 hours[1]. If either of these conditions is not met, it doesn’t necessarily mean you can’t use the system described below, but some additional analysis or discussion with the local health department is warranted. Also, in rare cases where your site is highly heterogeneous, the permitting authority’s inspector may request a second test pit.
Now let’s talk through how to construct the system. The drainpipe exiting your house will typically be 3” Schedule 40 PVC DVW pipe. You want it to drop 1/8” for every horizontal foot, and to connect to the septic tank at least 10 feet from the house. In between the septic tank and the house, install two PVC cleanouts in case of pipe clogs in the future. The septic tank should have a total volume of at least 1,000 gallons and two or more chambers inside of it to facilitate wastewater settling. You should place it on level, undisturbed or carefully compacted soil, because it will be very heavy when full. The outlet from the septic tank should be just slightly below the inlet, to maximize the usable volume of the tank. Make sure to bury the septic tank in a way that matches the existing grade while leaving the openings accessible from the surface.
The outlet from the septic tank should discharge into a four inch-diameter PVC drainpipe that connects to the leach field. The leach field should be a rectangular prism dug 10 feet wide by 50 feet long by 28” deep, with a level bottom. Line this area with geotextile fabric and fill it with enough septic rock to support the perforated drainpipe one above the bottom of the leach field. Place three 50’ runs of 4” perforated drainpipe, spaced 4’ on center, and connected in parallel on both ends using elbows for the outside pipes and tees for the inside pipe. Cover the perforated pipe with additional septic rock, place geotextile fabric on top of the septic rock to seal it off and cover the leach field with topsoil to the surrounding grade.
[1] This analysis is slightly more technical and based on local regulations, but you’re basically assessing the flowrate of water per horizontal square foot that the soil can accept per against the maximum anticipated flowrate. This latter parameter can be calculated by dividing the maximum flowrate expected from your house’s drain (in gallons per minute) by the area of your leach field (in square feet).