Conventional septic system
A conventional septic system is an onsite sewage system of one or more septic tanks whose effluent travels by gravity, pump or siphon to a subsurface drainfield that distributes it by gravity. Virginia's statute treats a treatment works that is not conventional and does not result in a point source discharge as an alternative onsite system.
Six buried parts in a row, ending in a field that runs by gravity
Everything in a conventional system happens in a fixed order, and almost none of it shows above the grass. The Virginia Department of Health describes a typical one as a run of six parts: a sewer line, a septic tank, a conveyance line, a distribution box, header lines and several absorption trenches.
Sewer line
Wastewater leaves the house through the sewer line and runs to the tank.
Septic tank
Virginia's statute counts septic tanks among the devices it calls treatment works, used in the storage, treatment or disposal of sewage.
Conveyance line
Tank effluent moves on toward the field. The statute lets that trip run by gravity, by a pump or by a siphon and still call the system conventional.
Distribution box
In VDH's list the box sits between the conveyance line and the header lines, dividing the flow among the lines that feed the field.
Header lines and absorption trenches
The header lines feed the absorption trenches, and the trenches are where effluent finally meets the soil.
Those trenches make up the drainfield, which the statute describes as a system installed within the soil and designed to accommodate treated sewage from a treatment works. What the conventional definition fixes is the end of the line: a subsurface drainfield, distributed by gravity.
In-ground trenches, and the Type I label for one house
A conventional drainfield is an in-ground system. Virginia's onsite sewage regulation defines an in-ground system as one whose infiltrative surface sits 18 inches or more beneath the original surface of the ground, and it names conventional septic tank drainfield systems among them. Where septic tank effluent goes into the soil, the 18 inches directly under the installation depth must show no limiting factor, and the regulation names bedrock, a seasonal or permanent water table, pans and other impervious strata.
The soil horizon(s) for the 18 inches immediately below the installation depth shall not show the presence of any limiting factor.
12VAC5-610-594, Virginia Administrative Code
A site without that much clear soil is not automatically the end of the question. Virginia's rules for alternative onsite systems allow less separation when the effluent is treated first; those designs belong to the alternative septic system, and whether a given lot qualifies for one is for a licensed onsite soil evaluator or engineer and the local health department to say.
The permit side uses a different label. The onsite regulation calls an individual system of a septic tank and subsurface drainfield serving a single residence a Type I sewage disposal system. A Type III system includes systems other than a septic tank absorption system, and any subsurface absorption system, regardless of design, with a design flow over 1,000 gallons per day, so the same tank and trench layout sized past that flow falls under Type III.
How the construction permit and the operation permit bracket all of this is laid out in septic systems explained.
The inspection, pumping and lifespan numbers come from VDH, and two are recommendations
Once a conventional system is running, the numbers people quote about it come from the Virginia Department of Health, and most of them are recommendations rather than rules.
- Inspection
- At least every three years by a septic service professional, for the average household system (VDH recommendation)
- Tank pumping
- Every three to five years for household tanks (VDH recommendation)
- Average lifespan
- 30 to 40 years, and longer if properly maintained (VDH)
- A legal pump-out
- VDH's example is systems in Chesapeake Bay Preservation Areas, pumped no less than every five years, where some localities accept an inspection report instead
None of the sources behind this entry places Carroll, Grayson, Wythe, Pulaski, Montgomery or Floyd County, or the City of Galax, in a Chesapeake Bay Preservation Area, so whether any local pump-out rule applies is for the county or city to confirm. The lifespan figure is an average across systems, not a rating for any one of them.
Selling a house on a conventional septic system?
Send the address and whatever paper you have on the tank and trenches, and Lowell will go through the septic records a buyer is likely to ask about before you list. No cost, no obligation.
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Questions about this term
Does a pump between the tank and the drainfield make a septic system alternative?
Not by itself. Virginia's statute defines a conventional onsite sewage system as one or more septic tanks with gravity, pumped or siphoned conveyance to a gravity distributed subsurface drainfield, so a pump on the line to the field fits inside that definition. VDH describes a typical alternative system as adding a treatment unit and a pump chamber. Whether a particular system was permitted as conventional or alternative is a question for the local health department.
Where this page's facts come from
Every rule, office and figure above traces to one of these. Rules change; check the office before you act on one.
- Va. Code 32.1-163, definitions of conventional and alternative onsite sewage systems, subsurface drainfield and treatment works Code of Virginia
- 12VAC5-610-594, in-ground systems and the 18 inches below the installation depth Virginia Administrative Code
- 12VAC5-610-250, Type I and Type III sewage disposal systems Virginia Administrative Code
- 12VAC5-610-340, what an operation permit does not guarantee Virginia Administrative Code
- Onsite sewage system owner responsibilities: inspection, pumping and lifespan Virginia Department of Health
- AOSS operation and maintenance FAQs, including the parts of a typical conventional system (PDF) Virginia Department of Health