Glossary

420-3-1: Ala. Admin. Code Chapter 420-3-1, Onsite Sewage Treatment and Disposal — the regulation governing septic systems in Alabama, adopted under Title 420, Alabama State Board of Health.

420-3-1-.25: Advanced Treatment Required — the rule listing the specific site conditions that mandate an advanced treatment system. Previously numbered 420-3-1-.36 and renumbered effective February 13, 2023, so older guidance citing .36 is out of date.

Advanced Treatment: Treatment beyond a conventional septic tank, required under 420-3-1-.25 where site conditions such as shallow bedrock, very slow or very fast percolation, shallow saturation, or mine spoil are present. Typically doubles the cost of a system.

ADEM: The Alabama Department of Environmental Management. Handles Underground Injection Control permits for community wastewater systems; separate from ADPH's onsite sewage program.

ADPH: The Alabama Department of Public Health, which writes and enforces Chapter 420-3-1. Enforcement runs through county health departments.

Aggregate / Drain Media: Hard, clean gravel or rock used to disperse effluent in the disposal field. Under 420-3-1-.04 it must have a hardness of 3 or greater on the Mohs scale — the rule's own test is aggregate that can scratch a copper penny without leaving residue — and be sized between 1/4 inch and 2.5 inches.

AOWB: The Alabama Onsite Wastewater Board, which licenses septic installers under Chapter 628-X-3. A separate body from ADPH. Homeowners may not install their own systems in Alabama.

Approval for Use: The authorization to use a system, issued by the local health department after all conditions of the rules and permits are satisfied. Distinct from the Permit to Install.

ASHES: The seasonal saturation indicator referenced in 420-3-1-.25(c). Advanced treatment is required where the depth to it from natural ground surface is less than six inches.

Conventional OSS: A standard septic tank and effluent disposal field that does not require engineer design. Anything else falls under the Engineered OSS definition.

COLE: Coefficient of Linear Extensibility, a measure of how much a soil shrinks and swells. Under 420-3-1-.04, high shrink-swell soils usually have COLEs of 0.09 or greater.

EDF: Effluent Disposal Field — the minimum area calculated under the rules into which sewage treated to at least primary effluent standards is dispersed into the soil. Called a drain field or leach field elsewhere.

Engineered OSS: Under 420-3-1-.04(36), all systems other than Conventional that require engineer design. Includes controlled fill mound systems, advanced treatment, drip irrigation, low pressure pipe, constructed wetlands, and any system where any portion of the disposal field protrudes above the unaltered natural soil surface.

High Shrink-Swell Soils: Soils with high clay content and minerals such as montmorillonite that swell when wet and shrink when dry. The regulation describes them as inherently slowly or very slowly permeable — the defining Black Belt condition.

LHD: Local Health Department — the county body that issues the Permit to Install (Repair) and the Approval for Use, and where you actually apply.

Mine Spoil: Material left from mining operations. Under 420-3-1-.25(g), advanced treatment is required where mine spoil is present at the natural ground surface to a depth of at least 48 inches within the proposed disposal field area.

MPI: Minutes Per Inch — the unit for percolation rate. Advanced treatment is required below 5 MPI (too fast) or above 240 MPI (too slow).

Natural Ground Surface: The naturally occurring surface of the earth, not significantly altered by cutting or filling. Depths in the rules are measured from it. Also referred to as in-situ soils.

Observation Pit: A soil inspection trench used in site evaluation. Under 420-3-1-.04, minimum 3 feet wide and 60 inches deep unless hard bedrock is encountered, safely accessible, parallel to the contour, and level.

OSS: Onsite Sewage Treatment and Disposal System — Alabama's regulatory term for a septic system. Defined as a system that collects, transports, treats and provides subsurface disposal of sewage.

Permit to Install (Repair): The permit required under 420-3-1-.06 before beginning installation or repair of a system or any component of one. ADPH may revoke it if lot conditions change, the system is not installed per the permit, or information submitted was misrepresented or inaccurate.

Responsible Person: Alabama's term for who signs the permit application. For a private dwelling it is the property owner or their authorized agent; for a corporation a principal executive officer; for a partnership a general partner; for a sole proprietorship the proprietor.

Riser: An extension bringing a buried tank access lid to grade. It pays for itself over a few pump-outs by removing the excavation charge.

Small-Flow OSS: A system with an average daily design flow of 1,800 gallons per day or less, or serving 12 bedrooms or fewer. Covers most residential systems.

Smectitic: A group of clay minerals including montmorillonite that cause soils to shrink and swell significantly. The mineralogy behind Alabama's most difficult septic soils.