Septic System Types Guide

Types of Septic Systems

Different site conditions, soil types, water table depths, and county requirements call for different system designs. Each page below explains how a system type works, what routine maintenance it may require, and what observable conditions to share when planning direct service.

Designs and local requirements vary. The information here is general - consult the applicable county and General Septic for property-specific service guidance.

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System designs, maintenance requirements, and regulatory obligations vary significantly by site, county, and installation. The pages below provide general educational information only. For guidance specific to your system, consult the applicable county health department and General Septic.

Conventional Gravity System

Also called: standard septic system, gravity-fed system

A conventional gravity system is the most common type of on-site sewage system. It uses a septic tank to separate solids from liquid effluent, then relies on gravity to move clarified effluent through a distribution box and into perforated pipes in a gravel-filled drain field trench. No pump or external power is required for the core treatment and dispersal process.

Routine tank pumpingEffluent filter cleaning

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Pressure Distribution System

Also called: pressurized septic system, low-pressure pipe system, LPP system

A pressure distribution system uses a pump to deliver timed, even doses of effluent through a network of small-diameter pressurized pipes to the drain field. Unlike gravity systems that allow continuous flow whenever effluent is available, pressure distribution ensures each section of the drain field receives a measured, uniform dose followed by a rest period - improving treatment efficiency and reducing the risk of overloading any single area.

Pump and float inspectionLateral flushing

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Mound System

Also called: raised septic system, elevated drain field system

A mound system places the drain field in an elevated, engineered bed of fill material constructed above the native soil surface. It is used on sites where shallow soil depth, high seasonal water tables, or low-permeability soil make a conventional in-ground drain field impractical or prohibited. The elevation creates the separation distance between the bottom of the drain field and limiting layers in the native soil that is required for effective treatment.

Pump and dosing systemMound surface and vegetation

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Sand Filter System

Also called: recirculating sand filter, intermittent sand filter, RSF

A sand filter system adds an engineered filtration step between the septic tank and the drain field. Effluent is distributed across a bed of specially graded sand, where it percolates slowly downward and receives biological and physical treatment before being collected and delivered to a final dispersal area. Sand filters are used where site conditions require a higher level of treatment than a conventional system provides, or where setback requirements from wells, water bodies, or property lines are restrictive.

Sand bed inspectionPump and timer maintenance

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Aerobic Treatment Unit

Also called: ATU, aerobic septic system, aerobic digester

An aerobic treatment unit (ATU) uses active aeration to accelerate biological treatment of wastewater before it reaches the dispersal system. Unlike a conventional septic tank - which relies on anaerobic (oxygen-free) bacterial activity - an ATU injects or mixes air into the treatment chamber, fostering aerobic bacteria that break down organic matter more rapidly and to a higher degree. ATUs typically produce cleaner effluent than a conventional tank, allowing them to be used on challenging sites or to meet more stringent treatment standards.

Routine service - often part of the system planAir pump and aeration equipment

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Drip Dispersal System

Also called: drip irrigation septic system, drip field, subsurface drip dispersal

A drip dispersal system delivers treated effluent through small-diameter drip tubing with pressure-compensating emitters installed just below the soil surface. Rather than filling trench voids with large quantities of effluent at once, drip systems deliver precise, frequent, small doses directly into the biologically active upper soil zone. This approach allows the system to treat and disperse effluent at a controlled rate across a larger area and in soil conditions where conventional trench systems may not be feasible.

Drip filter cleaningPump and control system

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Holding Tank vs. Septic System

Also called: holding tank, pump-and-haul, waste holding tank

A holding tank stores all household wastewater without any treatment or dispersal. Unlike a septic system - which treats and disperses effluent into the soil - a holding tank is a sealed vault that must be pumped out by an authorized septage hauler whenever it fills. Holding tanks are used only where no other on-site treatment option is feasible, or as a temporary measure, because the ongoing pumping demands are significantly higher than those of a treatment-and-dispersal system.

Monitoring the alarm and pump-out scheduleTank inspection for leaks

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Not sure what type of system you have?

Describe what you know about your system - any visible components, control panels, or alarm systems - so General Septic can plan direct service in King, Pierce, or Snohomish County.

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