Alternative Septic System Care: Pressure, Mound, Sand Filter and Aerobic Systems

Alternative septic systems use pumps, extra treatment or raised drain fields where the soil is too shallow for a gravity system. Washington requires an inspection at least once a year for them, against every three years for a tank and gravity drain field. Idaho adds an annual maintenance event and a report for aerobic units.

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Last reviewed: October 26, 2026

Why alternative systems exist

The Washington Department of Health says different systems exist because soil and water conditions vary. A gravity system, the simplest, needs at least three feet of native suitable soil beneath the drain field. Where there is less soil, other designs make up the difference. The Department lists them by the depth of native soil they can work with: pressure distribution when there are 2 to 3 feet, a mound when there are only 1 or 2 feet, a sand filter when there is 1.5 feet, and a biofilter as little as 1 foot. Aerobic treatment units are in the same category of systems for properties that do not have enough native suitable soil depth to provide adequate treatment alone.

How each type works

Pressure distribution: a pump chamber receives treated wastewater from the tank, and a pump and floats dose the drain field under pressure. See septic alarm and pump systems.
Mound: a drain field raised above the natural soil surface in a specific sand fill, covered by cover soil, with a gravel or gravelless bed and small-diameter pipes.
Sand filter: a lined or concrete box filled with a specific sand, with pipes on top, dosed by a pump tank. Treated water collects in an underdrain and moves on.
Aerobic treatment unit and biofilter: extra pretreatment that the district approves by product. A biofilter product requires designers and installers approved by the manufacturer, the Department says.

Idaho DEQ describes the aerobic version as an extended treatment package system, or ETPS, which gives secondary treatment to septic tank effluent before the drain field. It may be required by site features: shallow soil, coarse soils, shallow groundwater, or the outcome of a nutrient-pathogen evaluation. DEQ says an ETPS can reduce total suspended solids and carbonaceous biochemical oxygen demand by 85 percent or better.

How often they are inspected

WAC 246-272A-0270 requires the owner to obtain an inspection at least once every three years for a system consisting solely of a sewage tank and gravity drain field, and annually for all other systems unless the health officer requires more. The Department of Health says the same. EPA says systems with electrical float switches, pumps or mechanical components should be inspected more often, generally once a year, and that a service contract is important since alternative systems have mechanized parts.

Proprietary treatment products follow the manufacturer's manual. WAC 246-272A-0260 requires the inspection to follow the product manufacturer's recommendations, and WAC 246-272A-0270 requires an inspection of proprietary treatment products per the manufacturer's recommendations at property transfer, beginning February 1, 2027. The record drawing for a proprietary product includes the manufacturer's literature and its maintenance recommendations, under WAC 246-272A-0265. Keep it with the permit file.

Who inspects and services them

In Washington, local health officers authorize maintenance service providers. SRHD publishes a list of permitted operation and maintenance providers, and NETCHD certifies them. Neither district endorses a company. NETCHD says an owner may inspect a system after watching its videos, though transfer inspections must be done by a certified provider.

In Idaho, a service provider permit is required to perform operation, maintenance or monitoring of ETPS and other complex systems under IDAPA 58.01.03 section 006, and DEQ says the service must be done by a certified provider of the owner's choice. A complex system installer permit is needed to install ETPS, pressure distribution systems, sand mounds, intermittent sand filters and similar systems.

Idaho ETPS requirements in detail

A minimum of one operation and maintenance event each year for each unit.
Annual monitoring, after the maintenance, with effluent samples analyzed by a laboratory for total suspended solids and carbonaceous biochemical oxygen demand, and total nitrogen if required.
Effluent standards of 45 milligrams per liter of total suspended solids and 40 milligrams per liter of carbonaceous biochemical oxygen demand.
An annual report to the public health district by July 31, submitted by the owner or the contracted service provider.
Responsibility for meeting the permit conditions belongs to the owner.

DEQ adds that these units need a constant supply of electricity, and that shutting off the power adds operation, maintenance and monitoring costs for the owner.

What the owner does between visits

Keep the control panel, alarm and pump chamber lids clear and secure.
Keep a log of alarm events and the circumstances.
Use water steadily across the week, not in one large burst.
Do not shut off power to an aerobic unit for long periods.
Keep vehicles, livestock and structures off the drain field and reserve area. WAC 246-272A-0270 requires protection from cover, surface drainage, compaction and soil removal.
Keep the manufacturer's manual and the record drawing.

Buying a property with an alternative system

These systems are worth checking before a purchase. Ask for the permit, the record drawing, the last inspection reports, any service contract, the alarm history and whether the property has a reserve area. In Washington the seller must provide the buyer with all available maintenance and repair records. Operation and maintenance records matter more for these systems because the system depends on parts that wear. See the home sale inspection page.

What changes the price

This site does not quote prices. Service price depends on the system type, the number of components, access, travel, laboratory sampling in Idaho, and whether parts need replacing. A service contract is one option EPA mentions. Read the scope and what it excludes. See the pumping cost guide for general ranges.

Reasons to trust this page

Alternative system descriptions come from the Washington Department of Health.
Inspection rules come from WAC 246-272A-0260 and 0270, and ETPS rules from Idaho DEQ and IDAPA 58.01.03.
Your permit and the product manual govern your system.
No provider or product is recommended.

Frequently Asked Questions

What counts as an alternative septic system?

Any system other than a septic tank with a gravity drain field. The Washington Department of Health lists pressure distribution, mound, sand filter, aerobic treatment unit and biofilter systems, used where native soil is too shallow for a gravity system. In Idaho, DEQ calls the aerobic version an extended treatment package system.

How often must an alternative system be inspected?

At least once a year in Washington, unless the health officer requires more (WAC 246-272A-0270), compared with every three years for a tank and gravity drain field. EPA says alternative systems with pumps or mechanical parts are generally inspected once a year and recommends a service contract.

What does Idaho require for an aerobic treatment unit?

DEQ requires at least one operation and maintenance event a year by a certified service provider, annual effluent sampling for suspended solids and oxygen demand, and an annual report to the public health district by July 31. The unit needs constant electricity, and the owner is responsible for meeting the permit conditions.

Who can inspect an alternative system in Washington?

A maintenance service provider authorized by the local health officer, such as a permitted provider on SRHD's list or a certified provider in NETCHD counties. NETCHD also lets owners inspect routinely after watching its videos, but a transfer inspection requires a certified provider. Proprietary products follow the manufacturer's manual.

Do alternative systems have a reserve area?

Washington requires a reserve area in the design, and WAC 246-272A-0270 requires the owner to protect the system area and the reserve area from structures, impervious cover, drainage, compaction and soil removal. The record drawing under WAC 246-272A-0265 must show the reserve area's location and dimensions.

Sources

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