Department of
Environmental Quality

Many homes and other buildings that are not served by public sewer systems depend on "on-site" septic systems to treat and dispose of wastewater. On-site systems use a septic tank and underground (subsurface) drainfield to treat wastewater on-site. On-site systems are the most common wastewater treatment system used in rural areas of Idaho.

Septic systems dispose of household sewage, or wastewater, generated from toilet use, bathing, laundry, and kitchen and cleaning activities. A system that is not functioning properly will release nutrient-rich and bacteria-laden wastewater into the ground water and/or surface water. A properly designed, located, constructed, and maintained septic system is imperative to protecting human health and the environment.

Regulation of Septic Systems in Idaho

DEQ has established minimum standards, the Individual/Subsurface Sewage Disposal Rules, for the design, construction, siting, and use of individual and subsurface sewage disposal systems. This rule also establishes requirements for obtaining an installation permit and an installer's registration permit.

These rules are administered by Idaho's seven local Public Health Districts under a memorandum of understanding between DEQ and the Public Health Districts. ThePublic  Health Districts permit and inspect septic systems, and, for a fee, also conduct site evaluations to determine the suitability of a location for a septic system. DEQ conducts plan and specification review for central and large soil absorption systems and reviews Nutrient-Pathogen (NP) evaluations, which are scientific evaluations of the water quality impacts of septic systems, when required.

Types of Septic Systems

On-site wastewater systems discharge wastewater into an underground tank where solids and water are separated. Solids settle to the bottom of the tank. Bacteria in the tank break down organic matter. Water (effluent) flows from the tank and into a drainfield of underground pipes surrounded by gravel and soil. The pipes slowly release the water and the gravel and soil filter out remaining contaminants before the water reaches the ground water. Residual sludge in the tanks must be pumped out periodically.

Individual Septic Systems

An individual septic system is a decentralized system that serves one or two homes and usually consists of a 1,000-gallon concrete septic tank and a drainfield or leach field. N-P evaluations may be required for individual septic systems in new subdivisions. Some individual systems may be "enhanced" to provide extra filtration or treatment between the septic tank and the drainfield.

In order to have a septic system installed, the property owner must obtain a permit from the local Public Health District. The permit states the conditions and requirements necessary for the septic system. It is recommended that the property owner have a site evaluation performed by the Health District and a licensed septic system installer before applying for a permit and before purchasing property. Not all property is suitable for septic systems, so some permits may be denied. Depending on the location of the property and the type of system being installed, an NP evaluation may be required as part of the permit application. Once the permit has been issued, the system should be installed by a licensed installer and inspected by the local Public Health District.

Large Soil Absorption Systems

A large soil absorption system (LSAS) is a subsurface sewage disposal system designed to receive 2,500 gallons per day or more. Developments and facilities such as resorts, schools, subdivisions, and rest areas that are not connected to centralized systems often use LSASs to treat their wastewater. A LSAS is one type of central system, which is any system that receives wastewater from two or more homes or buildings or receives more than 2,500 gallons per day of wastewater.

As with an individual system, a property owner or developer wishing to install an LSAS must obtain a permit from the local Public Health District. In addition, the LSAS design must be prepared by a professional engineer licensed by the state of Idaho and must undergo plan and specification review by DEQ. The Public Health Districts provide the plans to DEQ for review and issue the LSAS permit after DEQ has completed the review and has issued a letter approving the system for construction. DEQ requires an NP evaluation as part of the permit application for all LSASs. Prior to issuing a permit, Health District staff will conduct an on-site evaluation and inspect test holes on the property in the area of the proposed drainfield.

Large soil absorption systems must be installed by a licensed complex installer and inspected by the Health District and DEQ. An annual operation and maintenance report must be sent to DEQ and the Health District each year. Some permits may also require monthly ground water monitoring.

Nutrient-Pathogen Evaluations

Septic systems have the potential to transport pollutants from sewage to ground water. To help prevent this, nutrient-pathogen evaluations are required for certain proposed on-site wastewater disposal systems. The evaluations consist of scientific analyses of potential water quality impacts of on-site subsurface sewage disposal systems. Learn more.

Technical Guidance Manual

The Technical Guidance Manual for Individual and Subsurface Sewage Disposal Systems provides guidance on the design, construction, alteration, repair, operation and maintenance of standard individual and subsurface sewage systems, their components, and alternatives. Access the manual and information on activities of the Technical Guidance Committee here.