Many homes and buildings that are not served by public sewer systems require on-site wastewater systems, commonly called septic systems, to treat and dispose of wastewater.
Septic systems discharge wastewater into an underground tank located on the property, where solids and water are then separated. Solids settle to the bottom of the tank where bacteria break down organic matter. Water (i.e., effluent) then flows from the tank 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 it reaches ground water. Residual sludge in the tanks must be pumped periodically.
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The “Individual/Subsurface Sewage Disposal Rules” (IDAPA 58.01.03) establish minimum standards for the design, construction, siting, and use of individual and subsurface sewage disposal systems. These rules also establish requirements for obtaining an installation permit and an installer’s registration permit.
These rules are administered by Idaho’s seven public health districts under a memorandum of understanding (MOU) with DEQ. The public 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.
Septage can present public health hazards and generate strong odors. IDAPA 58.01.03 establishes standards for pumping and cleaning septic systems and transporting and disposing of human excrement. To prevent public health hazards and nuisance conditions, septage must be collected, pumped, transported, stored, and disposed of according to these rules.
There are several types of septic systems and the design and size can vary depending on the facility, soil type, site slope, proximity water bodies, and local regulations. There are three primary types of septic systems in Idaho: Individual Septic Systems, Large Soil Absorption Systems (LSAS), and Alternative Systems.
Learn more about the types of septic systems on EPA’s website.
Individual Septic Systems – An individual septic system is a decentralized system that serves one or two homes and usually consists of a septic tank and a drainfield or leach field. To have a septic system installed, the property owner must obtain a permit from a public health district. The property owner should have a site evaluation performed by the public health district and a licensed septic system installer before purchasing property and applying for a permit. Once the permit has been issued, the system should be installed by a licensed installer and inspected by the public health district.
Large Soil Absorption Systems – A large soil absorption system (LSAS) is a subsurface sewage disposal system designed to receive at least 2,500 gallons of wastewater per day. Resorts, schools, subdivisions, and rest areas that are not connected to centralized systems often use LSAS to treat their wastewater.
As with an individual system, a property owner or developer wishing to install an LSAS must obtain a permit from a public health district and include a nutrient-pathogen (NP) evaluation with the permit application. The LSAS design must be prepared by a professional engineer licensed in Idaho and must undergo a plan and specification review by DEQ. A licensed complex installer is required for installation, which is inspected by the public health district and DEQ. At the end of each year, an annual operation and maintenance report must be sent to both agencies.
Alternative Systems – An alternative system is any system for which DEQ has issued design guidelines or designates as a simple modification of a standard system. Alternative systems include extended treatment package systems (ETPS) and proprietary wastewater treatment systems (PWTS).
An ETPS is a manufactured and packaged mechanical treatment device that provides secondary treatment to septic tank effluent before the effluent’s discharge to a drainfield. A PWTS is a subsurface sewage treatment system that incorporates proprietary wastewater system technology to provide additional treatment to a septic tank effluent system.
Septic systems can potentially transport pollutants from sewage to ground water. To help prevent this, DEQ’s “Ground Water Quality Rule” (IDAPA 58.01.11.006) requires a nutrient-pathogen (NP) evaluation for certain proposed on-site systems.
An NP evaluation is a technical review of a septic system’s potential impact on water quality. The evaluation seeks to determine the appropriate number of septic systems for a given parcel of land and ensure that the placement of an on-site system will not significantly degrade the quality of ground water or surface water resources.
- Guidance for conducting NP evaluations
- Level 1 Nutrient-Pathogen Evaluation Mass-Balance Spreadsheet (Use for non-central system or a subdivision)
- Level 1 Nutrient Pathogen Evaluation Domenico Spreadsheet (Use for central system/LSAS)
When An NP Evaluation is Required – DEQ requires an NP evaluation for proposed central septic systems located in nitrate priority areas, over sensitive resource aquifers, and for all proposed large soil absorption systems (LSAS).
DEQ, the public health district, or county agency may also require an NP evaluation on parcels of land where certain conditions may impact surface water or ground water quality:
- Nutrient and/or pathogen contamination already exists and has the potential to create a health risk
- Soil depth is shallow
- A predominance of gravel or coarse-grained sediment exists
- Ground water is close to the surface (10 feet or less)
- Fractured bedrock is close to the surface (10 feet or less)
The Technical Guidance Manual for Individual and Subsurface Sewage Disposal Systems (TGM) provides guidance on the design, construction, alteration, repair, operation, and maintenance of standard individual and subsurface sewage systems, their components, and alternatives. The TGM is updated periodically to adapt to the complex nature of small wastewater disposal systems.
An in-depth description of ETPS operation, maintenance, monitoring, and annual reporting requirements is found in the “Extended Treatment Package System” section of the TGM. Property owners are encouraged to review this section before installing an ETPS. Visit our Technical Guidance Committee web page to participate in an upcoming meeting.
Extended treatment package systems (ETPS), commonly known as aerobic treatment units, provide secondary treatment to septic tank effluent before it discharges to a drainfield. Enhanced treatment using ETPS may be required for certain parcels based on specific site features:
• Shallow soil depths
• Predominance of coarse- or very coarse-grained soils
• Shallow ground water depth
• Outcome of a nutrient-pathogen evaluation
ETPS can reduce total suspended solids (TSS) and carbonaceous biological oxygen demand (CBOD5) in septic tank effluent by 85% reduction or better and can reduce total nitrogen (TN) in septic tank effluent.
These units require a constant supply of electricity. Shutting off the power supply to any of an ETPS unit will result in additional operation, maintenance, and monitoring costs for the property owner.
Operation, Maintenance, and Monitoring – DEQ requires a minimum of one operation and maintenance event each year for each ETPS unit. Annual monitoring is also required to ensure the effluent discharged from an ETPS unit meets the required reduction levels.
Monitoring occurs after the annual operation and maintenance. Effluent samples are submitted to a laboratory and analyzed for concentrations of TSS, CBOD5, and TN, if required. All ETPS units must produce effluent meeting the quality standards of 45 milligrams per liter (mg/L) (parts per million [ppm]) TSS and 40 mg/L (ppm) CBOD5.
A certified service provider must perform the operation, maintenance, and monitoring, though property owners may use a certified service provider of their choice.
An annual report must be submitted to the public health district by July 31 each year, either by the property owner or their contracted service provider. It is the property owner’s responsibility to ensure the septic permit requirements are met on an annual basis.
Domestic septage is either liquid or solid material removed from septic tanks, cesspools, portable toilets, privy vaults, wastewater holding tanks, Type III marine sanitation devices, RV holding tanks, small wastewater treatment plants, or similar facilities receiving wastewater strictly from domestic sources. It does not include liquid or solid material removed from septic tanks, holding tanks, or similar treatment works that receive either commercial or industrial waste (i.e., grease trap waste, commercial wastewater, or industrial liquid).
Properly managing septage through disposal or beneficial reuse is critical to protecting public health. Septic tanks will function properly until the sludge fills over 40% of the volume of the tank (as measured from the bottom of the septic tank to the invert of the tank outlet), or the scum reaches the top of the inlet or outlet baffle in the tank.
Septic tanks require pumping every 3 to 5 years to remove the fats, oil, grease, and settled solids.
Domestic septage may only be disposed of using methods allowed by the IDAPA 58.01.03:
- Discharging to a public sewer
- Discharging to a sewage treatment plant
- Burying under the earth in a location and by a method approved by DEQ
- Drying in a location and by a method approved by DEQ
- Land application
Land application of domestic septage is regulated by the following state rules:
- Solid Waste Management Rules (IDAPA 58.01.06)
- Individual/Subsurface Sewage Disposal Rules and Rules Governing the Cleaning of Septic Tanks (IDAPA 58.01.03)
- Wastewater Rules (IDAPA 58.01.16)
- Ground Water Quality Rule (IDAPA 58.01.11)
- Water Quality Standards (IDAPA 58.01.02)