These are standard systems and usually consist of a 1000-gallon concrete septic tank and a drainfield or "leach" field. Septic tank effluent typically has nitrate levels at a range of around 45 mg/L. Nutrient-pathogen (NP) studies may be required for new subdivisions. These systems require Health Department approval and permitting, with possible DEQ review for NP study.

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These systems typically employ a type of filtration or other extended treatment after the septic tank to reduce nutrients or solids entering the drainfield and, therefore, the groundwater. (See TGM link above.) These systems require Health Department approval and permitting, with possible DEQ review for NP study.

Central systems are defined as any system that receives blackwaste or wastewater from more than two dwelling units or more than two buildings under separate ownership or receives blackwaste or wastewater in volumes exceeding 2,500 gpd. Large Soil Absorption Systems (LSAS) are subsurface sewage disposal systems designed to receive 2,500 gpd or more.

Both Health Districts and DEQ review central systems and LSASs. The responsibilities of each agency are outlined in the February 2000 Memorandum of Understanding (MOU) between the Public Health Districts and DEQ. An engineering report (including design calculations) is required for all central systems and LSASs. An NP study is required for all LSASs. DEQ conducts the NP study review.

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On-site systems also can be defined as "decentralized systems." For this discussion, decentralized systems involve those that do not discharge to an existing municipal wastewater treatment plant (WWTP) or publicly owned treatment works (POTW). Those are usually defined as centralized systems and are discussed below.

Wastewater collection systems typically include gravity, vacuum or pressure-piped systems from homes in the subdivision that discharge to a central wastewater treatment facility. Gravity collection is preferred from an operation and maintenance position, but is sometimes expensive to build in high groundwater or rock. Vacuum collection is preferred for these situations if the terrain is relatively flat and the number of homes is sufficient to make it cost effective. Vacuum collection is also preferred over pressure collection in cold weather situations. Otherwise, pressure collection may be selected when there are difficulties with gravity or vacuum collection systems. When proposed for use, all of these systems must meet various requirements including review by DEQ.

Many types of wastewater treatment systems are available for decentralized developments. These include lagoon systems, conventional activated sludge systems, oxidation ditches, sequencing batch reactors, micro-filtration using membrane bioreactors, and many others. Recent technology has favored technologies that produce higher quality effluent, utilize a smaller footprint, minimize odors, and provide for limited impact on groundwater when land applied.

When proposed for use, all of these systems must meet various requirements including review by DEQ. Each proposed system must have a proven track record, as determined by DEQ based on independent monitoring and testing of previous pilot or full-scale plants. Process approval is required prior to site-specific project approval. Also , each system approved for use must be operated by a licensed wastewater operator. DEQ approves entities that contract with one or more facilities to provide needed operation, maintenance, monitoring, testing, etc.

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The three main methods of treated wastewater effluent "disposal" are discharge to surface water, subsurface discharge, and land application for beneficial use.

Discharge to surface water requires an NPDES permit from the U.S. EPA. It can be difficult, although not impossible, for small dischargers to obtain an NPDES permit. Writing an NPDES permit is a time-consuming task, and because EPA's budget and resources are limited, priority is given to re-permitting larger existing facilities. Also, Idaho limits contaminant loads that particular reaches of streams can receive through the state's water quality improvement plans known as Total Maximum Daily Loads (TMDLs).

Subsurface discharge of effluent: See information on Central Systems and Large Soil Absorption Systems above.

Land application for beneficial use in Idaho is managed by the wastewater land application permit program administered by DEQ. Various requirements and treatment limits for effluent dictate the beneficial use of the effluent. Beneficial uses include growing crops such as hay or grain, growing trees, and watering golf courses, parks and other "common" areas of developments. Some states have similar rules that allow less restricted uses for very highly treated effluent such as individual lawn watering and ground water recharge. Idaho is considering adding these uses and associated additional limits and requirements.

Large-scale municipal wastewater treatment plants (WWTPs) or publicly owned treatment plants (POTWs) are generally defined as centralized systems. They serve larger populations of established cities and sometimes provide treatment and disposal services for neighboring sewer districts. They can have similar treatment processes and equipment as decentralized systems, but on a larger scale. Advantages of centralized systems over various decentralized systems include economies of scale, more controlled operations, one management and staff force instead of several, and one discharge to monitor instead of several.

Most of the POTWs in the U.S. were built in the 1970s and 1980s with EPA construction grants. In those years, EPA and others focused primarily on centralized systems. This belief is generally held today as well, although there are good reasons for use of decentralized systems and should be evaluated on a case-by-case basis.