Wastewater systems collect and dispose of household wastewater generated from toilet use, bathing, laundry, and kitchen and cleaning activities. Any structure with running water, such as a house or office, must be connected to some sort of wastewater disposal system.

Wastewater disposal systems can generally be divided into two categories: centralized and decentralized. Centralized systems are "public sewer systems" and usually serve established towns and cities and transport wastewater to a central location for treatment. Decentralized systems are systems that do not connect to a public sewer system. They may treat wastewater on-site or may discharge to a private treatment plant. View graphic of types of systems. 

Large-scale public sewer systems (municipal wastewater treatment plants) are centralized systems. They generally serve established cities and towns and sometimes provide treatment and disposal services for neighboring sewer districts.

Homes and other buildings that are not served by public sewer systems depend on decentralized septic systems to treat and dispose of wastewater. Most decentralized systems are on-site systems (wastewater is treated underground near where it is generated). On-site systems are the most common wastewater treatment system used in rural areas: 36% of Idaho's homes, or about 210,000 residences, use on-site septic systems to treat their sewage. These systems can be as small as a single septic system and drainfield serving one residence or as large as a large soil absorption system serving an entire subdivision. Wastewater in decentralized systems can also be treated by a private (usually small) wastewater treatment plant. These plants can have similar treatment processes and equipment as centralized systems, but on a smaller scale. Read more about on-site decentralized systems. 

Before it can be treated, wastewater must first be collected and transported. Wastewater can be transported in gravity, vacuum, or pressure-piped systems that carry wastewater from homes and other buildings to a central wastewater treatment facility. Gravity collection is generally preferred and is most common, but is sometimes expensive to build in rock or where ground water is near the surface. In these locations, vacuum collection is usually used if the terrain is relatively flat and the number of homes is sufficient to make it cost effective. Vacuum collection is better than pressure collection in cold weather situations. Pressure collection is usually the third option and may be selected when there are difficulties with gravity or vacuum collection systems. All of these systems must meet the requirements outlined in Idaho's Water Quality Standards (IDAPA 58.01.02) and Wastewater Rules (IDAPA 58.01.16).

Many types of wastewater treatment systems are available for centralized systems. These include lagoon systems, activated sludge systems, oxidation ditches, sequencing batch reactors, micro-filtration using membrane bioreactors, and many others. Recent technology has provided processes that produce high quality effluent, use a small footprint, minimize odors, and provide for limited impact on ground water when land applied. 

Once wastewater has been treated, it is "disposed" of by re-introducing it to the environment. The three main methods of disposing of treated wastewater effluent are surface water discharge, subsurface discharge, and land application for beneficial use.

Surface water discharge requires a National Pollution Discharge Elimination System (NPDES) permit from the U.S. Environmental Protection Agency. 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. 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 occurs on-site where wastewater is treated by discharging it underground where it leaches through a drainfield. Read more about on-site treatment systems.