Treated wastewater can be reused in industry (for example in
cooling towers), in artificial recharge of aquifers, in agriculture, and in the rehabilitation of natural ecosystems (for example in
wetlands). The main reclaimed water applications in the world are shown below:
Urban reuse In rarer cases reclaimed water is also used to augment
drinking water supplies. Most of the uses of water reclamation are non-potable uses such as washing cars, flushing toilets, cooling water for power plants, concrete mixing, artificial lakes, irrigation for golf courses and public parks, and for
hydraulic fracturing. Where applicable, systems run a dual piping system to keep the recycled water separate from the potable water. Usage types are distinguished as follows: • Unrestricted: The use of reclaimed water for
non-potable applications in municipal settings, where public access is not restricted. • Restricted: The use of reclaimed water for non-potable applications in municipal settings, where public access is controlled or restricted by physical or institutional barriers, such as fencing, advisory signage, or temporal access restriction.
Agricultural reuse Irrigation with recycled municipal wastewater can also serve to
fertilize plants if it contains nutrients, such as nitrogen, phosphorus and potassium. There are benefits of using recycled water for irrigation, including the lower cost compared to some other sources and consistency of supply regardless of season, climatic conditions and associated water restrictions. When reclaimed water is used for irrigation in agriculture, the nutrient (nitrogen and phosphorus) content of the treated wastewater has the benefit of acting as a
fertilizer. This can make the
reuse of excreta contained in
sewage attractive. The irrigation water can be used in different ways on different crops, such as for
food crops to be eaten raw or for crops which are intended for human consumption to be eaten raw or unprocessed. For processed food crops: crops which are intended for human consumption not to be eaten raw but after
food processing (i.e. cooked, industrially processed). It can also be used on crops which are not intended for human consumption (e.g. pastures, forage, fiber, ornamental, seed, forest and turf crops).
Risks in agricultural reuse In
developing countries,
agriculture is increasingly using untreated municipal wastewater for irrigation – often in an unsafe manner. Cities provide lucrative markets for fresh produce, so they are attractive to farmers. However, because agriculture has to compete for increasingly scarce
water resources with industry and municipal users, there is often no alternative for farmers but to use
water polluted with urban waste directly to water their crops. There can be significant health hazards related to using untreated wastewater in agriculture. Municipal wastewater can contain a mixture of chemical and biological pollutants. In low-income countries, there are often high levels of pathogens from excreta. In
emerging nations, where industrial development is outpacing environmental regulation, there are increasing risks from inorganic and organic chemicals. The
World Health Organization developed guidelines for safe use of wastewater in 2006,
Environmental reuse The use of reclaimed water to create, enhance, sustain, or augment water bodies including
wetlands, aquatic habitats, or stream flow is called "environmental reuse". For example,
constructed wetlands fed by wastewater provide both
wastewater treatment and
habitats for flora and fauna.
Industrial reuse Treated wastewater can be reused in industry (for example in
cooling towers).
Planned potable reuse Planned potable reuse is publicly acknowledged as an intentional project to recycle water for drinking water. There are two ways in which potable water can be delivered for reuse – "Indirect Potable Reuse" (IPR) and "Direct Potable Reuse". Both these forms of reuse are described below, and commonly involve a more formal public process and public consultation program than is the case with de facto or unacknowledged reuse. Some water agencies reuse highly treated effluent from municipal wastewater or resource recovery plants as a reliable, drought-proof source of drinking water. By using advanced purification processes, they produce water that meets all applicable drinking water standards. System reliability and frequent monitoring and testing are imperative to their meeting stringent controls. Nonetheless, the main health risk for potable use of reclaimed water is the potential for pharmaceutical and other household chemicals or their derivatives (
environmental persistent pharmaceutical pollutants) to persist in this water. This would be less of a concern if
human excreta was kept out of
sewage by using
dry toilets or, alternatively, systems that treat
blackwater separately from
greywater.
Indirect potable reuse Indirect potable reuse (IPR) means the water is delivered to the consumer indirectly. After it is purified, the reused water blends with other supplies and/or sits a while in some sort of storage, man-made or natural, before it gets delivered to a pipeline that leads to a water treatment plant or distribution system. That storage could be a
groundwater basin or a surface water reservoir. Some municipalities are using and others are investigating IPR of reclaimed water. For example, reclaimed water may be pumped into (subsurface recharge) or percolated down to (surface recharge) groundwater aquifers, pumped out, treated again, and finally used as drinking water. This technique may also be referred to as
groundwater recharging. This includes slow processes of further multiple purification steps via the layers of earth/sand (absorption) and microflora in the soil (biodegradation). IPR or even unplanned potable use of reclaimed wastewater is used in many countries, where the latter is discharged into groundwater to hold back
saline intrusion in coastal aquifers. IPR has generally included some type of environmental buffer, but conditions in certain areas have created an urgent need for more direct alternatives. IPR occurs through the augmentation of drinking water supplies with municipal wastewater treated to a level suitable for IPR followed by an environmental buffer (e.g. rivers, dams, aquifers, etc.) that precedes drinking water treatment. In this case, municipal wastewater passes through a series of treatment steps that encompasses
membrane filtration and separation processes (e.g. MF, UF and RO), followed by an advanced chemical oxidation process (e.g. UV, UV+H2O2, ozone). In 'indirect' potable reuse applications, the reclaimed wastewater is used directly or mixed with other sources.
Direct potable reuse Direct potable reuse (DPR) means the reused water is put directly into pipelines that go to a water treatment plant or distribution system. Direct potable reuse may occur with or without "engineered storage" such as underground or above ground tanks. In other words, DPR is the introduction of reclaimed water derived from domestic wastewater after extensive treatment and monitoring to assure that strict water quality requirements are met at all times, directly into a municipal water supply system.
Reuse in space stations Wastewater reclamation can be especially important in relation to
human spaceflight. In 1998,
NASA announced it had built a human
waste reclamation bioreactor designed for use in the
International Space Station and a crewed
Mars mission. Human
urine and
feces are input into one end of the reactor and pure
oxygen, pure
water, and
compost (
humanure) are output from the other end. The soil could be used for growing
vegetables, and the
bioreactor also produces
electricity. Aboard the International Space Station,
astronauts have been able to drink recycled urine due to the introduction of the
ECLSS system. The system costs $250 million and has been working since May 2009. The system recycles wastewater and urine back into potable water used for drinking, food preparation, and oxygen generation. This cuts back on the need to frequently resupply the space station.
De facto wastewater reuse (unplanned potable reuse) De facto, unacknowledged or unplanned potable reuse refers to situations where reuse of treated wastewater is practiced but is not officially recognized. For example, a
sewage treatment plant from one city may be discharging
effluents to a
river which is used as a drinking water supply for another city downstream. Unplanned Indirect Potable Use has existed for a long time. Large towns on the
River Thames upstream of
London (
Oxford,
Reading,
Swindon,
Bracknell) discharge their treated sewage ("non-potable water") into the Thames, which supplies water to London downstream. In the United States, the
Mississippi River serves as both the destination of
sewage treatment plant effluent and the source of potable water. == Design considerations ==