Used water, known as wastewater or sewage, is directed to a specialized facility called a wastewater treatment plant (WWTP). The primary purpose of a WWTP is to convert contaminated influent into a clean, reusable stream called effluent. This purification process ensures the water is safe for release back into the environment or for various reuse applications. The final destination is determined by regulatory standards, local water needs, and the level of treatment it receives.
Return to Surface Water Bodies
The most common destination for treated effluent is direct discharge into natural surface water bodies, including rivers, lakes, streams, estuaries, and oceans. This returns the water to the natural hydrological cycle. Before release, the water must meet quality parameters to protect aquatic life and downstream users. These requirements are managed through regulatory frameworks, such as the National Pollutant Discharge Elimination System (NPDES) permit program in the United States, established under the Clean Water Act.
The NPDES permits establish specific limits on the concentration and quantity of pollutants that can be discharged from a point source. These limits cover conventional contaminants like biochemical oxygen demand (BOD), total suspended solids (TSS), and fecal coliform, as well as non-conventional pollutants such as ammonia and nutrients. The receiving body of water dictates the stringency of the permit, ensuring the effluent does not compromise the water quality standards of the river or ocean it enters. Wastewater treatment plants are continuously monitored and inspected to ensure compliance.
Environmental and Groundwater Augmentation
Treated effluent is used to supplement natural water systems. One significant application is groundwater recharge, which involves replenishing underground aquifers depleted by heavy usage. This is accomplished through two main methods: surface spreading basins or direct injection wells.
Surface Spreading Basins
In surface spreading, the treated water percolates slowly through highly permeable soil. This provides an additional layer of natural filtration before reaching the aquifer below.
Direct Injection Wells
Injection wells are used when the aquifer is deep or separated from the surface by an impermeable layer. This allows for the controlled release of treated water directly into the underground reservoir. This practice is beneficial in coastal areas, helping to prevent saltwater intrusion into freshwater supplies. Treated water is also used in environmental restoration projects, such as augmenting stream flows during dry periods or sustaining new wetlands.
Non-Potable Water Recycling
Non-potable water recycling is a growing application for treated wastewater. This requires a level of treatment higher than simple surface discharge but less intensive than that required for drinking water.
The water is used for several purposes:
- Agricultural irrigation, supplying water to non-food crops, vineyards, and orchards.
- Landscape irrigation in urban settings, including watering golf courses, parks, and highway median strips.
- Industrial facilities utilize this water for processes like cooling towers and boiler feed water, reducing reliance on fresh potable supplies.
A distinct infrastructure is needed to ensure this water remains separate from the public drinking supply. Pipelines are frequently color-coded purple, a globally recognized standard that prevents cross-contamination.
Direct and Indirect Potable Reuse
Potable reuse involves introducing the purified water back into the public drinking water system. This practice is divided into two primary categories based on the delivery method.
Indirect Potable Reuse (IPR)
IPR involves discharging highly treated water into an environmental buffer, such as a reservoir or groundwater basin. It mixes with other water sources before being withdrawn and treated again at a conventional drinking water plant. This environmental buffer provides a natural retention time and an additional safeguard, which has made IPR a more publicly accepted practice for decades.
Direct Potable Reuse (DPR)
DPR bypasses the environmental buffer entirely, introducing purified water directly into the drinking water treatment process or the distribution system. Both IPR and DPR require advanced purification beyond conventional treatment, typically involving microfiltration, reverse osmosis, and an advanced oxidation process. This multi-barrier approach ensures contaminant removal, producing water that meets or exceeds drinking water standards. DPR is gaining traction in areas facing severe water scarcity, as it is a highly efficient way to augment local water supplies.