We continuously reuse water; it’s the basis of the hydrologic cycle. However, we can shorten the cycle by reusing treated water rather than pulling more water out of the cycle. Reuse is a means of dispersing high-quality effluent back into the environment while simultaneously doing something productive with the water. Treating wastewater to the appropriate level for the particular reuse is of obvious importance.
Wastewater reuse has increased in recent years and will continue in the future. Urban reuse systems provide reclaimed water for a wide variety of nonpotable purposes, including irrigation for ornamental landscapes, use in decorative water features, dust control, concrete production for construction projects, fire protection through reclaimed water fire hydrants as well as toilet and urinal flushing in commercial and industrial buildings. In environmental reuse, reclaimed water is used to create constructed wetlands, enhance natural wetlands and sustain or augment stream flows. Recreational reuse allows reclaimed water to be used in impoundments for fishing, boating and in some cases, human water recreational activities.
Reclaimed wastewater for nonpotable reuse must undergo some combination of primary, secondary and tertiary treatment to meet reuse requirements. The number and choice of treatment steps will vary based on how the water will be used. However, most recycled water will undergo some form of disinfection for the protection of public health. When disinfection is not used, the reuse area must be isolated from direct human or animal contact by fencing, signs or other means. The most commonly used nonpotable reuse applications are described below.
Irrigation reuse
Irrigation reuse is the direct use of reclaimed wastewater by applying it to agricultural crops or landscaped areas. Irrigation is a value-added means of dispersing water back into the environment. Spray distribution uses similar equipment and methods to apply the water. It is designed for the dispersal of effluent and does not have the “value-added” component of crop production.
It is important to remember that when a crop or landscape does not need irrigation, another means of reusing the reclaimed water must be identified. The two main categories of irrigation reuse are agricultural irrigation for things like crop fields or commercial nurseries, and landscape irrigation for parks, playgrounds, golf courses, freeway medians, landscape areas around commercial, office, industrial developments and residential landscape areas.
Both agricultural and landscape irrigation reuse may eliminate the cost of nutrient removal, which can be significant. Communities of all sizes can incorporate reuse of treated wastewater for landscape feature irrigation. Larger communities can produce sufficient water to make agricultural crop irrigation practical. Smaller communities or cluster-size systems only generate sufficient flow to satisfy smaller demands.
Industrial reuse
Industrial facilities use reclaimed water primarily for cooling system make-up water, boiler-feed water, process water and general washdown. It can also be used for concrete production on construction projects. Industrial reusers may require that the water undergo additional treatment. Softening is often done to protect the heat-transfer surfaces of industrial cooling towers. These additional treatment components are typically installed close to the point at which the reuse will occur.
Environmental/recreational reuse
Reclaimed water can be used to create constructed wetlands, enhance natural wetlands and sustain or augment stream flows. An impoundment of reclaimed water in which recreation is limited to fishing, boating and other noncontact recreational activities constitutes restricted recreational reuse. This form of reuse must be accompanied by appropriate signage. With unrestricted recreational reuse, reclaimed water is used in an impoundment of water in which no limitations are imposed on human recreational activities.
Urban reuse
In urban reuse, reclaimed water is used for various nonpotable purposes such as decorative water features, dust control, fire protection and toilet and urinal flushing in commercial, residential and industrial buildings. Irrigation of ornamental landscapes, parks and golf courses can also be a part of an urban reuse system.
Traditional urban water reuse systems have two major components: water reclamation treatment facilities and a reclaimed water distribution system. Infrastructure is needed to bring wastewater into the treatment facility (sanitary sewers) and a distribution system is needed to take the reclaimed water back out to potential users.
Nonpotable recycled water goes through a separate pipeline (purple pipe) system, which is completely separate from the drinking water distribution system. This “dual distribution” of potable and nonpotable waters is the most expensive component of a reuse system. The nonpotable distribution must be constructed to prevent cross connections with potable waterlines and ensure that nonpotable water is put to appropriate use. Periodic cross connection tests ensure that the nonpotable recycled water pipelines are not accidentally connected to the drinking water system. In addition, there is ongoing monitoring and testing of the nonpotable recycled water and drinking water systems to protect public health. To avoid cross connections, all aboveground appurtenances and equipment associated with reclaimed water systems must be clearly marked.
Aquifer recharge
Artificial aquifer recharge is the enhancement of natural groundwater supplies using constructed conveyances such as rapid infiltration basins or injection wells. Aquifer storage and recovery is a specific type of AR practiced with the purpose of both augmenting groundwater resources and recovering the water in the future for various uses. AR and ASR wells are found in areas of the U.S. that have a high population density and proximity to intensive agriculture; dependence and increasing demand on groundwater for drinking water and agriculture; and/or limited ground or surface water availability.
Toilet flushing
One beneficial reuse of treated wastewater is to flush toilets. Numerous decentralized treatment facilities use treated wastewater to flush toilets. Some large urban buildings have installed wastewater treatment facilities on their premises and reuse the water for toilets and fire protection.
The use of treated wastewater for toilet flushing is a water conservation strategy that typically uses graywater from nontoilet/kitchen sources like sinks, showers and washing machines. Graywater requires less intensive treatment than blackwater for nonpotable reuse. Blackwater from toilets and kitchen sinks can be treated and recycled, but this requires a more rigorous, multistage treatment process to meet safety standards for nonpotable indoor uses like toilet flushing. The wastewater must be treated and disinfected to meet strict public health and water quality standards for nonpotable use.
A typical treatment process will involve primary, secondary and tertiary treatment, resulting in water that is clear, odorless and safe for toilet flushing, but kept separate from the drinking water supply to reduce risk. A specialized plumbing system is required to safely deliver the treated water. Buildings using recycled water for toilet flushing must have two completely separate sets of pipes — one for potable water and one for the nonpotable recycled water. The nonpotable pipes and fixtures are often color-coded or clearly labeled to prevent cross connection with the potable water system. The recycled water is routed directly to the toilet tanks for flushing, an end-use that accounts for a substantial portion (often 30%-40%) of indoor household water consumption.
Potable reuse
The use of treated wastewater for drinking water, known as potable reuse, is a highly advanced and regulated practice. It involves purifying municipal wastewater to a standard that meets or exceeds all federal and state drinking water quality requirements. Potable reuse is generally implemented in two main ways, both relying on a multibarrier approach to ensure safety — either indirect or direct potable reuse.
Indirect potable reuse
IPR is the more common method and involves using an environmental buffer before the water enters the drinking water system. Wastewater is first treated at a municipal facility using primary, secondary and tertiary treatment (removing solids, organic matter and nutrients). The treated effluent then undergoes a rigorous purification process, often called Advanced Water Purification. A common “treatment train” includes:
Microfiltration or Ultrafiltration: Removes suspended solids, bacteria and protozoa.
Reverse Osmosis: A high-pressure process that pushes water through a semipermeable membrane to remove nearly all remaining dissolved salts, viruses, pesticides and trace contaminants.
Advanced Oxidation Process: Uses UV light combined with hydrogen peroxide to destroy any remaining trace organic compounds that may have passed through the RO membrane.
The highly purified water is then discharged into a natural body of water, which acts as a buffer. This can be to the groundwater where the purified water is injected into or percolates through a groundwater aquifer. A second option is surface water augmentation, where the purified water is released into a reservoir or river used as a source for a downstream drinking water treatment plant. The water is stored, blended with the native water supply in the buffer and finally withdrawn and treated again at a conventional drinking water treatment plant before being distributed to consumers.
Direct potable reuse
DPR is a newer, less common and more sensitive method that introduces the purified water directly into the drinking water system without an environmental buffer. The initial stages are similar to IPR, involving conventional wastewater treatment followed by the same highly protective sequence of purification (microfiltration/ultrafiltration, reverse osmosis and advanced oxidation). The purified water is then immediately and intentionally added into the raw water supply line leading to a drinking water treatment plant, or directly into the distribution system after blending with other potable sources. DPR requires extremely robust, real-time monitoring and an increased regulatory focus on process control and redundancy to ensure public health safety, given the absence of the natural buffer of time and dilution.
The paramount concern in potable reuse is public health. Both IPR and DPR rely on a sequence of treatment technologies where each step is designed to remove different types of contaminants, providing redundancy in case one process fails. The final product water must meet or exceed the same federal and state quality standards (such as the Safe Drinking Water Act in the U.S.) that apply to all drinking water sources. The advanced purification processes are specifically designed to remove contaminants of emerging concern, such as trace pharmaceuticals and personal care products, to levels often undetectable by analytical instruments. Potable reuse is a rapidly growing strategy in water-stressed regions because it is an effective way to create a reliable, droughtproof local water supply, thereby enhancing water security and sustainability.
Direct reuse systems are becoming increasingly used and will likely be part of your company’s future. Look for opportunities to build reuse into projects as appropriate.
