Recycled Water

Recycling, or reuse of previously used, water is becoming increasingly prevalent in areas where fresh potable water is scarce. Perhaps the most infamous type of water recycled is indirect potable reuse, sometimes referred to as “toilet-to-tap.” But not all recycled water is used for drinking water. Other uses of recycled water include direct nonpotable reuse and greywater.

Domestic wastewater is made up of blackwater and greywater. Blackwater is the waste-water from toilets. Greywater is the wastewater from washing laundry, baths and showers, and sink water with food wastes removed. The majority of the disease risk from untreated wastewater comes from blackwater, due to the large amount of pathogens naturally present in feces. Greywater can be reused for noncontact uses, which do not involve bathing or washing, such as irrigation or toilet flushing. A bathroom design that uses sink greywater for flushing toilets, popular in Japan, is now used in some homes in the United States. Reusing greywater not only saves potable water, but it reduces the amount of wastewater entering sewers through reuse. Greywater often requires filtration and disinfection for reuse.

Not all water treatment is the same. The requirements for the treatment of wastewater are not as stringent as for drinking water. Most wastewater today requires tertiary treatment, which includes solids removal, biological treatment, and additional processes to remove nutrients, metals, or chemicals. Tertiary-treated wastewater meets the requirements for release into the environment and can be utilized for nonpotable uses, such as irrigation. This water is often referred to as “reclaimed” water. Within the United States, areas experiencing sustained water shortages may reuse treated wastewater.

The state of California has mandated that, where treated wastewater is available, it should be utilized for green-belt irrigation instead of potable water. San Diego, which depends mostly on imported water, adopted a water reclamation ordinance in 1989. This program has issued official rules and regulations for the reuse of water in the city that are meant to prevent human consumption of the reclaimed wastewater, prevent cross contamination between potable water and reclaimed water systems, and isolate the reclaimed water from contamination by other sources. One of the most famous elements of these rules is the requirement that reclaimed water be conveyed in purple piping, providing a means of easily distinguishing the nonpotable reclaimed water from potable water. This program allows the irrigation of green spaces within San Diego, while reducing the demand on the precious resource of potable water.

In contrast to nonpotable recycling uses of water, recycled potable water must be treated to drinking water standards. After traditional tertiary wastewater treatment, additional [Page 409]treatment will most likely occur through the use of reverse osmosis membranes, which reject impurities at the molecular level, followed by advanced oxidation to inactivate remaining pathogens. Recycled potable water is generally not injected directly into drinking water distribution systems, but goes through indirect potable use (IPU). In the United States, IPU has been used as recharge water to augment drinking water sources, such as groundwater and reservoirs. This water is mixed with the background surface or ground water, which then undergoes the normal drinking water treatment process. Perhaps the most famous IPU project in the world is in Singapore.

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