Treated Wastewater
The term wastewater can refer to several forms of contaminated water resulting from use in industrial, farm, and household activities. Municipal wastewater, a mixture of sewage and wastewater from homes, businesses, and industries, is delivered via sewer lines to wastewater facilities where it is treated and then discharged or reused. Water discharged at this stage is considered safe for the environment in general, but it is still not considered high enough quality for drinking. You may have seen trucks carrying small tanks of water labeled non-potable. This is an example of water that’s safe enough for the environment but not safe for drinking.
In contrast, the process of reusing treated wastewater for drinking water is called potable water reuse. Drinking Water Treatment Plants are designed to take fully treated municipal wastewater through several additional filtration steps, including UV and carbon filtration, to remove harmful pathogens and bacteria. These specialized plants produce pure drinkable water that meets all EPA standards for potable water.
Once water at these facilities has completed the treatment process and has met standards for drinking water, it must be stored. Storage usually takes place in an underground storage tank called a “clear well”, or in elevated storage tanks that are typically visible around towns. Whichever storage type is employed, it must also meet requirements for storing potable water so that contaminants are not re-introduced. Underground storage tanks, above ground water towers or cisterns, and any other options must all use liners, coatings, or materials that are ANSI/NSF-61 certified for potable water storage.
Cisterns
When storing potable water, whether it’s municipal tap water, treated rainwater, or anything else, the water needs to be isolated from the environment so that it doesn’t pick up impurities from the storage environment. Depending on usage, the water itself may be used and replenished daily or it may be stored over long periods as a backup supply in the case of emergency. While small amounts of potable water may be safely stored in UV-resistant, food-grade plastic containers, a basement full of rows and rows of individual 5 gallon plastic containers quickly becomes impractical.
Cisterns and reservoir tanks are an ancient form of high-volume water storage, typically used today for purposes like drinking, toilet use, and even irrigation. Today’s cisterns are available in options that can store as little as 50 gallons, to huge cistern reservoirs capable of storing tens of thousands of gallons. Water storage tanks can be installed above or below ground and if space permits, additional cisterns can be added when greater storage volume is necessary.
Modern cisterns may be constructed from a variety of materials including cast-in-place reinforced concrete, cinderblock and concrete, brick or stone set with mortar and plastered with cement on the inside, ready-made steel tanks, precast concrete tanks, redwood tanks, and fiberglass tanks. Obviously, some of these materials aren’t appropriate for direct contact with drinking water.
Below ground cisterns are commonly available in a large variety of materials. Metal cisterns are the most common aboveground option, employing thin gauge steel for the structure itself and an impermeable liner for secure containment.
Cistern water can also be used for laundry, showering, toilets or watering. In the case of drinking water, though, cisterns should always be lined with an impermeable, chemically resistant, potable-grade liner to prevent contamination over time from the cistern itself. Since bleach is often added to the water in low concentrations to prevent growth of unwanted microorganisms, the use of a chemically inert liner is critical, not only to prevent breakdown of the liner itself, but to ensure no toxins are released into the stored water as the liner is exposed to the corrosive liquid.
