A community’s wastewater typically contains a wide range of microorganisms and chemicals, some of which could be harmful to public health or to ecosystems. Water managers can choose from a portfolio of treatment options to design a wastewater treatment system that reduces contaminants to levels that will be acceptable for the intended uses of the reclaimed water.

Major wastewater contaminants include:

Pathogens. Bacteria, viruses, and other infectious organisms enter wastewater from human excrement and other waste. Viruses with the potential to cause disease are of particular concern for potable reuse because they are very small, can be difficult to eliminate from water, and some can cause infection even at low concentrations.

Nutrients. Municipal wastewaters are rich in nitrogen and phosphorus. Some forms of nitrogen can present a health risk for potable reuse if not properly treated. Excess nutrients can also cause the overgrowth of algae when reclaimed water is used to augment lakes. On the other hand, some nonpotable uses, such as irrigation, are actually enhanced by higher nutrient levels.

Organic chemicals. Pharmaceuticals, natural hormones, household chemicals, and -byproducts formed during the treatment process are often present in wastewater. High levels of such chemicals could pose a health risk, particularly for potable uses, unless they are effectively removed or degraded by appropriate water treatment processes.

Other contaminants. Metals and salts are examples of other contaminants that could affect drinking water taste or pose a risk for human health and the environment.

Secondary sedimentation basin in a wastewater treatment plant.

Today’s advanced analytical methods can detect many contaminants at extremely low levels—but the presence of a contaminant at a low but detectable concentration doesn’t always mean that the water poses a significant human health or environmental risk. For example, the risk posed by a particular contaminant varies with the concentration of the contaminant, the intended use of the reclaimed water, and the degree to which people will be exposed to it. Water intended for drinking typically must meet higher quality standards than water intended for nonpotable uses. In addition, different nonpotable uses can have different treatment requirements—water used for industrial cooling might have different quality requirements than those for a lake where swimming is allowed.

Treatment Technologies

There are a number of technologies available for treating wastewater intended for reuse, many of which can be used in combination. To choose the appropriate combination of treatment options, water managers must consider the specific contaminants that are of concern, the intended use of the water, costs, and other factors such as energy use or waste disposal options.

The National Research Council report reviewed options for ensuring water quality in water reuse projects. Because protecting public health is of utmost importance for any drinking water system, the report’s authoring committee recommended potable water reuse systems include several redundant treatment elements to strengthen the reliability of the system.

In addition to redundant treatment processes, the committee recommended that water reuse systems incorporate plans for monitoring water quality and quickly responding to problems caused by equipment malfunctions, operator error, or changes in the quantity or quality of incoming wastewater. For nonpotable reuse systems, it is also important to prevent drinking water contamination from the inadvertent cross-connection of nonpotable water and potable water pipes.

Treatment processes commonly used in water reclamation. At each stage of processing, several treatment options are available. Water managers can choose one or more of these options to create a process that will treat water to the quality needed for its intended use. Water treatment begins with preliminary and primary treatment, which targets suspended and particulate matter, followed by secondary treatment to remove biodegradable organic carbon. Water managers can then choose to proceed to advanced treatment, which provides additional removal of nutrients, trace organic chemicals, and suspended solids. Engineered natural processes allow reclaimed water to blend with water from other sources and provide additional natural treatment to further remove contaminants from the water.

The Evolving Role of Environmental Buffers

In many potable reuse systems, environmental buffers such as aquifers, lakes, or wetlands serve a number of purposes. They are used to hold reclaimed water to provide additional time before it is introduced into a drinking water system and to allow it to blend with water from other sources. Environmental buffers may also provide additional natural treatment to further remove contaminants from the water, and they create psychological distance between the water’s source (treated wastewater) and its destination (drinking water) by incorporating natural environments into the process.

Until recently, environmental buffers were considered a core element of all potable reuse projects. However, the National Research Council committee concluded that environmental buffers do not provide any water quality services that cannot also be provided by the use of engineered processes, such as advanced treatment and constructed storage facilities. Although environmental buffers remain useful elements of water treatment systems that should be considered alongside other options, they are not essential elements to reach water quality goals. Since 2010, several potable reuse projects have been developed or proposed that do not incorporate environmental buffers.