Across the United States, local government agencies are typically responsible for the collection, treatment, and management of sewage and wastewater. For large cities and small towns alike, the main goal of managing wastewater is to ensure public health and the health of our waterways. In some cases, the goal extends to being able to use treated wastewater as part of a city’s water supply.

As the safe collection and treatment of wastewater can be disrupted by extreme weather events—and the frequency and intensity of some events is increasing—cities may be able to avoid unpleasant issues by checking and addressing weather- and climate-related vulnerabilities of their water treatment systems. 

Two types of sewers

In most U.S. cities, underground sewers collect sewage and wastewater from homes, businesses, and industries and deliver it to wastewater treatment facilities. After treatment, the water is discharged on land, into water bodies, or reused. Two main types of sewer systems are in use: combined sewers and separate sanitary sewers.

Combined sewers collect both sanitary sewage and stormwater runoff and convey it to a treatment plant in a single-pipe system. Heavy precipitation events can overwhelm the capacity of these systems, resulting in untreated effluent overflowing the system and pouring into nearby waterways. To prevent this, authorities have generally restricted construction of combined sewers since the second half of the 20th century.

Sanitary sewer systems carry wastewater from homes and businesses—they are not designed to collect or carry stormwater.

Sanitary sewers are also known as separate sanitary sewers or municipal separate storm sewer systems (MS4). These systems collect wastewater only—they do not facilitate widespread drainage for large amounts of runoff from precipitation events. Typically, sanitary sewers are built with some allowance to handle the higher flows that occur when excess stormwater enters the collection system, but they are not designed to handle large volumes of stormwater.

An imperfect solution

Though they are superior to combined sewer systems, sanitary sewers are not always perfect. These systems can have structural issues such as cracks, faulty seals, and/or improper connections, which can result in infiltration and inflow (I/I) into the system during wet weather. Large volumes of I/I can cause sanitary sewer overflows and/or operational problems at wastewater treatment facilities. System blockages, equipment failures, broken pipes, or vandalism can also cause sanitary sewer overflows. Solutions such as sustainable landscape design and appropriate land use planning can reduce the flow of stormwater into sewer systems and local waterways.

The Clean Water Act

The Gowanas Canal in New York City has a legacy of environmental problems that began with decades of heavy sewage flow into the canal. Environmental cleanup of the waterway is an ongoing effort.

To protect our water and environment, wastewater treatment plants must adhere to regulations concerning what they discharge to waterways. Permits from the National Pollutant Discharge Elimination System establish the volume of discharge and conditions under which discharge is allowed at each plant (see Resources for discharge requirements below). Overall, the nation has made substantial progress in implementing the Clean Water Act (CWA), yet changing conditions can impede the forward momentum in some locations. Stressors—such as population growth, aging infrastructure, increasingly complex water quality issues, limited resources, economic challenges, and climate change—can all interfere with efforts to operate within CWA guidelines.

When wastewater treatment plants struggle to meet CWA standards, they can take an integrated approach to begin moving toward a solution. For instance, municipalities can move toward meeting multiple CWA requirements by increasing the efficiency of their wastewater or stormwater programs while they build capacity for improving the other system, and they can prioritize investments to complete the most effective projects first. This approach can also lead to more sustainable and comprehensive solutions—such as the implementation of green infrastructure solutions—to improve water quality.

Reusing wastewater

Some breweries are using recycled water to produce their beers.

Recent advances in wastewater treatment have demonstrated the ability to reclaim surprisingly pure, reusable water from treatment facilities. In the United States and abroad, water scarcity resulting from drought or increased demand has been met with creative and efficient strategies to reuse wastewater. Where municipalities and regulatory agencies are nimble and supportive of innovative start-ups, corporations, or agencies that seek to use wastewater for new purposes, treated wastewater can become part of the water supply. For example, breweries in California and Oregon have recently begun using recycled water to make craft beers. Other companies are using bacteria and/or fungi to remove pollutants from wastewater, enabling them to reuse the water for their production needs.

Wastewater's part in climate change

The process of treating wastewater emits relatively large amounts of the heat-trapping gas methane (CH₄) to the atmosphere; among other processes that emit methane, wastewater treatment is the fifth largest anthropogenic source of the gas. Methane emissions can be reduced through improvements to infrastructure and equipment, but as the population increases, so too does the demand on wastewater treatment facilities. Coming up with funding for major improvements can be difficult. 

Innovations such as using wastewater to produce renewable energy can help treatment facilities become waste-neutral instead of waste-generating. Support of renewable energy programs at treatment plants can also result in more clean energy jobs and reduce greenhouse gas emissions by reducing the amount of energy produced by fossil fuel-based power plants.

For more information on building resilience of municipal water supply, see Water/Municipal Water Supply.

References

• U.S. Environmental Protection Agency, 2016: National Pollutant Discharge Elimination System (NPDES). Accessed August 2016.  
• U.S. Environmental Protection Agency, 2016: Climate Change. Accessed August 2016.
• U.S. Environmental Protection Agency, 2015: Summary of the Clean Water Act. Accessed August 2016.
• U.S. Environmental Protection Agency, 2016: Global Mitigation of Non-CO₂ Greenhouse Gases, 2010–2030. Accessed August 2016.