Stormwater Quality and Treatment
Recent Area of Emphasis for Civil Engineers and Other Disciplines
Jul 6, 2009
For decades, civil engineers were concerned with drinking water and wastewater as two major areas of emphasis. Stormwater was an issue of adequate capacity for conveyance, and flood control for larger rivers.
The quality of stormwater was seen as mostly a wastewater issue: adequately treating wastewater before discharging to a waterway and controlling industrial waste discharges were thought sufficient for cleaning up America’s rivers.
The Clean Water Act of 1972
Heightened environmental awareness in the late 1960s, along with an increasing amount of data and understanding of river ecology, convinced elected officials that something more needed to be done. In 1972 Congress passed the Clean Water Act (CWA), PL 92-500, which mandated that the EPA undertake a program to improve the quality of the nation’s fresh waters.
The CWA recognized that pollution can come from point sources (a single, identifiable, localized source of water pollution) or non-point sources (pollution from other than a point source). Point sources are easily recognized: a sewage outfall; an industrial waste pipeline; a large storm sewer; waste from a feedlot. Point sources are also more easily dealt with in terms of treating the pollution. The contaminated water is already collected. A treatment plant can be easily built. Hence the initial legislation was most concerned with point sources.
However, pollution from a non-point source, such as the run-off from a subdivision, industrial plant, or a parking lot, is much more difficult to collect and analyze and to treat. For years a debate raged as to the extent to which non-point sources contributed to pollution of the nation’s waters.
Quantifying Urban Non-Point Sources of Pollution
From 1977 to 1983, the EPA conducted an extensive study of twenty-two urban areas to quantify the amount of pollutants from urban non-point sources. Called the National Urban Runoff Program (NURP), about 2,300 rainfall events at eighty-one sites were sampled and analyzed. The resulting report proved that urban non-point sources did have significant pollution. As a result Congress passed amendments to the CWA in 1987, the amendments toughening the requirements for controlling pollution from non-point sources.
To make the amendments effective, the EPA extended the National Pollutant Discharge Elimination System (NPDES) permits to include requiring permits for runoff from various industrial facilities and urban centers. This originally targeted large cities (over 250,000 population), heavy industry, and large construction sites. Later amendments to the program reduced the population threshold for cities and the size for construction sites, as well as added many additional industries and large parking lots.
Requirements Vary By State
Although Federal regulations govern stormwater, States are given much latitude to set their own water quality standards for rivers and streams. Often standards are based on total maximum daily loads (TMDL) of pollutants. Sometimes they are based on best available technology, or best practicable technology without regard to existing water quality. Some States base their treatment requirements on a percentage removal of predicted pollutant loads. Some States have few requirements, and might limit stormwater treatment to a few streams that remain pristine or are severely impaired.
Current Status of Stormwater Clean-up Efforts
Despite the variation between states, stormwater pollution mitigation continues to expand. The dual focus is on prevention and treatment. Prevention includes such things a street cleaning and industrial site good housekeeping practices. Treatment efforts include both structural systems (small treatment units) and natural methods such as infiltration swales, rain gardens, ponds to mimic the cleaning of natural wetlands, etc.
The various methods to intercept stormwater and treat it before it reaches a stream require a combination of disciplines. Civil engineers have a large role, especially with structural methods of treatment. Landscape architects and environmental scientists are involved with the more natural methods of removing pollutants from runoff. However, even these methods involve civil engineers, who will provide flow calculations, inlet and outlet conveyances, and review process design even for natural methods.
While the ability to pay for stormwater treatment will vary with economic cycles, the trend is definitely toward more and more treatment. Most new commercial facilities will require some type of structural or natural stormwater treatment. Virtually all construction sites in the USA are now required to have minimal to extensive erosion and sediment control measures in place. Efforts continue to extend treatment to existing urban and suburban areas through retrofitting existing storm sewer systems and open channels.
