businesses, and recreational facilities rely on onsite wastewater systems, which serve approximately 60 million people (USEPA, 2002) (Table 1). About one-third of new homes rely on onsite systems. The fraction of the population served by onsite systems varies widely throughout the country, with the highest fraction, 55 percent, served in Vermont, and the lowest fraction, 10 percent, served in California. It is now recognized that the fraction of the population served by onsite systems in the United States is not likely to decrease (it has not changed significantly in the past three decades), because providing centralized collection and treatment for these dispersed populations is not economically feasible.

Unfortunately, many onsite wastewater systems are failing, due to inappropriate siting, design, or maintenance (USEPA, 2002). Failing onsite systems are recognized as sources of both groundwater and surface water contamination, posing a risk to public health (due to the presence of pathogens and nitrate) and the ecological health of lakes, rivers, and estuaries (due to nutrients that cause eutrophication). The regulation of onsite systems is currently undergoing important changes, and stricter and more uniform design and performance standards are expected in the future. Many existing systems will likely be required to upgrade.

The systems that are used for the onsite treatment and disposal of wastewater in the United States typically require substantial land area. As a result, communities with a higher population density tend to have centralized collection systems that transport the wastewater to a centralized treatment plant. However, there is no specific total population, or population density, at which it is necessary to provide a sewer system. Some communities that have historically relied on onsite treatment are now installing sewer systems. For example, Chico, California, with a population of 64,000, is beginning the installation of a sewer system that will collect the wastewater for about two-thirds of its population (the rest will continue to use onsite systems), with the aim to reduce nitrate contamination of groundwater.