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9,388

4,428

2,032

Sources: U.S. Public Health Service Municipal Wastewater Inventories and USEPA Clean Water Needs Surveys.

Notes: Less than secondary facilities = Primary + advanced primary; Greater than secondary facilities = Advanced secondary + advanced treatment; No discharge = Facilities that do not discharge effluent to surface waters.

"This total excludes 4,467 oxidation ponds and 142 land application facilities classified as secondary treatment facilities in USEPA's 1972 inventory of municipal wastewater facilities (USEPA, 1972). They were excluded because (1) USEPA did not categorize oxidation ponds as secondary treatment facilities in any other year covered in this analysis and (2) land application facilities are classified as "no discharge" facilities in subsequent years. To be consistent with data compiled after 1972, the 142 land application facilities were included in the "no discharge" category for 1972.

Sources: U.S. Public Health Service Municipal Wastewater Inventories and USEPA Clean Water Needs Surveys.

Notes: Less than secondary facilities = Primary + advanced primary; Greater than secondary facilities = Advanced secondary + advanced treatment; No discharge = Facilities that do not discharge effluent to surface waters.

"This total excludes 4,467 oxidation ponds and 142 land application facilities classified as secondary treatment facilities in USEPA's 1972 inventory of municipal wastewater facilities (USEPA, 1972). They were excluded because (1) USEPA did not categorize oxidation ponds as secondary treatment facilities in any other year covered in this analysis and (2) land application facilities are classified as "no discharge" facilities in subsequent years. To be consistent with data compiled after 1972, the 142 land application facilities were included in the "no discharge" category for 1972.

ponds designed for evaporation and/or infiltration of effluent. Other examples of "No Discharge" facilities include recycling, reuse, and spray irrigation systems.

Key observations from Table 2-4 and Figure 2-8 include the following:

• The total number of POTWs in the nation increased by about 36 percent between 1950 and 1996.

• POTWs providing only raw and less than secondary treatment decreased in proportion to facilities providing secondary and greater than secondary treatment during the 1950-1996 time period. In 1950, only 30 percent of POTWs nationwide (3,529 of 11,784 facilities) provided secondary treatment. In 1968, 4 years before the CWA, 72 percent of the POTWs (10,052 of 14,051 facilities) had secondary treatment or greater. By 1996, 24 years after the 1972 CWA, 99 percent of the Nation's 16,024 POTWs were providing either secondary treatment or greater or were not discharge facilities.

• In 1968, 72 percent of the nation's POTWs were providing secondary treatment and less than 1 percent were providing greater than secondary treatment. By 1996, 59 percent of POTWs were providing secondary treatment and 27 percent had greater than secondary treatment.

Figure 2-8 Number of POTWs nationwide for select years between 1940 and 1996 organized by wastewater treatment type. Sources: U.S. Public Health Service Municipal Wastewater Inventories and USEPA Clean Water Needs Surveys.

Trends in Population and Influent Wastewater Flow to POTWs

U.S. Public Health Service (USPHS) municipal wastewater inventories and the USEPA Clean Water Needs Surveys were the primary data sources used to document the population served by POTWs and the rate of influent wastewater flow to them between 1940 and 1996. Actual influent wastewater flow data were available from reports prepared for 1978, 1980, 1982, 1984, and 1986. For the years in which these data were not available, influent wastewater flow data were estimated based on the population served and an assumed constant normalized flow rate of 165 gallons per capita per day (gpcd). The influent wastewater flow rate of 165 gpcd is based on the mean of the total population served and wastewater flow data compiled in the USEPA Clean Water Needs Surveys for the five years for which actual wastewater flow data were reported (data ranged from 160 to 173 gpcd).

Influent wastewater includes residential (55 percent), commercial and industrial (20 percent), stormwater (4 percent), and infiltration and inflow (20 percent) sources of wastewater flow (AMSA, 1997). The constant per capita flow rate of 165 gpcd used in this study is identical to the typical U.S. average within the wide range (65 to 290 gpcd) of municipal water use that accounts for residential, commercial and industrial, and public water uses in the United States (Metcalf and Eddy, 1991).

Table 2-5 presents the population served by POTWs and the rate of influent wastewater flow to POTWs nationally for select years from 1940 to 1996. Figure 2-9 is a column chart displaying the population data.

TABLE 2-5 Population Served and Influent Wastewater Flow to POTWs by Wastewater Treatment Type, 1940-1996

Population Served by POTWs (millions)

TABLE 2-5 Population Served and Influent Wastewater Flow to POTWs by Wastewater Treatment Type, 1940-1996

Population Served by POTWs (millions)

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