One reason communities were slow to adopt secondary treatment into their urban water cycle was perception. There was no way to articulate the link between the organic wastes in wastewater and DO levels in natural waters. In the 1920s, these relationships became clearer with the development of an indicator called the biochemical oxygen demand (BOD). Performed in a laboratory, the BOD test measures the molecular oxygen used during a specific incubation period for the biochemical degradation of organic material, the oxidation of ammonia by nitrification, and the oxygen used to oxidize inorganic chemical compounds such as sulfides and ferrous iron.
Historically, the BOD was determined using an incubation period of 5 days at 20 degrees Celsius (C). For domestic sewage and many industrial wastes, about 70 to 80% of the total BOD is decomposed within the first five days at this temperature (Metcalf and Eddy, 1991). Because of the incubation period, BOD5 has been adopted as the shorthand notation for this measurement in the literature. Expressed as a concentration, the BOD5 measurement allows scientists to compare the relative pollution "strength" of different wastewaters and natural waters. The widest application of the BOD5 test, however, is for measuring the strength and rates of wastewater loadings to and from POTWs and evaluating the BOD5 removal efficiency of the treatment system.
Because of widespread problems with oxygen depletion in many urban rivers, several states, especially those in the more populated Northeast, Midwest, and far West, took a leadership role in the 1930s to encourage municipalities to upgrade from primary to secondary treatment. By 1950, 3,529 facilities, or about one-third of the 11,784 municipal treatment plants existing at that time, provided secondary treatment for 32 million people. At the same time, however, 35 million people were still connected to systems that discharged raw sewage, and 25 million people were provided only primary treatment (USPHS, 1951). Increasing the number of facilities that provided at least secondary treatment became a national issue as the technology was seen as a solution to the pervasive problem of low levels of DO.
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