Peter Maier, Ph.D., P.E.

The two waste components of sewage are carbonaceous (fecal) and nitrogenous (urine and protein). Fecal waste feeds heterotrophic bacteria, which causes a carbonaceous biochemical oxygen demand (C-BOD), while urine and protein are picked up by autotrophic bacteria, resulting in a nitrogenous demand or N-BOD. The latter also can be measured with the Total Kjeldahl Nitrogen (TKN) test.

The original technical literature assumed that, during the first five days of the BOD test, only heterotrophic bacteria are active and that the average strength of raw sewage has a BOD5 value of 200 milligrams per liter (mg/l) with a TKN value of 40 mg/l. The total BOD exerted by raw sewage, if discharged into open water, is 1.5 x 200 (300) + 4.6 x 40 (184) = 488 mg/l and not 200 mg/l as some still think.

This is well described in any older textbook or from the U.S. Geological Survey (pdf). Unfortunately, when regulations were initiated, many assumed that the oxygen required to stabilize all organic matter in sewage was equal to the BOD5 value or 200 mg/l, thereby ignoring not only part of the oxygen required by heterotrophic bacteria, but all of the oxygen required by the autotrophic bacteria. This was and still is commonly done in most countries.

The Clean Water Act sought to eliminate all pollution by 1985, which had to be achieved by demanding best available sewage treatment. Since "complete" or 100 percent treatment was not economically feasible, Congress settled for "secondary treatment," which was considered to be equal to 85 percent treatment. With the assumption that raw sewage contains 200 mg/l BOD5, the U.S. Environmental Protection Agency in 1972 used the BOD5 test in its definition of secondary treatment and set the BOD5 effluent standard at less than 30 mg/l (85 percent of 200), without realizing that the BOD5 test only represents 40 percent of the total BOD. In fact, instead of demanding 85 percent treatment, EPA actually settled for 34 percent (85 percent of 40 percent = 34 percent) BOD treatment in its NPDES (National Pollution Discharge Elimination System) permits.

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