The test behind our water pollution program failures

Sewage contains carbonaceous (fecal) waste and nitrogenous (urine and protein) waste, which both are used by two different types of bacteria. Treatment plants are actually bacteria feedlot operations to break down this waste, while containing them under controlled conditions. Since the two types are different and have different requirements, it is essential to know how much and what type of waste has to be broken down, thus what type of holding areas are needed and how much oxygen would be required for this process. This oxygen demand can be measured with the BOD (Biochemical Oxygen Demand) test.

Another important design criteria is the amount of sewage (flow rates) expected. This in turn will depend on in what type of sewer collection system this sewage is collected. Older cities will mostly have combined sewer systems, whereby both sanitary sewage and storm water is collected and which of course do require much larger hydraulic capacity as compared to separate sewer system, where only sewage is collected. To handle extreme high flows, combined systems also have CSOs (Combined Sewer Overflows), where during such periods both sewage and storm water is directly discharged into open water. Separate systems have the disadvantage that storm water in many cities is not as clean as was expected and that this storm water has become a major untreated pollution source.

The selection of sewage treatment therefore not only depends on how much carbonaceous and nitrogenous waste is in sewage, but also on the flow rates in what type of sewage collection system this sewage is collected.

The BOD test, developed in England around 1920, measures the oxygen use by heterotrophic bacteria using carbonaceous  (fecal) waste, exerting a C-BOD and autotrophic bacteria using nitrogenous (urine and protein) waste exerting a N-BOD.  Biochemical tests are slow, so it will take at least 30 days before all the waste is broken down into CO2, NO3 and water, while the test still does not show how much oxygen is used by the heterotrophic and how much by autotrophic bacteria. However, since very fresh raw sewage hardly contains autotrophic bacteria, the test during the first 6 to 8 days only measured the oxygen use by heterotrophic bacteria and since the oxygen used by autotrophic bacteria also can be measured by the faster TKN (Total Kjeldahl Nitrogen) test, it was possible to calculate the total BOD already after 5 days, by using the following formula:   Total BOD = 1.5 x BOD5 + 4.6 x TKN.

Average sewage (according the literature) will measure BOD5 = 200 mg/l and TKN = 40 mg/l, hence Total BOD = 300 + 184 = 484 mg/l.  Unfortunately, when EPA implemented the CWA, it used only the BOD5 value and since ‘secondary treatment’ was supposed to be 85% treatment, the BOD5 of secondary treated sewage should be less then (85% of 200) 30 mg /l.

Since autotrophic bacteria already are present in many sewage collection system and sewage treatment plants, there is no 5-day delay, as was found in 1920, so many sewage treatment plants in the seventies violated their discharge permits, while in fact the higher than 30 mg BOD5 /l reading was actually partly N-BOD5. In these cases, plants were violating their discharge permits, while in fact treating the sewage better than was required by their permits.

In 1984 the Environmental Protection Agency (EPA) acknowledged this problem and allowed the C-BOD5 test, which is the same BOD5 test, except that a certain chemical is added that will only kill the autotrophic bacteria. It was estimated that 60 percent of the plants in the 1970s that were out of compliance were brought into compliance by adding this chemical to their test.

However, by allowing this, EPA not only lowered the final goal of the CWA from 100 percent treatment (elimination of all water pollution by 1985) to a measly 35 percent treatment, but also officially ignored nitrogenous (urine and protein) waste, while this waste, besides exerting an oxygen demand, also is a fertilizer for algae, causing eutrophication often resulting in red tides and dead zones. This, while it still is impossible to evaluate the real treatment performance of a sewage treatment plant and establish what its effluent waste loading is on receiving water bodies.

Since we still also do not know how much carbonaceous and nitrogenous waste is in raw sewage, the chances are that sewage treatment plants are designed to treat the wrong waste, as most engineers still assume that the BOD5 in raw sewage represents its C-BOD5 value only, while in many cases (especially warmer climates) this is not the case.

The only correct way to use the BOD test is to use the C-BOD5 test in combination with the TKN test:  BOD = 1.5 x C- BOD5 + 4.6 x TKN.

This will provide the only correct data how much carbonaceous and how much nitrogenous food has to be broken down in these bacteria feedlot operations, called sewage treatment plants.

Consequences of incorrect use of the Biochemical Oxygen Demand (BOD) test:

• Government regulations failed to achieve any of its goals as 60% of the pollution Congress intended to prevent, was ignored.

• Since it still is impossible to evaluate the real treatment performances of sewage treatment plants, it is impossible to compare the different treatment processes and therefore makes any cost-benefit engineering analysis of different treatment processes worthless.

• Since it also is impossible to know the real effluent waste loading on receiving waters, this data is misleading when used in TMDL (Total Daily Maximum Load) watershed programs.

• Raw sewage can already be N-BOD5 and when design engineers still assume that it only represents C-BOD5, sewage treatment plants will be designed to treat the wrong waste.

• Better sewage treatment was accomplished by adding secondary and now tertiary treatment processes, only after stricter treatment criteria were required.

• When the oxidation ditch was developed in 1950, which contained all these additional treatment processes in one process, it was not recognized, because there was no correct test data available and engineers were solely focused in meeting effluent standards.

• Improving the oxidation ditch process by adding final clarifiers was a mistake, as it caused hydraulic limitations. The original concept allows inexpensive solutions to the problems now experience with combined sewer collection system, eliminating CSOs.

• The lack of correct test data and unwillingness to even discuss this, has created an atmosphere of denial, which promotes the status quo, sticking only to traditional systems, which were original only odor control facilities and also inhibits innovation.

• The fear of exposing what happened with the CWA, now interferes with other environmental issues (air pollution, climate change, acidification of oceans, forest fires, etc) impacted by the huge increase of reactive nitrogen in our biosphere.

Maier received a doctoral degree in civil engineering from the University of Delft in Holland, and has professional engineering licenses in New York and Utah. He worked for the State of New York, a large engineering consulting firm in Holland and an equipment manufacturing firm in Utah. He was involved in the design and R&D of municipal and industrial wastewater treatment plants in Europe, Brazil, Mexico and the United States. He can be reached at [email protected].

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