The BOD (Biochemical Oxygen Demand) test.
by: Peter Maier, PhD,PE
History of the test.
Since the BOD (Biochemical Oxygen Demand) test is the backbone of water pollution regulation and the process design basis of sewage treatment plants, its understanding and correct application is essential. The test was developed around 1910 in England, in order to predict the ‘oxygen sag’ when sewage is discharged into the river and how long it would take the river to ‘self purify’. ‘Oxygen sag’ is the lowering of dissolved oxygen in the river over a certain distance, downstreams of the discharge of sewage and ‘self purification’ is achieved when the dissolved oxygen level is again the same, as it was before the sewage was discharged in the river.
Description of the test.
A sample of sewage is put into a test bottle, supplied with oxygen and kept at a certain temperature (20 degrees Celsius). The BOD test then over time measures how much oxygen is used by bacteria, until all such activity cedes. Although there is only one reading, it actually is the summation of two oxygen usages, one of heterotrophic bacteria feeding on carbonaceous (fecal) waste or C-BOD and the second one that of autotrophic bacteria feeding on nitrogenous (urine and protein) waste or N-BOD.
Since early testing was done on very fresh sewage, with hardly any autotrophic bacteria present in the sample (urine is sterile), one found that the oxygen use by autotrophic bacteria only contributed to the test reading after 6 to 8 days and since the use of oxygen by autotrophic bacteria also can be calculated by using the faster TKN (Total Kjeldahl Nitrogen) test, it became common, in order to save time, to use the 5-day reading of the test (BOD5), representing part of the oxygen use by the heterotrophic bacteria, in combination with the TKN test representing the oxygen used by the autotrophic bacteria. As in the test, the depletion of dissolved oxygen in a river is caused by both carbonaceous (fecal) and nitrogenous (urine and protein) waste and this oxygen demand could be calculated by using the following formula:
Total BOD= C-BOD (1.5 x BOD5) + N-BOD (4.6 x TKN).
Technical literature assumes that average strength of municipal sewage has a BOD5 test value of 200 mg/l and a TKN test value of 40 mg/l ( milligram per liter). The total BOD value is:
Total BOD = 1.5 x 200 + 4.6 x 40 = 300 + 184 = 484 mg/l.
All still assuming that the autotrophic bacteria are not sufficient present in raw sewage, what in most cases is not the case.On the left the (uncommon) BOD graph of ‘very’ fresh sewage, whereby the autotrophic bacteria only become active after 6-8 days. On the right the more common BOD graph of a similar strength sewage already containing autotrophic bacteria.
BOD test as backbone of regulations.
While the goal of the Clean Water Act (CWA) was to eliminate all water pollution by 1985 and realizing that this, at that time, was not feasible, the Act demanded EPA should start demanding ‘secondary treatment’, what many believed was 85% treatment, hence a reasonable first step towards the 100% treatment.
Unfortunately, when EPA in 1972 set sewage treatment standards for its NPDES (National Pollution Discharge Elimination system) permits, it established ‘secondary treatment’ to be effluents containing less than (15% of 200 mg/l) 30 mg/l BOD5, thereby not only ignoring 33% of the C-BOD, but all the N-BOD, while this nitrogenous waste also is a fertilizer for algae or as it is now called a nutrient.
Since EPA also had assumed similar conditions (lack of autotrophic bacteria) in the effluents, many sewage treatment plants in the seventies violated their NPDES permits, as their BOD5 reading was exceeding the 30 mg/l, while part of this reading already was N-BOD5. Such plants actually violated their permits, while they treated their sewage better than was required by their permits.
EPA finally in 1984 acknowledges the problems and by administrative rule in 1984, instead of simply applying the BOD test correctly, allowed the inhibited BOD5 test or C-BOD5 test. This requires a special chemical to be added to the test selectively killing only the autotrophic bacteria (also called nitrification), thus no N-BOD reading. EPA lowered the C-BOD5 treatment requirement in the effluent to be less than 25 mg/l, while still ignoring part of the C-BOD and also now officially ignoring all the water pollution caused by nitrogenous waste. All without informing or asking approval of members of Congress.
It was estimated that 60% of the alleged permit violators got into compliance by adding this special chemical to their BOD5 test. No estimate of the number of plants that in that period were modified or replaced and now probably treat their sewage worse than it was treated before.
The only correct BOD test application.
With the proper knowledge of the BOD test, it should be obvious that only using it 5-day reading, without any nitrogen data, is not only technically incorrect, but also very misleading. Therefore, the only correct application of the BOD test is to test for C-BOD in combination with the TKN test and using the following formula:
Total BOD = 1.5 x C-BOD5 + 4.6 x TKN
This correct test data is also essential for the process designs of a sewage treatment plants.
Using the 30-day test value (BOD30) test, would give the Total BOD, but does not provide the essential C-BOD and N-BOD data, essential to properly design a sewage treatment plant.
EPA’s recent claim that Congress only intended to address the oxygen depletion under the CWA, is clearly false. With this BOD test knowledge, one actually can claim, that if indeed Congress intended to solely address the dissolved oxygen depletion, EPA should have addressed the nitrogenous waste, as over time, this depletion of oxygen (first direct as N-BOD, but later after the stimulated algae die off), is much larger than the oxygen depletion caused by carbonaceous waste. EPA’s regulatory program, solely based on BOD5 without any nitrogen data, was and still is technically faulty and has created many practical problems in this engineering field. Among them, that we still can not evaluate how sewage is treated (by comparing what goes out with what comes in) and the possibility exist that in many (most) cases, autotrophic bacteria are already in raw sewage, so that the assumed BOD5 value is not only C-BOD5, but already can be N-BOD5. In such cases the sewage treatment plants are designed to treat the wrong waste.
High time to hold EPA, but everybody else involved in our water pollution programs, accountable and insist that such programs are based on proper science and correct test data.