[mbeychok]

Anyone involved in the field of water quality or wastewater treatment will find the following discussion to be of interest.

Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes.  The result is that the oxygen content of the water will be decreased.  Basically, the reaction for biochemical oxidation may be written as:

Oxidizable material + bacteria + nutrient + O₂  ==>  CO₂ + H₂O + oxidized inorganics such as NO₃ or SO₄

Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:

S^--+ 2 O₂  ==>  SO₄^--
NO₂^-  + 0.5 O₂  ==>  NO₃^-

Since all natural waterways contain bacteria and nutrient, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above).  Those biochemical reactions create what is measured in the laboratory as the Biochemical or Biological Oxygen Demand (BOD).

Oxidizable chemicals (such as reducing chemicals) introduced into a natuaral water will similarly initiate chemical reactions (such as shown above).  Those chemical reactions  create what is measured in the laboratory as the Chemical Oxygen Demand (COD).

Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant.  Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.

The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period.  The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD.  The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.

There are also many different COD tests.  Perhaps, the most common is the 4-hour COD.

It should be emphasized that there is no generalized correlation between the 5-day BOD and the Ultimate BOD.  Likewise, there is no generalized correlation between BOD and COD.  It is possible to develop such correlations for a specific waste contaminant in a specific water stream ... but such correlations cannot be generalized for use with any other waste contaminants or any other water streams.

If you want more details, read "Aqueous Wastes from Petroleum and Petrochemical Plants" published by John Wiley & Sons in 1967.  It is available in most university libraries.

The laboratory test procedures for the determining the above oxygen demands are detailed in the following sections of the "Standard Methods For the Examination Of Water and Wastewater" available at www.standardmethods.org:

(a) 5-day BOD and Ultimate BOD: Sections 5210B and 5210C
(b) COD: Section 5220

Milt Beychok
(Visit me at www.air-dispersion.com)

[eugenedakin]

Hi mbeychok,

You are correct when saying that there is no correlation between COD and BOD.  There are definately rule-of-thumbs between the two.  I have been working with water quality and their indicator for approximately 8 years in an industrial setting.

What general rule-of-thumbs have you seen between the 5-day BOD and the ultimate BOD?

The problems that I had with these tests, is trying to legally defend them.  Lets say company xyz performs a BOD test and it is slightly above the regulated limit.  By the time that the test has been completed, the BOD of the water has changed (since 5 days have elapsed).  So, the legally reported value is not the actual value that you are discharging 5 days later.   I know, its semantics.

Any insight you have is greatly appreciated.

Sincerely,

Eugene

[Donaldson Tan]

Aren't these test data supposed to be evaluated against a normal distribution, by Central Limit Theorem?

[eugenedakin]

Hi geodome,

Yes, you are right.

Eugene

[mbeychok]

Eugene:

Most of my experience with waste waters has been in the petroleum refining and petrochemical industry.  In those types of facilities, the wastewater flows and compositions are rarely if ever at steady-state conditions.  A correlation between BOD and COD that was valid a few hours ago is no longer valid a few hours later.  As for generalized correlations that are valid from one refinery to another, that simply isn't to be had.

You might like to read a book that I wrote in 1967, "Aqueous Wastes From Petroleum and Petrochemical Plants" by Milton R. Beychok, published by John Wiley & Sons. It is available in many university libraries. It has a much more in depth discussion of the relationship between 5-day BOD and Ultimate BOD.