Appendix 8

Guidance on relationships between common acronyms related to effluent water: BOD, COD, ThOD, StOD, TOD, TOC, and DOC

1. Introduction

In the fields of effluent water treatment and assessment of the impact of discharges on the aqueous environment, there are many terms employed that relate to the oxygen demand and/or organic carbon content of the water. This array of acronyms with very similar definitions, often based on arbitrary assumptions, can be confusing to the non-specialist. However, these terms are of vital importance in the evaluation, design, operation, and monitoring of effluent treatment plants, in formal reporting, and in the assessment of the impact of effluent discharges on the aqueous environment.

The aim of this note is to clarify the definition of these terms and to establish the relationships between them.

2. Definitions

COD (Chemical Oxygen Demand), This is the amount of oxygen required for the chemical oxidation of compounds in water, as determined using a strong oxidant (most standard methods use dichromate). It should be noted that this method may not fully oxidise all atoms in a molecule (e.g., nitrogen may not be fully oxidised to nitrate) and that some organic molecules (e.g., benzene) are relatively resistant to dichromate oxidation and may give a falsely low COD.

BOD (Biochemical Oxygen Demand). This is the amount of oxygen required for biodegradation of compounds by micro-organisms growing under aerobic (oxygenated) conditions. The test is performed on an appropriately diluted sample (below any toxic effect concentration), at a defined temperature (normally 20°C) and for a standard period, which is usually 5 days (hence BOD₅) but can be longer for specific purposes. A very small inoculum of micro-organisms is added at the start of the test: usually this is sludge collected from an effluent biotreatment plant, although commercial freeze-dried inocula are available. Under standard test conditions, the small number of micro-organisms and the short time available may not be sufficient for much degradation to occur, even for a compound that is relatively well biodegradable. This difficulty can be addressed by measuring "ultimate BOD", i.e., continuing the test until degradation ceases. It should also be noted that, when ammonia is present in the test sample, there could be a significant increase in measured BOD as a result of microbial oxidation of the ammonia (ultimately to NO₃).

ThOD (Theoretical Oxygen Demand). This is the calculated amount of oxygen required to oxidise a compound to its final oxidation products. However, there are some differences between standard methods that can influence the results obtained: for example, some calculations assume that nitrogen released from organics is generated as ammonia, whereas others allow for ammonia oxidation to nitrate. Therefore in expressing results, the calculation assumptions should always be stated.

StOD (Stoichiometric oxygen demand). This was developed by ICI to give a standard basis for calculation in estimation of "Environmental Burden" ¹. It is a comprehensive ThOD calculation that assumes complete oxidation of the molecule (nitrogen to nitrate, sodium to Na₂O, P to P₂O₅, S to SO₂, and halides to their respective acids) and therefore gives a maximum potential oxygen demand.

TOD (Total oxygen demand). This is measured by the catalytic combustion of the substances in the aqueous stream into carbon dioxide and water plus oxides of any other elements present.

TOC (Total organic carbon). This is the mass of organic carbon in a sample. It is determined by combustion or strong oxidation of the test solution (after removal of inorganic carbonates) and measurement of the carbon dioxide produced. For single substances or where the composition of the waste stream is known, TOC may be calculated from the chemical formula of the substance(s) present.

DOC (Dissolved organic carbon). Measurement of TOC in a solution which has been filtered.

3. Units

For water samples, all oxygen demand and organic carbon parameters are normally expressed as mg/l (ppm). Concentration units should always be specified to avoid misunderstandings.

For the specification of the oxygen demand (COD, BOD, ThOD, StOD, TOD) of individual components, the unit is g O₂/g compound. For TOC it is g C/g compound. Since these are mass ratios, the unit is often omitted in speaking and writing.

4. How do the parameters relate to each other?

This relationship between many of the parameters is complicated and often empirical. Precise values will depend on test conditions employed, molecular structure, ease of chemical oxidation, etc. However, some quantitative correlations do apply and these are summarised in this section.

• For many compounds, COD should be the same as ThOD. Exceptions are: (i) compounds where chemical oxidation is inefficient (e.g., some simple hydrocarbons); and, (ii) cases where ThOD calculates oxidation of atoms beyond that actually achieved by the COD test method (e.g., N to NO₃, P to P₂O₅). The significance of case (ii) obviously depends upon the calculation method employed.

• StOD is a maximum ThOD and therefore relates to COD in the same way.

• TOD should be equivalent to StOD. TOD will be equivalent to COD except in those cases where there is oxidation beyond that actually achieved by the COD test method (e.g., N to NO₃, P to P₂O₅).

• BOD will be zero for non-biodegradable compounds. For biodegradable compounds, the value of BOD will tend to approach COD as the test period increases. However, ultimate BOD will be less than COD, since a proportion of the compound biodegraded is not oxidised but used for cell growth. In rare cases when a highly biodegradable compound is poorly oxidised in the COD test (e.g., benzene), BOD may be greater than COD.

• TOC can be calculated for known compounds from the molecular formula. For dissolved compounds TOC is equal to DOC. Relating TOC and DOC to COD and BOD can only be achieved by testing.

To summarise, depending on the test methods used:

StOD = TOD >= ThOD >= COD > BOD.

StOD and ThOD have the advantage of being simple to interrelate provided the ThOD calculation method is known. Converting between other (measured) parameters is extremely difficult since the results depend upon the precise test methods employed and, in the case of BOD, will be dependent upon the performance of the microbial population used.

For almost all industrial effluents and their components, the only reliable way to interconvert BOD, COD, TOC, ThOD, etc., is by testing and calculation.

For domestic effluent streams and some very easily biodegradable compounds, some rules-of-thumb do exist that permit interconversion with a reasonable degree of confidence, for example:

BOD₅ = 0.68 x COD

Ultimate BOD = 0.9 x ThOD (the other 10% is lost due to formation of new cell materials)

5. Uses of the parameters

The most commonly used parameters for process evaluation and design are BOD, COD and, when these are not available, ThOD. TOC and COD are widely used for process monitoring and can be performed on- or off-line.

Comparison of BOD with COD assesses whether the compound is readily biodegradable. For BOD₅, a rough indication is that a COD:BOD ratio of > 100 means that the compound is relatively non-biodegradable and a ratio of <10 that it is relatively readily degradable. However, low BOD₅ may merely mean that the test microbes need longer than the test period to begin breaking the compound down and therefore ultimate BOD or other biodegradation testing is generally much more reliable.

In view of the diversity in definition and application, the critical issue is not which parameters are used but that they are consistently applied within each situation.

6. Acknowledgement

This note is based on a document produced by David Milner (ICI).

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1. "Environmental Burden: The ICI Approach", ICI, UK (1996).