In view of the enormous number of different compounds present in most waste waters, it is totally impractical, if not impossible, to determine these individually. For this reason the concept of organic material is introduced, which is indicative for the combined concentration of all the organic compounds present in a waste water. To quantify the mass or concentration of organic material, it is possible to use the properties that practically all organic compounds have in common: (1) they can be oxidised and (2) they contain organic carbon. In sanitary engineering the property that organic material can be oxidised has found the widest application. There are two standard tests based on this property: the biological oxygen demand (BOD) and the chemical oxygen demand (COD). Both have standardised procedures that are described in several specialised texts (for example Standard Methods, 2002), thus the experimental details will not be discussed here. The TOC test is based on the determination of organic carbon. The following items will be discussed in this section:
- Decription of COD test
- Description of BOD test
- Description of TOC test
- Comparison of parameters for measurement of organic material
(1) Description of COD test
In the COD test, the organic material concentration is calculated from the oxidant consumption necessary for the oxidation of the organic material, using a very strong inorganic oxidant. For most organic compounds, oxidation is nearly complete. The theoretical COD value of a specific compound can be calculated from the structural formula. Several examples are given. Click here to download this section.
(2) Description of BOD test
In the BOD test micro-organisms are used for the oxidation of organic material. The BOD is determined from the observed decrease in oxygen concentration. Often the duration of the BOD test is limited to 5 days, although it is well known that oxidation of a significant part of the organic compounds found in wastewater will not yet complete. An empirical formula is then used to estimate the ultimate BOD from the BOD5 value. Click here to download this section.
(3) Description of TOC test
In the total organic carbon (TOC) test the production of carbon dioxide is measured upon complete oxidation of organic material through combustion at high temperature. The carbon dioxide mass is indicative of the mass of organic carbon initially present in the sample. The equipment for the TOC test is rather sophisticated but it has the advantage of taking only a few minutes, so that it permits virtually on line control. Similar to the COD test, it is possible to calculate the theoretical TOC value of a compound from the structural formula. To download this section, click here.
(4) Comparison of parameters for measurement of organic material
In this section the COD and TOC parameters are related to the free energy release upon oxidation. It will be demonstrated that the value of the free energy release is remarkably constant in the case of COD, when the number of electrons transferred per carbon atom is in the range between 3 and 8, as will be the case for the overwhelming majority of the compounds present in wastewater, refer to Figure 2.2.
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| Figure 2.2 | Value of the released free energy as a function of the number of transferred electrons per C-atom (the numbers in Figure 2.2 correspond to the compounds listed in Table 2.4) |
As the use of BOD5 has significant disadvantages, COD is selected as the most approriate quantitative parameter to indicate the concentration of organic compounds. Click here to download this section.