The term metabolism refers to the utilisation of a substrate such as organic material by micro-organisms. Invariably part of the metabolised organic material is transformed to chemically stable end products, which is an energy releasing process. The chemical transformation of the organic material is called catabolism or dissimilation. A second process, occurring simultaneously with catabolism is anabolism, the process of synthesis of new cellular mass. Depending on the type of micro-organisms involved, the source material for synthesis may be organic material (heterotrophic micro-organisms) or carbon dioxide (autotrophic micro-organisms). Nitrifiers, responsible for the transformation of ammonium to nitrate, are an example of autotrophic bacteria. The metabolism of nitrifiers will be discussed in the section on Nitrogen Removal. The following items will be discussed in this section:
(1) Aerobic metabolism
First the metabolism of heterotrophs in an aerobic environment will be considered. In this case the catabolic process will be the oxidation of organic material by oxygen, also called aerobic respiration. The anabolic process will be the synthesis of cellular material from organic material. It is concluded that the organic material represents both an energy source and a material source for the micro-organisms, as is displayed in Figure 2.3. The ratio between bacterial mass and its COD value is determined, which allows the yield coefficient for heterotrophic growth to be calculated. Click here to download this section
 |
|
| Figure 2.3 | Metabolism of organic material in an aerobic environment |
(2) Anoxic metabolism
In the absence of oxygen, other compounds may serve as an oxidant, such as nitrate, nitrite, sulphite and sulphate. Click here to download this section.
(3) Anaerobic metabolism
Certain micro-organisms are capable to metabolise organic material In the absence of an oxidant, a process called fermentation. It results in a rearrangement of the electrons in molecules of the metabolised compound in such a fashion that at least two new molecules are formed. Fermentations are of great importance in the food industry (e.g. for the production of cheese, yoghurt and beer). In sanitary engineering, the fermentation of particular interest is anaerobic digestion. This fermentation is characterised by the fact that the end products are methane and carbon dioxide. The particularity is that methane cannot be further reduced and carbon dioxide cannot be further oxidised, so that anaerobic digestion is the most complete of all fermentation processes. To download this section, click here.