In this text COD will be used as the parameter to indicate mass of organic material in the wastewater. Some advantages of its use over its alternatives BOD and TOC have been mentioned already in the section on organic material and bacterial metabolism. In this section another important advantage will be presented: the possibility to verify if the mass balance of the organic material closes. The following subjects wil be discussed
- Characterisation of the organic material in the wastewater into COD fractions
- Composition of the sludge mass in the activated sludge system
- Construction of a mass balance of the organic material
(1) Characterisation of the organic material in the wastewater into COD fractions
In Chapter 2 a first distinction was made between biodegradable and non-biodegradable material, the former being susceptible to metabolisation by the bacterial mass, whereas the latter was not affected by the biochemical actions of the micro-organisms. For a more refined description of the activated sludge system, both the biodegradable and the non-biodegradable fractions are divided into a dissolved and a particulate part. Figure 3.1 shows the resulting characterization of the influent COD. Click here to download this section.
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| Figure 3.1 | Characterisation of the influent COD in different fractions and their relation to the main processes in the activated sludge system |
(2) Composition of the sludge mass in the activated sludge system
In order to describe the behaviour of the activated sludge system the volatile suspended solids are divided in two basic fractions: (1) active sludge, composed of the living micro-organisms that act in the metabolisation of the influent organic material and (2) inactive sludge composed of organic material that does not exhibit metabolic activity. This inactive fraction is composed of accumulated inert suspended material from the influent and endogenous residue remaining after cell decay. Click here to download this section.
(3) Mass balance of the organic material
When an activated sludge system receives a constant load of organic material, a sludge mass will develop that is quantitatively and qualitatively compatible with this load and the prevailing operational conditions. Under steady state conditions there is no accumulation of influent organic material, therefore it will either be discharged with the effluent or the excess sludge, or it will be transformed into inorganic products by oxidation. Hence the daily applied mass or flux of influent organic material will be equal to the sum of the fluxes of (1) organic material in the effluent, (2) organic material contained in the excess sludge, and (3) the flux of oxidised material. Figure 3.2 shows the flow diagram of the ideal steady state activated sludge system.
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| Figure 3.2 |
Flow diagram of the ideal steady state activated sludge system and the associated COD fluxes
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Click here to download this section. The recovery factor for organic material is introduced: this value of this factor indicates whether the mass balance for organic material closes: this is demonstrated in Example 3.1.