The first important step towards modelling the activated sludge system is to simplify: i.e to select only those processes that may be considered as crucial to the observed system behaviour. In this section an ideal activated sludge system will be considered, that is with one completely mixed reactor and operating under conditions of constant flow and load. The term ideal indicates that all the biodegradable organic material is effectively metabolised in the process and that the final settler is a perfect liquid-solid separator in the sense that there are no suspended solids in the effluent and that the sludge hold-up in the final settler is negligible compared to the sludge mass in the biological reactor. Based on the selected processes taking place in the biological reactor, model equations are developed that:
- Predict the sludge composition
- Predict the division of the daily applied mass or flux of influent COD into
:
- The flux of organic material in the effluent
- The flux of organic material contained in the excess sludge
- The flux of oxidised material
In this section the following items will be discussed:
- Development of the steady state model for organic material removal in the activated sludge system
- Summary of the steady state model
- Calibration of the steady state model
(1) Development of the steady state model for organic material removal
When a waste water containing organic material is placed in contact with an activated sludge mass under aerated conditions, the following processes will occur:
- Metabolisation of biodegradable organic material
- Decay of active biomass
- Bioflocculation of inert particulate organic material
This is graphically depicted in Figure 3.3.
 |
|
| Figure 3.3 | Overview of the processes that develop in an ideal activated sludge system |
The sludge age, the average retention time of the biomass in the activated sludge system, is defined as the ratio between the total sludge mass in the system and the daily sludge mass discharged from it. The sludge age is introduced as the crucial parameter in activated sludge system design and operation. Model equations are developed to determine the different sludge fractions for different influent compositions. This allows calculation of the division of the daily applied mass of influent COD into the fluxes of (1) organic material in the effluent, (2) organic material contained in the excess sludge, and (3) the flux of oxidised material. Click here to download this section.
(2) Summary of the steady state model
In this section the basic equations of the steady state model of the activated sludge system are summarized. In the equations that are presented on a concentration basis the hydraulic retention time is present, erroneously giving the impression that this parameter is of fundamental importance to model activated sludge system behaviour. To demonstrate that this is not the case, the equations are rewritten in the form of mass equations, in which the hydraulic retention time is eliminated. These mass equations will be the core of the activated sludge model as used throughout this book. Click here to download this section.
(3) Calibration of the steady state model
To calibrate the steady state model, it will be necessary to attribute values to the model parameters. Only three model parameters can be considered as unknown factors: the temperature and the fraction of non biodegradable particulate- and non biodegradable soluble COD in the influent. The temperature can be determined by taking into consideration the climate in the region where the activated sludge system is to be constructed, while for some industrial waste waters it may be estimated from the temperature at which the effluent is produced. As for the two non biodegradable influent COD fractions, in this section a calibration method is presented to determine the values. Click here to download this section.