The steady state model developed in the previous sections can be used to characterize the performance of the activated sludge system. Several practical applications are given below:
- Determination of excess sludge mass and -composition
- Determination of required biological reactor volume
- Determination of nutrient demand
- Temperature effect
- Comparison of the design parameters sludge age and F/M ratio
- Control of the sludge age
(1) Determination of excess sludge production and -composition
Using the mass equations developed in the previous section, it is easy to calculate the daily amount of produced excess sludge and the daily oxygen demand. Refer to Example 3.2. It is also interesting to evaluate the active fraction of the biomass. Click here to download this section.
(2) Determination of required biological reactor volume
In the previous section it was established that in a steady state situation a sludge mass will develop in the activated sludge system that is compatible with the daily applied COD load. When the sludge mass of the system is known, the reactor volume can be calculated after selection of the sludge concentration in the aeration tank. This is shown in Example 3.3. To download this section, click here. The method used to select the optimal sludge concentration, resulting in lowest total cost or lowest total volume of the aeration tank and final settler, will be discussed in the section on final settler design.
(3) Determination of nutrient demand
Once the daily excess sludge production is known, it is easy to calculate the daily demand of nitrogen and phosphorus, taking into account the concentrations of these nutrients in the influent. Refer to Examples 3.4 and 3.5. To download this section, click here. Nutrient removal is discussed in the sections on nitrogen removal and phosphorus removal.
(4) Temperature effect
The temperature influences the steady state activated sludge model through its effect on the active sludge decay rate. This is shown in Example 3.6. To download this section, click here.
(5) Comparison of the design parameters sludge age and F/M ratio
In the development of the steady state model, the sludge age appeared as the fundamental process variable and design parameter. However, in practice a different parameter is amply used in design and analysis of activated sludge systems: the F/M ratio. This parameter seeks to express the ratio between the influent organic material (F for “food”) and the bacterial mass available to metabolise it (M for “mass of micro-organisms ”). It will be demonstrated that the F/M ratio is a rather ambiguous parameter: for the same sludge age it is significantly effected by the value of the non biodegradable, particulate COD fraction. Therefore the sludge age should be selected as the parameter for activated sludge design and operation. Click here to download this section.
(6) Control of the sludge age
In this section it will be demonstrated that hydraulic control of the sludge age, i.e. direct discharg from the aeration tank, is much better than discharge from the return sludge line, even though the concentration of suspended solids is higher than in the latter. To download this section, click here.