So far, various examples of designing and optimising the different units of the activated sludge systems have been discussed. In this section a conceptual method is presented, which can be used for the integrated cost-based design optimisation of the activated sludge treatment configurations presented in the previous section. This optimisation method uses the same body of theory as already presented in the earlier sections, but for the benefit of the reader the whole procedure is now presented in an integrated form, considering all components included in the design. The following configurations will be discussed:
- Configuration A1: Conventional secondary treatment
- Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration
- Configuration C2: Tertiary treatment - nitrogen and phosphorus removal in an UCT configuration
Furthermore it will be demonstrated that selection of the minimum sludge age required to meet the treatment objectives will indeed result in a minimal costs design. Finally, an example is given of the application of the optimised design procedure to optimisation of existing systems.
- Influence of the sludge age on treatment costs
- Operational optimisation of existing activated sludge systems
(1) Configuration A1: Conventional secondary treatment
The most elementary configuration of the activated sludge system, shown in Figure 10.1, consists of a completely mixed aerobic reactor treating the influent, followed by a final settler for solids/liquid separation and equipped with a gravity thickener and anaerobic digester for stabilisation of the produced excess sludge. In practice this system will also be equipped with a pre-treatment capable of removing large debris (rags, paper, plastics), sand and if required oil, fat and grease. For optimised system design the following data are required:
- Sludge age at which the system should be operated;
- Values of the parameters of the ideal model of the activated sludge system;
- Influent characteristics;
- Cost and financial parameters.
Click here to download the optimised design procedure of configuration A1 and here to download Example 10.2
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| Figure 10.1 | Basic process flow diagram of system configuration A1 |
(2) Configuration C1: Tertiary treatment - nitrogen removal in a Bardenpho configuration
If nitrogen removal is to be achieved, the system configuration has to be modified to include non-aerated zones for denitrification, as is shown in Figure 10.2.
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| Figure 10.2 | Basic process flow diagram of system configuration C1 |
Another important change is that the sludge age is no longer set by the requirements for organic material removal: now the sludge age will depend on the constraints from the nitrification and denitrification processes. In the section on optimized design for nitrogen removal, a method was presented to calculate the minimum sludge age required to achieve complete removal of nitrate in an activated sludge system. Click here to download the optimised design procedure of configuration C1 and here to download Example 10.5.
(3) Configuration C2: Tertiary treatment - nitrogen and phosphorus removal
In order to effect biological removal of phosphorus, it is required to incorporate a completely anaerobic zone in the system configuration, as shown in Figure 10.3.
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| Figure 10.3 | Basic process flow diagram of system configuration C2 |
Based on the theory presented in the sections on bio-P removal and optimization of nutrient removal, it was concluded that a relatively small anaerobic zone is sufficient and that preferably the sludge age is low. On the other hand, the necessity to remove nitrogen as well requires a relatively long sludge age. There is no analytical solution for this problem: one has to find an optimised solution iteratively while using expert judgment regarding the values of several operational and design variables. Click here to download the optimised design procedure of configuration C2 and here to download Example 10.6.
(4) Influence of the sludge age on treatment costs
In the optimised design procedure as presented in the previous examples, it was assumed that the selected sludge age is always equal to the minimum required sludge age required for proper functioning of the processes involved. The design procedure will now be used to demonstrate that this is in fact a correct assumption. For several sludge ages the system parameters will be calculated in order to determine the quantitative effect of the sludge age on system design and costs. To download this section, click here.
(5) Operational optimisation of existing activated sludge systems
The previous examples all refer to the design of an activated sludge system based on an expected or experimentally determined waste water flow or composition. Once the treatment system has been designed, the actual quantity and quality of the waste water will probably differ from those expected, as well as the values of the operational parameters. In this case the theory presented in this book can be used for another type of optimisation: for a given configuration determine the optimal operational conditions, characterised by production of the specified effluent quality at minimal costs. To download this section, click here.