TECHNICAL BULLETIN 153 Optimization of Diffuser Systems
Diffused Aeration System Performance Optimization
Operation of aeration systems for biological treatment is well documented as requiring the bulk of energy in wastewater treatment plants (WWTP). Figure 1 shows the electricity requirements for the primary operation in an activated sludge facility. Aeration contributes 50-70%% of the energy requirements in the plant. With so much of the energy use in a WWTP concentrated at the aeration system, it is a logical target for energy conservation.
FIGURE 1 Figure 1. Energy Usage at a Typical WWTP
Historically aeration systems have been designed with general guidelines of system performance. For diffused aeration, design criteria of 5.0 to 6.7% per meter (1.52 to 2.05% per foot) has been applied universally with little consideration of total cost of ownership (TCO). The following are key factors in TCO, and may affect the optimized design of an aeration system:
Proper optimization of WWTP operating cost for minimum TCO requires a thorough evaluation of diffuser system technology and selection of most appropriate diffuser type and diffuser design for EACH application. General rules of design for diffuser efficiency are no longer acceptable as thorough analysis results in major savings to the end user.
To date no effective method or easy to apply procedure for integrated aeration system optimization is available for routine application by design engineers. Process model programs, such as BioWin, use general rules of design for mechanical performance. They also give no indication to total operating cost or payback on investments. An aeration system analysis that integrates the key variables affecting cost can deliver major savings in annual operating costs and savings for minimum TCO.
EDI has recognized the opportunity for savings for WWTP operation by developing and applying "rational design" procedures for optimizing total aeration system design. EDI’s Integrated Diffused Aeration Design Procedures incorporate a 4-phase analysis to develop engineered solutions for type of diffusers, number of diffusers, and blower sizing.
Phase I: Organize Design Parameters
To perform the optimized design calculations, it is necessary to have process design information and physical site information. EDI Aeration Design Form provides a summary of many of the necessary design criteria (Appendix A).
In addition to the project design data, a clear confirmation of factors such as biological treatment process is required to establish a rational design. The treatment process selected has a major impact on aeration system optimization. Examples may include the following common treatment applications:
Phase II: Select Diffuser Type and Establish SOTE Performance
To employ the most appropriate diffuser, a comparison of performance of various diffuser platforms must be considered in the design. The most important performance values include operating pressure and standard oxygen transfer efficiency (SOTE). This requires rational clean water test results under ASCE or equivalent testing procedures. Figure 2 shows a typical SOTE performance graph for a 9-inch (230 mm) diameter diffuser generated by clean water testing in accordance with ASCE oxygen transfer procedures. For each project a type of diffuser (disc, tube, etc) must be selected then the optimization process will incorporate data for calculations that are diffuser platform specific. A separate set of calculations is required when comparing different diffuser platforms, i.e., disc versus tube, versus MiniPanel™, versus StreamLine™.
Figure 2. Typical SOTE Performance Graph
Note: Proper SOTE performance will incorporate multiple factors influencing performance such as:
Phase III: Perform Aeration System Sizing Calculations
The initial aeration system optimization calculations begin with a EDI’s Design Brief computation. This computation uses the project data in Phase I to find the overall oxygen demand in the system. The second half of the Design Brief uses the diffuser performance found in Phase II to calculate the airflow requirements and proper diffuser count required in the aeration basin. This calculation creates one design point of diffuser numbers, one air flow rate/diffuser; and total system air flow.
Output from this initial Design Brief calculation can confirm the basic system feasibility for aeration to meet O2 demands of the process. Mixing criteria are also established based on the process and basin geometry. It is now possible to use multiple iterations of Design Brief calculations to establish an aeration performance envelope for the project.
Phase IV: Create Diffuser Performance Profiles
The Design Brief is the work vehicle used to establish performance profiles for SOTE, air volumes, and number of diffusers as described below. This set of aeration profiles can be particularly useful in developing blower sizing, motor and electrical switch gear sizing, plus the basis of future evaluations of total cost of ownership.
Multiple designs from Phase III (design briefs) can be used to create an SOTE performance profile when SOTE is plotted versus number of diffuser units. These same multiple designs from Phase III also allow us to establish the air flow Performance Profile with air volume plotted versus the number of diffusers. Both curves are typically plotted together as shown in Figure 3. Note: Curves are independent and they do not show the optimum design point where they intersect. Separate calculations or evaluations are required to find the optimum total aeration solution.
Figure 3. Optimization of Fine Pore Aeration Diffusers
Using the curves of Figure 3, it is easy to determine:
EXAMPLE: Optimize 9” inch Disc Design
Figure 4. Annual Energy Cost as a function of diffuser design.
The integrated aeration system optimization allows the project "Total Cost of Ownership" to be developed. This incorporates present worth or net present value calculations over the design life of the project. The cost of ownership calculations again require iterative procedures to create the NPV "Opportunity" for savings, and is an in-depth analysis beyond the basic diffuser/blower optimization program shown here.
Appendix A. Aeration Design Form Example
Appendix B. Design Brief Example
Appendix B. Design Brief Example - Continued
Appendix B. Design Brief Example - Continued
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