Optimization of Diffuser Systems

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:

1. Today’s cost of energy (cost/kWh)
2. Long-term escalation of energy cost
3. Advancements in diffused aeration technology
4. Opportunity cost for major TCO savings by aeration system optimization
5. Relative impact of TCO from aeration system optimization versus more traditional energy conservation measures, such as high efficiency blowers.
6. Carbon credits
7. Aeration system optimization requires an integrated analysis of all major system components with close attention to how they interface:
   • Blower size, efficiency and cost
   • Electrical switch gear size and cost
   • Piping sizes and cost
   • Optimization of diffuser type or numbers for energy savings
   • Relative impact of diffuser system design on other components (i.e., increasing diffuser efficiency may reduce the size and cost of items A through D above!)

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:

1. Activated Sludge
2. Extended Aeration
3. SBR
4. Nutrient Removal (BNR)
5. Lagoon
6. MBR
7. MBBR
8. Oxidation Ditch
9. Digester

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:

1. Standard test conditions. Multiple standards exist, including ASCE and German ATV standards.
2. Density of diffusers (number of diffusers per area of tank, or membrane area per area of tank).
3. Airflow rate per diffuser.
4. Water depth and diffuser submergence.
5. Type of diffuser platform (Disc, Tube, Panel, StreamLine™).
6. Diffuser Arrangement, such as grid, roll, or other

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:

1. Range of SOTE or Air Volume design opportunities by changing the quantity of diffuser applied.
2. Total air volume based on a preliminary selection of operating SOTE or preferred number of diffusers.
3. Comparison of energy mixing limits (i.e., airflow per basin floor area, or airflow per basin volume) versus possible operating airflows.
4. Allows selection of energy efficiency to match existing compressor/blower units or size limits. Also enables sizing blowers to stay within unit Hp (Kw) limits for the site, by selecting the number of diffusers to deliver the desired air volume.

EXAMPLE: Optimize 9” inch Disc Design

1. Utilize design data from Aeration Design Form (Appendix – A)
2. Process oxygen required for extended aeration at 1.4 kg of O2 /kg BOD (1.4 lbs/lbs) plus 4.6 kg of O2/kg of ammonia (4.6lb)
3. No O2 recovery for denitrification (DN) in this design
4. Establish SOTE performance for 9-inch disc at 5.75 m diffuser submergence (6m SWD less the height of diffusers). SOTE curves plotted versus air flow per disc, as shown in Figure 2. Note: These performance curves for each diffuser platform are imbedded into the Proprietary EDI Process Design Brief calculations. If running a separate or manual set of iterations these SOTE curves must be established or obtained from the diffuser manufacturer.
5. Using data from Figures 2 and 3 above run the Design Brief (Appendix B) to determine one set of design conditions.
6. After review of the initial Design Brief, run several iterations of the design brief by varying air flow per diffuser while holding the design load constant and solving for number of diffusers, efficiency, air flow, etc. Plot Figure 3 and establish the diffuser performance envelope using the Design Brief calculations.
   • Create Curve of SOTE % versus number of 9-inch disc units for design diffuser submergence.
   • Create curve of total air volume versus number of 9-inch discs for process oxygenation (Note mixing limited cases may need to be reviewed).
   • Compute the energy cost/year based on $10/Kw and assumed blower efficiency of 70% plotted versus number of 9-inch discs.
7. Select optimum diffuser design point to meet the project design objectives (i.e., Select the number of diffusers).

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