Field Performance of Fine and Ultra Fine Bubble Diffusers

History – Coarse, Medium, Fine Bubble Diffusers

Diffuser efficiencies are generally measured in clean water to allow basis of comparison of similar products and prediction of actual performance in a wastewater treatment plant application. Before standard testing procedures were implemented, SOTE data were managed by the various suppliers of aeration equipment with optimistic results from conducting their own clean water test procedures. Introduction of the ASCE clean water aeration testing procedures in the USA and the ATV standards in Europe standardized testing methods and allows rational comparisons of clean water test results between diffusers or diffuser systems. These standard results are generally reported as SOTE (Standard Oxygen Transfer Efficiency) measured at sea level in clean water and at 20°C. These procedures offer excellent comparison of performance of similar diffusers systems in clean water and can generally predict comparative energy efficiency of the devices when applied in a wastewater treatment plant.

When applying the clean water test values for plant design, it must be recognized these test results are only for clean water and significant adjustments must be made to predict or measure performance in wastewater. A key factor for adjustment of performance of diffusers to field conditions is the alpha factor (α) which is representative of the ease or difficulty of transferring air/O₂ across the air liquid interface on the bubble. To compare aeration devices for their performance in the field, some value of α must be recognized and employed. History has established that α factor determining field O₂ varies substantially depending on type of diffuser or more specifically, size of the air bubbles created by the diffuser. For example, coarse bubble diffuser units are generally assumed to create bubbles of 4 mm of Ø or larger and credited with an α factor between 0.65 and 0.85. The large Ø bubble efficiency is not impacted by the air/liquid interface to the same degree as systems with medium or fine bubbles. Performance evaluations and testing for medium bubble units suggest α factors are reduced as bubble size is reduced. Medium bubble diffusers typically deliver 2 mm to 4 mm mean bubble Ø with α factors of 0.5 to 0.7 in wastewaters. Fine bubble diffusion devices are generally assigned mean bubble Ø’s of 1 mm to 2.5 mm and α factors decline to a typical range of 0.4 to 0.6 in wastewater. See Figure 1.

The above general relationships of α to diffuser bubble Ø’s are generally well accepted and have proven reliable for performance comparisons of diffuser systems for over 40 years. The impact of bubble Ø on a is clearly shown in Figure 1 plotting typical α factors vs mean bubble Ø created by the diffuser system. Surprisingly, the effect of α vs mean bubble Ø plots as almost a straight line suggesting if the mean bubble Ø created by an aeration system is known, a reasonable α factor can be assigned.

Figure #2 gives typical values of efficiency of diffuser systems vs bubble Ø. Clearly, SOTE is improved with finer or smaller Ø bubbles!

Any full analysis of diffuser energy efficiency must consider the two competing factors:

1. Increased SOTE with small bubble Ø.
2. Reduced field transfer with smaller bubble Ø.

Actual energy performance is represented by Table 1 showing typical SOTE of Figure 2 adjusted by typical α from Figure 1 to show expected FIELD efficiency. Table 1 shows major energy savings by applying fine bubble diffusers to a system. Fine bubble typically offers about 50% energy savings vs coarse bubble.

New – Ultra Fine Bubble Diffusers

Aeration diffuser manufacturers have devoted significant resources to improving diffuser efficiency. Geometry of devices was evaluated to determine their impact on diffuser efficiency. Perforation patterns, type of perforations, and density of perforations were evaluated to determine their impact on diffuser efficiency. Density (membrane area) of the diffuser systems in the aeration zone were also evaluated for their impact on efficiency. This detailed research and evaluation produced a new configuration of diffusers designated as ultrafine bubble diffusers (typically panels) optimized to deliver maximum SOTE using the clean water testing standards for clean water protocol. Results from optimization of clean water efficiency appear very promising, showing use of high density panel diffusers with very small perforations to deliver smaller (ultrafine) bubbles could increase measured SOTE values substantially by creating ultrafine bubbles of 0.5 to 1.5 mm Ø. These ultrafine diffuser systems are now available and marketed for delivering increased clean water efficiency vs more traditional fine pore aeration systems. A quick analysis comparing SOTE values of ultrafine bubble vs conventional fine pore systems shows a significant advantage on SOTE for the ultrafine diffusers which seems to suggest major energy savings. Many economic evaluations have been conducted using persuasive clean water data to select ultrafine pore systems as an economical aeration device for energy conservation in the wastewater treatment plant based on energy savings driving improved NPV.

In diffuser economic analyses, it is important to evaluate all options for energy savings and select an optimum diffuser system delivering maximum field performance on any application. Clearly, it is also very important to consider all variables in comparison of the high SOTE ultrafine bubble technology to historical fine bubble systems.

A review of several Energy NPV analyses suggests a significant reality is being overlooked and the following facts and variables should be reviewed:

1. Ultrafine bubble diffuser units are optimized for clean water SOTE. They are quite successful in maximizing SOTE.
2. Ultrafine bubble systems by definition produce a new category of diffusers with smaller mean Ø bubbles.
3. Ultrafine bubble diffuser vendors typically take advantage of the small Ø bubble efficiencies in their analysis. These efficiencies are demonstrated in Figure 2.
4. Ultrafine bubble diffuser systems do not recognize the second part of the equation outlined previously where α factors are reduced as bubble Ø is reduced! Ultrafine bubble diffuser systems should be considering the Figure 3 attached showing the continued deterioration of α as bubble Ø is reduced.
5. Systems that utilize ultrafine bubble diffuser technology MUST utilize the attendant α factor to deliver a rational engineering comparison.

In the analyses that we have reviewed, ultrafine fine bubbles achieve beneficial higher SOTE with the smaller Ø bubbles; however, the tendency is to assign typical fine pore α factors for this economic analysis rather than the proper α factor that attends to ultrafine pore technology!

Ultrafine bubble systems are by definition different from typical fine pore diffuser systems. All variables that pertain to these ultrafine pore diffuser systems based on SOTE must be considered or field O₂ transfer results and energy consumption will be disappointing.

A proper selection of the α factor is critical in any comparison of energy operating efficiency from diffuser systems. Using the same α factor for a bubble Ø of 0.5 to 1.5 mm, i.e. ultrafine diffuser and a bubble of 1.0 to 2.5 mm of traditional fine pore diffusers cannot be supported by the industry experience.

A general graph of mean bubble Ø vs α factors in Figure 3 shows a proper α factor for ultrafine technology and shows a major reduction in performance can be expected in the field by a continued reduction in alpha as the bubble size has reduced. Net energy efficiency gains from high SOTE values of ultrafine bubble diffusers must be tempered by use of appropriate α for the system being evaluated. Table 2 enclosed compares traditional fine bubble with ultrafine. The impact of ascribing the enhanced O₂ transfer efficiency of the ultrafine bubble diffusers from the smaller bubble Ø plus the impact of the proper α on the field O₂ transfer efficiency and the resulting energy kw is enlightening.

A close evaluation of the results in Table 2 shows the push for higher clean water O₂ transfer efficiencies by developing ultrafine bubble Ø is may be misplaced. Table 2 shows clearly the impact of the increase in O₂ transfer from the fine bubble is more than offset by the application of a proper α factor with these ultrafine bubble diffuser systems. Net effect of the use of typical or median values for the ultrafine bubble system on SOTE and α is to deliver a negative efficiency compared to the performance of traditional fine pore diffuser systems! Figure 2 shows the ultrafine bubble diffusers delivering a very high efficiency of 2.4% per foot (7.87% per meter) for SOTE. Applications on the 0.4 typical α factors reduces this net to approximately 0.96% O2 transfer per foot of submergence in the field (3.15% per meter). The typical fine pore diffusers at 2.1% per foot in clean water (6.89% per meter) and a field transfer of 1.03% per foot (3.38% per meter) delivers greater O2 transfer per kw than the ultrafine pore diffuser systems.

Notes:

1. Use of ultrafine design shows LOSS of field efficiency of about 6.8% i.e. actually requires about 6.8% MORE air vs typical fine bubble.
2. Continuing push for higher SOTE also reduces α and reduces efficiency over 24%!
3. To get same field performance as proper design pore diffusers requires SOTE (0.3) = 1.03%/ft i.e. 3.43%/ft SOTE (11.25%m). This SOTE will not be practical.
4. Greater pressure of ultrafine diffusers makes energy comparisons ugly.

It should be noted that not only does the field O2 transfer efficiency go down with the ultrahigh efficiency diffusers, the impact on kw is even more negative! The ultrahigh efficiency diffusers by definition have quite high pressure losses with pressure losses across the membranes approximately two times the pressure losses for traditional fine pore diffuser assemblies. Adding the differences in the pressure and the α (SOTE) differences together would produce a loss in efficiency from use of the ultrafine bubble diffuser systems would be considerably above 10%. Chasing even higher SOTE can result in energy penalties vs proper fine pore aeration as much as 24%.

Selecting different values in the typical range of operations for the various diffusers can influence these values slightly, but the directional impact of this analysis is clear: you cannot continue to reduce bubble size for aeration and expect a net benefit in energy conservation. There are some limitations on the optimum bubble size for overall system performance with many of the following variables to be considered:

1. Operating pressure of the system.
2. Maintenance aspects of high pressure vs low pressure (tiny opening vs traditional fine pore opening in the membrane).
3. Capital cost of the ultrafine pore diffuser systems.
4. Mixing requirements of the process.

A summary of the overall operating results would suggest that optimizing the traditional fine pore aeration system to take full advantage of its capabilities will generally provide the best value diffuser system. To take full advantage of any fine pore aeration system it the following factors must be recognized which apply to all of the fine pore systems:

1. Airflow rate per unit of membrane needs to be minimized. Low flux rate gives higher O2 transfer efficiency.
2. Greater density of the traditional fine pore aeration systems can deliver greater O2 transfer to the liquid in the field, both clean water and dirty water.
3. Take advantage of the low pressure losses associated with traditional fine pore aeration systems.
4. The capital cost of the for traditional fine pore systems are much more manageable and competitive with multiple vendors.

Field operating history of actual systems installed with fine pore aeration vs ultrafine pore aeration have provided comparisons that confirm the analysis listed above. EDI replacement of ultrafine panels at Neusiedlersee Austria delivered an unexpected energy savings of over 30%. This savings was NOT predicted when comparing SOTE values. Parallel operation of three fine bubble types in parallel with one ultrafine bubble diffuser system was projected to deliver energy savings of about 20% based on SOTE. Field results show traditional fine bubble actually delivered better field performance by 6% to over 20%! For detailed analysis of these field data where ultrafine pore systems were anticipated to provide substantial improvements but actually delivered less performance than from traditional fine pore aeration systems, please contact your local EDI representative.