Pros & Cons Of High Capacity Fine Bubble Diffusers

By: Tom Frankel
Post Date: April 26th 2017

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Post Tags: We classify wastewater aeration diffusers into two types – Fine Bubble and Coarse Bubble. Within Fine Bubble, there are needle-perforated panels, standard disc and tubes with ~1mm perforations, and larger perforations, typically 2mm, sometimes referred to as medium bubble and other times as fine bubble diffusers.

Performance

The standard fine bubble diffusers' performance is well understood in the market. Typically, they provide 2%/ft – 6.5%/meter standard oxygen transfer efficiency (SOTE), have a pressure drop of approximately 12" WG – 30 mbar when new, and are usually installed in a full floor grid, at airflux rates of 2.5-6.0 SCFM/sqft – 50-120 Sm3/hr-m2, producing bubbles in diameters ranging from 1 to 3 mm.

The performance of fine bubble diffusers with needle or large perforations is less understood, occasionally referred to as ultra-fine bubble and high capacity or medium bubble, respectively. This discussion will focus on the high capacity products and save the panel/strip diffusers for another post.

When To Use Fine Bubble Diffusers

High capacity diffusers are preferable in specific situations. For instance, if severe fouling is anticipated, larger slits may extend the time before failure or cleaning is needed. In very deep tanks (over 25 ft – 8m), small perforations may be less efficient. If converting from coarse bubble to fine bubble diffusers while re-using existing blowers, larger slits may facilitate the reuse of pressure-limited blowers.

Standard fine bubble diffusers are typically chosen by default when there is any uncertainty due to their versatility.

The orifice size of the diffuser determines the uniformity of air distribution across the grid. Standard fine bubble diffusers, with smaller orifices, allow significant airflow turn-down to meet lower oxygen needs overnight. There is a nonlinear relationship between orifice size and minimum airflow rate per diffuser for uniformity, where even a 1 mm difference is impactful. For instance, standard 9"/270mm disc diffusers may have a 5mm orifice, while high capacity diffusers have a 10mm orifice. The former requires a minimum airflow rate for uniform distribution of 0.5 SCFM – 0.9 Sm3/hr, while the latter requires 2.5 SCFM – 4.3 Sm3/hr. Choosing high capacity diffusers could reduce turndown by a factor of 2.5, even if fewer are used.

The modulus of elasticity of an elastomer measures the force with which stretched rubber tries to return to its original shape. Fine bubble membranes benefit from this property, as the slit releases a bubble, closes, and repeats the process, aided by the hydrophobicity of the rubber or biofilm. Manufacturing tolerances in membrane compounding, mixing, molding, and slit cutting ensure small bubbles. Large, deep slits with high air flux create large bubbles that rise quickly, resembling coarse bubbles with low aggregate surface area.

Hydraulic Considerations

Hydraulic considerations also play a role. Maintaining an aeration system with a single large diffuser can lead to poor oxygen distribution and fast bubble ascent, reducing oxygen mass transfer. Ideally, the entire floor should be covered with membranes to ensure uniform air distribution without dead zones. This can create a desirable environment with laminar flows from top to bottom and allow bubble columns to promote oxygen mass transfer efficiently. Leaving gaps for maintenance is necessary, but larger gaps may result from oversized tanks, leading to suboptimal diffuser placement.

Mechanical Benefits Of Fine Bubble Diffusers

Using diffusers within their standard range of operation offers mechanical benefits. Fine bubble membranes have been in use for over 20 years, with many older than 10 years still functioning well. They usually do not present serious issues with membranes, holders, or pipe attachments. However, increasing airflow rates can lead to stress cracking, tearing, drying, and different wear patterns on the membrane.

In most cases, selecting standard fine bubble diffusers provides a system with significant turn-down and turn-up capabilities, high oxygen transfer efficiency, sufficient mixing to keep solids suspended, and long-lasting performance. When evaluating both capital and operating costs, high capacity fine bubble diffusers are often hard to justify except in specific scenarios. The high capacity system may incur a 20%+ penalty in lifetime operating costs due to lower turndown and efficiency, impacting the plant's energy consumption. Owners should seek products and systems that offer long-term value.

Mr. Frankel co-founded SSI in 1995 with experience in designing and distributing engineered systems. He oversees sales, marketing, and operations, and holds multiple US patents related to diffusers. He graduated from Washington University in St. Louis.