Fine bubble diffusers are a pollution control technology used to aerate wastewater for sewage treatment.

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Description

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Fine bubble diffusers produce a plethora of very small air bubbles which rise slowly from the floor of a wastewater treatment plant or sewage treatment plant aeration tank and provide substantial and efficient mass transfer of oxygen to the water.[1] The oxygen, combined with the food source, sewage, allows the bacteria to produce enzymes which help break down the waste so that it can settle in the secondary clarifiers or be filtered by membranes. A fine bubble diffuser is commonly manufactured in various forms: tube, disc, plate, and dome.[2]

Bubble size

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The subject of bubble size is important because the aeration system in a wastewater or sewage treatment plant consumes an average of 50 to 70 percent of the energy of the entire plant.[3] Increasing the oxygen transfer efficiency decreases the power the plant requires to provide the same quality of effluent water. Furthermore, fine bubble diffusers evenly spread out (often referred to as a 'grid arrangement') on the floor of a tank, provide the operator of the plant a great deal of operational flexibility. This can be used to create zones with high oxygen concentrations (oxic or aerobic), zones with minimal oxygen concentration (anoxic) and zones with no oxygen (anaerobic). This allows for more precise targeting and removal of specific contaminants.

The importance of achieving ever smaller bubble sizes has been a hotly debated subject in the industry as ultra fine bubbles (micrometre size) are generally perceived to rise too slowly and provide too little "pumpage" to provide adequate mixing of sewage in an aeration tank. On the other hand, the industry standard "fine bubble" with a typical discharge diameter of 2 mm is probably larger than it needs to be for many plants. Average bubble diameters of 0.9 mm are possible nowadays, using special polyurethane (PUR) or special recently developed EPDM membranes.[3]

Fine bubble diffusers have largely replaced coarse bubble diffusers and mechanical aerators in most of the developed world and in much of the developing world. The exception would be in secondary treatment phases, such as activated sludge processing tanks, where 85 to 90 percent of any remaining solid materials (floating on the surface) are removed through settling or biological processes. The biological process uses air to encourage bacterial growth that would consume many of these waste materials, such as phosphorus and nitrogen that are dissolved in the wastewater. The larger air release openings of a coarse bubble diffuser helps to facilitate a higher oxygen transfer rate and bacterial growth. One disadvantage of using fine bubble diffusers in activated sludge tanks is the tendency of floc (particle) clogging the small air release holes.[4]

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References

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Fine Bubble vs. Coarse Bubble Diffusers

By: Tom Frankel
Post Date: July 19th

Table Of Contents

In the primary and secondary treatment of wastewater, it is common to use aeration as a method of removing organic matter. Submerged bubble aeration, in particular, works effectively by diffusing small air bubbles to mix with the wastewater and facilitate the work of aerobic bacteria in breaking down organic waste. Generally, submerged bubble aeration provides a more advanced system than surface aeration, and many wastewater treatment facilities have moved in this direction.

However, there are two different types of bubble diffusers: fine bubble and coarse bubble diffusers. Choosing the one that&#;s best suited to your facility will depend on your needs and priorities.

Fine Bubble and Coarse Bubble Diffusers: What&#;s the Difference?

Though fine bubble diffusers and coarse bubble diffusers both work through bubble aeration, it&#;s important to identify some distinguishing features of their design and function to determine which is appropriate.

Fine bubble membrane diffusers produce a smaller-diameter bubble for the aeration process. These bubbles typically measure 1 to 3 mm in diameter. This fine bubble aeration usually provides 2% or more Standard Oxygen Transfer Efficiency (SOTE) during aeration. Fine bubble diffusers come in a range of diameters themselves. At SSI Aeration, Inc., we offer 7-, 9- and 12-inch fine bubble diffusers designed for performance and durability in municipal and industrial use.

Coarse air bubble diffusers, in contrast, produce a larger diameter of bubble to displace, churn and mix the wastewater effectively. Coarse bubble aeration typically provides 1% or less SOTE during aeration. At SSI, we offer a range of coarse bubble diffusers at a variety of price points to ensure our long-lasting products, engineered to the highest standards, can fill every type of need.

Fine Bubble Diffusers: Types

Fine bubble diffusers come in a few different styles of function and complexity. Below, you will find a short list of some fine bubble diffuser designs:

1. Disk

Disk diffusers consist of an EPDM rubber membrane mounted onto a plate.

SSI Aeration, Inc., offers a unique PTFE coating on the rubber membrane that is highly effective at combating bacterial coating and fouling. As compressed air flows through the membrane, the rubber expands. This expansion opens the constellation of small holes and releases thousands of bubbles into the water.

Disk diffusers are very effective in their function but can be more complex to install than other types of diffusers.

2. Tube

Tube diffusers consist of a long, narrow plastic or ceramic cylinder surrounded by an EPDM rubber membrane. Again, the rubber expands as compressed air flows through it to open the tiny holes and release small-diameter bubbles into the wastewater.

This type of fine bubble diffuser tends to have more surface area than disk diffusers and produces more bubbles. They require lower initial expenses than disk diffusers because they are simpler to install, though they are not always less expensive to maintain over the long term.

3. Perforated Tubing/Bubble Tubing

Though disk diffusers and tube diffusers make up two primary types of fine bubble diffusers, other types of fine bubble diffusers are available too, such as perforated tubing, or bubble tubing, diffusers. These consist of perforated plastic panels. Compressed air flows through and leaks out through the perforations.

Unlike with disk and tube diffusers, there is no expansion or contraction of the material to open the holes.

Fine Bubble Diffusers: Pros

Below, you will find a discussion of some of the features that make fine bubble diffusers, both disk-type and tube-type, an effective choice for wastewater treatment.

1. Efficient Aeration

Standard Aeration Efficiency (SAE) is measured by the number of pounds of oxygen mixed into the wastewater per horsepower-hour as produced by the blowers that power the treatment system.

Fine bubble diffusers disperse more oxygen into the water column for more efficient aeration, typically producing at least twice as much oxygen mass as comparable coarse bubble diffusers. Especially during secondary treatment, when wastewater must be purified to meet the EPA standard of removing 85% of organic matter, efficient aeration is a strong asset.

2. Efficient Oxygen Transfer

Standard Oxygen Transfer Efficiency (SOTE) refers to how efficiently the dispersed oxygen combines with the water in order for the aeration process to occur. A high number of tiny bubbles, like those produced with fine bubble diffusers, are able to transfer more oxygen into the water column than a smaller number of larger bubbles.

This increased oxygen transfer efficiency of aerators is the result of two factors:

• Surface area: A mass of many small bubbles has a much larger cumulative surface area than a few larger ones. This increased surface area allows for a higher rate of oxygen transfer as the air and water come into contact across the surfaces of the bubbles.
• Speed: Smaller bubbles rise at a slower rate than larger ones. This slow rate of speed results in a greater amount of contact time between air and water and permits more oxygen to be distributed throughout the water column. The increased time for oxygen transfer results in greater transfer efficiency.

All in all, the smaller bubbles produced by a fine bubble diffuser can accomplish the same amount of oxygen transfer with about half as much air as a coarse bubble diffuser would require.

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3. Energy Efficiency

Energy efficiency is one of the most attractive enhancements fine bubble diffusers offer and is a primary reason why fine bubble diffusers have been so widely adopted.

In a typical wastewater treatment facility, 50% of the power requirements come from the aeration system. An energy-efficient diffuser leads to a reduction in power needs and results in significant savings for any treatment plant. Fine bubble diffusers generally cost less to operate than other types of aeration, like coarse bubble aeration, in terms of energy used. This is because the smaller amount of compressed air needed for a fine bubble diffuser makes fewer demands on the blowers that power the aeration system.

Fine Bubble Diffusers: Cons

Though fine bubble diffusers have a variety of benefits, they are not suited to every application. A few of the drawbacks of fine bubble diffusers are outlined below:

1. Low Alpha Factor

Alpha factor refers to the diffuser&#;s resistance to fouling. Ultra fine pore oxygen diffusers tend to become fouled more quickly than coarse bubble diffusers because the tiny pores are more susceptible to becoming clogged with bacteria and organic waste.

This low alpha factor has several consequences. First, the sludge buildup impairs the diffuser function and reduces its efficiency in aeration and oxygen transfer. Second, it sends back-pressure through the system, which can lead to increased wear and tear on the blowers. Finally, the quick rate of fouling means the diffusers must be cleaned and replaced more often, causing losses to the treatment facility in terms of both money and time.

2. Weaker Mixing

The smaller bubbles produced by fine bubble diffusers, while exceedingly efficient for aeration and oxygen transfer, are poorer at mixing. Their smaller volume in comparison to coarse bubble diffusers means they cannot displace the same amount of liquid.

In wastewater treatment, mixing is vital for keeping biosolids in suspension so they cannot settle in the water and form sludge, which makes the treatment process less efficient and is costly and time-consuming to remove. Mixing is also necessary for bringing microorganisms into contact with solid organic matter in order to break it down. Fine bubble diffusers provide less power to facilitate this process.

3. Complex Installation and Cleaning

Often, when it is necessary to install or clean the diffusers, the entire tank must be dewatered and desludged. This costs time &#; at least a week per cleaning &#; plus labor and money for the wastewater treatment facility. In some cases, if the diffusers are attached to floating laterals, the installation must occur while the diffuser is submerged in the wastewater, which can be more difficult and dangerous.

Coarse Bubble Diffusers: Types

Now that we&#;ve discussed fine bubble diffusers and their pros and cons, we can investigate some of the benefits and drawbacks of coarse bubble diffusers. Coarse bubble diffusers, which can produce bubbles as large as 50 mm, come in a few different types, such as those listed below:

1. Disk Diffusers

The ReliaBall diffuser from SSI Aeration, Inc. is a coarse bubble disk diffuser. This type of diffuser typically consists of a wide band with perforated holes in it. Sometimes, this type of diffuser includes a cap with a membrane. Compressed air flows through the perforations and into the wastewater, producing large bubbles.

2. Tube Diffusers

Coarse bubble tube diffusers consist of a pipe or tube. This tube may have holes drilled into it or, in some cases, compressed air flows through the bottom of the tube. Orifices may be static or open and close with the flow of compressed air.

Coarse Bubble Diffusers: Pros

Below, you will find a discussion of the best selling points of coarse bubble diffusers for wastewater treatment systems:

1. High Alpha Factor

The larger holes of coarse bubble diffusers mean the diffusers are less susceptible to fouling than fine bubble diffusers. Bacteria and biosolids are less likely to become trapped and encrusted in the perforations. This keeps the diffusers running efficiently and reduces back-pressure that would otherwise put wear and tear on the blowers.

2. Strong Mixing

The larger bubbles produced by coarse bubble diffusers have more volume, displace more waste liquid, and consequently produce more power for mixing. Biosolids are kept in suspension and are less likely to form unwanted sludge. The effective mixing of coarse bubble diffusers facilitates the breakdown of organic waste thanks to its powerful combination of oxygen, bacteria and waste material.

3. Low Maintenance

Coarse bubble diffusers&#; larger pores mean the diffusers are less prone to clogging and require cleaning less frequently with a pressure washer or manual brush. This low maintenance saves the treatment facility money and time.

4. Durability

Enhanced resistance to fouling means the diffusers wear out and require replacement less frequently. The lessened burden of replacement likewise saves money.

Coarse Bubble Diffusers: Cons

Although coarse bubble diffusers offer strong benefits in their power and durability, it&#;s important for treatment facilities to consider the drawbacks of these diffusers, which are discussed below:

1. Less Efficient Oxygen Transfer

Coarse bubble diffusers produce fewer, larger bubbles that travel faster than those produced by fine bubble diffusers. Larger, faster bubbles transfer less oxygen into the water column. Typically, the SOTE of coarse bubble diffusers is less than 1%. To achieve the oxygen transfer efficiency of a comparable fine bubble diffuser, a coarse bubble diffuser would require twice as much air.

2. Less Efficient Aeration

Similarly, despite their increased vertical mixing power, coarse bubble diffusers disperse less oxygen into the water for less efficient aeration, typically achieving 50% or less of the efficiency achieved with fine bubble diffusers.

3. Less Energy Efficiency

Coarse bubble diffusers are generally less energy-efficient than fine bubble aeration. The amount of compressed air necessary for a coarse bubble diffuser to run puts greater demands on the blowers and a higher drain on the electrical system.

Choose the Right Aeration Diffusers for Your Application

Depending on your requirements, fine bubble or coarse bubble diffusers may have clear advantages for the work you need to do. Knowing how fine bubble and coarse bubble diffusers can be applied to the wastewater treatment process will help you reach a beneficial choice.

Deep Tanks

For deep, voluminous tanks, coarse bubble diffusers typically provide superior performance. Their premium vertical power means the organic material within the wastewater will be mixed sufficiently for proper breakdown and will remain in suspension without forming unwanted sludge.

Heavy Solids

For wastewater with a high composition of biosolids, coarse bubble diffusers perform well. The strong mixing performance enhances the breakdown of solids, and they are better suited than fine bubble diffusers to resisting the severe fouling that is likely to result from high biosolid composition. In locations such as equalization basins and grit chambers, coarse bubble diffusers outperform fine bubble diffusers.

Capital Cost Reduction

For companies where keeping costs low is a high priority, economy-line styles of coarse bubble diffusers provide a good fit while maintaining the high production quality that wastewater treatment facilities count on. In some cases, the innovative, economy-minded design is capable of reducing costs by 75%.

An Energy-Conscious Environment

For a company focused on shrinking its carbon footprint or pursuing a greener image, fine bubble diffusers make an ideal match, provided the facility has the resources necessary for more frequent and more painstaking cleanings of its diffusers. The efficient aeration and comparatively low energy use of fine bubble diffusers provide a high-performing fit for a forward-looking treatment facility.

Fine and Coarse Bubble Diffusers Recognized Worldwide at SSI Aeration, Inc.

Whether a wastewater treatment facility is best suited for fine bubble diffusers, coarse bubble diffusers or a combination of the two, SSI Aeration, Inc. is a global leader for superior solutions and service. Our products have innovative designs, world-class engineering and time-tested quality to keep wastewater treatment systems functioning smoothly and efficiently. Contact us today to learn more about why we&#;re trusted and respected worldwide.

Mr. Frankel co-founded SSI in with experience in design and distribution of engineered systems. He is in charge of sales, marketing and operations in the company. Mr. Frankel holds multiple US patents related to diffusers. He is a graduate of Washington University in St. Louis.

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