Flexible membrane vs. ceramic coarse bubble diffusers: Who's winning?
By Linda Chaloux
Environmental Analyst, Frost & Sullivan
Contents
Diffusers….What's out there?
Cleaning course is more intensive than fine pore
Energy consumption less with fine pore
And the late entry is the ultra-fine…
Who is the winner?
Diffusers….What's out there?
The size, shape, and materials of construction of diffused aeration devices may vary; however, they all tend to be classified as either fine or coarse bubble. Coarse bubble diffusers produce a bubble diameter of 6–10 millimeters (mm), whereas fine bubble diffusers produce 2–5 mm bubbles.
The first fine bubble aeration systems were composed of a ceramic type media, either a fused alumina oxide or a silicon oxide material. Those aeration systems have been around for 80 years and until the 1950s and '60s they were fairly efficient. However, over time, they have developed problems due to media fouling. Fouling consists of buildup of biofilms and/or deposition of inorganic precipitates that can alter the operating characteristics of the diffusers. Four general shapes of fine pore diffusers exist: plates, tubes, domes and discs.
Porous plastic media was the next step in fine pore diffusers, claiming lighter weight, durability, cost effectiveness and ease of manufacturing. Porous plastic media was made in rigid or nonrigid form. These plastic diffusers were installed in the 1980s, but have lost popularity due to several problems such as media fouling, lack of quality control in the manufacturing process (no uniform air distribution) and emerging cost competitiveness of other fine pore products.
The membrane diffusers that have shown the most potential are the flexible membranes made of elastomers or thermoplastic material. Polyvinyl chloride (PVC) is the most common thermoplastic material, producing a soft, flexible membrane. Elastomers are made from rubber or synthetic rubber materials, and claim to be more resistant than other media to fouling and debris infiltration. Flexible membrane diffusers have better operability and flexibility. Their operability is improved because they are die-cut to create small slits. Since the size of the slits is controlled, the quality of the product and the air distribution on the product back flow prevention create a drastically reduced foul rate. This operating flexibility dominates over the ceramic technology at the present time.
Coarse bubble aeration systems were developed in the 1960s and competed against the fine bubble systems because they appeared to be virtually maintenance free and never lost efficiency. The coarse bubble diffusers seemed to be fine until energy costs became a major issue in the mid 1970s and 1980s, and that is how the predominant focus in the wastewater treatment industry began to turn toward fine bubble devices.
In the last 10 years, fine pore membrane technologies made of elastomer, the flexible membrane type, have become more popular. Their popularity stems from the idea that the membranes are less susceptible to fouling and operate at maximum efficiency longer than the ceramic type of media. Early membrane products had a very short life and the media deteriorated on its own. Only within the last five years has the technology of the elastomers reached maturity. Most membrane products are now have live spans in excess of five years, often extending to nearly 10 years before any maintenance or replacement of the product is necessary. (Back to top)
Cleaning course is more intensive than fine pore
The ceramic coarse bubble diffusers are like a grinding stone, in that they have a very porous surface, which must be cleaned by high pressure hosing in combination with a muriatic acid-based application. This cleaning method is much more intensive than the fine pore membrane, where the membrane, in most cases, needs only to be cleaned by hosing or scrubbing the media. (Back to top)
Energy consumption less with fine pore
One other consideration in choosing methods of aeration is energy consumption. This has typically been the number one reason for replacing the traditional coarse bubble aeration systems with fine pore aeration devices. Fine pore aeration devices maintain much higher oxygen transfer efficiency. Since aeration energy consumption does represent 50 to 90 percent of the total energy required by a wastewater treatment plant, and at least 30 percent of an activated sludge plant, it has become essential to minimize energy costs. (Back to top)
And the late entry is the ultra-fine…
The ultra-fine bubble diffuser is the newest addition to the diffuser product line. Utilizing a homogenous thermoplastic membrane, the ultra-fine diffuser is able to generate bubbles of approximately 1 mm in diameter, as opposed to the 2–10 mm diameters found in the conventional coarse or fine bubble systems. This particular diffuser product offers oxygen transfer efficiencies of almost two times that of conventional fine bubble ceramic or membrane discs.
The ultra-fine bubble diffusers also come in a different form: a flat plate or panel version. This much larger model of aeration diffuser products encompasses an increased aeration basin, offering greater oxygen dispersion. Additionally, the smaller bubbles and the larger surface area combine to produce a longer retention time of the bubbles with the suspended solids. This technology has been proven to last at least 10 years before replacements are required, which is much greater than the 3–5 year range originally given to other fine bubble diffuser products. Additionally, although clean water tests only put this device at 30 percent greater efficiency than fine bubble products, in-situ, or dirty water tests reveal efficiencies as great as 200 percent better than earlier fine bubble products. The largest disadvantage of this type of system is the relatively high initial cost compared to other aeration devices. However, in areas where energy costs are extremely high, this system will prove its economic benefits within a very short time-span. (Back to top)
Who is the winner?
Many large wastewater treatment plants and large cities continue to choose the coarse bubble ceramic products because of replacement costs. Very large plants have maintenance budgets, but they don't have a large capital expenditure budget where they can afford to replace the diffusers every few years. However, smaller installations are finding the benefits and energy savings of the flexible membrane diffusers far outweigh the cleaning and/or replacement costs. Some industry participants believe that flexible membrane diffusers are the best technology, capturing as much as 75 percent of the diffuser market, and the choice of pick for replacements and retrofits. Additionally, most competitors today only manufacture the membrane type, so that is another reason the industry has been swinging toward the membrane. With the addition of the ultra fine bubble technology, diffuser product manufacturers are able to offer the wastewater treatment industry a product in almost every cost and size category. The primary factor that end-users need to consider is the life cycle cost of the product. It is never just the initial installation figure, but it is the operations and maintenance, replacement fees, labor required, and effluent quality. The evolution of the diffuser product is simply one example of how the face of the wastewater treatment industry is changing, and should prompt end-users to calculate with their limited dollars, which system is the best fit, and will provide the greatest result for the longest period of time.
Recently, a diffuser manufacturer entered a competitive bid with their new ultra-fine bubble diffuser, with the following set of criteria. They would receive only half of the bid dollars up front, and upon completion of a one-year operation, if the output still matched the specifications, the diffuser manufacturer would receive the remaining half of their bid price. This particular manufacturer's product not only matched the original specification, but also performed better than expected. The question for you, the end-user, at this point is how did your product function after one operating year, and how will you choose your products in the future. (Back to top)
(For a comprehensive view of the U.S. Wastewater Treatment Equipment Markets please contact Rolf Gatlin at 210.348.1017 and reference Frost & Sullivan report #7808-15)