Wastewater Treatment
Flexible membrane vs. ceramic coarse bubble
diffusers - Who's winning the wastewater war?
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 bub
ble. Coarse bubble diffusers produce a bubble diameter of 6-10 millimetres
(mm), whereas fine bubble diffusers produce 2-5 mm bubbles. The first fine bubble aeration systems
were composed of a ceramic type me dia, either a fused alumina oxide or a silicon oxide material. Those aeration
systems have been around for 80 years and until the 1950s and 1960s they were fairly efficient. However,over time,they have developed problems due to media fouling. Fouling consists of build-up 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 non-rigid form. These plastic diffusers were in stalled in the 1980s, but have lost popu larity due to several problems such as media fouling, lack of quality control in the manufacturing process (no uniform air distribution) and emerging cost com petitiveness of other fine pore products. Membrane diffusers that have shown
The evolution of the diffuser product is simpiy one exampie of how the face of the wastewater treatment industry is changing. ate 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 mainte nance 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
the most potential are the flexible mem
turn toward fine bubble devices.
branes made of elastomers or thermo
plastic material. Polyvinyl chloride (PVC)is the most common thermoplas tic material, producing a soft, flexible
In the last 10 years, fine pore mem brane technologies made of elastomer, the flexible membrane type, have be come more popular. Their popularity
membrane. Elastomers are made from
stems from the idea that the membranes
rubber or synthetic rubber materials, and
are less susceptible to fouling and oper ate at maximum efficiency longer than the ceramic type of media. Early mem brane products had a very short life and the media deteriorated on its own. Only within the last five years has the tech nology of the elastomers reached matu rity. Most membrane products now have lifespans in excess of five years, often extending to nearly 10 years before any maintenance or replacement of the prod uct is necessary. Cleaning
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 ere-
By Linda Chaloux, Environmental Analyst, Frost & Sullivan, San Antonio, Texas 56
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. Energy consumption One other consideration in choosing methods of aeration is energy consump tion. This has typically been the number one reason for replacing the traditional coarse bubble aeration systems with fine pore aeration devices. Fine pore aera tion devices maintain much higher oxy gen 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 mini mize energy costs. And now the ultra-fine... The ultra-fine bubble diffuser is the
newest addition to the diffuser product line. Utilizing a homogenous thermo plastic membrane,the ultra-fine diffuser is able to generate bubbles of approxi mately 1 mm in diameter, as opposed to the 2-10 mm diameters found in the con
ventional coarse or fine bubble systems. This particular diffuser product offers oxygen transfer efficiencies of almost
Environmental Science & Engineering, June 2001