Rubber diffuser membranes expand when air flows through them. Each tiny perforation on the diffusers is a variable orifice that expands to admit a bubble of air into the wastewater liquid. These air bubbles rise through the water column and transfer oxygen into the liquid to assist with aeration processes. Larger bubbles can also assist in the mixing process. When the airflow is turned off, the perforations close and prevent backflow of liquid into the aeration system.
Different diffusers have different perforation patterns. Disc diffusers can be punched in a circular pattern all the way around, and this punching pattern allows for low headloss and high oxygen transfer efficiency, or OTE. It also maximizes the number of perforations, but it puts a modest amount of strain on the membrane. It’s also common to punch a disc membrane in discrete perpendicular sections. Though this punching pattern increases headloss and lowers OTE, it puts less stress on the membrane.
Most disc diffuser membranes are punched with perforations of 1 millimeter. Holes smaller than 1 millimeter provide higher OTE but also raise headloss. Larger holes, conversely, lower headloss but diminish OTE. In Europe, tube diffuser membranes are likewise punched with perforations of 1 millimeter, but in North America, they are generally punched with perforations of 2 millimeters.