Wastewater Technology Fact Sheet Fine Bubble Aeration

DESCRIPTION In wastewater treatment processes, aeration introduces air into a liquid, providing an aerobic environment for microbial degradation of organic matter. The purpose of aeration is twofold: 1) to supply the required oxygen to the metabolizing microorganisms and 2) to provide mixing so that the microorganisms come into intimate contact with the dissolved and suspended organic matter. The two most common aeration systems are subsurface and mechanical. In a subsurface system, air is introduced by diffusers or other devices submerged in the wastewater. A mechanical system agitates the wastewater by various means (e.g., propellers, blades, or brushes) to introduce air from the atmosphere. Fine pore diffusion is a subsurface form of aeration in which air is introduced in the form of very small bubbles. Since the energy crisis in the early 1970s, there has been increased interest in fine pore diffusion of air as a competitive system due to its high oxygen transfer efficiency (OTE). Smaller bubbles result in more bubble surface area per unit volume and greater OTE. The Saukville Wastewater Treatment Plant (SWTP) uses fine bubble aeration to increase treatment efficiency and oxygen transfer, as well as reduce power costs. Initially, the plant used stainless-steel coarse bubble diffusers requiring two centrifugal blowers. Although these diffusers required little maintenance, oxygen transfer was inadequate to meet process needs, and power requirements were excessive. In an attempt to reduce power costs and increase treatment plant efficiency, the plant was retrofitted with a combination of coarse bubble and fine bubble ceramic diffusers in 1985. One of the two treatment cells was retrofitted with fine bubble ceramic diffusion, and the second retained its coarse bubble diffusion system. This reduced power requirements by eliminating the need for one blower. After start-up, it was noticed that the mixed liquor dissolved oxygen (DO) levels were not as high in the fine bubble cell as in the coarse bubble cell, due to uneven air distribution between cells. To correct this, a positive displacement blower was added to the fine bubble cell. To reduce fouling of the ceramic diffusers, an in-place gas-cleaning system was installed to inject anhydrous hydrochloric acid into the air stream while the system was in operation. In 1990, the plant switched to fine bubble membrane diffusers, which were interchangeable with the ceramic diffusers. The ceramic and membrane efficiencies were comparable, so few adjustments in air rate were needed. The SWTP was awarded the U.S. …