Fine bubble diffusers
Fine bubble diffusers are a pollution control technology used to aerate wastewater for sewage treatment.
Description
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 and other factors affecting efficiency
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. Bubble size is only one factor. The diffuser airflux rate, diffuser density, and submergence also play a significant role in oxygen transfer efficiency. Fine bubble aeration is efficient because of gentle horizontal mixing that takes place between the bubble columns. The driving force for horizontal mixing is the difference in specific gravity between the bubble column (lower SG) and the surrounding water (higher SG). The larger the gaps between diffusers, the stronger theses driving forces are. In such cases, there is a dramatic increase in bubble coalescence and bubble ascent velocity, causing a decrease in system efficiency.[4]
Fine bubble diffusers have largely replaced coarse bubble diffusers and mechanical aerators in most of the world. The biological process uses air to encourage bacterial growth. The bacteria secrete enzymes which break convert the waste into a settle-able form. The larger air release openings of coarse bubble diffusers are useful in some cases, such as mixing tanks, where oxygen transfer efficiency is not important. One disadvantage of using fine bubble diffusers in activated sludge tanks is that there can be an increase in dynamic wet pressure over time, due to fouling. Fouling can occur as a result of biological growth, but more often, it is a result of chemical precipitation. Diffuser cleaning systems, such as liquid acid dosing, scrubbing, or hosing may be useful, however, not all deposits can be easily scrubbed off, many are not very soluble in acid, and sometimes fouling causes irreversible damage to the diffuser membrane, necessitating a membrane replacement. [5]
See also
References
- ^ Dr. B.C. Punmia; Ashok Kr. Jain; Arun Kr. Jain (1998-01-01). Waste Water Engineering. Firewall Media. ISBN 978-81-7008-091-6.
- ^ Fine Bubble Aeration (PDF) (Report). Wastewater Technology Fact Sheet. Washington, D.C.: U.S. Environmental Protection Agency (EPA). September 1999. EPA 832-F-99-065.
- ^ Frank R. Spellman (2013-03-12). Water & Wastewater Infrastructure: Energy Efficiency and Sustainability. CRC Press. pp. 300–. ISBN 978-1-4665-1785-1.
- ^ "Tom Frankel - SSI Aeration, Inc."
- ^ "Tom Frankel - SSI Aeration, Inc."
