Aeration Design
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When laying out an aeration grid a lot of questions come up regarding header and diffuser spacing. This usually happens when the design calls for either a high quantity of diffusers (high density) or very few diffusers (low density). When dealing with a low density of diffusers it is important to keep in mind the Four Foot (1.2 m) Rule. The Four Foot Rule says that the maximum distance allowed between a wall or column be no more than 4 ft. to the centerline of the nearest diffuser. Additionally, there should be no more than 4 ft. between headers or between adjacent diffusers on the same header. Studies have shown that using a maximum spacing of 4 ft. reduces solids deposition and improves mixing and oxygen transfer efficiency.
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On the other side of the spectrum are high density grids. In that case each manufacturer will have limits as to what their equipment will allow. With Sanitaire equipment, in general 12” (30.5 cm) spacing between the centerlines of disc diffusers on the same header is the minimum allowable. Likewise with header spacing, the closest the headers can be is 12” (30.5 cm) on center. After that offsetting the diffuser holders on adjacent lines is possible to get the lines slightly closer together, but this type of layout should only be used in select applications. When the lines are 12” apart it leaves approximately 2” between the outer edges of the diffuser holders on lines that are next to each other. As a result, once the header spacing is less than about 18” it becomes more practical to group the headers to allow for larger gaps called access alleys. Access alleys make it easier to install the system, to perform maintenance, change membranes, and to clean out the tank.
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The available spacing for both the headers and diffusers are on 1 inch intervals. Experience has shown that almost any size tank and any layout are capable of being met with these limitations. If there is an application where the diffusers are not the primary method of mixing and oxygen transfer is only required to keep the liquid oxygenated, such as a sludge holding tank downstream of a digester, header and diffuser spacing greater than 48” is allowable.
