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Fine Bubble Aeration Grid Design
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Grid Design Basics
It is often said that ‘x’ thing is more of an art than a science. The same could be said about laying out an aeration grid. While there definitely is a science to aeration grid design, there is also a balancing act that occurs between cost, efficiency, and engineering factors that gets weighed every time a design is proposed.
The basic premise guiding aeration grid design is that longer headers and shorter manifolds result in the most cost effective use of materials. That is why most ‘stock’ designs will be rectangles with long headers. There are instances when a longer manifold and shorter headers will make sense, but rotating the grid is typically made on a case-by-case basis.
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One important factor of oxygen transfer efficiency is how even the diffusers are spread out across the floor of the tank. Highly efficient aeration systems spread the air more evenly over the tank floor. The measure of the evenness is captured in a number called At/Ad or the ratio of the area of the tank floor to the area of the diffusers, where a lower At/Ad indicates a higher density of diffusers.
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Grid Design Issues & Troubleshooting
To calculate an At/Ad the tank area in square feet (or square meters) is divided by the square footage of the membrane area. In the case of a 9” diameter disc diffuser the area is 0.41 ft2 (0.038 m2) per diffuser. Typical diffuser densities will range from approximately 4.5-20, with an allowable range of 4-40 without the need of special consideration. The percent floor coverage is simply the inverse of the At/Ad, so an At/Ad of 4.0 will equate with a floor coverage of 25%, while an At/Ad of 40 equates to 2.5% floor coverage. High density grids are generally considered zones with an At/Ad of less than ~5 while low density grids are zones with an At/Ad of greater than ~15. Any layout with an At/Ad greater than 40 is not considered ‘full floor coverage’, and therefore would be prone to solids separation and tank stratification without supplemental mixing.
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