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Surface vs.
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Subsurface Sand Filtration
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The big goal of this research is to understand the difference between surface and depth filtration and the parameters that determine which regime is operative. We suspect that subsurface injection of the water to be filtered shifts the regime to depth filtration. The head loss and effluent turbidity were measured and compared between a control filter, where water is added above the filter in a conventional downflow design, and a subsurface injection filter in which water is injected into the middle of sand bed through a smaller tube modelling a slotted pipe in the Stacked Rapid Sand Filter. |
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Spring 2013
Depth filtration likely occurs when the fluid forces on the flocs that bridge across a pore in the filter bed exceed the strength of the flocs. Thus the dimensionless parameter that determines whether depth or surface filtration occurs must include both floc strength and pressure drop through a thin layer of flocs. Pressure drop through a thin layer of flocs is influenced by the porosity of the flocs which is a function of their fractal dimension. Small flocs are less porous than large flocs and thus small flocs are less likely to produce surface filtration.
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Experimental Apparatus Schematic
Experimental Apparatus in Real Life
Example of Results for Effluent Turbidity
Example of Results for Head Loss
Example of the Difference in Particle Capture Location
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