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Control & Operation of a Moving Bed Sand Filter

Moving bed sand filters are primarily used in wastewater treatment, although a form of this type of filtration can also be found in the hot gas clean-up of fine particles. They are sometimes also referred to as moving bed granular filters. Moving bed filters in wastewater treatment allow a continuous supply of filtered water without requiring backwash cleaning cycles. Moving bed filters typically either have a downflow or upflow configuration.

  1. Downflow Configuration

    • In the typical downflow moving bed filter configuration, influent enters at the top of the filter bed and flows through layers of increasingly fine sand. The filtered water collects in a central chamber then exits the filter. An air-lift pump draws solids held in the filter bed down into the sand and provides a scrubbing action that separates sands and solids, which are discharged into the washbox, a baffled chamber. The filter washbox facilitates gravity separation of clean sand from waste solids. From there, sand is returned to the top of the filter bed and the waste solids are piped out or removed for disposal.

    Upflow Configuration

    • In an upflow moving bed filter, influent enters into the bottom of the filter and flows upward in a series of tubes. The wastewater flows upward through a downward-moving sand bed, which removes solids while clean water overflows to exit the filter. While the clean water overflows the filter, the sand bed and collected solids are drawn downward by an air-lift pipe located in the center of the filter. A small amount of compressed air is released in the bottom of the airlift, causing the sand, dirt and waste to surge up through the pipe at a fast speed, which scours the impurities from the sand. At the top of the air-lift the slurry spills into a reject compartment, whereupon a gravity washer separator allows faster-settling sand but not the dirty liquids to penetrate and return to the sand bed.

    Control Options

    • One of the ways to optimize filter performance is to modify the air-lift, or the amount or rate of dirty sand and water forced upwards through the central pipeline or similar structure. This modification controls the sand circulation speed. The filter media size can also be adjusted to best suit individual requirements.

    Related Systems

    • Like moving bed sand filters, pulsed bed filters are automatic backwashing filters. Pulsed bed filters operate quasi-continuously and media besides sand can be selected for filtering action. This system type incorporates low pressure air that creates a rolling motion above the bed. This motion holds large particles in suspension and when clogged, pulses force any settled solids back above the bed surface.

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