Filter beds use materials like sand to remove oils and solids from liquid streams. They can be used for industrial wastewater and residential septic systems. The filtration media can vary, and efficiency depends on surface area. Residence time and uniform distribution are important. Sand filter beds can break down solid waste and require occasional sand replacement. Fluidized bed filters are designed for chemical filtration.
A filter bed is a material such as sand that is used to remove certain materials, including oils or solids, from liquid streams. They are commonly used to clean industrial wastewater streams and can be installed as residential septic systems. Filter bed technology is most commonly a gravity fed system, where liquids enter at the top and travel down to drains located at the base.
Industrial filter bed technology can use a variety of materials as the filtration media, including sand, porous earth, or carbon balls. Filtration efficiency depends on several factors, including the surface area of the filter medium. Surface area is the total surface area of the filter media, which is the total area of all particles or beads used for the bed. The mounts are carefully designed to provide large surface areas without becoming too small, which could cause clogging.
Residence time, or how long the liquid remains in the bed, can be important for filter systems that remove chemicals from industrial process streams. The distribution of the liquid should be uniform across the top of the bed, and the bed itself should be level to prevent the liquid from flowing down one side rather than across the entire area. The bed should be packed loosely to allow free circulation of the liquid; a tightly packed bed has less available surface area and can become clogged more quickly.
Filter beds can operate continuously, as may occur in an industrial process, or they can operate in a discontinuous or intermittent application. An example of batch filtration is a filter bed used to treat domestic wastewater. When water drains from the house, it first enters a septic tank where the solids are separated. The liquid leaving this tank often goes to a pump tank, which is a smaller concrete tank with a submersible pump located at the bottom.
When a float switch indicates that the pump tank is filling up, the pump activates and empties the contents into a bed of sand. This bed is often built above ground and contains layers of sand and gravel, with waste pipes located at the bottom and sprinkler or distribution piping located at the top. The injected water passes through the distribution pipe, where it is sprinkled or dripped onto the sand bed.
Sand filter beds are useful for wastewater because they not only separate more solids, but the bacteria and air in the bed can help break down any solid waste that enters the septic tank. The drainage system collects the water at the bottom, and is normally sent to an underground distribution field where the water can enter the ground. Sand filters require occasional sand replacement as it fills with solids, but these systems can provide years of reliable service.
Another type of filter bed is a fluidized bed filter. These systems can be designed in two ways; liquid can be pumped from the bottom of the filter to lift and agitate the bed, and air or another gas can be bubbled into the bottom of the filter tank to agitate the media. Fluidized systems are primarily designed for chemical filtration, where a large surface area is required to extract contaminants or chemicals from the liquid stream. They are not effective for filtering solids, as the agitation effect would prevent the filter from trapping solid material.
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