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 move downward 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 beads. Filtration efficiency depends on several factors, including the surface area of the filter media. Surface area means the total surface of filter media, which is the total area of all the particles or beads used for the bed. Media are carefully designed to provide large surface areas without becoming too small in size, which could lead to plugging.
The residence time, or how long the liquid remains in the bed, can be important for filter systems removing chemicals from industrial process streams. Liquid distribution should be even across the top of the bed, and the bed itself should be level to prevent liquid running down one side instead of through the entire area. The bed should be loosely packed to allow free movement of liquid; a tightly packed bed has less surface area available and can clog more quickly.
Filter beds can be operated continuously, as may occur in an industrial process, or they can operate in a batch, or intermittent, application. An example of batch filtration is a filter bed used to process home wastewater. As water drains from the home, it first enters a septic tank where solids are separated out. The liquid leaving this tank often goes to a pump tank, which is a smaller concrete tank with a submerged pump located at the bottom.
When a floating switch indicates the pump tank is getting full, the pump activates and empties the contents into a sand bed. This bed is often built above ground, and contains layers of sand and gravel, with drain pipes located at the bottom and spray or distribution piping located at the top. The water pumped in goes through the distribution piping, where it is sprayed or dripped onto the sand bed.
Sand filter beds are useful for wastewater because they not only separate more solids, but bacteria and air present in the bed can help breakdown any solid waste that enters from the septic tank. The drainage system collects the water at the bottom, and it normally is sent to an underground distribution field where the water can enter the ground. Sand filters require occasional replacement of the sand as it fills with solids, but these systems can provide years of dependable service.
Another type of filter bed is a fluidized bed filter. These systems can be designed in two ways; liquid can be pumped up 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 designed primarily for chemical filtration, where a high surface area is needed to extract contaminants or chemicals from the liquid stream. They are not effective for filtering solids, because the agitation effect would prevent the filter from trapping solid material.