The bacteria and air present in sand filters can help break down solid waste that enters the septic tank.
A filter bed is a sand-like material 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 descend to drains located at the bottom.
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 area of the filter medium, which is the total area of all the particles or granules used for the bed. The media is carefully designed to provide large surface areas without becoming too small, which can cause clogging.
Residence time, or how long the liquid stays in the bed, can be important for filtration systems to remove 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 from running to one side rather than all over the area. The bed must be packed loosely to allow free movement of the liquid; a compacted bed has less surface area available and can clog more quickly.
Filter beds can be operated continuously, as can be done in an industrial process, or they can be operated in batch or intermittent applications. An example of batch filtration is a filter bed used to process domestic wastewater. As the water leaves the house, it first enters a septic tank where the solids are separated. The liquid that comes out of this tank usually 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, the pump activates and empties the contents onto a bed of sand. This bed is usually built on top of the ground and contains layers of sand and gravel, with drainage pipes located at the bottom and sprinkler or distribution pipes located at the top. The pumped water passes through the distribution pipe, where it is sprayed or dripped onto the sand bed.
Sand filters are useful for wastewater because not only do they separate more solids, but the bacteria and air present in the bed can help break down the 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 soak into the ground. Sand filters require occasional replacement of the sand as it fills with solids, but these systems can provide years of reliable service.
Another type of bed filter 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 other 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 large surface area is required to remove contaminants or chemicals from the liquid stream. They are not effective for filtering solids, because the effect of agitation would prevent the filter from trapping solid material.