The granular media filters used for the purification of water typically comprise a graded bed of gravel and a filter bed of particulate media such as sand, anthracite (carbon) or other filtration materials well known in the art. Underdrain blocks are typically disposed beneath the filter bed and provide an outlet for filtered effluent exiting the bed. Thus, water to be filtered flows downwardly through the filter bed and suspended solids are removed and retained in the filter media. These solid contaminants tend to decrease the effectiveness of the filtration system by decreasing the rate of flow through the filter. The filter bed must therefore be cleaned periodically to remove the contaminants that have collected in the media by a process known as "backwashing." In a backwash operation, the flow of liquid through the underdrain is reversed and backwashing liquid is pumped from the bottom of the filter bed to the top, flowing upward through the media and carrying away the contaminants. In some instances, a backwash gas such as air is supplied in addition to or in conjunction with the backwash liquid. The addition of backwash gas aids in the loosening and removal of solids from the filter media.
Although both backwash liquids and gases can be effectively distributed by underdrains found in the prior art a number of problems remain. First, it is desirable to keep the height of the filtrate collection/backwash distribution as low as possible. This maximizes the volume available within a given filter for filter media. However, many older underdrains do not have the capability of providing a separate air distribution system to permit the controlled introduction of air or another gas during backwash. Although it is possible to remove the filter media and place an air distribution system atop existing underdrain blocks, such a retrofit will lose some filter media depth due to the volume occupied by the air distribution system. Secondly, most prior art designs adapted to distribute both backwash fluid and liquid cannot distribute both fluids evenly over the entire area of the filter. Inherent compromises in the design result in less than optimal performance. Finally, most existing underdrain designs utilize cast or extruded clay or plastic blocks as their primary component. Assembly of the blocks on the floor of the filter results in an interconnected set of passageways to collect and channel filtrate and distribute backwash fluids. Such an underdrain is not easily leveled, a requirement for proper operation.
Thus, it would be desirable to provide an underdrain system capable of collecting filtrate and distributing backwash liquid and backwash gas. It would be further desirable that such a system distribute liquids and fluids in separate streams to permit control over the parameters relating to each type of backwash fluid. It is therefore an object of the present invention to provide an improved underdrain system. It is a further object of the present invention to provide an underdrain that requires a minimum amount of height above the floor of the filter system. It is also an object of the present invention to provide an underdrain that is modular and easily constructed, as well as a system that is easy to assemble on site. Finally, it is another object of the present invention to provide an underdrain system that is simple to level and adjust so that the distribution of liquid and gases is even.