Following the old adage of “what goes up must come down,” manufacturers of water filtration underdrains are adding water recovery features to their underdrain blocks. The water recovery channels broaden the airflow range from 3-5 scfm to 1-5 scfm, improve air stability under all operating conditions and lower water maldistribution to less than 6 percent of total for laterals up to 40 feet end feed and 80 feet center feed.
Because the upward flow of air through an underdrain creates low pressure zones in the underdrain, some orifices become starved for flow and even reverse flow. It was found that allowing water to reenter the underdrain to equalize the low pressure areas dramatically improves the performance of the underdrain and provides superior results particularly with the dual lateral design.
Other features to look for when specifying underdrains are more forgiving installation tolerances and simpler installation procedures. For instance, older underdrains required a levelness of +/- 1/8 inch. Newer underdrains require a levelness of +/- _ inch, which is more forgiving in existing, older installations where the construction tolerances were not as rigid.
Retaining clips have been replaced with simple, integral snap tabs that readily snap together. These snap-lock joints, together with O-rings, provide a positive seal between blocks, reducing leaks and making distribution more efficient. Integral alignment tabs in the block help to ensure proper alignment.
Today’s underdrain provides closely spaced orifices of 2 inches on center as compared to other types of underdrains that have orifices on 8 inch centers. The close spacing prevents the formation of dead spaces and enhances scouring and cleaning action to prevent mudball formation. Some underdrain blocks are also manufactured of high-density polyethylene for strength and corrosion resistance. The surface of the block is sooth, reducing the potential for calcification.
Some manufacturers are offering a porous plate that is fixed to the top of the underdrain. Made of plastic beads sintered together, this type of plate eliminates the need for support gravel. By eliminating the support gravel, the overall depth of the filter can be reduced which could also allow for deeper media in existing filters. The porous plates are lightweight and easy to handle. Additionally, they allow for easy removal and change of media such as activated carbon.
So, when specifying today’s underdrain, look for a water recovery channel for high performance, new high-density polyethylene for strength and corrosion resistance and porous plats for increasing media depths.
