By Rod Asbrey
Water treatment operators using the Monolithic (one-piece) filter floor system should expect long-term filter efficiency and minimal maintenance given that the improved design includes no sealing joints or grout. Available from Orthos Liquid Systems Inc. of Bluffton, S.C., the Monolithic system was adapted from the suspended floor underdrain system design.
Many modern designs of underdrains are in use, including pipework lateral systems, perforated tiles and blocks, and suspended false floors - all constructed in a wide range of materials. In the past, suspended floors were comprised of pre-cast concrete slabs, typically 8 square feet and 4 to 6 inches thick. These slabs are cast with a pattern of threaded sleeves/tubes. This allows the fitting of threaded filter nozzles when the installation is complete. The slabs are placed on a support structure and fixed and sealed to produce a suspended floor over a "plenum" chamber approximately 3 feet deep.
Two drawbacks are problematic for this technically sound system. Leveling the floor is awkward since typically the slabs weigh in excess of 400 lbs. so handling and manipulation requires an adequate lifting apparatus. In addition, the slabs require fixing and sealing/grouting, leaving a potential weak area in latter years.
Recent developments adapted the suspended floor concept to produce the Monolithic filter floor. The system begins with the production of a glass reinforced concrete (GRC) panel, which acts as an in-situ formwork onto which a layer of concrete will be poured, producing a Monolithic concrete slab. GRC panels are constructed in two configurations.
The flat GRC panels, 4 square feet by 5/8 inch thick, are supported on 6-inch PVC pipes cut to the height required for the filter floor. The tubes have steel starter bars within them, set into the base floor. The GRC panels are fitted with threaded riser tubes 6 inches tall. These are capped but will receive ultimately the threaded filter nozzles. Two layers of steel rebars are set above the GRC panels, placed between the riser tubes. Concrete is poured across the floor to complete the structure.
Corrugated GRC panels, typically 14 square feet by 5/8 inch thick, are supported on dwarf walls and can be constructed to any reasonable dimensions up to a 4 foot span. After the panels are laid, construction is similar to that of the flat panel version.
The two versions generally are designed to produce a similar end product, however the flat paneled design lends itself to retrofit, refurbishment and projects in which the head space employed may be critical. The corrugated version is suited to "new build" projects or where larger plenum chambers are required. There is, however, a substantial overlap when specifying the preferable design for a particular application.
The use of GRC for panel construction has several technical advantages over other materials:
- It is light while retaining a very high degree of rigidity. The deflection across the flat panel at maximum load is less than 3/32 inch. Deflection does not exceed 1/8 inch over a 3 foot span with corrugated panels.
- GRC panels can be trimmed or shaped on site with no loss of strength. This may be needed to allow for variation in civil dimensions, giving the floor a fit with no gaps to be filled or sealed.
- When the concrete is poured onto the GRC formwork, it will produce a bond that creates a one-piece structure that requires no sealing.
The density of filter nozzles per square foot across the filter floor can be engineered to specific requirements, although standard spacings are 6 inch or 8 inch centers. A filter nozzle density of approximately 2.25 per square foot is the norm in the U.S. market, however the international and U.S. markets are starting to move towards a higher density of nozzles.
A marked reduction in problems, such as "rat holing" and "mud balling," results from a higher density of filter nozzles. In addition, specialized support medias are not required as often. This higher density is particularly useful with applications in which finer medias are in direct contact with filter nozzles, and refurbishment requires deeper filtration media.
The use of activated carbon as a post filtration adsorption media to solve taste, odor and pesticide removal problems is a growing trend. The Monolithic filter floor with a higher density of filter nozzles is suited for this application.
The rigidity and accuracy of the formwork panels result in the finished filter floor being flat and strong. The total "out of level" condition of the finished floor should not exceed 3/16 inch across the whole floor. This should be attainable without the shimming of filter nozzles. To neglect this specification will affect detrimentally the air scour pattern, and is probably the single largest factor with poor filter performance and media loss.
The finished Monolithic filter floor's strength is due to the absence of any sealing joints or grout. For this reason, no long-term degradation of ultimate strength and efficiency is to be expected. The floors are designed to a standard net loading of 1,500 pounds per square foot (psf) for the flat paneled version and 2,000 psf for the corrugated version; however designs up to 2,500 psf are achievable with little modification.
About the Author: Rod Asbrey is president of Orthos Liquid Systems Inc., located in Bluffton, SC.
