Over the past decade, there has been much documented evidence that concludes filter backwash systems that incorporate air scour capabilities provide superior cleaning efficiency and reduced water consumption over water-only or surface-wash assisted systems. Gradually, these air/water nozzle underdrain systems have become the industry standard.
In the early 1970s, Infilco Degremont, Inc. (IDI) began using air scour nozzle underdrains in a pre-cast concrete slab system. This technology was replaced in the late 80s by a lower-cost alternative referred to as the Monoflor® Underdrain system. This underdrain uses a single, poured-in-place concrete floor, in lieu of multiple components, to form a filter floor. It was recognized that this single monolithic style floor could provide superior strength for high flow backwash rates, while eliminating the need for sealants or grout. The development of the Monoflor® represented a true monolithic underdrain system, with the reinforced concrete false floor and support posts cast in one common pouring.
In order to remain at the forefront of the underdrain market and to keep abreast of the latest regulations and trends in the filtration industry, IDI has developed and only recently begun marketing its latest version of the monolithic underdrain, the Monoflor® HD Underdrain System. The Monoflor HD was developed to provide a new monolithic nozzle underdrain system that incorporates the strengths of its predecessor, yet provides superior performance.
The name “HD” stands for “High Density,” and relates to the number of nozzles per square foot of filter area. The new HD version of the Monoflor was designed to incorporate 4.8 nozzles/ft.2 vs. the industry standard 2.25 nozzles/ft.2 The result is an underdrain system that provides more even distribution of both backwash water and air.
As regulations are changing and common industry practice is leaning toward mixed tri-media or layered dual media filters, the need for a better (i.e. less pumping) nozzle has become evident. The Monoflor HD features the D-20 nozzle, which has a better distribution profile than its predecessor and also has a lower “basket” height resulting in less support material (i.e. torpedo sand) required.
Another added benefit of the new system is that less installation time is required due to a 25 percent larger form and subsequently less pipe support columns to level and shim. This increase in form size and support column spacing also allows the capability to retrofit existing filters that have unusual gullets or channels. In addition, the shipping, storage and handling costs associated with other systems are reduced since concrete and rebar is supplied on site by the contractor. The typical installation procedure is as follows:
The Monoflor HD underdrain is cast at the site using specially designed 24-inch x 30-inch high-impact polystyrene forms, with 6-inch Schedule 40 PVC pipes serving as support columns. The forms contain preformed cylinders spaced on 6-inch and 5-inch centers, designed to accept a threaded nozzle adapter that snap-locks into place without the need for gluing or adhesives. The nozzle adapters, which include a disposable cap to prevent concrete from fouling the cylinder, become a permanent part of the finished filter floor.
Complete structural integrity of the floor is made possible by stainless steel threaded rods which are inserted inside each pipe support column and chemically anchored into the base concrete floor. Reinforcing rods are doweled into the sidewalls of the filter and tied to rebar which is spaced according to the design load of the underdrain system.
After the concrete is poured and the disposable caps removed, the filter nozzles are simply screwed into the adapters. The standard design typically requires a 2-inch ledge (minimum) to support the slab forms around the periphery of the filter cell. This is formed by the contractor when the cell walls are poured. For retrofit designs or conventional filters without wall ledges, an alternate support system consisting of 2-inch x 2-inch stainless steel angles anchored to the walls is available.
The new IDI Monoflor HD Underdrain System represents the latest in monolithic concrete filter underdrain technology, and perhaps most importantly, reflects a 5 percent to 10 percent cost savings over its predecessors. This system is appropriate for use in potable water treatment and advanced wastewater treatment, and is available in six standard designs (net loadings): 600 psf, 1200 psf, 1400 psf, 1600 psf, 1900 psf, & 2100 psf.
