Underdrain system

Filtrate is collected in the underdrain system, which may be as simple as a network of perforated pipes covered by graded gravel or a complex structure with slotted nozzles or conduits that will retain the finest sand media while maintaining high flow rates. This latter design allows the use of both air and liquid for the backwashing and cleaning operations. 

The original perforated collector pipes in landfills were made of concrete like those used in highway underdrain systems. As landfills became higher, the strength of such pipes became inadequate. Today, perforated PVC pipes are commonly used, as are HDPE pipes. New regulations require that all materials be tested for chemical resistance as part of the permit-approval process. 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

In the petroleum industry, mixed-media filters are usually of the pressure type. Either vertical or horizontal pressure vessels are used. Fig. 5 provides a cross-section of a typical horizontal filter system using mixed media. Note that a pipe lateral-underdrain system is used with silica gravel, providing support for the filter bed. An all-water backwash is used typically the backwash flow rate b 15 to 20 gal/min/sq ft, depending upon the water temperature. A surface wash device b used to aid in the cleaning cycle. It provides increased agitation of the upper portion of the filter bed, improving backwash efficiency. 

Figure 15.20. Fixed bed ion exchange vessels and arrangements, (a) Typical design of a water softener, showing bed support, distributor, and effluent collector, (b) Vessel with radial-type distributors and collectors (Illinois Water Treatment Co.), (c) A double-dish underdrain system (Permutit Co.), (d) Some arrangements of vessels for cation and anion exchange.

One of the major difficulties encountered in municipal wastewater treatment plant application of GAC has been inadequate column underdrain system design and construction. This problem has been the primary factor in the shutdown... 

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