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Two pass lateral design
The two pass lateral underdrain has been designed to overcome the problems of the more commonly used systems. The problems of even distribution are solved by using two parallel laterals. The central (feeder) lateral has orifices along its length extending to a second parallel lateral called a compensating lateral. During backwash, water enters the central feeder lateral from a pipe or pressurised flume and is then distributed into the compensating lateral through orifices in this lateral. As in any single lateral, the highest flow rate will occur through the orifices furthest from the entry point. This variation in flow rate is evened out in the compensating lateral, enabling the system to provide an even distribution of backwash water across the entire filter bottom while minimising headloss.

Practical application of this design uses preformed blocks made from high-density polyethylene which incorporate the feeder and compensating laterals within the blocks. The blocks interlock and are laid end to end in rows so that the laterals are aligned continuously across the entire filter (Figure 5). The rows of blocks are placed adjacent to one another across the entire width of the filter, and the blocks over the flume are anchored down with special anchor rods and the small space between the rows filled with grout. This has the effect of locking the blocks together to form a flat, level floor.
Compared to traditional underdrain designs, the two pass lateral system provides more even distribution of backwash water and air; the filter compartment’s simple construction makes allows for a plan, flat floor; there is no plenum floor to fail and no nozzles or strainers to block; and installation is simple.

Use of air scour
Trapped solids and water treatment chemicals can adhere strongly to the filter media. The traditional method of cleaning a filter has been to pump clean water up from the bottom of the bed at a velocity sufficient to fluidise the media. The attached solids are sheared off and removed. The problem with this method is that the shearing action may not be sufficient to remove chemical floc from all of the media. If the media is not completely cleaned, the accumulations of solids can create mudballs, which can cause blockages in the filter and result in a deterioration in performance. One way to overcome this is to include an air scour or combined air and water wash in the backwash cycle.

In the two pass block design, air and water enter the feeder lateral. The orifices near the top of the feeder lateral discharge backwash air into the compensating lateral. Larger orifices located lower in the feeder lateral discharge backwash water into the compensation lateral. The mixture of air and water is then discharged from orifices at the top of the compensating lateral.

Gravel support layers
The gravel support layers prevent the fine filter medium from entering the underdrain and blocking it and help distribute the backwash water and air in the filter. Normally several layers of gravel are used. In the simplest form the largest size gravel (about 20 mm) is at the bottom. Above this are layers of finer gravel down to 2 mm at the top. Each layer is about 50 mm thick, and total gravel depth may be up to 300 mm. Where air scour is used in backwashing it is customary to use an “hourglass” configuration. This configuration is effective in restricting penetration of media into the underdrain and in preventing gravel mounding due to hydraulic shock during backwashing.

Recently, media retention plates have been developed which are able to perform the functions of the gravel layers in a much smaller depth. Media retention plates are porous moulded plates made from high-density polyethylene and fitted to the underdrain block using self-tapping screws. The edges of the plate are sealed with an approved polyurethane sealant, and the exposed edges are also sealed to prevent side leakage. The plate is about 30 mm thick and is used to replace gravel layers up to 300 mm thick. As a result either the filter can be shallower or a greater depth of media can be used. The headloss across a media retention plate is the same as or less than for the gravel it replaces.

Three types of retention plate are available. The single porosity plate is available from two manufacturers. It has a single porosity layer of 500 or 700 microns. A bi-plate consisting of two layers of 300 microns (top layer) and 500 microns and a tri plate with a layer of 300 microns sandwiched between two outer layers of 500 micron material are available from one manufacturer. The larger pore size is used to protect the finer material from blockage. The larger pore size is excellent for most filtration applications and is especially suited to applications with fine garnet and activated carbon.

Commercial Utilisation
The dual lateral underdrain was first developed in the United States in the 1970s. The original blocks consisted of a single primary lateral and two secondary laterals and measured 300 mm high by 250 mm wide.

The current design was developed in the late 1990s and is called a low profile block (LP block). This is a single block measuring 412 mm wide by 225 mm high. The block is split into two channels.The block’s lower profile allows shallower filter or greater media depth. In addition, the block’s primary lateral area is larger than other blocks, so distribution is excellent at longer lengths up to 10m. The wider block requires less installation time and less grout between rows compared to other blocks, too. The dual lateral underdrain block design can be adapted to retrofitting applications as well as new build.

Operating results
The extent of maldistribution in a filter is determined by measuring the pressure from tappings in the secondary chambers of the blocks along the length of a lateral. The percentage maldistribution is calculated from the formula: Maldistribution (%) = (1-√(Min Press/Max Press)) X 100. The percentage maldistribution should be less than 10%.

Conclusions
The dual parallel lateral underdrain is a modular block type underdrain that provides users with several benefits including its ease of installation, evenness of distribution, more compact filter design and improved operator safety.

www.stwater.co.uk

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