Developments in Filter Floor Design for Rapid Gravity
Sand Filters

While the proper filter media is important to the performance of rapid gravity filters, evidence suggests that filtration and backwashing performance is highly dependent upon the design of the filter underdrain and the support for the filter medium. The recent development of our low profile (LP Block™) dual parallel lateral underdrain has been proven to provide an even distribution of backwash water and air. In the dual parallel lateral underdrain, sintered plastic plates (media retention caps) are used to replace gravel support layers in the filters. These plates come in three different configurations of  different porosities based on specific applications. .

The Filter Underdrain: Functions and Types
The main components of a potable water rapid gravity sand filter are the filter media, the gravel support layers and the filter underdrain. The underdrain serves to support the filter medium and gravel; to collect filtered water evenly from the bottom of the filter; and to distribute air and water evenly across the bottom of the filter during backwashing. Key to these functions is  the evenness of filtration and the distribution of backwash air and water.
In the United Kingdom, the primary types of underdrains used in potable water are the header and lateral type, the plenum floor or nozzle type and the two-pass dual lateral block underdrain, a relatively new method that is gaining recognition.

Header and Lateral Type
The simplest type of underdrain is the header and lateral type (Figure 1). In this type of underdrain the backwash water enters the filter bottom through a pipe or pressurized flume called a header. Pipes called laterals are connected at right angles to the header and are buried in the filter gravel layers. The laterals distribute the backwash water through a series of orifices.

The biggest drawback of the header and lateral underdrain is the difficulty in obtaining even distribution of the backwash water. The high velocity of the backwash water in the header causes the header’s static pressure to be highest at the end. Because of this, the laterals at the end of the header receive most of the flow. This can be overcome by redistributing the head loss from the header to the orifices in the laterals. For this to be effective, the head loss across the orifices must be in the order of 2 to 3m – a rate that increases pumping costs. Another problem with this type of underdrain is that there are no orifices in the header. With no backwash  flow,  this area is not properly cleaned and can potentially lead to dead spots within the filter bed.

Plenum Floor Type
Plenum floor or nozzle type underdrains consist of a false floor penetrated by nozzles or strainers (Figure 2). Nozzle systems have large orifices and require the use of gravel to keep the media out of the nozzle. Strainer systems use fine openings to retain the filter media. Like the header and lateral designs, plenum floor designs can suffer from hydraulic distribution problems. Unless the plenum is very large, the velocity of the incoming water is such that the flow is greatest at the inlet and near the sides (Figure 3). In addition, the spacing of the nozzles is often quite wide (up to 200mm), comprising 40 to 60 per m3 of filter floor space, and dead spots may occur during backwashing.

There are other challenges involved in using the plenum floor type underdrain. The false bottom of the underdrain is inherently susceptible to structural failure. During a backwash, considerable upward pressure is exerted on the underside of the plenum. This is exacerbated if the strainers become blocked with suspended solids or bits of sand and gravel, which may be in the backwash water. Repeated backwash cycles can cause rupture of the floor due to repeated flexing. Cleaning the strainers can only be done by removing the filter medium. Personnel access to the plenum area is a health and safety risk, complicating routine maintenance. Finally, the plenum floor underdrain’s nozzles often become damaged during system installation.

Two–Pass, Dual Lateral Design
The dual lateral underdrain was first developed in the United States in the 1970s, and the current design was developed in the late 1990s. The LP block™ is a single block measuring 412mm wide by 225mm high and is designed to overcome the problems of the more commonly used systems. The problems of even distribution are solved by using two parallel laterals (Figure 4). The central lateral, the feeder, 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 pressurized flume and orifices distribute it into the compensating 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 minimizing head loss.

The LP block’s lower profile allows a 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 LP block requires less installation time and less grout between rows compared to other blocks. The dual lateral underdrain LP block design can be adapted to retrofitting existing filters as well as new build construction.

Practical application of this design uses preformed blocks made from high-density polyethylene (Figure 5), 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 6). 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.

The LP Block™ also features the patented grout-grip™, which anchors this block more securely into the grouted filter floor and requires a significantly higher pull-out (lbs or N) than conventional dual-parallel underdrain blocks.

The LP Block™ dual lateral underdrain provides users with several benefits over more traditional underdrain designs, including ease of installation, evenness of distribution and, more compact filter design.

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