VIDEO NEWS: Backwash Air and Water Distribution System Boosts Filtration Efficacy at Desalination Plants
Posted: Wednesday 17th June 2009
The dual lateral filter underdrain system, a key development in pre-treatment filtration at desalination plants since its introduction in the 1970s, has gone through significant enhancements over the past 10 years. Improvements to the design of these systems have provided significant cost savings and improved operational efficiency.
New desalination plants in Europe, South America and Africa showcase one of the newest underdrain system components, the TETRA® FlumeFlow™ from Severn Trent Services. The backwash air and water distribution system, employed with the TETRA® LP (Low Profile) Block filter floor system, offers capital cost savings and improved efficiency of the filtration process.
Role of rapid gravity filter floors in desalination process
The design and performance of pre-treatment filters in removing solids is a critically important aspect affecting the performance of today’s desalination plants. While a number of different rapid gravity filter floor technologies have been employed in potable water treatment, the dual parallel lateral filter floor design offers significant advantages. The TETRA LP Block filter floor system, for example, features robust plastic components, which is especially important in desalination plants, where high levels of chlorides can cause extreme corrosion to metal components. In addition, the nozzle-less design ensures there are no blockages that could potentially affect the performance of the filter.
Developed in the late 1990s, the LP Block design features a single block measuring 412 mm wide by 225 mm high that is split into two channels. The block’s lower profile allows for 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 block requires less installation time and less grout between rows compared to other blocks, too, and can be adapted to retrofitting applications as well as new build.
The main components of a 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. Keys to these functions are the evenness of filtration and of the distribution of backwash air and water. The evenness of backwash water distribution is particularly important, as filter efficiency depends upon the effectiveness of the backwash cycle.
The problem of even distribution common with other filter underdrain systems is solved by the TETRA LP Block’s 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. Variations in flow rate are 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.
In the two parallel lateral filter floor design, preformed blocks made from high-density polyethylene which incorporate the feeder and compensating laterals within the blocks are used. The blocks interlock and are laid end to end in rows so that the laterals are aligned continuously across the entire filter. 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 locks the blocks together to form a flat, level floor.
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.
Recently, media retention plates have been developed which are able to perform the functions of the gravel layers in a much lesser depth. 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.
New system design yields capital cost savings, greater operational effectiveness
The TETRA FlumeFlow is a backwash air and water distribution system that works in tandem with the TETRA LP Block design to offer savings in capital costs by negating the need for a separate air header for the distribution of backwash air and water from a central culvert. Additional benefits of the TETRA FlumeFlow system include increased flexibility when positioning laterals in the filter bed; prevention of air and water leakage; grout grip technology for additional stability; and even distribution between laterals, ensuring efficient and effective cleaning of dirty media. The TETRA FlumeFlow design accommodates the same flow regimes as the traditional LP Block design: filtration, air scour, water only backwash and combined air scour and water backwash.
The FlumeFlow comprises a FlumeFlow interface plate; TETRA LP Block system; sealant; air L-pipe; and grout. The interface plate is designed to fit into the LP Block and is sealed to provide an air/water seal and to prevent loss through the end of the lateral. It is fitted to the end of each LP Block lateral.
The FlumeFlow is installed onto the lateral prior to fitting it into the cell. Each row of laterals is positioned to coincide with apertures that pass through into the enclosed chamber. The lateral is pushed into a grout bed and is fixed in place using the unique grout grip technology employed by TETRA LP Blocks. The new FlumeFlow interface plate has similar grout grips to the LP Block to firmly anchor the system in place. The smooth edge of the FlumeFlow interface plate is then sealed against the wall to create an air and water seal, which does not affect the integrity of the grout and hence maintains the high adhesion to the floor and subsequent strength of the floor.
The FlumeFlow interface plate is then grouted into position to ensure that it does not move and maintains the seal. The grout is angled above the plate as this helps ensure that the whole filter is used with no dead space.
The FlumeFlow design removes the need for a separate air header and utilises Air L-Pipes to control air to the upper chamber of the interface plate from the enclosed channel.
A baffle within the FlumeFlow interface plate creates two chambers within it; an air chamber and a water chamber. It provides sufficient stability and velocity to ensure even distribution into each LP Block lateral for effective backwashing.
During filtration, water collected in the LP Block laterals passes through the slot and water chamber to the enclosed channel where the output from each lateral is collected, combined and taken away.
If an air scour is required at the start of the backwash cycle, air is pumped into the top of the enclosed chamber, forcing the water in it through the slots until the top of the L-Pipes is exposed to the air. After purging the water out of these air pipes, the air passes through the L-Pipes and into the different laterals, giving an even air scour.
For a water-only backwash, the air is stopped (if applied), and the water is started, which raises the level in the enclosed channel again, forcing water through the slots in the FlumeFlow interface plate and into the laterals.
With a combined air and water backwash, the air is not stopped, and the water is started. The level again rises until the pressure between the air and the water balances. Air continues through the L-Pipes and into the top chamber in the FlumeFlow interface plate while water passes through the slots in the lower chamber. This balanced pressure is the key to providing a good distribution between the laterals, and the FlumeFlow interface plate creates this even pressure.
Conclusion
As the worldwide need for effective desalination solutions expands, new technologies will be sought to cost effectively remove solids in the pre-filtration stage. The TETRA FlumeFlow backwash air and water distribution system has proven to produce capital cost savings and to ensure operational efficiency through more effective cleaning of dirty media.
