Due to widespread global water shortages, population growth, urban and suburban sprawl, and more, the world is struggling to maintain access to a pure, potable supply of water for both municipal and industrial applications. Technological advancements within the water treatment market have transformed desalination into a competitive purification technology for the production of potable water from seawater.
A critically important aspect affecting the performance of today’s desalination plants is the design of pre-treatment filters in removing solids. While a number of different rapid gravity filter floor technologies have been employed in the pre-treatment of potable water, the dual parallel lateral filter floor design offers significant advantages. And now, a new underdrain system component is providing impressive operational and cost efficiencies to the already proven design of a dual parallel lateral filter floor.
The main components of a rapid gravity sand filter are the filter media, the gravel support layers and the filter underdrain. The underdrain, or filter floor, 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. The evenness of backwash water distribution is a particularly important function, as filter efficiency depends upon the effectiveness of the backwash cycle.
The TETRA® LP (low profile) Block™ filter floor system from Severn Trent Services 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 10 m. The wider block requires less installation time and less grout between rows compared to other blocks.
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 lateral from a pipe or pressurized flume and is then distributed into the compensating lateral through its orifices. 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 minimizing headloss.
Injection molded blocks, made from high-density polyethylene, 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. The blocks over the flume are anchored down and the small space between the rows filled with grout, forming a flat, level floor.
Gravel support layers prevent the fine filter medium from blocking the underdrain
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 at 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. 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 system is installed prior to fitting the LP Block laterals into the filter cell. The lateral is pushed into a grout bed and is fixed in place using the TETRA LP Block grout grip technology. The 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, maintaining excellent adhesion to the floor ensuring its strength. The FlumeFlow interface plate is then grouted into position to maintain the seal.
The FlumeFlow design removes the need for a separate air header and utilizes 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 are 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 provided by the FlumeFlow interface plate
is the key to providing a good distribution between the laterals.
In recent installations at plants in South America and Africa, the TETRA FlumeFlow backwash air and water distribution system is responding to the worldwide demand for effective desalination solutions.
Ninety-four percent of the world’s water resides in the sea. So it’s easy to see why desalination will continue to provide water for domestic purposes, industrial processing, parks and agricultural irrigation, power plant applications and recharging of groundwater supplies on a global scale.
For more information, e-mail info@severntrentservices.com.