Filtration specialist asks: How clean are your filters?

The effective removal of solids collected in water filtration plants is key to the success of a filtration process; comparing his experience with both UK and U.S. solutions Peter Noyce, filtration specialist at GB Municipal, explains why...

Aug 18th, 2008

By Peter Noyce, GB Municipal

The effective removal of solids collected in water filtration plants is key to the success of a filtration process; comparing his experience with both UK and U.S. solutions Peter Noyce, filtration specialist at GB Municipal, explains why.

Background
Overloaded and dirty filters have a knock on effect in a filtration system. A dirty filter will pass less water and thereby impose a greater load on other filters. Ultimately, the filters will become mud-balled and clogged with solids resulting in a number of unwanted consequences:
• an increased risk of turbidity and cryptosporidium break-through
• channelling through the media bed
• decreased capacity
• shorter filter run times
• less water suitable for distribution

Air Scour Systems
In the UK Water Industry the benefits of air scouring filters during backwash are well known and widely utilised. In recent years the trend has been for the air scour to be applied with the backwash water to provide a combined air/water wash, or 'collapsed pulse'. This process is well documented in AWWA (American Water Works Association) sponsored research¹.

The problem in the UK has been that whilst we have existing air scour systems, the air is introduced in the same underdrain system as the water, with the air scour process taking place before the wash water. Many underdrain systems are simply not designed for simultaneous air/water backwash. Attempts to introduce air and water together in these underdrain systems are likely to lead to failure because the presence of air in the water can create standing waves in the laterals leading to an uneven release of air into the media bed. This can cause the support gravels to mix and in some cases will cause the bed to turn over and allow filter media back into the underdrain system. This can have disastrous effects because nozzles then become blocked creating areas of unwashed media, forcing more air out of less nozzles. In some cases underdrain failure can occur, rendering the filter unusable.

In the past, the preferred solutions have been either to replace the existing underdrain system in its entirety, or to install a separate air lateral system above the existing filter floor. Both are viable methods, but each represents a very expensive solution because, in order to undertake the work, the filter has to be taken out of service and the media has to be removed and discarded.

U.S. Experience
In the United States, an innovative solution to the problem of air scour installation is now commonplace; the Roberts Filter Group has developed the Aries® managed air system which is assembled on the surface of the media. A backwash is then instigated which causes the Aries® modules to settle through the fluidised media and rest on the underdrain system or support gravel before connection to the air pipework. As a result, the conversion process can be performed quickly and with very little disruption to the process.

The Roberts Filter Group is the largest US held manufacturer of municipal and industrial filtration systems in the US, and its products are now available in the UK through Chelmsford based GB Municipal.

Experience in the United States has shown that even badly mud-balled filters can be re-habilitated by this method, returning the filter media to a re-usable condition in a very short period of time.

The system has been shown to provide reductions of up to 60% in the use of wash water required to clean the filter media bed, thereby enabling a greater quantity of water to pass through the plant into distribution, delivering savings in both water consumption and time, and therefore money.

For situations in which new-build is being considered or where circumstances dictate that new underdrain systems must be installed, careful consideration must be given to the design of the new installation; a wide range of options exist, including conventional header and lateral with nozzles, block lateral underdrain systems, plenum chambers (usually new build only) etc.

New-build and refurbishment options
Of these, block lateral underdrain systems offer a very efficient and cost effective solution because they are easy to install and provide excellent distribution of backwash air and water. This is achieved as a result of the effective orifice density being much greater than conventional nozzle systems that create a much more efficient media cleaning operation. A traditional nozzle floor may typically have 40 nozzles per m2 whereas in the United States, the Roberts Trilateral® and Infinity® employ an effective density of over 230 orifices per m².

The block underdrain system has been available in the UK for some time and traditionally consists of 1200mm blocks that must be joined together on site to provide a continuous lateral. In contrast, many U.S. systems are installed as a one-piece extrusion in UPVC. For example, the Infinity® underdrain is delivered on site ready for installation with all cut outs and end stops fitted.

These one-piece extrusions dramatically reduce installation time and avoid the wastage that often occurs when blocks are damaged during installation. Furthermore, jointless construction eliminates failure points, it has a superior burst strength and it provides superior distribution characteristics.

Latest Innovation
As a result of market demand in the United States, a new version of the Infinity has been developed as an air/water underdrain in order to provide excellent distribution characteristics in a low profile underdrain. The overall height of the underdrain is 180mm, which is 160mm lower than the standard profile, offering either additional media depth, or additional freeboard above the media. This is highly significant because it could make the difference between being able to fit a specific media profile into an existing structure, or having to incur significant cost and time penalties in raising the structural height.

Media Retention Options
Most, if not all, block-based underdrain systems are available with media retention plates of one type or another and the U.S. one-piece extrusions are no exception; employing a porous plate manufactured in a sintering process that fuses HDPE beads, providing a unique media retention system. These media retention plates attach to the Infinity® underdrain using a moulded rail system that allows the plate to be removed or changed without recourse to screw or other fixing systems.

Summary
At GB Municipal, we believe that there is tremendous potential within the UK wastewater and potable water sectors to utilise the U.S. technologies that are now available domestically. These opportunities include underdrain systems for both new-build and refurbishment applications, and managed air scour systems which can be installed in rapid gravity filters without the need for media removal.

Bibliography
1. Amirtharajah, Appiah, et al. Optimum Backwash of Dual Media Filters and GAC Filter-Adsorbers with Air Scour. AWWA Research Foundation and American Water Works Association, 1991.

Peter Noyce is a filtration specialist with GB Municipal (Essex, UK). He can be contacted at +44 (0) 1245 240880 or via e-mail at info@gb-filters.com. For more information: www.gb-filters.com


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