How clean are your filters?

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:

• Increased risk of turbidity and cryptosporidium
• Channelling through the media bed
• Decreased capacity
• Shorter filter run times
• Less water suitable for distribution

In the UK water industry, the benefits of air-scouring filters during backwash are well known and widely used. 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 American Water Works Association sponsored research (1). The problem in the UK has been that, while we have existing air scour systems, the air is introduced in the same under-drain system as the water, with the air scour process taking place before the wash water.

Many under-drain systems are simply not designed for simultaneous air/water backwash. Attempts to introduce air and water together in these under-drain 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 under-drain 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, under-drain failure can occur, rendering the filter unusable.

In the past, the preferred solutions have been either to replace the existing under-drain system in its entirety, or to install a separate air lateral system above the existing filter floor. Both are viable methods. But each represents an 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.

In the US, an innovative solution to the problem of air scour installation is now commonplace. The manufacturer 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 under-drain system or support gravel before connection to the air pipework. As a result, the conversion process can be performed quickly and with little disruption to the process.

Experience in the US has shown that even badly mud-balled filters can be rehabilitated by this method, returning the filter media to a reusable condition in a 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. This enables a greater quantity of water to pass through the plant into distribution, delivering savings in water consumption and time, and therefore money.

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

Of these, block lateral under-drain systems offer an 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 square metre, whereas, in the US, the Roberts Trilateral and Infinity employ an effective density of more than 230 orifices per square metre.

The block under-drain system has been available in the UK for some time and traditionally consists of 1,200mm blocks that must be joined together on site to provide a continuous lateral. In contrast, many US systems are installed as a one-piece extrusion in UPVC. For example, the Infinity under-drain 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.

As a result of market demand in the US, a new version of the Infinity has been developed as an air/water under-drain in order to provide excellent distribution characteristics in a low-profile under-drain. The overall height of the under-drain is 180mm, which is 160mm lower than the standard profile. This offers either additional media depth, or additional freeboard above the media.

This is 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.

Most, if not all, block-based under-drain systems are available with media retention plates of one type or another and the US one-piece extrusions are no exception. They employ a porous plate manufactured in a sintering process that fuses HDPE beads. This provides a unique media retention system. These media retention plates attach to the Infinity under-drain using a moulded rail system, which allows the plate to be removed or changed without recourse to screw or other fixing systems.

There is tremendous potential within the UK wastewater and potable water sectors to use the US technologies that are now available domestically. These opportunities include under-drain 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.

1) Amirtharajah, Appiah, et al. Optimum Backwash of Dual Media Filters and GAC

Filter-Adsorbers with Air Scour. AWWA, 1991