Scouring for a solution

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 clogged with solids, resulting in a number of unwanted consequences:

• An increased risk of turbidity and cryptosporidium breakthrough
• 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 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 documented in American Water Works Association

sponsored research.

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. This means the air scour process takes place before the wash water. Many under-drain systems are not designed for simultaneous air/water backwash.

Standing waves

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. This leads to an uneven release of air into the media bed. It 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. This forces 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 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.

In the US, a solution to the problem of air scour installation is commonplace. The Roberts Filter Group – its products are available in the UK through GB Municipal – has developed the Aries-managed air system that 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.

The conversion process, Roberts says, can be performed quickly and with little disruption to process.

Roberts says: “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 both 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 wide range of options exist, including conventional header and lateral with nozzles, block lateral under-drain systems, and plenum chambers.

Of these, block lateral under-drain systems offer a 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.

A traditional nozzle floor may, typically, have 40 nozzles in a square metre, whereas, in the US, the Roberts Trilateral and Infinity employ an effective density of 230-plus orifices per square metre.

The block under-drain system 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 – the Infinity under-drain is delivered on site ready for installation with cut outs and end stops fitted.

Roberts says: “These one-piece extrusions reduce installation time, and avoid the wastage that often occurs when blocks are damaged during installation. Also, jointless construction eliminates failure points – it has superior burst strength, and it provides superior distribution characteristics.

“A new version of the Infinity has been developed as an air/water under-drain 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.

Media retention

“This 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. The US one-piece extrusions are no exception. They employ a porous plate, manufactured in a sintering process, that fuses HDPE beads, providing a media-retention system.

“These media-retention plates attach to the Infinity under-drain using a moulded rail system, allowing the plate to be removed or changed without recourse to screw or other fixing systems.

“We believe there is potential within the UK wastewater and potable water sectors to use the US technologies 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.”

Peter Noyce is a filtration specialist at GB Municipal.

T: 01245 240880