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- 6 June 2007 -

Rapid gravity sand filters: Developments in filter floor design

There are different designs for underdrains – important when removing suspended solids from potable water. David Shepherd of Severn Trent Services Ltd takes us through some recent developments.

The performance of potable water filters in suspended solids removal is crucially dependent upon the design of the filter underdrain and the support for the filter medium. There are a number of different rapid gravity filter floor technologies available at present, including the TETRA LP Block (Low Profile), which has increasing installations across Europe.

Total media cleaning is the key requirement for a rapid gravity filter to function effectively and efficiently under all loading conditions. Backwashing gravity filters is necessary to remove the solids collected by the media during the filtration run. An upward flow of water or a combination of air and water to fluidise and scour the media is required.

While the proper filter media is important to the performance of rapid gravity filters, evidence suggests that filtration and backwashing performance is highly dependent upon the design of the filter underdrain and the support for the filter medium. The recent development of a dual parallel lateral underdrain has been proven to provide an even distribution of backwash water and air. In the dual parallel lateral underdrain, sintered plastic plates are used to replace gravel support layers in the filters. Composite plates of different porosities also are utilised.

The filter underdrain: functions and types
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. Key to these functions are the evenness of filtration and of the distribution of backwash air and water. The evenness of distribution of the backwash water is of particular importance. The efficiency of the filter is dependent on the effectiveness of the backwash cycle.
In the United Kingdom, the primary types of underdrains used are the header and lateral type, the plenum floor or nozzle type, and the dual parallel lateral type block underdrain. The latter is a relatively new development on the market that is increasingly being used.

Header and lateral type
The simplest type of underdrain is the header and lateral type. In this type of underdrain the backwash water enters the filter bottom through a pipe or pressurised flume called a header. Pipes called laterals are connected at right angles to the header and are buried in the filter gravels. The laterals distribute the backwash water through a series of orifices.
The biggest drawback of the header and lateral underdrain is the difficulty in obtaining even distribution of the backwash water. The high velocity of the backwash water in the header causes the header’s static pressure to be highest at the end. Because of this, the laterals at the end of the header receive most of the flow. This can be overcome by redistributing the head loss from the header to the orifices in the laterals. For this to be effective, the headloss across the orifices must be in the order of 2 to 3 m – a rate that increases pumping costs. Another problem with this type of underdrain is that there are no orifices in the header. With no backwash flow, this area is not properly cleaned.

Plenum floor type
Plenum floor or nozzle type underdrains consist of a false floor penetrated by nozzles or strainers. Nozzle systems have large orifices and require the use of gravels to keep the media out of the nozzle. Strainer systems use fine openings to retain the filter media. Plenum floor designs can suffer from similar hydraulic distribution problems as header and lateral designs. The velocity of the incoming water is such that the flow is greatest at the inlet and near to the sides unless the plenum is very large. In addition, the spacing of the nozzles is often quite wide (up to 200mm) comprising 40 to 60 per m3 of filter floor space and dead spots may occur during backwashing.

There are other challenges involved in using the plenum floor type underdrain. The false bottom of the underdrain is inherently susceptible to structural failure. During a backwash, a considerable upward pressure is exerted on the underside of the plenum. This is exacerbated if the strainers become blocked with suspended solids or bits of sand and gravel which may be in the backwash water. Repeated backwash cycles can cause rupture of the floor due to repeated flexing. Cleaning the strainers can only be done by removing the filter medium. Maintenance is complicated by the fact that personnel access to the plenum area is a health and safety risk. Finally, the plenum floor underdrain’s nozzles often become damaged during system installation.

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