BOURNEMOUTH & West Hampshire Water provides clean drinking water to around half a million people in an area from Poole to Beaulieu and from Bournemouth to Salisbury – an area of more than 1,000km2. The company “has an excellent record for water quality”, and provides on average about 150Ml of drinking water every day, through nearly 3,000km of water mains.

With a new pumping station to be installed at its Knapp Mill site in Christchurch the company contacted Centristic, of Newton Abbott, to facilitate the re-siting of the existing Linatex sand filter bed treatment plant.

A comprehensive review revealed that elements of the present Linatex Sand Plant were too small for the feed requirements. Linatex has a long history in sand plant design and has enjoyed a long-standing relationship – more than 25 years – with Bournemouth & West Hampshire Water. As a result, the company was tasked to supply a 25tph sand processing system, with Centristic acting as main contractor on the plant build.

Skimmed sand

Maintenance of the sand filter beds at Knapp Mill treatment works is performed on a continuous cycle with 30mm being skimmed off the beds about every four months. To maintain effective operation the filter beds must retain a minimum of 300mm of sand above the carbon, therefore after skimming the Knapp Mill team ensures that 100mm of clean sand is replaced on the filter bed.

This continual process is effectively a recycling operation as new sand is only introduced about every ten years.

As it is essential that the sand quality is maintained in this operation the skimmed sand requires scrubbing to release any impurities – silts, clays, weeds – before the Linatex sand plant effectively washes and dewaters the material.

The material, being essentially wet filter bed sand (largest particle size 5mm) is fed by wheel loader into a hopper which feeds on to a 15m-long inclined conveyor that delivers the material into a 1.2m diameter x 3 m long trommel (sand scrubber barrel) which is lined with Linatex rubber and lifters.

Sand and water discharging from this barrel is then flumed down a Linatex lined chute to the centre of a 2.1m diameter Linatex feed regulating sump with any overflow from the sump gravitating to waste.

From the base of the sump a 100/100mm bore Linatex lined centrifugal pump draws the slurry and feeds via 100mm bore pipework to a Linatex lined HK 100 extended sand separator with the overflow from the separator returning to the vicinity of the feed sump for balancing the system or disposal to waste. The Linatex HK 100 separator is constructed with mild steel and is Linatex lined in all wear areas.

Fitted with extended feed box, discharge regulator and pressure flange assembly the separator incorporates two pressure gauges to measure inlet pressure and vacuum that are key to performance of the unit to maintain operational efficiency and maximise sand recovery.

The sand separator, which is mounted in a supporting tower, incorporates three discharge chutes providing approximately 2,000 tonnes stockpile capacity.

High standard

With the need to retain as much fine material as possible (95% above 50 micron) the Linatex extended separator ensures that the quality is achieved, proving itself to be a cost effective method of providing clean sand to a very high standard.

Since supplying a system for the Bournemouth & West Hampshire Water Alderney water treatment works some 19 years ago, Linatex has worked closely with the client consistently providing the technology and expertise in terms of sand processing plant.

Tony Primmer, Bournemouth & West Hampshire Water’s production manager, comments: “It is pleasing to continue our long-term relationship with Linatex. They have always provided us with first-class sand washing plants and we look forward to working with Linatex for many years to come.”

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