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Wessex Water has bought a materials-storage and handling system from Braby to expand the recycling options for the sewerage sludge produced at its Avonmouth processing plant.

The Braby system consists of two aluminium silos with pneumatic-transfer conveying systems, a common bagging plant to bag off material in case of over capacity, a support structure with stair access to entablature levels, all-site wiring and a local control panel.

If sewage sludge is dried to a concentration of 92-94 per cent, the higher bulk density and calorific content allow it to be used as a fuel.

Once the product is dried, the materials-handling and storage system discharges the product into tankers.

The end product is bought by weight, so Wessex Water wanted to utilise all available space in the silos and tankers by conveying a product of uniform size and weight.

Following drying, the material is presented in uneven noodle shapes from the belt dryer, causing problems when storing the product.

The uneven noodles can interlock with each other, resulting in a lower loading density and meaning all available storage space is not utilised.

Braby tackled this problem by installing a kibbler between the belt dryer and pneumatic conveying transfer system.

The noodles are now forced through a mesh to produce a uniform size.

This produces a shape that is easier to empty from the silos and increases the loading density from 300kg to 650kg for each metre cubed, reducing the storage-space needed.

The dried sewage sludge is transferred via two pneumatic conveying systems into two aluminium storage silos.

Each silo rests on load cells that measure the weight of product being released into the tankers.

While one silo discharges, the other can receive material from the dryer, allowing the system to operate continuously.

A rotary discharge device then drops the product through flexible bellows into the tanker below.

Because the dried sewage sludge is abrasive, the pipework had to be made in stainless steel at schedule-40 thickness.

Stainless steel extends the life of the pipework and protects against corrosion and wear.

The self-heating nature of the dried sewage-sludge presented explosion risks and further challenges to the Braby engineering team.

All elements had to comply with ATEX regulations and required earthing and lightning protection.

Michael Green, process-system sales manager at Braby, said: ‘All the conditions needed for an explosion (except for the actual ignition source) exist within the Wessex Water silos, so Braby designed-in explosion panels and calculated all other possible risks against ATEX regulations.

‘Whereas carbon steel used to be the material of choice, aluminium has very good heat-dispersing qualities, essential when storing a material that has the potential to self-heat’ added Green.

To prevent explosions, the temperature in the silos is constantly monitored at five points and is linked to an argonite (a mixture of 50 per cent argon and 50 per cent nitrogen) gas-inerting system.

Four explosion panels also help prevent explosions.

Calculated on the volume of the silo and the explosivity of the material, the panels are built in to the roof of the aluminium silos to reduce the pressure to 0.7barg.

All components selected and fitted are ATEX certified and compliant to zone 20 internal and zone 22 external.

Static produced from blowing the sewage sludge through the system can also pose a threat.

The pneumatic conveying system, pipes and silos are earthed to reduce the static risk.

Lightning protection is provided through a common ring conductor compliant with the latest British Safety Standards (BS EN 62305).

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