Sewerage sludge materials handling package

Paul Boughton

Wessex Water recycles the sewerage sludge produced at its Avonmouth, UK, processing plant for agricultural use.

Utilised at a low solids rate the material is suited for farming fertilizing processes, but Wessex Water is looking to broaden the range of available recycling opportunities. If sewage sludge is dried to a higher concentration of 92-94 per cent, it creates a product with higher bulk density and calorific content, the characteristics of which provide Wessex with a wider variety of recycling options, including energy recovery as a waste derived fuel.

To complete a new and extensive sewerage sludge drying operation at the Avonmouth plant, Wessex Water put out to tender for a materials storage and handling system. The contract was won by Braby, one of the UK’s largest manufacturers of bulk storage and handling systems.

Once the drying of the product was complete, Wessex Water needed the materials handling and storage system to discharge the product efficiently into waiting tankers. The end product is purchased by weight not volume, so Wessex Water wanted to utilise all available space in the silos and tankers through conveying a product of uniform size and weight. This would also save on transport costs for the customer.

Braby’s solution consisted 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.

Following completion of the drying process, 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, meaning all available storage space is not utilised due to a lower loading density.

Braby tackled this problem by installing a kibbler between the belt dryer and pneumatic conveying transfer system. The installation of the kibbler met Wessex Water’s objective to produce an end product with a higher loading density for their customers. The noodles are forced through a mesh to produce a uniform size. This produces a new shape which is easier to empty from the silos and increases the loading density from 300kg to 650kg per metre cubed, offering significant cost savings by reducing the size of storage silo required.

The dried sewage sludge is transferred via two pneumatic conveying systems into two aluminium storage silos. Each silo rests on load cells which measure the weight of product being released into the tankers. Whilst one silo discharges, the other is able to 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.

Conveying the material brought its own issues. The dried sewage sludge is abrasive, so the pipework had to be constructed in stainless steel at schedule 40 thickness. The use of stainless steel gives the pipework a longer life than if carbon steel was used, protecting against both corrosion and wear.

The self-heating nature of the dried sewage sludge presented explosion risks and further challenges to Braby’s engineering team. All elements had to comply with highly specific ATEX regulations and required earthing and lightning protection.

“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,” comments Michael Green, Process System Sales Manager at Braby.

“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. The marine grade aluminium not only helps to protect the silo from risk of explosion by preventing any concentrated ignition points on the silo surface but it is also highly resistant to coastal weathering and will not require re-painting in so many years to come.”

To help solve the explosion risks, the temperature within 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. Carbon dioxide was the previously preferred gas inerting choice but since 2001, industry regulations (BS5306-4) stipulate it is not acceptable to use CO2 within an explosive environment due to the risk of electrostatic discharge.

Four explosion panels are also key to preventing 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 0.7 bar g (Pred). 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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