Slashing costs in gully waste

The implementation of the Landfill Directive has made many industries reappraise their waste disposal activities.

The directive bans certain wastes from landfill, including liquids, certain clinical wastes, explosives, corrosives, highly flammable materials and tyres. Since July 2005, compliance with the stringent Waste Acceptance Criteria has been mandatory. Waste for disposal at landfill must meet strict criteria including limits on total organic carbon (TOC) content and heavy metal contamination.
The effects of the Landfill Directive, together with the annual increase in landfill tax, have been to significantly increase the cost of disposal of gully waste (currently at around £50 per tonne) and as a consequence increase the operating costs for companies providing gully emptying services. The effects have been felt not only in the UK but also by other countries in the EC.
Over the years, Huber Technology has been active in many industries developing waste treatment technologies in order to reduce the impact of waste on the environment and to facilitate recycling. With respect to gully waste disposal, Huber has been able to design gully waste treatment plants which reduce the operator's waste disposal costs and facilitate the recycling of the mineral content of the waste as a recycled sand aggregate.

Typical composition
Gully waste typically comprises small stones, sand, gravel, grit, cinders, silt, metal fragments, organic material (decaying plant matter), oil and litter. Figure 1 shows the typical composition by weight. However, the main constituent of gully waste (even from which free water has been allowed to drain) is water - 60% of gully waste is water and hence 60% of the landfill costs arise from the landfilling of water.

Huber's gully waste treatment process (RoSF5)
The philosophy behind the design of the RoSF5 plant has been to clean and dewater the coarse mineral material within the gully waste (which is typically more than 70% of the dry waste) to a level such that it can be used as a recycled aggregate. The result is a process that allows you to transform a large fraction of the waste into a product with a tangible value.
Gully waste and road sweepings are discharged into reception pits. The waste is then loaded into a feed screw, which transports the waste into a Rotamat Wash Drum (RoSF9). The wash drum separates coarse material (more than 10mm) typically stones, plastic, paper, metal fragments and wood from the fine mineral and organic material.

Oversize material
The oversize material is conveyed along the slowly rotating drum and discharged into a skip (this product has no commercial value and is destined for landfill, figure 3).
The agitation and washing of the waste material releases entrained sand and silt, which is discharged into a collection sump underneath the wash drum. The sand and silt is pumped from this sump into the Rotamat Grit Washer (RoSF4). The grit washer lies at the heart of the plant.
The innovative technology within this piece of kit allows us to wash out all the organic material form the sand producing a clean dewatered product with an organic content less than 3% and a dry solid content around 90% (figure 4).
Wastewater from the RoSF4 gravitates through a 1mm wedge wire screen which removes fine organic material and the screened wastewater is recycled as washwater for the wash drum. Surplus wastewater is discharged to a sludge sedimentation tank. Clarified water is passed through a dissolved air flotation plant to remove oil contaminants and the settled sludge is dewatered in a Rotamat Sludge Dewaterer (RoS3) providing a sludge cake of 50-60% dry solid for disposal (figure 5).

Sludge dewaterer (RoS3)
Apart from the water needed for polymer dilution (the polymer is used to flocculate the sludge prior to dewatering), all the water requirements of the plant are met by recycling treated wastewater.
Huber currently has five plants in operation: three in Switzerland, one in the UK and one in Finland. These plants are able to treat a mixture of gully waste, road sweepings and sewer grit.
Each plant treats up to 10,000 tonnes of waste per annum, reducing waste disposal costs and reducing the impact on the environment. The incorporation of this treatment technology has led to reduction in waste going to landfill of up to 75%.
Tel: 01249 765002
www.huber.co.uk

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