Gully Waste Treatment: Cost Saving Recycling
Implementation of the European Landfill Directive has mad many industries re-appraise their waste disposal activities. The Directive bans certain wastes from landfills including liquids, certain clinical wastes, explosives, corrosives, highly flammable materials and tyres. Since 2005 compliance with the stringent waste acceptance criteria has been mandatory. Waste for disposal at landfills must meet strict criteria including limits on total organic carbon (TOC) content and heavy metal contamination. In effect, solids containing more than 5 % organics can no longer be disposed on landfills, but must be incinerated.
An effect of the Landfill Directive together with annual increases of landfill fees has been that the costs of disposal of gully waste, currently at around € 100 per ton, are significantly increasing. The operating costs of municipalities or companies providing gully emptying services have consequently risen steeply. These effects have been felt in all European countries.
Over the years HUBER 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 that reduce the operators’ waste disposal costs by facilitating recycling of the mineral content of the waste as recycled sand aggregate.
Gully waste is comprised of small stones, sand, gravel, grit, silt, metal fragments, organic material (decaying plant matter), oil and litter. Figure 1 shows a typical composition of dried gully waste by weight. Even worse, the main constituent of gully waste is water, even if it has been allowed to drain. About 60 % of gully waste is water and hence 60 % of the disposal costs arise from its water content.
The philosophy behind the design of the HUBER gully waste treatment system RoSF 5 is to clean and dewater the coarse mineral material that is contained in the gully waste such that it can be reused in the construction industry as recycled aggregate. This fraction typically amounts to over 70 % of the dry weight. The result is a process which allows its operators 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 (See Fig. 2). The waste is then loaded into a feed screw which transports the waste into a ROTAMAT® wash drum RoSF 9. The wash drum separates coarse material (> 10 mm), typically stones, plastics, paper, metal fragments and wood from the fine mineral and organic material. The oversize material is conveyed along the slowly rotating drum and discharged into a skip. This product has no commercial value and is disposed as solid waste (See Fig. 3). Agitation and washing of the waste within the wash drum releases entrained sand and silt, which are discharged into a collection sump underneath the wash drum, wherefrom it is pumped into a ROTAMAT® grit washer RoSF 4.
The grit washer RoSF 4 is the core of the system. Its innovative technology allows us to wash out virtually all organic material from the sand and to produce a clean and dewatered product with an organic content < 3 % and a water content of around 10 % (See Fig. 4) Wash water from the RoSF 4 grit washer flows through a wedge wire screen with a spacing of 1 mm. The screen removes fine organic material and the screened wastewater is recycled and reused as wash water in the RoSF 9 wash drum. Surplus wastewater is discharged to a sludge sedimentation tank. Its supernatant is passed through a dissolved air flotation unit HUBER HDF where oil contaminants are removed. The settled sludge is dewatered in a screw press ROTAMAT® RoS 3 to a sludge cake with 50 to 60 % dry solids for disposal (See Fig. 5).
Apart from the potable water needed for polymer dilution (the polymer is used to flocculate the sludge prior to dewatering), all water required for system operation is treated and recycled wastewater.
HUBER has several gully waste treatment systems in operation. These plants are able to treat a mixture of gully waste, road sweepings and sewer grit. Each plant treats up to 10,000 tons of waste per year, significantly reducing waste disposal costs and, at the same time, reducing impacts on the environment. These systems reduce the amount of waste by up to 75 %.