Waste for Waste Treatment
With 270,000 employees operating in 75 countries, Veolia Environnement is a world leader in environmental services. In the
In the past, most waste was simply collected and delivered to the nearest landfill. Today, given suitable transport, collection is the relatively easy task, while recycling, recovery and disposal has become a specialised art engaging the expertise of the full spectrum of scientific and engineering disciplines.
Where collection journeys are relatively short and there is a reliable content of sufficiently large volumes of residual waste, it is possible to convert the waste into electricity for the benefit of the local community at an energy-from-waste (EfW) plant.
During the process ferrous metals are screened out and sent for recycling. The remaining residual material is loaded into a hopper that continually feeds into the furnace and is heated to approximately 940 degrees Centigrade.
Hot gases produced in the combustion process pass through a water tubed boiler, where they are cooled to produce the steam that drives the turbines and then go through an extensive cleaning process. The turbines produce electricity for the National Grid. Veolia Environmental Services, Veolia Environment’s waste management arm, operates six such plants in the
Although EfW is a very clean process, Veolia Environmental Services still has to dispose of the residual fly ash. This ash, which contains small traces of lead, cadmium and other metals found in everyday household products, is not suitable for block-making and is too light and abrasive to go direct to landfill where it would be dispersed by the lightest breeze.
The solution is to utilise the fly ash in fulfilment of contracts to collect and dispose of waste from the metal finishing industries. In the form of chemical sludge this waste contains residual zinc, tin, nickel and other base metals used in plating and other processes.
At Aldridge, in the
The plant, designed, fabricated, installed and commissioned by bulk handling engineers Taylor Woolhouse of Rotherham, comprises storage silos and tanks, a mixing station, output conveyors and a load-out bunker.
The fly ash is brought by tankers to the plant where it is blown into one of six, 9 metre high, 4 metre diameter silos with a combined capacity of 678 cubic metres. The chemical sludge arrives separately in sealed tankers and is pumped into four stock feed tanks. These tanks are also 9 metres high and 4 metres in diameter with a maximum holding capacity of 113 cubic metres each.
Fabricated from mild steel, the silos and the tanks are fitted with volume indicators supplied by WAM Engineering.
With the composition of trace elements varying from delivery to delivery, the Aldridge process has the essential support of an on-site lab which tests each delivery for contaminants and viscosity before setting the blending recipe for each batch.
Fly ash is then screw fed to the three WAM plough share mixing stations while four high torque, centrifugal slurry pumps inject the sludge.
After mixing, the neutralised cake is dispensed onto a 17 metre horizontal troughed conveyor and from there onto a 12 metre 30 degree inclined troughed conveyor which off-loads into a 250 cubic metre storage bunker.
From there a CAT 938G loading shovel loads the cake into 20 tonne tippers for its final journey to the landfill site.
Owned and managed by Veolia Environmental Services, the landfill site is fully lined to prevent leaching and, as each cell is filled, it is capped with clay and top-soil and eventually safely returned to its future use.
Working closely with Veolia Environmental Services, Taylor Woolhouse has delivered a simple and efficient materials handling and processing system capable of producing up to 700 tpw of neutralised cake. Including site preparation, completed within six months, the Aldridge facility meets its design specification and is currently outputting cake at an average of rate of 200 tonnes per day.
Taylor Woolhouse Ltd, Carr House,
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