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GP PLANNING LTD ENVIRONMENTAL STATEMENT
S007-03 ES / AB Appendix July 2012
APPENDIX 2: Grit Separation Process
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SITA UK GRIT SEPARATION PROCESS
HOW DOES IT WORK?
Loading the Street Sweepings
Waste arrives at the site either directly in street sweepers or
via transfer stations on Roro and six-wheel tipper vehicles. These
vehicles are directed to tip into a dedicated receipt bay which is
specially designed in order to control the high liquid content of
this waste type. The waste is then transferred using a loading
shovel into a reception hopper which is connected to a variable
speed screw feeder.
A conveyor transports the waste to the ‘AggMax’ where stage 1 of
the segregation takes place.
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Stage 1: Large Particle Separation
The first stage of the SITA UK grit separation process is at the
log washer or AggMax. The AggMax is a heavy-duty machine, fitted
with twin counter rotating shafts to break up any contaminated
material.
At this stage water is added to the waste to wash the aggregates
by abrasion and rotation.
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Organic material and any other light weight wastes are removed.
The organics are screened, dewatered and transported to the
organics bay for composting. Large particles of washed aggregate
(greater than 6mm) are dewatered and discharged into the aggregate
bunker. This aggregate can then be reused in construction projects
such as road building.
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Stage Two: Secondary Particle size separation – The
Hydro-cyclone
Remaining material (less than 6mm) is collected and pumped to a
variable concentration hydro-cyclone, which separates sand from
finer silt. The sand is discharged onto a dewatering screen and
placed into the sand bunker. The sand can then be used in a variety
of projects from the manufacture of concrete to pipe-bedding
material for utilities projects. The silt (less than 75 micron)
passes directly into the Aqua-Cycle thickener for the first phase
of water treatment.
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Stage Three: Fine particle separation – The Aqua Cycle
The silt material is dosed with a small volume of chemical
flocculants and coagulants which make the fine silt particles stick
together.
The material then enters into the AquaCycle tank where it
rapidly coagulates producing a thick sludge which settles to the
bottom of the tank.
Process water overflows the top of the tank and is sent to the
recycled water storage tank for re-use within the system.
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Stage Four: Filter Cake Production
The sludge is then pressed to a produce a ‘filter cake’. The
filter cake is a clay-type material, rich in nutrients that can be
blended with soils in land remediation projects.
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End Products
Depending on nature of waste received and contamination levels,
we anticipate between 75% and 95% recycling:
- 35% Sand (>75 micron 4mm non-organic fraction); - 10%
Pressed filter cake (
