The rough and the smooth
Inflow pre-treatment system toughened-up to tackle rising grit levels
Demand for Jones and Attwood’s Washpactor system for washing, removing and compacting
raw sewage screenings has been driven by higher performance requirements for pre-treatment
of STW inflows.
At the heart of the system is a tank in which an impeller creates high-rate
turbulence which liquifies organic solids in the screenings. Meanwhile the displacement
created by the incoming water combined with energy surges from the impeller
causes the water and washed solids to overflow into an exit channel.
Dirty water and washed solids are delivered to a screw compactor housed in
a perforated U-trough which acts as a fine screen and drains off the dirty water.
The retained screenings are rinsed then forwarded to an inclined chute. Material
accumulated within the chute provides the backpressure for satisfactory dewatering
of the clean screenings.
The unit needs to be very robust. Screening raw sewage removes sizeable solids
which can include metal, bricks, stones and vast quantities of grit. Indeed
the quantity of grit increases exponentially as screen-hole diameters decrease,
which has been a significant trend in screen design over the last few years.
Much of the development work undertaken with Washpactor relates to improving
the resilience of the system to increasing levels of grit. The impeller housing
is subject to high levels of abrasive attack from grit impacting its surface
at speed. To reduce wear on this component, Jones and Attwood has developed
an abrasion-resistant plastic lining.
The impeller is also vulnerable to erosion, and a two-section model has been
introduced to help resolve the problem of excessive wear. The vaned impeller
disc is ultra-hard and durable, while the shaft and mounting plate remain in
place with the mechanical seal during impeller replacement, helping making the
task quicker and easier.
The screw compactor, which dewaters the screenings, works for long periods
under backpressure and in an abrasive environment. As a result, the manufacturer
has developed a specially-designed thick-wall sleeve bearing which has vastly
reduced wear on the screw and in turn prolonged the life of associated components.
To reduce further stone and grit wear, Jones and Attwood has introduced preventative
as well as curative measures. The company has designed traps, which are fitted
to the launder feed system supplying material from the screens to the Washpactor
tank. The traps remove heavy debris and can be emptied manually or automatically.
The Washpactor range varies from the Mini version, which processes 0.68m3/hr
of screened solids to the No. 6, which can deal with 12m3/hr. Independent analysis
has shown the average result for product dryness and BOD are 52% dry solids
and 10g/l BOD.
To get the most efficient operation from the Washpactor it is important to
size the equipment according to site-specific conditions, especially the volume
of screened solids to be processed. To help with this, Jones and Attwood’s technical
department can design the right system for a client’s application. The configuration
and layout of each Washpactor can be tailored to suit the STW design. It is
important the client provides the manufacturer with throughput data along with
site-specific design and layout requirements.
When sizing equipment it is generally assumed the delivery of screened material
is as near continuous as possible. Where the flow varies or the equipment is
at rest for long periods, it is usually necessary to provide a system tailored
for the largest instantaneous flow. Multiple units are used to deal with extra
high volume requirements and in other instances a single machine has dedicated
duty to an individual screen.
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