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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