WET News

TECH KNOW • Hydrocyclones accurately recover inert grit down to 50-70microns • Vibrations move the screen through the sludge, and the surface tension is broken • The retention rate of rag on the vibrating screen is high The efffective removal of rag and grit can save money on unnecessary maintenance and downtime Through the force of vibrations alone, rag is moved forward towards the discharge point JUNE 2016 WET NEWS 9 Furthermore, through the force of vibrations alone, rag is moved forward towards the discharge point. Now that vibrations can be used to "convey" screenings forward, there is no need for brushes or mechanical scrapers, which removes one more point of failure within conventional systems. Retention rate The retention rate of rag on the vibrating screen is high. With the prior concerns of flowrate and blinding removed, the specification of the screening size, and type, can be relaxed. This allows for two-dimensional screening (i.e. square apertures), rather than the spiral slots previously required to assist in achieving the required flowrate. Within a square aperture screening arrangement, particles greater than the intended screening size are not allowed to pass through with the sludge, which prevents downstream issues, and justifies the screens inclusion to begin with. The consequence of improved rag capture, at higher screening rates, and with a much reduced potential for solids build-up on the screen, has been noted Europe-wide. The UK in particular is now using this technology widely for inter-site imported sludge screening, indigenous sludge screening and inlet screening within pumped inlet works. The development of high frequency vibrations is not limited to rag recovery. CDEnviro has recently launched a bolt-on component to recover grit in a simultaneous process. Many of the grit recovery systems and vortex chambers in use today will achieve a grit recovery cut point around 400-500microns. However, while this removes some grit there is a cut point misconception – much of our grit problems come from particles under this limit. NEED TO KNOW 1 Rag and grit, and the associated detrimental effects have been tolerated in treatment works 2 The development of high frequency vibrations is not limited to rag recovery 3 Much of the grit problems come from particles under THE VERDICT "The UK in particular is now using this technology widely for inter-site imported sludge screening, indigenous sludge screening and inlet screening within pumped inlet works" Philip McGlade Most of the residual grit in any system is between 100-500micron in size; grit that is larger would be removed by current screening systems, and any grit that is smaller would take a long time to settle, ultimately remaining in suspension and being removed through the final dewatering processes. However, the "mid-fraction" is having a damaging effect on many works, building up in any area where the flow is slow moving. Ultimately once it is in the process, it will have to be specially cleaned, which can be a costly and unforeseen exercise. The only way to prevent this is to stop these 100-500micron particles getting into the slurry stream in the first place. Therefore, providing a homogenous, consistent flow is supplied; Hydrocyclones accurately recover inert grit down to 50-70microns. This is the grit recovery medium that is the solution to many of the industry's current grit problems. Hydrocyclones accept a high pressure flow on a tangential inflow to create a continuous vortex within the main chamber. High density particles like grit are thrown to the outer wall and forced down (to the underflow), whereas the residual slurry and water is forced to the overflow, and on to downstream processes within the works. Recovered grit can be finally dewatered on a separate section of the high frequency screen that is in place to screen rag. The S:MAX G from CDEnviro can recover both rag and grit in a simultaneous, in-line process within the same system. n Philip McGlade is senior technical engineer at CDEnviro.

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