Advanced fine screening provides front line defence for waste water plants

Advanced fine screening technology is being deployed in Australasia that can substantially extend the lifespan and reduce maintenance of waste water plant

The local projects employ Brackett Green Band Screens that are being increasingly adopted internationally to meet the expanding need for waste water fine screening to capture solids that either clog conventional screens or bypass them entirely.   Australasian applications currently under way will feature mesh apertures down to 5mm, with even smaller apertures possible. This permits near-total capture of solid waste and protects plant that could otherwise be high maintenance, including pumps, impellers and mixers. A further benefit of the fine screening is prevention of needle stick injuries from sharp waste and an overall need for less hands-on maintenance, says Eimco Water Technologies-AJM Environmental Services (EWT-AJM) National Sales Manager, Industrial/Municipal, Mr John Koumoukelis. Brackett Green Limited, founded in 1899, was the original designer of all forms of screening equipment and currently has about 9,000 installations in some 85 countries worldwide.   “Brackett Green Band Screen technology is a real advance in screening systems that has been extensively proven in Europe as an effective alternative to slotted screens,” says Mr Koumoukelis. “By using circular apertures and screening in two dimensions instead of one, they can virtually double screen efficiency and capture fine objects that otherwise may pass through an equivalent size slot. Based on the Brackett Green Band Screen technology supplied for raw water applications and utilizing features developed for the Brackett Sewage Drum Screen, the band screens introduced to Australasia have proved to be a reliable and efficient fine screen both as a retrofit and for new installations on raw sewage and treated effluent. The Brackett Green Band Screen has an endless band of screening panels through which the water passes. Debris collected on the mesh panels is raised to deck level and removed by back washing. “Screening, the first line of defence in many treatment plants, is also often the most overlooked and neglected part of the treatment process, detrimentally affecting nearly all downstream processes, says Mr Koumoukelis.   “Effective screening at the front-end of a treatment plant can make the grit removal, clarification, aeration, filtration and digestion processes much more efficient by taking out rags, plastic, sticks and fibrous material before they have a chance to clog up tanks and pumps or entangle in mixers and other mechanical equipment. “Screening technology has come a long way since the days where coarse bar screens were considered adequate. In these systems, the screens are fixed, semi permanent structures and cleaning was accomplished either manually or by elaborate mechanical linkages. Needless to say, the removal of screenings and routine maintenance on the screens would not have been amongst the more “likable” jobs in the treatment plant. Later, slotted screens and other types of traveling screens were introduced where the screening bars themselves moved in a frame and the screen cleaning was automated by far simpler devices. The evolution of the screen has now progressed to the band screen, a screen offering unparalleled capture rates, durability and ease of maintenance.” The Brackett Green company (acquired by Eimco Water Technologies in 2005) has been at the forefront of screening technology for over a century. They have seen bar screens and step screens come and go, and now they almost exclusively deal in band screens for sewage treatment works. Their CF200 and CF100 band screens have been tested independently at the UK National Screen Evaluation Facility and found to offer the highest capture rates of the many screens tested, up to 93 per cent The exceptional capture rate is due to the two dimensional, inside-to-outside method of screening which eliminates carryover typical of step-type and other screens: anything passing the screens must pass through the orifices. Their unique and patented screening panels use conical holes that effectively prevent hair pinning. Screen cleaning is accomplished by low pressure jets and screenings are removed by a flooded launder, virtually eliminating maintenance and manual handling.”   Patented Mesh Panel and Seal   Band Screens such as the Brackett Green CF200 incorporate thick, molded polyurethane mesh panels with round tapered holes.  Sizes range from 2mm. diameter to 6mm. diameter apertures.   (US Patent No. 5407563) This design of mesh panel has become an established feature of Brackett Green sewage screens, eliminating the problem of hair pinning and blinding experienced with conventional mesh materials, such as perforated plate or wire mesh. To prevent solids bypassing the Band Screen, the Brackett Green contact seal (UK Patent No. 002661) is utilized. Brackett Green Band Screens are designed to suit each individual application, with alternative flow patterns and construction enabling tailoring to fit existing arrangements. They can be constructed in a wide range of widths and depths and can be readily installed in existing channels with the minimum of civil work change. “By utilizing the Brackett Green Center Flow Band Screen, the problem of solids bypassing or carrying over experienced with conventional in-channel band screens is overcome,” says Mr Koumoukelis.     “With the ever increasing demand for the improvement of discharge standards and membrane applications, all unacceptable solids need to be retained by the wastewater screening system rather than passing downstream and creating more expensive problems there. A relatively simple and comparatively smaller investment upstream can prevent major problems and considerable expense downstream.