By August 2000, the U.S. Environmental Protection Agency will promulgate a regulation governing the recycling of filter backwash wastewater in Public Water Systems. This regulation is being developed to address concerns regarding recycling of concentrated contaminants back into the water treatment plant train during filter backwash wastewater recovery operations.The ballasted flocculation process (e.g. ACTIFLO?/MICROSEP? from USFilter) is an innovative treatment system that has been successfully applied to treat and recover high quality water from filter backwash wastewater. The principal benefits of ballasted flocculation include vastly reduced space requirements, rapid start-up and response times, relative insensitivity to fluctuations in raw water quality, and removal rates far greater than conventional treatment systems.The key behind the physical-chemical ballasted flocculation process is the use of continuously recycled ballast or fine sand, which attaches to a flocculated particle through a polymer bridge. The addition of ballast provides two benefits: the high specific gravity sand weights flocculated particles leading to much faster solids settling, and the increase in collision frequencies between sand and flocculated particles dramatically increases flocculation kinetics, resulting in very short mixing times. In backwash recovery operations, this combination promotes well over 90% solids removal capability in a total process residence time of just 11 minutes.Due to intermittent generation of filter backwash wastewater from conventional filters, the ballasted flocculation process is also capable of treating a side stream of raw water through the same equipment when filter backwash recovery is not required. This dual mode treatment approach provides an additional advantage of continuously treating a portion of the plant influent flow for reduced solids loading onto the filter.The ballasted flocculation process was introduced as a dual mode treatment system at a groundwater-sourced water treatment plant in Nebraska. Although treating only a portion of the plant influent flow, the high quality recovered water from the ballasted flocculation process has dramatically improved filter effluent quality and increased filter run lengths. Process performance has demonstrated that ballasted flocculation provides a highly responsive and effective small footprint solution for recovering high quality water from filter backwash wastewater streams.
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