Tale of Two Screens

The more operating time that membranes bioreactors (MBRs) have, the more knowledge that is obtained on their operational and maintenance characteristics. Fine screens have typically been a requirement for all MBRs. Being able to screen return activated sludge (RAS) in an MBR system also has several advantages. Screening RAS with its high solids content, 6,000 to 12,000 mg/L, adds some complexity to the proper selection of fine screens.Retrofitting an existing operating facility with a fine screen posed several operational, engineering, and construction challenges. The satellite scalping reuse facility was only down for 12 hours for the installation and connection of the new fine basket screen. Startup was flawless —the entire RAS recycle and screening system was started in less than 8 hours. Chemical cleaning was reduced from once per week to once every 3 months. Average permeability was increased by approximately 38%. The new fine screen captured 1-1/2 cubic yards of material per week that had previously entwined itself and fouled the membrane units. All screens are not created equal.