Floating Solids

Floating bead media filters should not be applied to the removal of floating solids, as the separation strategy presumes the solids captured will ultimately sink. The units have a moderate tolerance for oil and grease, but should be protected from heavy loadings by a clarifier equipped with a surface skimmer, or by a dissolved air floatation (DAF) unit.

Units placed behind a DAF should be thoroughly degassed as bubble formation under or in a floating bead bed will dislodge captured fines and lead to impaired removal efficiency for the smallest particle categories.

Managing Filter Performance

Management can have a significant impact on filter performance. The actual removal efficiency varies according to the size of the particles found in the water, the velocity of flow through the bed (flux in gpm/ft2 or feet/sec), and the depth of the bed. When standard spherical or EN media is used, single pass removal efficiencies are typically very high (95%+) for particles above 50 microns and 30 percent for the 5-10 micron size classification. Biofilm development in applications improves fine solids removal, as do flow reductions, and extension of backwash intervals. Smaller particles (<20 microns) generally do not adversely impact TSS levels since they do not contain much mass. They can adversely impact turbidity levels as they display extraordinarily high surface areas that deflect light. Applications that demand fine solids capture turbidity are usually attacked with recirculation (RCPG models) or by pretreatment with a coagulant.

Media Selection

Selection of media can also impact solids capture. Floating bead beds packed with standard media are slightly superior to EN media in their ability to capture fine solids. Thus, units designated to treat waters for solids only are often packed with standard media. However, the EN media has a higher porosity that lowers head loss and stores more solids between backwashes. EN media is the packing of choice for all clarifying filters operating with an airlift. The media is also often selected for clarifier applications where the solids loading is heavy and extending backwash frequencies permit better sludge consolidation in the internal sludge storage compartment.

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