93Filter Loading Rate
The filter loading rate is a measure of the filter production per unit area and is typically expressed
in gpm/sf. Typical filter loading rates range from 2 to 4 gpm/sf; however, higher filter loading rates,
4 to 6 gpm/sf, are becoming more common at full-scale.
This can be a critical parameter because it determines the water velocity through the filter bed and
can impact the depth to which particles pass through the media.
The filter run time describes the length of time between filter backwashes during which a filter is in
production mode. The filter run time is not only an indicator of the effectiveness of prior treatment
(i.e., the ability of the coagulation and clarification steps to remove suspended solids), but also
plays a role in the effectiveness of the filter itself.
Filter Performance
Filter performance, particularly with regard to particulate contaminants, is often poorest immediately
following a backwash. As the filter run time increases and the concentration of solids in the media
increases, the filtration process often performs better with regard to particulate contaminant
removal.
Spent Backwash
Residuals generated by the conventional treatment process include coagulation solids (sludge) and
spent backwash. Spent backwash is often returned to the treatment process as a means to
minimize water loss. Sludge may also be recycled to minimize coagulant and coagulant aid doses
and improve process performance.
Process solids (i.e., coagulation sludge and filtered solids) will contain elevated concentrations of
contaminants removed during the treatment process. Depending on the source water concentration
of a particular contaminant and any disposal limitations, it may be necessary evaluate the disposal
of process solids with respect to state and local hazardous waste regulations.