|(a) Reasons for Use.
(1) Permit Requirements. A wastewater treatment facility
that requires tertiary effluent limitations must use filtration as
a unit of operation to supplement suspended solids removal. Tertiary
effluent limits are any limits less than or equal to five milligrams
per liter for five-day biochemical oxygen demand or total suspended
(2) Specific Water Quality Requirements. A wastewater
treatment facility designed for secondary or advanced secondary treatment
effluent limitations may use filtration as a unit of operation to
supplement suspended biological floc removal. Filters may be designed
for intermittent operation if filters are not necessary to meet permitted
(1) A wastewater treatment facility using filtration
to provide tertiary treatment to comply with a permit requirement
must have a minimum of two independent, fully functional filter basins
with independent, fully functional equipment.
(2) The design of a filtration system must specify
the submerged filter surface area required to treat the peak flow
with the largest filter unit out of service.
(3) If a filter is provided solely to polish wastewater
in situations where permit compliance does not depend on the use of
a filter, such as in some cases of reclaimed water use, one filter
(c) Source of Backwash Water. A filtration system must
use filtered effluent as the source of backwash water.
(d) Disposition of Backwash Water. A filtration system
must return backwash water containing material cleaned from a filter
to the headworks of the wastewater treatment facility or to the influent
lift station for processing.
(e) Sequence of Treatment Units.
(1) A final clarifier must precede a filter.
(2) A filter system may be used in conjunction with
a disinfection tank to provide additional detention time, provided
the filter is backwashed to the headworks of the wastewater treatment
facility or to the influent lift station.
(f) Overload Conditions. A design must prevent effluent
or partially treated effluent from overflowing from any filtration
(g) Control of Slime Growth. A filtration system must
provide periodic disinfectant in the influent stream to control slime
growth in the filter and backwash storage tank.