(a) Classification. A trickling filter or other attached-growth
treatment unit in series with a suspended-growth process is considered
a dual treatment process that is classified as an Activated Biological
Filter (ABF) System, a Trickling Filter/Solids Contact (TF/SC) System,
a Roughing Filter/Activated Sludge (RF/AS) System, an Activated Biological
Filter/Activated Sludge (ABF/AS) System, or a Trickling Filter/Activated
Sludge (TF/AS) System.
(1) ABF System. An ABF system consists of a tricking
filter and a final clarifier. An ABF system recirculates settled solids
from the final clarifier through the trickling filter with no separate
aeration basin or solids contact basin.
(2) TF/SC System. A TF/SC system consists of a trickling
filter sized to remove the majority of the soluble five-day biochemical
oxygen demand (BOD5 ), followed by an
aerated solids contact basin sized to provide polishing and improved
sludge settleability, followed by a final clarifier. A TF/SC system
recirculates activated sludge to a solids contact basin. The design
of a TF/SC system may include a sludge re-aeration basin.
(3) RF/AS System. An RF/AS system consists of a trickling
filter sized to perform primary treatment, followed by an aeration
basin sized to remove the majority of the soluble BOD5 , followed by a final clarifier. An RF/AS
system circulates activated sludge to the aeration basin.
(4) ABF/AS System. An ABF/AS system consists of a trickling
filter sized to perform primary treatment, followed by an aeration
basin sized to remove the majority of the soluble BOD5 , followed by a final clarifier. An ABF/AS
system recirculates activated sludge to the trickling filter.
(5) TF/AS System. A TF/AS system consists of a trickling
filter sized to perform roughing and concentration dampening, followed
by an intermediate clarifier, followed by an aeration basin sized
to remove the majority of the soluble BOD5 ,
followed by a final clarifier. A TF/AS system circulates activated
sludge to the aeration basin.
(b) Process Design.
(1) Attached and suspended growth sub-processes in
a dual treatment system must be designed through an integrated process
that includes the effluent quality from the first stage in determining
the design basis of the second stage.
(2) The design of a dual treatment system must include
an estimate of the performance of the second stage of a dual system
using data from existing similar installations or applicable pilot
studies.
(3) For a dual treatment system design in which activated
sludge is recycled to first-stage trickling filters, the design must
not include the reduction of oxygen demand to the second-stage aeration
basin because of sludge recirculation to the trickling filters.
(4) The design of a dual treatment system may include
estimates of the applicable design equations and methodology used
for a single stage process.
(c) Treatment Unit Design. The design of a suspended
and attached growth system must include all of the features and operational
capabilities required for the same treatment unit used for single-process
treatment. The design of a suspended and attached growth system must
also include the design for pretreatment, snail control, return sludge,
aeration, sludge age, hydraulic residence time, and nitrification
design.
(1) Pretreatment. Pretreatment for a dual treatment
system must conform to the requirements for a first-stage process.
(2) Snail Control. A dual treatment system must include
a low-velocity channel between the first-stage and second-stage treatment
units for control of snails.
(3) Return sludge.
(A) A dual treatment system that includes recirculation
of activated sludge or sloughing to trickling filters must prevent
recirculation of pieces too large to pass through the distributor
nozzles or the trickling filter media voids.
(B) The trickling filters in a dual treatment system
that recirculates sludge to the trickling filters must be of a high-rate,
vertical flow design that uses fully corrugated media.
(C) Sludge must be incorporated into the influent prior
to application to trickling filters, and must be incorporated into
the effluent from first-stage processes prior to being introduced
into second-stage aeration basins.
(4) Aeration. An aeration system for second-stage treatment
units in a dual system not designed for nitrification must transfer
at least 1.2 pounds of oxygen per pound of first stage effluent BOD5 per day. An aeration system for second-stage
treatment units in systems designed for nitrification must transfer
sufficient oxygen to meet stoichiometric requirements for:
(A) biomass growth;
(B) respiration for both carbonaceous material oxidation
and nitrification; and
(C) oxygen demand due to biomass sloughing events from
the first-stage.
(5) Sludge Age.
(A) A second-stage suspended growth process must operate
in a way that varies the age of the sludge.
(B) The mean cell residence time must be:
(i) at least 1.5 days for the suspended growth process
for TF/SC systems; or
(ii) at least 3.0 days if the second process is an
activated sludge aeration basin.
(C) A nitrifying dual treatment system must maintain
a total combined mean cell residence time in the attached and suspended
growth systems of at least 10.0 days with capability to provide at
least 6.0 days mean cell residence time in the suspended growth process
alone.
(6) Hydraulic Residence Time. A design of second-stage
processes must have a minimum hydraulic residence time of:
(A) 0.5 hour if the second process is an aerated solids
contact basin; or
(B) 3.0 hours if the second process is an activated
sludge aeration basin.
(7) Nitrification Design. A wastewater treatment facility
designed for nitrification using a dual treatment system must include:
(A) a sludge re-aeration basin if the second process
is an aerated solids contact basin; or
(B) an intermediate clarifier if the second process
is an activated sludge aeration basin.
|