| (a) A volume-flux design must size an aeration basin
and clarifier on the relationship between the volume-flux of solids
in the secondary clarifier, the sludge volume index (SVI), and the
sludge blanket depth. The following design approach may be used as
an alternative to the traditional design approach. If the volume-flux
design approach is used, it must be used consistently throughout the
design. No other design method may be used in combination with the
volume-flux design method.
(1) A design may base the aeration tank volume and
the clarifier volume on a mixed liquor suspended solids (MLSS) and
floc volume (at SVI of 100) for the required minimum solids retention
time.
(2) Larger values of MLSS require less aeration tank
volume and greater clarifier volume.
(3) By examining a range of values of the MLSS and
the floc volume, the most favorable arrangement for a wastewater treatment
facility may be selected.
(4) When using the volume-flux design method, the size
of an aeration basin and a clarifier must be in accordance with the
requirements of this section.
(b) Design approach.
(1) Determine the solids retention time (SRT) needed
to meet the permit requirement for five-day biochemical oxygen demand
(BOD5 ) and ammonia-nitrogen (NH3 -N) effluent limitations.
(2) Select a trial value mixed liquor floc volume (for
example, MLSS at an SVI of 100).
(3) Using the design organic loading rate, the required
SRT and yield, and the trial MLSS, determine the aeration tank volume.
(4) Using the trial value of mixed liquor flow volume,
determine the clarifier area.
(5) For clarifiers overloaded in thickening at the
peak flow, determine the final MLSS during storm flow and the resulting
sludge blanket depth.
(6) Observing effluent limitations, determine the side
water depth (SWD) and volume of the clarifier.
(7) Repeat the steps in paragraphs (2) - (6) of this
subsection at different mixed liquor floc volumes and select the most
favorable conditions for the wastewater treatment facility design.
(c) Aeration Basin Sizing.
(1) For a wastewater treatment facility that does not
require nitrification, the minimum SRT is as follows:
(A) for a wastewater treatment facility with an effluent
BOD5 monthly average limitation of 20
milligrams per liter (mg/l), the minimum SRT is three days;
(B) for an extended aeration wastewater treatment facility
with an effluent BOD5 monthly average
limitation of 20 mg/l, the minimum SRT is 22 days;
(C) for a wastewater treatment facility with an effluent
BOD5 monthly average limitation less
than 20 mg/l, the minimum SRT is 4.5 days; and
(D) for an extended aeration wastewater treatment facility
with an effluent BOD5 monthly average
limitation of less than 20 mg/l, the minimum SRT is 25 days.
(2) For a wastewater treatment facility that requires
nitrification, the minimum SRT is based on the winter reactor temperature
as set forth in §217.154(b) of this title (relating to Aeration
Basin and Clarifier Sizing--Traditional Design) and the values of
SRT and net solids production (Y), as listed in Table F.8. in Figure:
30 TAC §217.164(c)(3). The maximum BOD5 loading
limitation for a single-step aeration process is 50 pounds (lb) BOD5 per 1,000 cubic feet (cf) and for the first
step of multi-step aeration process facilities is 100 lb BOD5 /1,000 cf.
(3) An above-ground steel or fiberglass tank requires
2 degrees Celsius lower minimum operating temperature than a wastewater
treatment facility utilizing a reinforced concrete tank. A wastewater
treatment facility must be designed for an MLSS concentration of at
least 2,000 mg/l but no more than 5,000 mg/l. The net solids production,
(Y), in the following table includes both coefficients for yield and
endogenous respiration:
Attached Graphic
(4) To calculate the SRT, divide the safety factor
by the maximum growth rate as shown in the following equation. The
safety factor includes the design factor for the ratio of average
to maximum diurnal ammonia loading. A value of 3.0, as recommended
in the United States Environmental Protection Agency manual Nitrogen Control, is used in calculating
the values in Table F.8. in Figure: 30 TAC §217.164(c)(3).
Attached Graphic
(5) To determine the aeration basin volume, select
a trial value of MLSS. The aeration basin volume is calculated as
the maximum value from the following equations:
Attached Graphic
(d) Clarifier Sizing.
(1) A clarifier basin size is based on volume-flux
from the floc volume of solids entering the clarifier.
(2) Biological solids may occupy different volumes
for the same mass of solids as indicated by the SVI.
(3) For purposes of determining weir overflow rates
for clarifier sizing, the design flow and the peak flow must include
any return flows from units downstream of the clarifier, including
flow from skimmers, thickeners, and filter backwash.
(4) A clarifier must be sized to prevent overloading
under any design condition.
(5) The settling velocity of the mixed liquor solids
must equal or exceed the two-hour peak weir overflow rate.
(6) A clarifier must be sized to prevent overloading
in the thickening process at the design flow.
(7) The wastewater treatment facility's operation and
maintenance manual must state the design maximum mixed liquor floc
volume.
(e) Determine Weir Overflow Rate and Area. The values
in Table F.9 in Figure 1: 30 TAC §217.164(e)(2)(I) determine
the maximum surface loading rates. The MLSS concentration must include
the same concentration used for sizing the aeration basin. The design
must be based on the underflow rate. The design must include calculations
for maximum weir overflow rate for the clarifier at the peak flow
(Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I)), the aeration
basin MLSS concentration, and a selected underflow rate. The area
of the clarifier is determined by the following equation:
Attached Graphic
(1) Determine Volume of a Clarifier. The volume of
a clarifier must exceed the values determined from the minimum side
wall depth (SWD) in Equation F.9. in Figure: 30 TAC §217.164(e)(1)
or the minimum detention time in Equation F.10. in Figure: 30 TAC §217.164(e)(1):
Attached Graphic
(2) Dimensions for Clarifiers Designed for Solids Storage
Capabilities. The design of a clarifier that may be overloaded in
thickening at the design flow must include the ability to store solids
during peak flow events. The design must be based on the values in
Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I), Table F.10.
in Figure 2: 30 TAC §217.164(e)(2)(I), and Table F.11. in Figure
3: 30 TAC §217.164(e)(2)(I). The process for designing a clarifier
based on this concept must be completed as follows:
(A) Determine the area of a clarifier. The area calculations
must be based on the trial MLSS value selected for the sizing of the
aeration basin in paragraph (1) of this subsection. The area of a
clarifier must exceed the greater of the areas determined by Equation
F.11. or Equation F.12. in Figure: 30 TAC §217.164(e)(2)(A):
Attached Graphic
(B) The final MLSS value must be the result of the
transfer of solids from an aeration tank to a clarifier at the peak
flow. A clarifier design must allow for rates of flow that will transfer
solids from an aeration tank to a clarifier if the clarifier becomes
overloaded in thickening until the mixed liquor solids are reduced
to the concentration that no longer causes the overload.
(C) Using Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I)
and the selected underflow rate, the MLSS concentration at peak flow
is determined using the following equation:
Attached Graphic
(D) Determine depth of sludge blanket at peak flow.
The depth of a sludge blanket is determined by the aeration basin
volume, the change in MLSS, the area of the clarifier, and the concentration
of the blanket solids at the selected underflow rate as shown in the
following equation:
Attached Graphic
(E) Determine the SWD. The SWD of a clarifier is the
maximum value resulting from the following conditions:
(i) 10 ft, unless a lower depth is allowed by §217.152(g)
of this title (relating to Requirements for Clarifiers);
(ii) 3.0 times the sludge blanket depth; and
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