Current through Vol. 41, No. 13, March 15, 2024
(a)
General. Suspended growth wastewater treatment systems generally
consist of one or more basins where incoming wastewater is mixed with mixed
liquor suspended solids and aerated for a period of time. The mixed liquor
suspended solids are then separated from the mixture where a portion is
returned to the mixing basin and the remainder diverted to other units for
additional treatment before beneficial re-use by land application or landfill
disposal. The liquid after separation from the solid is discharged or diverted
to other units for additional treatment before discharge. Suspended growth
systems covered by these standards are commonly know as the Activated Sludge
process including the Sequencing Batch Reactor ("SBR") process. The activated
sludge process includes several modifications. The most common is the extended
aeration process which includes the oxidation ditch and SBR variations. Submit
a complete design analysis for all suspended growth systems to DEQ for review.
Contact stabilization is not recommended as the only secondary treatment
process, but may be considered where equalization of flow is provided or where
other treatment units follow.
(b)
Primary treatment. The conventional activated sludge process must
be preceded by primary treatment in the form of a primary clarifer(s) in
accordance with 252:656-17. Provide equipment necessary to adequately remove
sludge as it accumulates and transport it to sludge treatment
facilities.
(c)
System
Design. Submit a comprehensive discussion of all functional design
calculations used to size activated sludge treatment facilities. Include the
following:
(1) influent wastewater
characteristics,
(2) temperature
range of wastewater,
(3) primary
treatment of the waste,
(4)
hydraulic and organic loading applied to the aeration basin,
(5) anticipated mixed liquor suspended solids
level to be maintained in the aeration basin,
(6) aeration time,
(7) oxygen and mixing requirements for
average and peak flows,
(8)
recirculation and sludge wasting,
(9) degree of treatment anticipated,
and
(10) equation(s) used to
compute treatment efficiency.
(d)
Aeration basins.
(1)
Capacities and permissible
loadings. The minimum design criteria for activated sludge systems are
listed in Appendix A, Design Tables.
(2)
Arrangement of aeration
basins.
(A)
Basin
dimensions. Design each unit to:
(i)
Maintain effective mixture and use of air.
(ii) Prevent unaerated sections and
noticeable channeling.
(iii)
Maintain velocities sufficient to prevent deposition of solids.
(iv) Restrict short-circuiting through the
tank.
(B)
Basin
lining. Line earthen aeration basins with concrete, asphalt or
equivalent material below the maximum water elevation. Do not use plastic
liners in aeration tanks.
(C)
Number of units. Divide the total aeration basin volume into at
least two units, capable of independent operation.
(D)
Inlets and outlets.
(i)
Controls. Provide inlet and
outlet devices to control flow and maintain constant water level in all
aeration basins. Design the system to allow for the maximum instantaneous
hydraulic load with any single unit out of service.
(ii)
Channels. Design channels
and pipes to maintain a velocity sufficient to hold solids in suspension or
provide a mechanical means for suspending the solids. Provide for draining each
channel when it is not being used.
(E)
Freeboard. Provide at least
18 inches of freeboard.
(e)
Aeration equipment.
(1)
Common elements. Aeration
equipment must be capable of maintaining at least 2.0 mg/l of dissolved oxygen
in the mixed liquor at all times and provide thorough mixing.
(A)
CBOD removal. Where data is
not available, the design oxygen requirement for the activated sludge process
is 1.1 lb O2/lb peak BOD applied to the aeration basins. For the extended
aeration process, the requirement is 1.8 lb O2/lb peak BOD.
(B)
Nitrification. For
nitrification the oxygen requirement for oxidizing ammonia must be added to the
requirement for carbonaceous BOD removal. The nitrogen oxygen demand (NOD)
shall be taken as 4.6 lb O2/lb NH3 at peak diurnal flow. Assure sufficient
alkalinity to maintain pH as required by
252:656-16-3(b)(3).
If the alkalinity is not sufficient, then chemical addition must be
required.
(2)
Diffused air systems.
(A)
Common elements. Normal air requirements for all activated sludge
processes, except extended aeration, is 1,500 ft3/lb
peak BOD for aeration basin loading. For the extended aeration process the
value is 2,000 ft3/lb peak BOD loading.
(B)
Blowers. Design the blower
system to account for temperature extremes ranging from 4 degrees F to 104
degrees F.
(C)
Multiple
units. Provide multiple units with enough capacity to meet the maximum
air demand with the largest unit out of service. The design must also allow the
volume of air delivered to be varied in proportion to the load demand of the
plant.
(D)
Diffusers.
Systems must be capable of providing the diurnal peak oxygen demand or 200% of
the design average oxygen demand, whichever is larger. Design air piping
systems where the total head loss from blower outlet (or silencer outlet where
used) to the diffuser inlet does not exceed 0.5 psi at average operating
conditions. The spacing of diffusers must be in accordance with the oxygen
requirements through the length of the channel or basin, and designed to allow
spacing adjustment without major revisions to the air header piping. All plants
using less than four aeration basins must be designed to incorporate removable
diffusers that can be serviced and/or replaced without dewatering the
basin.
(E)
Filters.
Provide all blowers with air filters.
(3)
Mechanical aeration systems.
The design requirements of a mechanical aeration system shall meet the
following:
(A) Maintain all mixed liquor
suspended solids in suspension;
(B)
Meet maximum oxygen demand and maintain process performance with the largest
unit out of service. A minimum of two units shall be provided;
(C) Provide for varying the amount of oxygen
transferred in proportion to the load demand on the plant; and
(D) If depth of submersion is an important
criteria, the aerators must be adjustable or the basin liquid levels must be
easily controlled with regard to depth.
(f)
Sequencing batch reactor
systems.
(1)
Reactor
design. Provide at least three (3) reactors. Design each reactor to
operate in a cyclic mode with sufficient time to fill, aerate, settle and
remove the clarified liquid.
(A) Organic
loading shall be between 5 to 20 pounds of BOD per thousand cubic feet per day.
Design the system using food to mass (F/M) ratios of 0.05 to 0.30. The total
reactor volume must provide at least 18 hours of hydraulic detention time. Size
the reactor volume on the hydraulic retention time and decant volume.
(B) The design operating levels shall be 10
to 20 feet with at least two feet of freeboard.
(C) Design for no more than four operating
cycles per day per reactor at average design flow.
(D) Sludge production depends on the mode of
operation. For extended aeration mode (24 hours retention time), base sludge
handling design on a minimum sludge production of 0.5 lbs. per lb. of BOD
removed. For conventional activated sludge mode, or for systems using more than
two cycles per day, base sludge production on 0.75 to 0.95 lbs. per lb. of
BOD.
(E) Base sludge storage
requirements on a concentration of 8,000 mg/l with a specific gravity of 1.02
for the settled sludge. Base the calculated sludge volume on the liquid depth
after decanting.
(2)
Aeration equipment. Aeration equipment must provide at least 1.4
lbs. of oxygen per lb. of BOD removed at a minimum residual dissolved oxygen
level of 2.0 mg/l during the aeration period. Where nitrification is required,
the aeration equipment shall have the capacity to provide an additional 4.6
lbs. of oxygen per lb. of ammonia nitrogen.
(3)
Decanter systems. Design the
decanter system to draw effluent from 12 to 18 inches below the surface and to
prohibit floating scum from entering the system during fill and aeration
periods. The design must not create currents that pull solids from the settled
zone at the lowest point in the cycle. The entrance velocities into the
decanter shall not exceed 1.0 fps at the maximum design flow
condition.
(4)
Scum
management. Provide resuspension or removal equipment to control
excessive scum build-up.
(g)
Oxidation ditches. An
oxidation ditch may take any linear shape as long as it forms a closed circuit,
and does not produce any eddies or dead spots.
(1)
Pretreatment. Bar screens
and grit removal facilities are required. Primary settling is not necessary
except for high strength waste.
(2)
Aeration basin.
(A) The volume
of the oxidation ditch must provide 18 to 24 hours hydraulic detention time at
average dry weather flow. Organic loading may range from 12 to 15 pounds BOD
per 1,000 ft3/day.
(B) Depth shall be at least 3 feet.
(C) Freeboard shall be at least one foot at
maximum water depths.
(D) Aeration
equipment shall maintain at least 1 fps velocity throughout the
ditch.
(E) Construct the ditch with
reinforced concrete at least 4 inches thick for ditches up to 5 feet deep, and
6 inches thick where deeper.
(F)
Rotor weight shall not be supported directly by gear reduction or motor
equipment. Protect motors, gear reduction equipment and bearings from
inundation and rotor spray.
(3)
Rotor aerators.
(A) Install at least two complete rotor
units. Design the system so a single rotor can provide the average design
oxygen demand and minimum velocity of 1 fps throughout the basin.
(B) Place rotors before a long, straight
ditch section.
(C) Provide a method
to control rotor submergence.
(4)
Miscellaneous.
(A) Introduce raw sewage and returned sludge
immediately upstream of the rotor that is farthest from the effluent control
weir.
(B) Provide elevated walkways
for rotor maintenance.
(h)
Return sludge equipment.
(1)
Return rate. Design all
return pumping systems for the capability to be operated at the following
return rates:
(A) Standard Rate:
(i) 15% minimum to
(ii) 75% maximum
(B) Carbonaceous Stage of Separate Stage
Nitrification:
(i) 15% minimum to
(ii) 75% maximum
(C) Step Aeration:
(i) 15% minimum to
(ii) 75% maximum
(D) Extended Aeration:
(i) 50% minimum to
(ii) 150% maximum
(E) Nitrification Stage of Separate Stage
Nitrification:
(i) 50% minimum to
(ii) 200% maximum.
(2)
Return pumps.
Maintain the maximum return sludge requirement with the largest pump out of
service. Provide a positive head on all pumps' suctions under all operating
conditions. Provide a minimum pump's suction and discharge opening of at least
3 inches. Air lift systems shall be at least 3 inches in diameter. Further, air
compressors shall be of sufficient capacity to supply design air requirements
plus a 25% safety factor.
(3)
Return piping. Provide 4-inch discharge piping designed to
maintain a minimum velocity of 2 fps at normal return rates. Provide mechanisms
for observing, sampling and controlling return sludge flow from each
clarifier.
(i)
Waste sludge facilities. Waste sludge control facilities shall
have a maximum capacity of not less than 25 percent of the average rate of
sewage flow and function satisfactorily at rates of 0.5 percent of average
sewage flow or a minimum of 10 gpm, whichever is larger.
(j)
Measuring devices. Install a
means to measure flow rates of raw sewage, primary effluent, waste sludge,
return sludge, and air to each basin unit.
Added at 23 Ok Reg 937,
eff 6-15-06; Added at 28 Ok Reg 1282, eff
7-1-11
