| (A) General design consideration. Aeration systems
shall be designed to maintain a minimum dissolved oxygen concentration
of 2.0 mg/liter throughout the basin at the maximum diurnal organic
loading rate and to provide thorough mixing of the mixed liquor. The
design oxygen requirements for activated sludge facilities are presented
in the following table. The minimum air volume requirements may be
reduced with appropriate supporting performance evaluations from the
manufacturer.
Attached Graphic
(i) Minimum air volume requirements are based upon
a transfer efficiency of 4.0% in wastewater for all activated sludge
processes except extended aeration, for which a wastewater transfer
efficiency of 4.5% is assumed.
(ii) Value in parentheses represents the minimum oxygen
requirement for ditch type systems which will achieve nitrification.
(B) Diffused air systems.
(i) Volumetric aeration requirements. Volumetric aeration
requirements shall be as determined from the preceding table unless
certified diffuser performance data is presented which demonstrates
transfer efficiencies greater than those used in the preparation of
the table. Wastewater transfer efficiencies may be estimated for:
(I) coarse bubble diffusers by multiplying the clean
water transfer efficiency by 0.65%;
(II) fine bubble diffusers by multiplying the clean
water transfer efficiency by 0.45%. The maximum allowable wastewater
transfer efficiency shall be 12%. Plants treating greater than 10%
industrial wastes shall provide data to justify actual wastewater
transfer efficiencies. Wastewater oxygen transfer efficiencies greater
than 12% are considered innovative technology. See §317.1(a)(2)(C)
of this title (relating to General Provisions) for performance bond
requirements. Clean water transfer efficiencies obtained at 20 degrees
Celsius shall be adjusted to reflect field conditions (i.e., wastewater
transfer efficiencies) by use of the following equation.
Attached Graphic
(ii) Mixing requirement. Air requirements for mixing
should be considered along with those required for the design organic
loading. The designer is referred to Table 14-V, aerator mixing requirements
in Wastewater Treatment Plant Design, a joint publication of the American
Society of Civil Engineers and the Water Pollution Control Federation.
(iii) Blowers and compressors. Blowers and compressors
shall be of such capacity to provide the required aeration rate as
well as the requirements of all supplemental units such as airlift
pumps. Multiple compressor units shall be provided and shall be arranged
so the capacity of the total air supply may be adjusted to meet the
variable organic load to be placed on the treatment facility. The
compressors shall be designed so that the maximum design air requirements
can be met with the largest single unit out of service. The blower/compressor
units shall automatically restart after a period of power outage or
the operator or owner shall be notified by some method such as telemetry
or an auto-dialer. The specified capacity of the blowers or air compressors,
particularly centrifugal blowers, should take into account that the
air intake temperature may reach 104 degrees Fahrenheit (40 degrees
Celsius) or higher and the pressure may be less than standard (14.7
pounds per square inch absolute). The capacity of the motor drive
should also take into account that the intake air may be 10 degrees
Fahrenheit (-12 degrees Celsius) or less and may require oversizing
of the motor or a means of reducing the rate of air delivery to prevent
overheating or damage to the motor.
(iv) Diffusers and piping. Each diffuser header shall
include a control valve. These valves are basically for open/close
operation but should be of the throttling type. The depth of each
diffuser shall be adjustable. The air diffuser system, including piping,
shall be capable of delivering 150% of design air requirements. The
aeration system piping should be designed to minimize headlosses.
Typical air velocities in air delivery piping systems are presented
in the following table.
Attached Graphic
(5) Mechanical aeration systems. Mechanical aeration
devices shall be of such capacity to provide oxygen transfer to and
mixing of the tank contents equivalent to that provided by compressed
air. A minimum of two mechanical aeration devices shall be provided.
Two speed or variable speed drive units should be considered. The
oxygen transfer capability of mechanical surface aerators shall be
calculated by the use of a generally accepted formula and the calculations
presented in the engineering report. Proposed clean water transfer
rates in excess of 2.0 pounds per horsepower-hour shall be justified
by performance data. In addition to providing sufficient oxygen transfer
capability for oxygen transfer, the mechanical aeration devices shall
also be required to provide sufficient mixing to prevent deposition
of mixed liquor suspended solids under any flow condition. A minimum
of 100 horsepower per million gallons of aeration basin volume shall
be furnished.
(h) Nutrient removal.
(1) Nitrogen removal. Biological systems designed for
nitrification and denitrification may be utilized for the conversion/removal
of nitrogen. Various physical/chemical processes may be considered
on a case-by-case basis.
(2) Phosphorus removal.
(A) Chemical treatment. Addition of lime or the salts
of aluminum, or iron may be used for the chemical removal of soluble
phosphorus. The phosphorus reacts with the calcium, aluminum, or iron
ions to form insoluble compounds. These insoluble compounds may be
flocculated with or without the addition of a coagulant aid such as
a polyelectrolyte to facilitate separation by sedimentation. When
adding salts of aluminum or iron, the designer should evaluate the
wastewater to ensure sufficient alkalinity is available to prevent
excessive depression of the wastewater or effluent pH. This is of
particular importance when the system will also be required to achieve
nitrification. The designer is referred to Nutrient Control, Manual
of Practice FD-7 Facilities Design, published by the Water Pollution
Control Federation and the Process Design Manual for Phosphorus Removal,
published by the Environmental Protection Agency, for additional information.
(B) Biological phosphorus removal. Biological phosphorus
removal systems will be considered on a case-by-case basis for systems
which can produce operating data which demonstrate the capability
to remove phosphorus to the required levels. All biological systems
which are required to meet a 1.0 mg/liter effluent phosphorus concentration
shall make provision for standby chemical treatment to ensure the
1.0 mg/liter is achieved.
(i) Aerated lagoon.
(1) Horsepower. Mechanical aeration units in aerated
lagoons shall have sufficient power to provide a minimum of 1.6 pounds
of oxygen per pound of BOD5 applied with
the largest unit out of service. If oxygen requirements control the
amount of horsepower needed, proposed oxygen transfer rates in excess
of two pounds per horsepower-hour must be justified by actual performance
data. The amount of oxygen supplied or the pounds of BOD5 per hour that may be applied per horsepower-hour
may be calculated by the use of any acceptable formula. The combined
horsepower rating of the aeration units shall not be less than 30
horsepower per million gallons of aerated lagoon volume.
(2) Construction. Earthen ponds shall have large sections
of concrete slabs or equivalent protection under each aeration unit
to prevent scouring of the earth. Concrete scour pads shall be used
in all areas where the velocity exceeds one foot per second. Earthen
ponds shall have protection on the slopes of the embankment at the
water line to prevent erosion of the slopes from the turbulence in
the lagoon. Where the horsepower level is more than 200 horsepower
per million gallons of lagoon volume, the pond embankment at the water
line shall be protected from erosion with riprap which may be concrete,
gunite, a six-inch thick layer of asphalt-saturated or cement-stabilized
earth rolled and compacted into place, or suitable rock riprap. The
crest and dry slopes of embankments shall be protected from erosion
by planting of grass.
(3) Subsequent treatment, discharge systems. Aerated
lagoon effluent will normally be routed to additional ponds for secondary
treatment and to provide sufficient detention time for disinfection.
The secondary ponds system shall consist of two or more ponds. Secondary
pond sizing shall not exceed 35 pounds of BOD5 per
acre per day. Hydraulic detention time in a combined aerated lagoon
and secondary pond system shall be a minimum of 21 days (based on
design flow) in order to provide adequate disinfection. In designing
the secondary ponds, BOD5 removal efficiency
in the aerated lagoon(s) may be calculated using the following formula.
Attached Graphic
(j) Wastewater stabilization ponds (secondary treatment
ponds).
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