| (1) Organic loading rates. Aeration tank volumes should
be based upon full scale experience, pilot scale studies, or rational
calculations based upon commonly accepted design parameters such as
food to microorganism ratio, mixed liquor suspended solids, and the
solids retention time. Other factors to be considered include size
of the treatment plant, diurnal load variations, return flows and
soluble organic loads from digesters, or sludge dewatering operations
and degree of treatment required. Temperature, pH, and dissolved oxygen
concentration are particularly important to consider when designing
for nitrification. As a general rate, minimum aeration tank volumes
shall be as set forth in the following table. Calculations must be
submitted to fully justify the basis of design for any aeration basins
not conforming to these minimum recommendations.
Attached Graphic
(A) The conventional activated sludge process is characterized
by having a plug flow hydraulic regime wherein particles are discharged
in the same sequence in which they enter the aeration basin. Plug
flow may be approximated in long tanks with a high length-to-width
ratio.
(B) The contact stabilization process divides the aeration
tank volume between the reaeration zone and the contact zone. The
ratio of reaeration volume to contact volume ranges from 1:1 to 2:1.
The hydraulic detention time in the contact zone shall be sufficient
to provide removals of soluble substrates to the required levels.
For domestic flows normally two hours is sufficient in the contact
zone. Contact zone volume shall be based upon acceptable removal kinetics
for soluble BOD5 and ammonia nitrogen.
(C) Oxidation ditches (which are organically loaded
consistent with this paragraph) shall have a minimum hydraulic retention
time of 20 hours based on design flow. These oxidation ditch systems
shall provide final clarification and return sludge capability equal
to that required for the extended aeration process. There shall be
a minimum of two rotors per ditch, each capable of supplying the required
oxygenation capacity and maintaining a minimum channel velocity of
1.0 foot per second with one rotor out of service. The ditch shall
be lined with reinforced concrete or other acceptable erosion-resistant
liner material. Provision shall be made to easily vary the liquid
level in the ditch to control the immersion depth of the rotor for
flexibility of operation. A motor of sufficient size to maintain the
proper rotor speed for continuous operation shall be provided. Rotor
bearings should have grease fittings that are readily accessible to
maintenance personnel. Gear housing and outboard bearings should be
shielded from rotor splash.
(2) Aeration basin general design considerations. Aeration
tank geometry shall be arranged to provide optimum oxygen transfer
and mixing for the type aeration device proposed. Aeration tanks must
be constructed of reinforced concrete, steel with corrosion-resistant
linings or coatings, or lined earthen basins. Liquid depths shall
not be less than 8.0 feet when diffused air is used. All aeration
tanks shall have a freeboard of not less than 18 inches at peak flow.
Access walkways with properly designed safety handrails shall be provided
to all areas that require routine maintenance. Where operators would
be required to climb heights greater than four feet, properly designed
stairways with safety handrails should be provided. The shape of the
tank and the installation of aeration equipment should provide a means
to control short circuiting through the tank. For plants designed
for design flows greater than 2.0 mgd the total aeration basin volume
shall be divided among two or more basins. Each treatment facility
shall be designed to hydraulically pass the design two-hour peak flow
with one basin out of service.
(3) Sludge pumps, piping, and return sludge flow measurement.
The pumps and piping for return activated sludge shall be designed
to provide variable underflow rates of 200 to 400 gallons per day
per square foot for each clarifier. If mechanical pumps are used,
sufficient pumping units shall be provided to maintain design pumping
rates with the largest single unit out of service. Sludge piping and/or
channels shall be so arranged that flushing can be accomplished. A
minimum pipe line velocity of three feet per second should be provided
at an underflow rate of 200 gallons per day per square foot. Some
method shall be provided to measure the return sludge flow from each
clarifier.
(4) Aeration system design.
(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 Cont'd... |
