| (A) Pretreatment. RBC units shall be preceded by pretreatment
to remove any grit, debris, and excess oil and grease which may hinder
the treatment process or damage the RBC units. The design engineer
should consider primary clarifiers with scum and grease collecting
devices, fine screens, and oil separators. For wastes with high hydrogen
sulfide concentrations, preaeration shall be provided.
(B) Organic loading. The organic loading for the design
of RBC units shall be based on total BOD5 in
the waste going to the RBC, including any side streams. The design
engineer should consider a maximum loading rate of five pounds BOD5 per day per 1,000 square feet of media in
any stage, depending on the character of the influent wastewater.
The maximum loading rate shall not exceed eight pounds BOD5 per day per 1,000 square feet of media in
any stage. The design engineer should also consider the ratio of soluble
BOD5 to total BOD5 and
its possible effect on required RBC media area. Allowable organic
loading for the entire RBC system shall not exceed the following criteria.
Attached Graphic
(C) Stages of treatment. The number of RBC units in
series (stages) for BOD removal only shall be a minimum of three stages.
For BOD removal and nitrification, there shall be a minimum of four
stages. If the plant is designed with less stages than noted in the
previous sentences of this subparagraph, the engineer must provide
justification based on either full-scale operating facilities or pilot
unit operational data. Any pilot unit data used in the justification
must take into consideration an appropriate scale-up factor.
(D) Drive system. The drive system for each RBC unit
shall be selected for the maximum anticipated media load. A variable
speed system should be considered to provide additional operator flexibility.
The RBC units may be mechanically driven or air driven.
(i) Mechanical drives.
(I) Each RBC unit shall have a positively connected
mechanical drive with motor and speed reduction unit to maintain the
required rpm.
(II) A fully assembled spare mechanical drive unit
for each size shall be provided on-site.
(III) Supplemental diffused air should be considered
for mechanical drive systems to help remove excess biomass from the
media and to help maintain the minimum dissolved oxygen concentration.
(ii) Air drives.
(I) Each RBC unit shall have air diffusers mounted
below the media and off-center from the vertical axis of the RBC unit.
Air cups mounted on the outside of the media shall collect the air
to provide the driving force and maintain the required rpm.
(II) Blowers shall provide enough air flow for each
RBC unit plus additional capacity to double the air flow rate to any
one unit while the others are running normally.
(III) The blowers shall be capable of providing the
required air flow with the largest unit out of service.
(IV) The air diffuser line to each unit shall be mounted
such that it can be removed without draining the tank or removing
the RBC media.
(V) An air control valve shall be installed on the
air diffuser line to each RBC unit.
(E) Dissolved oxygen. The RBC plant shall be designed
to maintain a minimum dissolved oxygen concentration of one milligram
per liter at all stages during the peak organic flow rate. Supplemental
aeration may be required.
(F) Nitrification. The design of an RBC plant to achieve
nitrification is dependent upon a number of factors, including the
concentration of ammonia in the influent, effluent ammonia concentration
required, BOD5 removal required, minimum
operational temperatures, and ratio of peak to design hydraulic flow.
Each of these factors will impact the number of stages of treatment
required and the allowable ammonia nitrogen loading (lb NH3 /day/1,000 ft2 media)
required to achieve the desired levels of nitrification for a given
facility. The engineer shall submit appropriate data supporting the
design.
(G) Design flexibility. The designer of an RBC plant
should consider provisions to provide additional operational flexibility
such as controlled flow to multiple first stages, alternate flow and
staging arrangements, removable baffles between stages, and provision
for step feed and supplemental aeration.
(g) Activated sludge facilities.
(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.
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