BIOLOGICAL TREATMENT
CHECKLIST – SUSPENDED
Water Quality
GROWTH – ACTIVATED
Wastewater Technical Review and Guidance
SLUDGE, OXIDATION
DITCHES
Water/Wastewater/#5.10, April, 2001
FACILITY NAME DATE
CONSULTING ENGINEER SITE INSPECTION (DATE & INSPECTOR)
PLANNING OR DESIGN PHASE
INTRODUCTION
The activated sludge process is an aerobic biological process, which uses the metabolic
reactions of microorganisms to attain an acceptable effluent quality by removing substances
exerting an oxygen demand. There are many modifications of activated sludge that can be
used to meet specific treatment requirements. These modifications consist of and are defined
as follows:
Conventional Activated Sludge - The conventional activated sludge process consists of an
Contents: aeration tank, a secondary clarifier and a sludge recycle line. Both influent sewage and
recycled sludge enter the tank at the head end, move through the tank in plug flow and are
General 3 aerated for a period of time.
Pretreatment 4
Aeration 4 Step Feed Activated Sludge - The Step feed process (sometimes called Step Aeration) is a
Aeration Equipment - Diffused Air 6
Aeration Equipment – Mechanical 9
modification of the activated sludge process in which sewage is introduced at two or more
Return Sludge 10 points in the aeration tank while return sludge is introduced only at the head end of the
Measuring Devices 11 aeration tank. In this modification, the oxygen demand is more uniformly spread over the
Oxidation Ditches 11 length of the aeration tank.
Protective and Service Facilities 13
Tapered Aeration Activated Sludge - The tapered aeration activated sludge process is a modification of the conventional activated
sludge process. Tapered aeration affects only the arrangement of the aeration devices in the aeration tank. In tapered aeration the
wq-wwtp5-10
diffusers or aeration devices are spaced closer together at the head end of the tank to match the oxygen demand.
Complete Mix Activated Sludge - The complete mix activated sludge process is characterized by distributing the
influent sewage, return sludge and air equally throughout the aeration tank to form a nearly homogeneous mixture in the
tank. The organic load and thus oxygen demand is uniform from one end of the tank to the other.
Minnesota Pollution Control Agency, 520 Lafayette Road North, St. Paul, Minnesota 55155-4194
(651) 296-6300, toll-free (800) 657-3864, TTY (651) 282-5332 or (800) 657-3864
This material can be made available in alternative formats for people with disabilities.
Printed on recycled paper containing at least 20 percent fibers from paper recycled by consumers.
BIOLOGICAL TREATMENT
CHECKLIST – SUSPENDED
Water Quality
GROWTH – ACTIVATED
SLUDGE, OXIDATION
Wastewater Technical Review and Guidance
DITCHES
Water/Wastewater/#5.10, April, 2001
Contact Stabilization Activated Sludge - The contact stabilization activated sludge process utilizes two aeration
compartments to divide the two phases of BOD removal. The first phase, adsorption, takes place in the first aeration tank
(contact tank). Mixed liquor from the contact tank then flows to the clarifier. Return sludge flows to the second aeration
tank (reaeration tank) where the second phase, absorption, occurs. The reaeration tank provides detention time before
flowing into the contact tank.
Extended Aeration Activated Sludge - The extended aeration activated sludge process operates in the indigenous
respiration phase of the growth curve, which necessitates a relatively low organic loading (low F/M) and long aeration
time.
Oxidation Ditch Activated Sludge - The oxidation ditch activated sludge process is an extended aeration process
utilizing a ring-shaped channel and aeration than provides continuous flow around the channel.
Pure Oxygen Activated Sludge - Pure oxygen activated sludge is characterized by the use of high purity oxygen instead
of air for aeration. 30
I. DESCRIPTION OF FACILITY – Provide a short description of the proposed facility:
Contents:
General 3
Pretreatment
Hydraulic 4and Organic Loading / Design Flow
Aeration 4
Average- WetDiffused
Aeration Equipment WeatherAir(AWW) 6 gals/day
Aeration Equipment – Mechanical
Peak Hourly 9
Wet Weather (PHWW) gals/day
Return Sludge 10
Peak
Measuring Devices
Instantaneous
11
Wet
Weather (PIWW)
Oxidation Ditches 11 gals/day
Organic
Protective and Loading,
Service FacilitiesCBOD5
13 lbs/day
Explain Primary modification of activated sludge
Type of activated sludge process chosen
wq-wwtp5-10
Type of Aeration
Tank Dimensions and Volume
Detention time in aeration basin hrs
Aeration tank organic loading (CBOD/day/1000ft3
Minnesota Pollution Control Agency, 520 Lafayette Road North, St. Paul, Minnesota 55155-4194
(651) 296-6300, toll-free (800) 657-3864, TTY (651) 282-5332 or (800) 657-3864
This material can be made available in alternative formats for people with disabilities.
Printed on recycled paper containing at least 20 percent fibers from paper recycled by consumers.
BIOLOGICAL TREATMENT
CHECKLIST – SUSPENDED
Water Quality
GROWTH – ACTIVATED
Wastewater Technical Review and Guidance
SLUDGE, OXIDATION
DITCHES
Water/Wastewater/#5.10, April, 2001
Design F/M ratio
Design MLSS
Process Preceded by (specify process)
Process Followed by (specify process)
Contents:
General 3
Pretreatment 4
Aeration 4
Aeration Equipment - Diffused Air 6
Aeration Equipment – Mechanical 9
Return Sludge 10
Measuring Devices 11
Oxidation Ditches 11
Protective and Service Facilities 13
wq-wwtp5-10
Minnesota Pollution Control Agency, 520 Lafayette Road North, St. Paul, Minnesota 55155-4194
(651) 296-6300, toll-free (800) 657-3864, TTY (651) 282-5332 or (800) 657-3864
This material can be made available in alternative formats for people with disabilities.
Printed on recycled paper containing at least 20 percent fibers from paper recycled by consumers.
Final effluent limitation mg/L
mg/L
mg/L
conc
mg/L
GENERAL
Calculations and/or documentation shall be submitted to justify the basis of design for the
following:
a. Process efficiency.
b. Aeration tanks.
c. Aeration equipment (including oxygen and mixing
requirements)
d. Secondary clarifiers
e. Return sludge equipment
f. Waste sludge equipment
g. Sludge handling facilities
Wastewater must be amenable to biological treatment. Check
for industrial discharges that may cause a nutrient imbalance,
variable pH or temperature, or toxic or slug discharge.
Certain dairy wastes, paper wastes, potato wastes particularly
should be examined for nutrient deficiencies.
City has sufficient equipment, trained perator, financial
resources, etc. to properly operate and mange this system:
Efficient use of energy is provided?
Particular attention should be given to initial operation when
oxygen demands may be significantly less than the design
oxygen demand. The design should always maintain the
minimum mixing levels; mixing may control power
requirements at low oxygen demands. Energy conservation
measures should be considered in design of aeration systems.
For diffused aeration systems, the following should be
considered:
a. Use of smaller compressors and more units.
b. Variable-speed drives on positive-displacement
compressors.
c. Intake throttling on centrifugal compressors.
d. Use of high-efficiency diffusers.
For mechanical aeration systems, the following should be
considered:
a. Use of smaller aerators.
b. Variable aeration tank weirs.
c. Multiple speed motors.
d. Use of timers.
BIOLOGICAL TREATMENT CHECKLIST – SUSPENDED GROWTH – ACTIVATED SLUDGE, OXIDATION PITCHES Page 4
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Duplication of critical units (pumps,
aerators, motors) as well as standby
units (tanks etc.) have been provided.
See “Reliability Guidelines.”
Standby or auxiliary power is provided for aeration and
pumping equipment. TYPE
Flexibility allows for at least three modes of operation (two
for extended aeration). Flexibility refers to providing several
modification, series and parallel operation, munerous return
and waste sludge flow options, etc. If feasible, aeration tank
systems, except extended aeration, should be designed to
accommodate at least three (3) modes of operation (such as
plug flow, complete mix, contact-stabilization, step feed etc.).
Two (2) modes of operation for extended aeration systems
larger than 1893 m3/day (0.5 MGD) should be considered.
Design of aeration systems should provide adequate
flexibility to vary the oxygen transfer capability and power
consumption in relation to oxygen demands. MODES
Protection of aeration tanks, clarifiers and equipment from
freezing. Winter protection for the aeration tanks may consist
of windwalls, earthen bank insulation, and extra freeboard,
etc. Other equipment such as blowers, pumps, etc. should
also be protected. Secondary clarifiers and aerobic digesters
should be covered under certain conditions. TYPE
PRETREATMENT
Grit removal, scum and comminution or screening
AERATION
Number of Units – The requirements for dual aeration tanks shall be as listed in MPCA’s
Reliability for Mechanical Wastewater Treatment Plants Guidelines.
Additional aeration tanks designed specifically for nitrogen removal shall not be used for satisfying the requirement of
dual aeration tanks for secondary treatment activated sludge.
a. Plants with initial flows of less than one-half design should have at least two (2) aeration basins. All flow splitting
devices shall have vertical inlet into the splitter and split by overflowing horizontal surfaces at the same elevation.
b. When two (2) or more tanks are installed, both series and parallel flow schemes should be available. In addition each
train of aeration tank(s) and clarifier(s) shall have its own separate return and waste sludge pumping facilities, as well
as intergroup return and wasting sludge capabilities.
ARRANGEMENT OF AERATION TANKS
a. Dimensions – Deep aeration tanks utilizing surface aeration should be avoided without assurance of adequate mixing.
Spiral flow refers to the circular flow pattern that results what the diffusers are located along one wall of the aeration
tank. Fillets should be provided around the bottom of aeration tanks where the wall and bottom meet to eliminate
dead spots.
Do any unusual factors exist that will affect design
such as excessive diurnal load (>4:1, <2:1)
variations, stricter degree of treatment than 25 mg/1
BOD, 30 mg/l TSS, temperatures, pH, etc. Wastes
with high concentrations of BOD diurnal load ratios
or peak hourly BOD5 to average BOD5 of greater
than 4:1. may be handled best by complete mix
activated sludge because BOD is dispersed
completely through tank and makes maximum use of
dilution. Contact Stabilization may not be practical
for wastewater with a high soluble BOD such as
dairy waste. Lab tests should be performed on
mixtures of industrial and domestic wastes.
Conventional activated sludge may be designed for
slightly higher organic loadings if the system also has
the capability to operate in a mode that can handle
higher loadings. For step feed plants, influent
hydraulic design should permit all flow to enter any
single pass. The flow to each pass must be
controllable.
CRITERIA
Liquid depth in aeration tank, should not be less than
10 or more than 30 feet. Horizontally mixed aeration
tanks shall have a depth of not less than 5.5 (as an
exception). FT
Shape of tank and installation of aeration equipment
shall avoid short-circuiting.
Access must be available to allow for observing aeration
tank contents, mixing pattern, sampling, etc., in aeration
tank and settling, scum collection, level weirs, etc., in
clarifier.
Each compartment has load-bearing walls for individual
BIOLOGICAL TREATMENT CHECKLIST – SUSPENDED GROWTH - ACTIVATED SLUDGE, OXIDATION DITCHES Page 6
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compartment dewatering. Slope bottom to assure complete
drainage. Aeration tank drainage should be discharged
upstream of the activated sludge process.
Piping for bypassing and dewatering each individual
compartment is provided?
Pressure relief valves in each compartment provided? Tanks
with freeboard greater than 18’ shall provide rails, steps or
other means of egress from each aeration tank. Additional
freeboard or windbreak may be necessary to protect against
freezing or windblown spray. Walkways should be located
and/or designed to preclude spray and foam from freezing
on their surfaces. Locating walkways away from or
sufficiently elevated, above water levels and using grated
walkways are examples of methods used to prevent ice build
up.
Aeration Tank freeboard, minimum 18 inches. IN
If a mechanical surface aerator is used, the freeboard should
not be less than 3 feet to protect against windblown spray
freezing on walkways, etc.
AERATION EQUIPMENT-DIFFUSED AIR
In diffused air systems, a minimum air flow volume of 20-
30 scfm/1000 ft3 than volume is required for adequate
mixing velocities and to avoid depositing of solids. For a
grid aeration system, mixing rates of 10 to 15 ft3/min/1000ft3
is sufficient.
A bypass to the atmosphere should be installed avoid
surging. The bypass should be arranged to be open at
starting and at all times when the airflow is less than that at
the surge point.
Aluminum pipe should be avoided due to corrosion
problems.
Centrifugal blowers should and rotary positive displacement
blowers must be provided with both inlet and discharge
silencers to reduce the high pitched whine that is transmitted
to the piping.
Surging will result If the blower is throttled on the discharge
side or by adjustment of the air valves at the aeration thanks
to capacity equal to or less than that at the peak of the head-
capacity curve.
Total aeration tank volume shall be divided between two or
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more units, capable of independent operation, for reliability.
Both parallel and series operation are available when two or
more aeration tanks exist.
Inlets and outlets of aeration tank shall be equipped with
valves, gates, plats, weirs or other devices to control flow to
any unit and to maintain constant liquid level.
The effluent weir for a horizontally mixed aeration tank
system must be easily adjustable by mechanical means and
shall be sized based on the design peak instantaneous flow
plus the maximum return sludge flow.
Design peak instantaneous flow can be carried with any
single aeration tank unit out of service.
All channels and pipes designed to maintain self-cleansing
velocities. Min.2’/Sec FT/SEC
Channels not being used during alternate flow patterns can
be drained
Minimum dissolved oxygen concentration that can be
maintained by aeration equipment. Should be at least 2 mg/l
D.O. Mg/l
Design oxygen requirements in lbs. O2/lb. Peak BOD5
applied to aeration tanks (Minimum 1.1 lbs. O2/lb. BOD5, LBS 02
except extended aeration the value shall be 1.5). DAY
For nitrification oxygen demand of recycle flow must be
added to determine 02 requirement LBS 02
Total oxygen requirement, BOD recycle plus nitrification in LBS 02
lbs. 02/lbs. BOD. DAY
Alpha and Beta factor (50% of clean water efficiency where
90 % + waste is domestic sewage, higher industrial ALPHA
percentages should use less that 50 % of clean water
efficiencies and calculations need be submitted to justify) BETA
Certified aeration device transfer efficiency.
Air requirements for diffused air systems, ft 3/lb BOD peak
aeration tank loading. (500 ft3/lb BOD except extended
aeration which shall be 2050 ft3/lb BOD.) CUBIC FT/LB BOD
Air requirements for additional air for channels, pumps,
digesters, etc. CUBIC FEET/MIN
Total air requirements, ft3/Min. CUBIC FEET/MIN
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High air intake temperature for blower (should be designed
for 115ºF (45ºC)). Fº
Low air intake temperature for motor drive (should be
designed for -20ºF (-29ºC)) Fº
Blowers in multiple units and can meet maximum air
demand with single largest unit out of service
Air rate shall be variable or adjustable. For energy
conservation
Adequate workspace shall be provided around each blower
and other equipment (2’ minimum). For easy maintenance
access. FT
Diffuser systems designed for diurnal peak oxygen demand
or 200% of design average oxygen demand, whichever is
greater. PEAK OR 200%
Total head loss from blower outlet of silencer to diffuser
inlet (should not exceed 0.5 psi at average operating
conditions. psi
Spacing of diffusers should be in accordance with oxygen
requirements through length of tank and designed to
facilitate adjustment of aeration throughout tank.
Diffusers are removable without dewatering aeration tank.
Hoist provided to raise diffusers.
Individual diffusers are equipped with control valves with
indicator markings for throttling, or for complete shutoff.
Air filters with easy access provided for blowers.
Soundproofing provided between blower room and other
work areas.
Airflow rate meters provided for each aeration compartment
in addition to a total air flow rate meter.
AERATION EQUIPMENT – MECHANICAL
Design Requirements – Provide mixing with use of a draft tube. Mixing requirements vary from .75 to 1.5 hp/1000ft 3
tank volume for complete mixing. Adjustable aeration tank outlet weirs are desirable with mechanical aerators for
varying submergence of the aerator. Aeration devices in small plants may have time minute increments.
Reliability – An additional motor (and bearings for brush aerator systems) shall be provided for single mechanical aerator
systems.
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Oxygen requirements, lbs. 92/Hr. DAY
Certified testing of mechanical aerator and drive unit shall
verify lbs. 02/Hr.
In the absence of specific design information, the oxygen
requirements shall be calculated using a transfer rate not to
exceed 2 lbs. Of oxygen per horsepower per hour in clean
water under standard test conditions.
Minimum D.O. maintained by mechanical aeration system
(2 mg/l). mg/l
Mechanical aeration system can maintain all biological
solids in suspension (for a horizontally mixed aeration tank
system an average velocity of 1 ft. per second must be
maintained).
Can mechanical aeration system meet maximum oxygen
demand and maintain process performance with largest unit
out of service.
Provide for varying the amount of oxygen transferred
(adjustable mechanism, adjustable aeration tank outlet weirs,
time clocks for smaller facilities, etc.)
Provide that motors, gear housing, bearings, grease fittings,
etc., and be easily accessible and protected from inundation
and spray as necessary for proper functioning of the unit.
Aerators shall be removable without dewatering tanks on
shutting off air supply to other tanks.
Protection from freezing and excessive ice build up.
RETURN SLUDGE EQUIPMENT
Return Sludge Rate – Timers should be considered only as a last resort. Timers should allow for operation in five (5)
minute increments. Airlift pumps used for return sludge and waste sludge shall have air flow regulators with gauged
handles for control of return sludge and waste sludge rates.
Return Sludge Pumps - Air lifts commonly plug up at top, so access must be provided at top of airlift to remove
accumulated debris.
Return Sludge Piping - Observation and sampling of waste sludge, as well as return sludge, should be provided from a
safe vantage point. Return sludge should be added to the influent wastewater prior to entering the aeration tank or added
to the aeration tank in a manner and location that assures adequate mixing with the influent waste.
Minimum return sludge rate as percentage of design average
flow (DAF). %
Maximum return sludge rate (%) %
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Method of varying rate of sludge return (variable speed
motors, drives or timers; timers for small facilities and as a
last resort only) obtained with largest return pump. TYPE
Can maximum return sludge capacity be obtained with
largest return pump blower out of service?
Size of suction and discharge openings on return sludge
pumps (three-inch minimum). IN
Do airlifts allow rapid and easy cleaning?
Diameter of airlift (minimum three inches). IN
Diameter of return sludge discharge (four-inch minimum). IN
Design velocity of return sludge piping at normal return
sludge rtes (minimum two feet per second). FPS
Devices provided for observing, sampling, measuring and OBS.
controlling return sludge flow from each settling tank SAMP.
MEAS.
CONT.
Type of waste sludge handling.
Type of waste sludge control (manual, timer, % of return
sludge, etc.)
Maximum capacity of waste sludge control facilities as
percentage of average rate of sewage flow (not less than
25%). %
Minimum capacity of waste sludge control facilities as
percentage of average rate of sewage flow (0.5 % or 10
gal/min, whichever is larger). %
Devices provided for observing, sampling, controlling and OBS.
measuring waste activated sludge flow (list type separately). SAMP.
MEAS.
CONT
Where is waste sludge discharge?
MEASURING DEVICES
Measurement devices should be located to measure flows to each tank unit. This means if flow splitting is utilized,
measurement should be located downstream of the flow splitting device to measure flows after splitting. When a totalizer
and recorder are available to measure flows prior to flow spilling, a weir may be sufficient to satisfy flow measurement
after splitting.
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For facilities over 1 MGD, can measuring devices for
return and waste sludge also total and record?
OXIDATION DITCHES
In addition to the above criteria concerning activated sludge facilities, oxidation ditches should conform to the following
recommendation.
Sidewalls and bottoms of oxidation ditch channels are
tied together with reinforcing bar or are poured as on
slab to eliminate frost heave?
There must be something holding the walls and bottom
together to prevent frost heaving causing separation. The
weight sufficient to prevent this separation.
Type of backfill for oxidation ditch channel (should be sand
with drainage).
Type of cover ground between channels.
The areas between the channels should be covered with an
impermeable material to prevent rainwater from entering the
ground between the channels. TYPE
Location of influent and return sludge lines with respect to
rotor (lines should be upstream for best mixing).
Location of ditch discharge to final clarifier with respect to
rotor (should be upstream of rotor and far enough upstream
of influent and return sludge to prevent short-circuiting).
The discharge weir shall be designed to withdraw mixed
liquor from below the channel water surface such as through
the use of a baffle in front of the weir. A baffle in front of
the ditch overflow weir will prevent decanting clear liquid
from the ditch. A floating draw off weir should be
encouraged.
Water level control in aeration channel (should be adjustable
weir).
Weir length of channel discharge (use maximum raw flow
plus maximum recirculated flow to determine weir length to
prevent excessive rotor immersions. FT
Type of access to rotor for maintenance (should be at least
walkways located upstream of rotor and located to prevent
spray from rotor on walkway).
Horizontal baffles should be located within 15 feet
downstream of brush aeration in channels with water depth
over six feet. This provides proper mixing of the entire
depth of the basin. Channels using jet aeration may be
deeper and should have a water depth as recommended by
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the manufacturer.
In a single channel, rotor drive assembly should be on
outboard side.
Drive and gear assembly elevated out of water for safe and
easy access.
Equipment requiring normal maintenance should be housed
and /or heated.
PROTECTIVE AND SERVICE FACILITIES
Ladders, hoists and walkways provided for easy access to
equipment, pipes and valves for normal operation, routine
maintenance or repair.
Tanks with freeboard greater than 18” shall provide rails,
steps or other means of egress from each aeration tank.
Additional freeboard or windbreak may be necessary to
protect against freezing or windblown spray. Walkways
should be located and /or designed to preclude spray and
foam from freezing on their surfaces. Locating walkways
away from or sufficiently elevated, above water levels and
sing grated walkways are examples of methods used to
prevent ice build up.
Hoisting equipment available for blowers, pumps and other
heavy equipment
Adequate remote and local controls provided for mechanical
equipment, especially waste sludge pump, return sludge
control, sludge loadout, etc.
Chlorination flexibility provided for chlorination of return
sludge.
Laboratory equipment to be provided, should be MLSS,
settlemeter, settleabilty and dissolved oxygen. Consider
minimum “Process Control” Tests.
Railings or other protective walls shall be placed around all
aeration tanks.
List safety equipment to be provided near aeration tanks and
clarifiers. Should include safety vests, lifelines, and rings
and safety poles.
Sufficient lighting should be provided to permit safe
working conditions near aeration tanks and clarifiers at
night.
BIOLOGICAL TREATMENT CHECKLIST – SUSPENDED GROWTH - ACTIVATED SLUDGE, OXIDATION DITCHES Page 14
