Extended Aeration Handout

Extended Aeration - The extended aeration process is one modification of the activated sludge
process which provides biological treatment for the removal of biodegradable organic wastes
under aerobic conditions.

• Air may be supplied by mechanical or diffused aeration to provide the oxygen required to
sustain the aerobic biological process.
• Mixing must be provided by aeration or mechanical means to maintain the microbial
organisms in contact with the dissolved organics.
• In addition, the pH must be controlled to optimize the biological process and essential
nutrients must be present to facilitate biological growth and the continuation of biological
degradation.

Process:
1) Wastewater enters the treatment system and is typically screened immediately to
remove large suspended, settleable, or floating solids that can damage or the equipment.
2) Wastewater may then pass through a grinder to reduce large particles that are not
captured in the screening process.
3) If the plant requires the flow to be regulated, the effluent will then flow into equalization
basins which regulate peak wastewater flow rates.
4) Wastewater then enters the aeration chamber, where it is mixed, and oxygen is provided
to the microorganisms.
5) The mixed liquor then flows to a clarifier or settling chamber where most microorganisms
settle to the bottom of the clarifier and a portion are pumped back to the incoming
wastewater at the beginning of the plant.
• This returned material is the return activated sludge (RAS). The material that is not
returned, the waste activated sludge (WAS), is removed for treatment and
disposal.
6) The clarified wastewater then flows over a weir and into a collection channel before being
diverted to the disinfection system.
Applicability
Extended aeration plants are typically used in:

• small municipalities
• suburban subdivisions
• apartment complexes
• highway rest areas
• trailer parks
• small institutions
• and other sites where flow rates are below 0.1 MGD.
These systems are also useful for areas requiring nitrification.

Advantages and Disadvantages

Advantages:

• Plants are easy to operate, as many are manned for a maximum of two or three hours per
day.
• Extended aeration processes are often better at handling organic loading and flow
fluctuations, as there is a greater detention time for the nutrients to be assimilated by
microbes.
• Systems are easy to install, as they are shipped in one or two pieces and then mounted
on an onsite concrete pad, above or below grade.
• Systems are odor free, can be installed in most locations, have a relatively small footprint,
and can be landscaped to match the surrounding area.
• Extended aeration systems have a relatively low sludge yield due to long sludge ages, can
be designed to provide nitrification, and do not require a primary clarifier.

Disadvantages:

• Extended aeration plants do not achieve denitrification or phosphorus removal without

additional unit processes.
• Flexibility is limited to adapt to changing effluent requirements resulting from regulatory
changes.
• A longer aeration period requires more energy.
• Systems require a larger amount of space and tankage than other "higher rate" processes,
which have shorter aeration detention times.
Design Criteria

Extended aeration plants are typically constructed from steel or concrete. If the system is small
enough, the entire system will arrive as one unit that is ready to be installed. If the system is
larger, the clarifier, aeration chamber, and chlorine tank are delivered as separate units, which
are then assembled on-site (WEF, 1985).
Key internal components of extended aeration treatment plants consist of the following:

• transfer pumps to move wastewater between the equalization and aeration zones
• a bar screen and/or grinder to decrease the size of large solids
• an aeration system consisting of blowers and diffusers for the equalization, aeration, and
sludge holding zones
• an airlift pump for returning sludge
• a skimmer and effluent weir for the clarifier
• and UV, liquid hypochlorite, or tablet modules used in the disinfection zone
Design Consideration
• Typical contact time for extended aeration plants is approximately 18-24 hours.
• The contact time, daily flow rate, influent parameters, and effluent parameters determine
the size of the aeration tank
• Systems should be installed at sites where wastewater collection is possible by gravity
flow.
• In addition, the site should be stable, well drained, and not prone to flooding.
• The facility should be installed at least 30 meters (100 feet) from all residential areas and
be in accordance with all health department regulations or zoning restrictions
The Oxidation Ditch Process and Carrousel process
Treatment of wastewater using an oxidation ditch is relatively similar to
wastewater treatment in a packaged plant. But the oxidation ditch replaces
the aeration basin and provides better sludge treatment.
The only pretreatment typically used in an oxidation ditch system is the bar
screen. After passing through the bar screen, wastewater flows directly into
the oxidation ditch.
The oxidation ditch is a circular basin through which the wastewater flows.
Activated sludge is added to the oxidation ditch so that the microorganisms
will digest the B.O.D. in the water. This mixture of raw wastewater and
returned sludge is known as mixed liquor.
Oxygen is added to the mixed liquor in the oxidation ditch using rotating biological contactors (RBC's.)
RBC's are more efficient than the aerators used in packaged plants. In addition to increasing the water's
dissolved oxygen, RBC's also increase surface area and create waves and movement within the ditches.
Once the B.O.D. has been removed from the wastewater, the mixed liquor flows out of the oxidation
ditch. Sludge is removed in the clarifier. This sludge is pumped to an aerobic digester where the sludge
is thickened with the help of aerator pumps. This method greatly reduces the amount of sludge
produced. Some of the sludge is returned to the oxidation ditch while the rest of the sludge is sent to
waste.

Prepared by: John Eric Altura

An oxidation ditch is a modified activated sludge biological treatment process that utilizes long solids
retention times (SRTs) to remove biodegradable organics. Oxidation ditches are typically complete mix
systems, but they can be modified to approach plug flow conditions. (Note: as conditions approach plug
flow, diffused air must be used to provide enough mixing. The system will also no longer operate as an
oxidation ditch). Typical oxidation ditch treatment systems consist of a single or multichannel
configuration within a ring, oval, or horseshoe-shaped basin. As a result, oxidation ditches are called
“racetrack type” reactors. Horizontally or vertically mounted aerators provide circulation, oxygen
transfer, and aeration in the ditch.
Preliminary treatment, such as bar screens and grit removal, normally precedes the oxidation ditch.
Primary settling prior to an oxidation ditch is sometimes practiced, but is not typical in this design.
Tertiary filters may be required after clarification, depending on the effluent requirements. Disinfection
is required and reaeration may be necessary prior to final discharge. Flow to the oxidation ditch is
aerated and mixed with return sludge from a secondary clarifier.

Comparison to a Packaged Plant

The treatment of wastewater in an oxidation ditch is similar to treatment in a packaged plant. The two
main differences between the processes are the retention time and the type of organisms which digest
the wastewater.
Retention time is much longer in an oxidation ditch. A packaged plant usually has a retention time of
two to four hours while an oxidation ditch retains the wastewater for two days.

Prepared by: John Eric Altura

Since the D.O. is higher in the oxidation ditch than in a packaged plant, a greater variety of
microorganisms live in the oxidation ditch. In contrast, packaged plants usually depend upon only a few
types of microorganisms to eat the sewage.

Advantages and Disadvantages of Oxidation ditch and Carrousel process

The greatest advantage of an oxidation ditch is the efficiency of sludge removal.
In an oxidation ditch, only about 15% of the original B.O.D. ends up as sludge,
compared to a packaged plant where about 60% of the B.O.D. becomes sludge.
However, oxidation ditches are expensive to maintain. The monetary cost is very
high per ton of B.O.D. removed. In some cases, the cost may reach nearly 350
dollars per ton.
Oxidation ditches have an additional environmental drawback. The water is
moved through the ditches using rotors, and these rotors in turn use electricity.
The electricity used to operate the plant causes sulfur dioxide and other
contaminants to be released into the atmosphere from coal-burning electrical
plants.

Prepared by: John Eric Altura

Oxidation ditches provide the most thorough process for treating sewage, but
oxidation ditches are also one of the most costly forms of treatment.

References:
• https://www.yokogawa.com/ph/library/resources/application-
notes/oxidation-ditch-type-treatment-process
• https://water.mecc.edu/Oxidation-ditch-treatment-process
• https://www.youtube.com/watch?v=AKvRu88YhZ8
• https://www.oxymem.com/blog/why-is-aeration-important-for-
wastewater-treatment
• https://kenkidryer.com/2020/03/16/oxidation-ditch-method/
• https://www.thewatertreatments.com/wastewater-sewage-
treatment/carrousel-system/

Prepared by: John Eric Altura

 Tuesday, 4 October 2022

SEWAGE TREATMENT: PLANNING AND

DESIGN

STEP-FEED AND TAPERED AERATION

FSTPLNS (ESE 226)

(October 2022)

Bungabong, Brian P.

Engr. Manny Anthony M. Taguba

Professional Lecturer
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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

OUTLINE
• Introduction
• Typical Wastewater Treatment Plant Scheme
• Working Terminologies
• Background on Conventional Activated Sludge

• Step-Feed Process (Step Aeration or Step Loading)

• Tapered Aeration

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Typical Wastewater Treatment Plant

Preliminary Primary
Treatment Treatment Tertiary Treatment
Secondary Treatment

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

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 Tuesday, 4 October 2022

Working Terminology
Term Definition

Activated Sludge Biological treatment process that involves the conversion of organic matter and/or other
constituents in the wastewater to gases and cell tissue by a large mass of aerobic
microorganisms maintained in suspension by mixing and aeration. The microorganisms
form flocculent particles that are separated from the process effluent in a sedimentation
tank (clarifier) and subsequently returned to the aeration process or wasted.
Aerobic (Oxic) Process Biological treatment process that occurs in the presence of free dissolved oxygen. Oxygen
is consumed by aerobic microorganisms to drive metabolic reactions.
Attached Growth Process Biological treatment process in which the microorganisms responsible for the conversion
process of organic matter or other constituents in the wastewater to gases and cell tissue
are attached to an inert medium, such as rocks, slag, or specially designed ceramic or
plastic materials. Attached growth processes are also known as fixed-film processes.
Anaerobic Process Biological process that occurs in the absence of oxygen and oxidized compounds.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Working Terminology
Term Definition

Anoxic Process Biological treatment process that occurs in the absence of free dissolved oxygen, where
oxidized compounds such as nitrate and sulfate are used to drive metabolic reactions.
Biochemical Oxygen Demand The amount of oxygen required by the microorganisms to break down organic matter in a
(BOD) wastewater.
Conventional Treatment Technologies such as activated sludge and trickling filter that remove BOD and suspended
Technologies solids from wastewater.
Denitrification The biological process by which nitrate is reduced biologically to nitrogen gas under anoxic
conditions.
Food to Microorganism Ratio (F/M) Ratio of influent BOD/day to mass of microorganisms.

Mixed Liquor The mixture of wastewater and microorganism

Nitrification The two-step biological process by which ammonia (NH3) is converted first to nitrite
(NO2)and then to nitrate (NO3)
Solids Retention Time (SRT) The average period of time during which the biomass has remained in a biological
treatment system.
Suspended Growth Process Biological treatment process in which the microorganisms responsible for the conversion of
organic matter or other constituents in the wastewater to gases and cell tissue are
maintained in suspension within the liquid.
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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Conventional Activated Sludge (CAS)

Aerobic Biological Treatment
Process

Growth of
Microorganisms

Suspended Attached Hybrid

Growth Growth Process

Conventional
Activated Sludge

• In the presence of Oxygen, microorganisms convert organic matter to Carbon dioxide

(CO2), Ammonia (NH3) and new microorganism.
COHN + O2 → CO2 + NH3 + C5H7NO2

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

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 Tuesday, 4 October 2022

Process Modification
• Step Feed
• Tapered Aeration
• Complete Mix
• Contact Stabilization
• Extended Aeration
• High Rate
• Oxidation Ditch
• Kraus Process
• High Purity
• Batch Fine-pore Diffuser

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Step-Feed CAS Process

• Also known as “step
Step-feed aeration or step
loading”
• Originally developed by
Kessener in 1937 and
in treatment plants in
Holland. But, other
sources (Metcalf and
Eddy, 1979) that it was
Gould pioneered the
process and
incorporated the
principle at the
Tallman’s Island WWTP
in New York.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Step-Feed CAS Process

Operational problems with Plug Flow CAS
Step-feed process
• Uneven oxygen requirements in the aeration tank.
o High load of organic wastes at the beginning of the
reactor (aeration tank)
o Aeration capacity < the required oxygen.

• Step feed is a modification of the conventional

plug flow process in which settled wastewater is
introduced at three to four feed points in the
aeration tank to equalize the food to
microorganism ratio thus lowering peak oxygen
demand.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

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 Tuesday, 4 October 2022

Step-Feed CAS Process

Advantages of Step-feed CAS Process
Step-feed
1. Better equalization of the waste load.
2. Operation flexibility.
3. More uniform oxygen demand along the
aeration tank, with lower peak demand.
4. Allows operational control of the sludge age and
hydraulic residence time.
5. Aeration tank size may be reduced considerably.
6. Can be used in preventing gross process failure
due to hydraulic overloading or sludge bulking.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Step-Feed CAS Process

Comparison of Step-feed to other CAS
modification process

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Step-Feed CAS Process

Design and Operational Parameters
1. Mixed Liquor Suspended Solids 2. Dissolved Oxygen Concentration
(MLSS) (DO)
• A mixture of raw wastewater and the RAS from • DO concentration must be sufficient for good
the clarifier. BOD removal.
• Typically 2000 – 4000mg/L • Typically set at ~ 2mg/L
• Higher MLSS – smaller volume of aeration • If too high DO favor filamentous bacteria that
basin, smaller footprint. do not settle in the clarifier.
• Waste of energy.
• Maximum concentration are limited by
• If MLSS is maximum with smaller aeration basin, • If too low DO, insufficient BOD removal
oxygen cannot be supplied efficiently to the • It will turned anaerobic (odorous)
microorganism.
• If MLSS is maximum solids will not settle easily
in the clarifier. Bigger clarifier is needed to settle
the suspended solids

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 Tuesday, 4 October 2022

Step-Feed CAS Process

Design and Operational Parameters
3. Hydraulic Retention Time (HRT) 4. Sludge Retention Time (SRT)
• The time the liquid stays in the system. • Also known as Mean Cell Residency Time
(MCRT).
• Typically 4hours to 24hours.
• The time the microorganism stays in the
• It should be design properly so that the aeration basin.
microorganism will have an ample time to do its
job especially during peak flow during storm • Typically 2 to 8 days for BOD removal.
event. • It should be design properly to allow the
microorganism grow and complete its
reproduction cycle before they will removed in
the system.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Step-Feed CAS Process

Design and Operational Parameters
5. Food to Mass Ratio (F/M)
• Ratio of influent BOD/day to mass of
microorganisms
• lbs.BOD/day/ lbs biomass reactor
• Range 0.2 to 0.5

• If too High
• Microorganism can’t handle all the food
(substrate)
• Insufficient BOD removal.
• Viscous bulking – poor settling.

• If too Low
• Favors filamentous microorganism – poor
settling.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Aeration Systems

Horizontal Surface Brush

Aerators Vertical Turbine Surface
Aerators

Coarse Bubbles Diffusers Blowers

Fine Bubbles Diffusers
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 Tuesday, 4 October 2022

Tapered Aeration Process

• Since oxygen demands

decrease along the length of
the plug-flow reactor, the
tapered aeration process
attempts to match the oxygen
supply to demand by adding
more air at the influent end of
the aeration tank than at the
effluent end.

• Obtained by varying the

spacing of the diffuser.
• 55% to 75% of the total air
supply is provided at the first
half of the aeration basin

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

Tapered Aeration
Advantages of Tapered Aeration
• Reducing below-capacity and operational
costs.
• Providing better operational control.
• Inhibiting nitrification if desired.

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

THANK YOU!

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

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 Tuesday, 4 October 2022

References

• Biological Wastewater Treatment, Principles, Modelling and Design by Henze, Loosdrecht,

Ekama, Brdjanovic
• Design of the Step-Feed Activated Sludge Process by Oswaldo Moreno
• Water Reuse, Issues, Technologies and Applications, Metcalf and Eddy/AECOM
• Wastewater Engineering Treatment and Reuse, Metcalf and Eddy Inc.
• Waster and Wastewater Calculations Manual, Second Edition by Shun Dar Lin

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methods, without the prior written permission of the owner, except for personal academic use and certain other noncommercial uses permitted by copyright law.

7
MODIFIED AERATION

Activated Sludge - refers to a flocculent culture of organisms developed in aeration tanks under
controlled conditions. Activated sludge is typically brown in color. Activated sludge is also known as
waste activated biosolids or waste activated solids.

Activated Sludge Process- is a treatment technique in which wastewater and reused biological sludge
full of living microorganisms are mixed and aerated. The biological solids are then separated from the
treated wastewater in a clarifier and are returned to the aeration process or wasted. The micro-
organisms are mixed thoroughly with the incoming organic material, and they grow and reproduce by
using the organic material as food.

Activated Sludge Modifications

First developed in 1913, the original activated sludge process has been modified over the years to
provide better performance for specific operating conditions or with different influent waste
characteristics.

Types of Activated Sludge Process

1. Tapered Aeration
2. Step Aeration
3. Modified Aeration
4. Two-Stage Aeration
5. Activated Aeration
6. Reaeration
7. Contact Stabilisation
What is Modified Aeration Process?
❑ It is like the Conventional Activated Sludge Process with the only difference that it operates
on low aeration times.
❑ As compared to other activated sludge processes, modified aeration process has relatively low
mixed liquor suspended solids (MLSS) concentration, high volumetric organic loading, low
percentage of sludge return, and lesser air requirements.
❑ In this method, less quantity of returned sludge is used, therefore, it is called high-rate
treatment. In high-rate treatment, shorter detention period (about 2 hours), lesser amount of
compressed air and small quantity of returned sludge is used.

What are the Main Characteristics of Modified Aeration Process?

i. Less quantity of return sludge about 10% to 25% of the quantity of sewage is used in this
method.
ii. Detention period of 2-3 hours is provided.
iii. Quantity of air is about 3.15 m3/cu.m of sewage quantity is used.
iv. Smaller sludge age 0.2 to 0.5 days or smaller concentration of suspended solids 500 to 1000
mg/litre is provided in the mixed liquor.
v. This process removes 70 to 80% of suspended solids and 80 to 85% of B.O.D. from the raw
sewage

What are the Advantages of Modified Aeration Process?

i. Smaller volume of aeration units, secondary settling tanks and lesser area of the land is
required. Therefore, it gives overall economy.
ii. Due to lesser amount of air for aeration and lesser amount of return sludge, the operational
cost is reduced.
iii. Larger volume of the gas is obtained in the digester.

What is the Main Disadvantages of Modified Aeration Process?

i. The main disadvantage is that the sludge becomes septic soon and requires early removal and
disposal from the settling tanks.