View other drafts Aerated lagoons are a type of wastewater treatment system that uses artificial aeration to promote the biological oxidation of wastewaters. They are relatively simple and inexpensive to construct and operate, and they can be effective in removing a wide range of pollutants from wastewater, including organic matter, nutrients, and pathogens.

1. Aerated Lagoons
Tanvir Siddike Moin
BUET

2. Introduction
An aerated lagoon is a suspended-growth process in wastewater treatment unit.
The aerated lagoon water treatment system consists of a large earthen lagoon or basin that is equipped with mechanical aerators
to maintain an aerobic environment and to prevent settling of the suspend biomass.
It is provided with inlet at one end and outlet at the other end to enable the wastewater to flow through and to retain for the
specified detention time.
An aerated lagoon may be a complete-mix lagoon or a partial-mix aerated lagoon. Complete-mix lagoons provide enough aeration
or mixing to maintain solids in suspension.
Power levels are normally between 20 and 40 horsepower per million gallons.
The partial-mix aerated lagoon is designed to permit accumulation of settleable solids on the lagoon bottom, where they
decompose anaerobically.

3. • Suspension mixed lagoons flow through activated sludge systems where the effluent has the same composition
as the mixed liquor in the lagoon.
• Typically, the sludge will have a residence time or sludge age of 1 to 5 days.
• This means that the chemical oxygen demand (COD) removed is relatively little and the effluent is therefore
unacceptable for discharge into receiving waters.
Types
Suspension mixed lagoons
Suspension mixed lagoons, where
there is less energy provided by the
aeration equipment to keep the
sludge in suspension.
Facultative lagoons, where there is
insufficient energy provided by the
aeration equipment to keep the
sludge in suspension and solids
settle to the lagoon floor. The
biodegradable solids in the settled
sludge then degrade as in
an anaerobic lagoon.

4. Fine bubble
aeration
• Fine bubble diffusers
generally produce bubbles of
about an eighth of an inch in
diameter or less that are fairly
consistent in size.
• These smaller bubbles have
more contact area per
volume of air, and they tend
to climb the water column
more slowly.
Fine bubble diffusers are efficient at transferring oxygen to
water, but can fail to provide sufficient mixing.

5. Coarse bubble
aeration:
• Coarse bubble diffusers produce
bubbles larger than a quarter of an
inch in diameter.
• They are usually irregular and
inconsistent, sometimes producing
very large bubbles of an inch or two,
interspersed with smaller ones.
• Generally, coarse bubbles are
regarded as effective mixers;
however, they are less efficient than
fine bubbles at providing the
necessary oxygen.
Coarse Bubble Static Tube Aerator

6. Surface
aeration:
• Mechanical surface aerators
sit on the surface of a lagoon
and are designed to mix and
churn the water into the air.
• This air/water contact
promotes oxygen transfer.
Surface aerators are among
the least efficient of lagoon
aeration methods due to the
high horsepower and energy
requirements.

7. Depending on how the microbial mass of solids is handled in
the aerated lagoons the same are classified as:
• (i) Facultative aerated lagoons and
• (ii) Aerobic aerated lagoons.
(i) Facultative Aerated Lagoons:
• Facultative aerated lagoons are those in which some solids may leave with the effluent stream, and some settle down in
the lagoon since aeration power input is just enough for oxygenation and not for keeping all solids in suspension.
• As the lower part of such lagoons may be anoxic or anaerobic while the upper layers are aerobic, these are termed as
facultative aerated lagoons.
(ii) Aerobic Aerated Lagoons:
• Aerobic aerated lagoons are those which are fully aerobic from top to bottom as the aeration power input is sufficiently
high to keep all the solids in suspension besides meeting the oxygenation needs of the system.
• No settlement of solids occurs in these lagoons and under equilibrium conditions the new (microbial) solids produced in
the system equal the solids leaving the system.

8. Methods of aerating lagoons or basins
There are many
methods for aerating
a lagoon or basin:
Motor-driven
submerged or
floating jet aerators
Motor-driven floating
surface aerators
Motor-driven fixed-
in-place surface
aerators
Injection of
compressed air
through
submerged diffusers

10. A Typical Surface-Aerated Basin (using motor-driven floating aerators)
Ponds or basins using floating surface aerators achieve 80 to 90% removal of BOD with retention times of 1 to 10 days.
The ponds or basins may range in depth from 1.5 to 5.0 meters. In a surface-aerated system, the aerators provide two functions:
they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing
the air and for contacting the reactants (that is, oxygen, wastewater, and microbes).
Typically, the floating high speed surface aerators are rated to deliver the amount of air equivalent to 1 to 1.2 kg [[O2]]/kWh.
However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do
not achieve the same performance level as activated sludge units.

11. Submerged diffused
aeration
Submerged diffused air is essentially a form of
a diffuser grid inside a lagoon.
There are two main types of submerged diffused
aeration systems for lagoon applications:
floating lateral and submerged lateral.
Both these systems utilize fine or medium
bubble diffusers to provide aeration and mixing
to the process water.
The diffusers can be suspended slightly above
the lagoon floor or may rest on the bottom.
Flexible airline or weighted air hose supplies air
to the diffuser unit from the air lateral (either
floating or submerged).

12. Design of the Aerated Lagoons:
• For facultative aerated lagoons,
the dispersed flow model gives
the relation between influent
and effluent substrate
concentrations, S0 and S,
respectively and other variables
such as the nature of the waste,
the detention period and the
mixing conditions, as shown in
the Wehner-Wilhem equation
given below-

13. Mixing Conditions:
• The mixing conditions in a lagoon are reflected by the term d which is known as the dispersion number and equals
(D/UL) or (Dt/L2). It is affected by various factors.
• Observed results have shown the (D/UL) values to be in the approximate range given in Table for different length-
width ratios of lagoons.
Construction Details:
• Lagoons are generally rectangular in shape though it is not absolutely essential.
• Natural land contours may be followed to the extent possible to save on earthwork.
• Lagoon units may be built with different length-width ratios and arrangement of internal baffles to promote
desired mixing conditions.

14. Discharge Design: A Design Feature That Can Distinguish Lagoons Is How
They Discharge Wastewater
• Continuous Discharge Lagoons.
• These lagoons release wastewater continuously to a holding pond, so the rate of output
roughly equals the rate of input.
• The hydraulic flow pattern in the lagoon is designed so the wastewater remains in the lagoon
long enough to receive treatment before it reaches the outlet.
• Controlled Discharge Lagoons.
• In these lagoons, wastewater is discharged in controlled amounts, usually once or twice per
year.
• This method is common in cold climates where discharges typically occur after spring thaw
and again in fall.

15. Hydrograph Controlled Release Lagoons.
This design can be used for lagoons that discharge directly to surface water.
It includes devices that measure the level and quality of the wastewater and receiving water and the
velocity of the receiving water to determine when conditions are most favorable for discharge.
This method can sometimes eliminate the need for further treatment.
Complete Retention Lagoons.
These lagoons are only practical in very dry climates where evaporation rate greatly exceeds rainfall
amounts. Wastewater is never released from this type of lagoon. Instead, it is allowed to evaporate.

16. 2. SAE
• SAE, or Standard Aeration Efficiency, is intended to help the designer compare the operating
costs of different aerators in a particular aerated lagoon application.
• Measured in terms of pounds of oxygen per horsepower hour, it incorporates both SOTE and
blower horsepower.
• As a result, it is a more complete metric that allows different aerators’ energy efficiencies to be
compared side by side.
1.SOTE
• SOTE stands for Standard Oxygen Transfer Efficiency, the measurement of how much oxygen is
transferred by a given aerator in clean water under the American Society of Civil Engineers’
testing standards.
• It is commonly used in aeration calculations to determine how much air is required to provide
the necessary pounds/kilograms of oxygen needed for aerated lagoon treatment. The higher the
percentage SOTE number, the less air is needed.

17. Advantages of
Aerated Lagoons:
The various
advantages of
aerated lagoons are
as indicated below:
(i) The aerated lagoons are simple and rugged in operation,
the only moving piece of equipment being the aerator.
(ii) The removal efficiencies in terms of power input are
comparable to some of the other aerobic treatment methods.
(iii) Civil construction mainly entails earthwork, and land
requirement is not excessive. Aerated lagoons require only 5
to 10 percent as much land as stabilization ponds.
(iv) The aerated lagoons are used frequently for the treatment
of industrial wastes.

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