3. Introduction
An aerated lagoon or aerated basin is a holding and/or treatment pond
provided with artificial aeration to promote the biological oxidation of
wastewaters. There are many other biological processes for treatment
of wastewaters, for example activated sludge, trickling filters, rotating
biological contactors and biofilters. They all have in common the use of
oxygen (or air) and microbial action to biotreat the pollutants in
wastewaters.
Aeration of wastewater lagoons
FUCHS OXYSTAR Aerators outstandingly meet the requirements of
aerated lagoons. If they are arranged appropriately in hydraulically well
designed lagoons, they generate an even, circulating flow throughout
the whole lagoon. OXYSTAR Aerators provide thorough mixing and
oxygen supply in the entire water body at minimal power input. For
information concerning design and operation of these machines please
see our brochure "Fuchs OXYSTAR Aerator".
In large, round or square shaped lagoons a Fuchs CENTROX Aerator is
placed in the centre. CENTROX Aerators generate a vertical flow
pattern and complete the circular flow created by the OXYSTAR
Aerators.
Both aerator types are very sturdy and virtually maintenance-free. Due
to the low weight installation is easy and needs little time.
For use in lagoons, the aerators are preferably mounted on float
assemblies.
4. The aerators have the following advantages:
# High circulation and mixing capacity
# Virtually maintenance free
# No risk of clogging, even at intermittent operation or power failure
TYPES OF AERATED 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.
METHODS OF AERATING LAGOONS
• 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.
5. PROCESS DESCRIPTION
• A treatment plant for domestic wastewater should consist of a fine
screen, two aerated lagoons in series and at least one polishing lagoon.
First, a fine screen removes coarse matter. A paved inlet zone in the
first lagoon allows settlement of heavy sludge and grit. A scum baffle
separates inlet zone and lagoon and prevents floating matter from
entering the lagoon. Floating matter has to be removed manually once
or twice a week with a rake.
6. • According to our experience, sludge in the inlet zone of the first
aerated wastewater lagoon has to be removed at regular intervals of
several years. Liquid manure-vacuum-tankers are used to pump off the
sludge.
• Dissolved and suspended organic pollutants are distributed in the
whole first lagoon
. • Decomposable organic matter should mainly be stabilized
aerobically to avoid odours or anaerobic sludge rising to the water
surface.
• The treatment process in aerated lagoons is similar to the natural
process in flowing water bodies. Biological degradation of pollutants is
based on attached growth.
6. • The biofilm needs continuous supply with oxygen and organic
pollutants. In addition to sufficient oxygen transfer, circulating flow in
the whole water body and effective mixing is required. The circulation
of wastewater and dissolved oxygen ensures optimal conditions for
aerobic growth at the lagoon bottom. Thus, organic pollution is highly
reduced and dead zones can be avoided. • Polishing lagoons further
improve the water quality. In particular, suspended light flocs of
biomass settle down.
7. Design of the lagoons
Aerated lagoons are often designed in rectangular shape. To enable a
good circulation and mixing at low power demand, the corners should
be rounded and the slope should not exceed a 2:1 (width to height)
ratio.
It is also possible to adapt the shape of the lagoon to the surrounding
ground. However, in this case more aerators of smaller size may be
needed.
The depth of the lagoons should range between 2.00 - 3.00 m; mostly a
water depth of 2.50 m is chosen. The angle of the slope corresponds to
the local soil conditions. Normally it is 1:1.5 to 1:2. Natural or clay lining
should be used wherever possible. Plastic lining should be applied only
in highly permeable soil in water protection areas.
The inlet zone of the first aerated lagoon should have a depressed floor
level and it should be reinforced or paved, so that settled sludge and
grit can easily be pumped off without risk of damage at normal
waterlevel.
To prevent damages caused by muskrats, plants or erosion the slope
should be reinforced at the height of normal water level.
The shape of the final clarification or polishing lagoon may vary in a
wide range.
8. However, short circuiting has to be avoided. The depth of the polishing
lagoon should be approx. 1.20 m.
ADVANTAGES
• Lagoon systems can be cost-effective to design and construct in areas
where land is inexpensive.
• They use less energy than most waste water treatment methods
• They are simple to operate and maintain and generally require only
part-time stuff.
• They can handle intermittent use and shock loadings better than
many systems, making them a good option for campgrounds, resorts,
and other seasonal properties.
DISADVANTAGES
• Lagoon systems require more land than other treatment methods
• They are less efficient in cold climates and may require additional
land or longer detention times in these areas.
• Odour can become a nuisance during algal blooms or with anaerobic
lagoons and lagoons that are inadequately maintained.
• Unless they are property maintained, lagoons can provide a breeding
area for mosquitoes and other insects.
9. SUMMARY
• Aerated lagoons are an efficient and cost-effective system for
primary and secondary wastewater treatment in small communities.
They can be integrated very well into the surrounding landscape.
• If appropriate aerators are used the power input is similar to a
comparable activated sludge plant. In addition to sufficient oxygen
transfer the aerators have to provide for mixing and circulation.
• Expenditures for maintenance and operation are restricted to a
minimum
Size and Shape
The exact dimensions of lagoons vary depending on the type of
processes they use for treatment, the amount of wastewater that
needs to be treated, the climate, and whether other lagoons or other
types of treatment are also being used. The size and shape of lagoons is
designed to maximize the amount of time the wastewater stays in the
lagoon. Detention time is usually the most important factor in
treatment.
10. In general, facultative lagoons require about one acre for every 50
homes or every 200 people they serve. Aerated lagoons treat
wastewater more efficiently, so they tend to require anywhere from
one-third to one-tenth less land than facultative lagoons. Many partial-
mix aerated lagoons are simply former facultative lagoons that have
been adapted to receive more wastewater.
Lagoons can be round, square, or rectangular with rounded corners.
Their length should not exceed three times their width, and their banks
should have outside slopes of about three units horizontal to one unit
vertical. This moderate slope makes the banks easier to mow and
maintain. In systems that have dikes separating lagoon cells, dikes also
should be easy to maintain. Interior bank and dike slopes are
determined by the size and depth of the lagoon, potential wave action
and other factors.
11. The bottoms of lagoons should be as flat and level as possible (except
around the inlet) to facilitate the continuous flow of the wastewater.
Keeping the corners of lagoons rounded also helps to maintain the
overall hydraulic pattern in the lagoons and prevents dead spots in the
flow, called short-circuiting, which can affect treatment.
Facultative lagoons are designed to hold wastewater anywhere from 20
to 150 days, depending on the discharge method and the exact size and
depth of the lagoon. Aerated lagoons tend to require shorter detention
times to treat the same amount of wastewater. In cold weather,
however, biological treatment processes in all lagoons slow down,
making longer detention times necessary.
Facultative lagoons are usually 3 to 8 feet deep, so they have enough
surface area to support the algae growth needed, but are also deep
enough to maintain anaerobic conditions at the bottom. Water depth
in lagoons will vary, but a minimum level should always be maintained
to prevent the bottom from drying out and to avoid odors.
Partial-mix aerated lagoons are often designed to be deeper than
facultative lagoons to allow room for sludge to settle on the bottom
and rest undisturbed by the turbulent conditions created by the
aeration process.
12. In conclusion
When designing an efficient aerated lagoon, it is important to be awareof the
pitfalls and tradeoffs involved during design. SOTE does not equal energy
efficiency, and, while SAE is a way to compare the relative energy efficiencies of
each aeration unit, it ultimately fails to account for the importance of mixing. In
addition, the interplay between depth, airflow per diffuser unit, and blower
backpressureallplay a role in determining blower horsepower and energy
consumption. By understanding the determining factors and tradeoffs involved,
you will better be able to balance them in order to design an optimized aerated
lagoon.
