The Use of an Aeration System to Prevent Thermal Stratification of Water Bodies: Pond, Lake and Water Supply Reservoir

Abstract

Water bodies such as lakes, ponds or water supply reservoir show vertical stratification of their masses, at least for some extended time periods. Density differences facilitate the process of stratification. Temperature and dissolved substances contribute to density differences in water. As a result, thermal stratification can be established during the warm season if the water bodies are sufficiently deep. On the contrary, during the cold season surface cooling force the vertical circulation of water as a result turnover will occur. In warmer weather regions such as Bangladesh, India etc. this annual cycle spending more time in late summer and early fall. Thermal stratification is a common phenomenon here. Failure to identify and control, stratification, can cause low oxygen level, fish die-offs, excessive growth of plankton, failure of meeting regulatory standards, customers' expectations. So, identify the early onset of stratification and developed operational response procedures that introduce appropriate control measures such as aeration to avoid potential impacts and minimize any adverse effects.

Full text

Applied Ecology and Environmental Sciences, 2014, Vol. 2, No. 1, 1-7
Available online at http://pubs.sciepub.com/aees/2/1/1
© Science and Education Publishing
DOI:10.12691/aees-2-1-1
The Use of an Aeration System to Prevent Thermal
Stratification of Water Bodies: Pond, Lake and Water
Supply Reservoir
Khairul Hasan, Kaosar Alam, Md. Saidul Azam Chowdhury*
Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet, Bangladesh
*Corresponding author: tuhin_st7@yahoo.com
Received November 15, 2013; Revised November 29, 2013; Accepted January 02, 2014
Abstract Water bodies such as lakes, ponds or water supply reservoir show vertical stratification of their masses,
at least for some extended time periods. Density differences facilitate the process of stratification. Temperature and
dissolved substances contribute to density differences in water. As a result, thermal stratification can be established
during the warm season if the water bodies are sufficiently deep. On the contrary, during the cold season surface
cooling force the vertical circulation of water as a result turnover will occur. In warmer weather regions such as
Bangladesh, India etc. this annual cycle spending more time in late summer and early fall. Thermal stratification is a
common phenomenon here. Failure to identify and control, stratification, can cause low oxygen level, fish die-offs,
excessive growth of plankton, failure of meeting regulatory standards, customers’ expectations. So, identify the early
onset of stratification and developed operational response procedures that introduce appropriate control measures
such as aeration to avoid potential impacts and minimize any adverse effects.
Keywords: thermal stratification, aeration system, destartification system, epilimnium, hypolimnium, thermocline
Cite This Article: Khairul Hasan, Kaosar Alam, and Md. Saidul Azam Chowdhury, “The Use of an Aeration
System to Prevent Thermal Stratification of Water Bodies: Pond, Lake and Water Supply Reservoir.” Applied
Ecology and Environmental Sciences 2, no. 1 (2014): 1-7. doi: 10.12691/aees-2-1-1.
1. Introduction
Humanity needs to be prepared for the changes it
imposes upon the Earth, especially in times of global
change and of direct human impact on the hydrological
cycle. The anthropogenic impact of the last decades on our
aquatic environment has shown that a responsible use of
our natural resources is mandatory to guarantee
sustainable conditions [2]. Stratification is a natural
occurrence, in any static body of water. It occurs when the
surface layer of water, warmed by the sun, becomes less
dense than the water underneath it. The development of
layers within the profile of a water body is a common
phenomenon of stratification [9]. Without intervention, the
severity of stratification commonly increases, along with
the possibility of drawing poor quality water from a
storage reservoir as well as lower level of dissolved
oxygen, excessive growth of algae.
To avoid these problems, the widely used tool is
aeration. The purpose of aeration in lake management is to
increase the dissolved oxygen content of the water and
Aeration can also increase fish and other aquatic animal
habitat, prevent fish kills and improve the quality of
domestic and industrial water supplies and decrease
treatment cost [13]. In some cases, nuisance algae blooms
can be reduced. Again, aeration of lakes and reservoirs has
been used for a number of years as a method of improving
water quality. Aeration can be used for improvement of
drinking water supplies, providing enhanced fisheries
habitat and treatment of symptoms of eutrophication.
2. Water Bodies
In last few years the demand of water supply for
agriculture, livestock and fish production has increased
tremendously whereas ponds, lakes and water supply
reservoirs are the reliable source of water to fulfill the
demand.
Aeration can be arbitrarily divided into two basic
categories. Destartification systems, as the name implies,
uses compressed air to cause vertical water movement and
mixing of the lake water column. Hypolimentic systems
add oxygen to the lake without destroying the thermal
stratification [6].
The term lake collectively refers to reservoirs (man-
made impoundments), natural lake systems and smaller
ponds (man-made or naturally created) Water from the
surrounding watershed enters a pond or lake as stream
flow, surface runoff and groundwater.
2.1. Lake
A lake could be a body of comparatively still water of
considerable size, localized in an exceedingly basin, that's
enclosed by land with the exception of a watercourse,

2 Applied Ecology and Environmental Sciences
stream, or alternative variety of moving water that serves
to feed or drain the lake. Water temperatures in lakes
throughout summer months aren’t uniform from high to
bottom. 3 distinct layers develop: the highest layer stays
heat at around 65-75 degrees F (18.8-24.5 degrees C). The
center layer drops dramatically, typically to 45–65 degrees
F (7.4-18.8 degrees C). Very cheap layer is that the
coldest, staying at around 39-45 degrees F (4.0-7.4
degrees C). Since light-weight doesn't penetrate to very
cheap, chemical process is proscribed to the highest layer.
Attributable to the hotter waters and additional plentiful
food provide the majority creatures pay the summer
months within the higher layer. During spring and fall the
lake temperature is additional uniform. Fish and
alternative animals’ are found throughout the layers of the
lake.
2.1.1. Biological Zones
Lakes contain several distinct zones of biological
activity, largely determined by the availability of light and
oxygen. The most important biological zones are the
euphotic, littoral and benthic zones [3].
Euphotic zone
The Upper layer of water through which sunlight can
penetrate is called euphotic zone. All plant growth occurs
in this zone. In deep water, algae are the most important
plants, while rooted plants grow in shallow water near the
shore. The depth of euphotic zone is determined by the
amount of turbidity blocking sunlight penetration. In most
lakes, the turbidity is due to algal growth, although color
and suspended clays may substantially reduce sunlight
penetration in some lakes. In the euphotic zone, plants
produce more oxygen by photosynthesis than they remove
by respiration. Below the euphotic zone lies the profundal
zone. The transition between the two zones is called the
light compensation level. The light compensation level
corresponds roughly to a depth at which the light intensity
is about one percent of unattenuated sunlight. It is
important to note that the bottom of the euphotic zone
only rarely coincides with the thermocline.
Littoral zone
The shallow water near the shore in which rooted water
plants can grow is called littoral zone. The extent of the
littoral zone depends on the slope of the lake bottom and
the depth of the euphotic zone. The littoral zone cannot
extend deeper than the euphotic zone.
Benthic zone
The bottom sediments comprise the benthic zone. As
organisms living in the overlying water die, they settle to
the bottom where they are decomposed by organisms
living in the benthic zone. Bacteria are always present.
The presence of higher life forms such as worms, insects
and crustaceans depends on the availability of oxygen.
Figure 1. Biological zones in a lake
2.1.2. Classification of Lake Based on Stratification
The thermal characteristics of lakes are a result of
climatic conditions that provide a useful physical
classification which is based upon the stratification and
mixing characteristics of the water bodies. Reference [5]
classified lakes as follows:
Dimictic lakes
Occur in the cool temperate latitudes. Overturn occurs
twice a year, normally in the spring and autumn. Heating
in the spring results in stratification with a warm water
epilimnion during the summer. The autumn overturn
results in homothermal conditions (at approximately 4°C)
which then cool to create a cold water inverse
stratification during the winter months. Spring warming
results in mixing and a re-establishment of the annual
cycle. The stratification and mixing processes for a large
dimictic lake are illustrated in Figure. This type of lake is
the most common form of lake. Since the cool temperate
latitudes encompass most of the world’s industrial nations
they have been subjected to the most intensive study and
represent the greatest part of our limnological knowledge.
Cold monomictic lakes
Occur in cold areas and at high altitudes (sub-polar).
The water temperature never exceeds 4°C and they have a
vertical temperature profile close to, or slightly below,
4°C. They have winter stratification with a cold water
epilimnion, often with ice cover for most of the year, and
mixing occurs only once after ice melt.
Warm monomictic lakes
Occur in temperate latitudes in subtropical mountains
and in areas strongly influenced by oceanic climates. In
the same way as their cold water counterparts, they mix
only once during the year with temperatures that never
falls below 4°C.
Figure 2. Lake thermal structure and classification based on mixing
characteristics [5]

Applied Ecology and Environmental Sciences 3
Figure 3. The global distribution of thermal lake types in relation to
latitude and altitude [12]
Polymictic lakes
Occur in regions of low seasonal temperature variations,
subject to rapidly alternating winds and often with large
daily (diurnal) temperature variations. These lakes have
frequent periods of circulation and mixing and may be
subdivided into cold polymictic, which circulate at
temperatures close to 4°C, and warm polymictic which
circulate at higher temperatures. As defined above, all
shallow lakes fall within this category.
Oligomictic lakes
Occur in tropical regions and are characterized by rare,
or irregular, mixing with water temperatures well above
4°C.
Amictic lakes
Occur in the Polar Regions and at high altitudes. They
are always frozen and never circulate or mix. Waters
beneath the ice are generally at, or below, 4°C depending
on the amount of heat generated from the lake bed or by
solar radiation through the ice. These lakes show inverse
cold water stratification.
2.2. Pond
In general, a pond could be a body of standing water,
either natural or semisynthetic, that's typically smaller
than a lake. Wet ponds accommodate a permanent pool of
standing water that promotes a stronger atmosphere for
gravitate sinking, biological uptake and microbe activity.
Runoff from every new storm enters the lake and partly
displaces pool water from previous storms. Water
temperature is fairly even from high to bottom and
changes with air temperature. There’s very little wave
action and also the bottom is typically coated with mud.
Plants can, and sometimes do, grow on the pool edge. The
number of dissolved element could vary greatly
throughout every day. In extremely cold places, the
complete pool will freeze solid.
The water quality of the surface waters of the state, as
well as all lakes and ponds, is regulated through statutes
(RSA 485-A) and rules (Env-Ws 1700). These laws and
laws create no distinction between lakes and ponds. Each
ought to meet all an equivalent water quality standards.
In earth science (the study of midland waters), surface
waters are divided into lotic (waters that flow in a very
continuous and definite direction) and lentic (waters that
don't flow in a very continuous and definite direction)
environments. Waters within the lentic category gradually
fill in over geologic time and the evolution is from lake to
pond to wetland. This evolution is slow and gradual, and
there is no precise definition of the transition from one to
the next.
2.3. Reservoir
A reservoir is a natural or artificial lake, storage pond
or impoundment from a dam which is used to store water.
Reservoirs may be created in river valleys by the
construction of a dam or may be built by excavation in the
ground or by conventional construction techniques such as
brickwork or cast concrete.
3. Thermal Stratification
The thermal stratification of lakes, ponds or reservoirs
refers to a change in the temperature at different depths,
and is due to the change in water's density with
temperature. Anyone who has taken a summer swim and
gone through the warm surface water to feel the icy water
a few feet underneath, has felt the most obvious effect of
thermal stratification.
Figure 4. Typical temperature profile from a stratified lake in the
temperate zone, showing the division of the water into three layers of
different density
The surface layer remains on top and the lower layer,
deprived of surface contact and insulated from the sun,
continues to get colder. This increases the difference in
density between the two layers and makes it even more
difficult for them to mix together. Once strongly
established, this stratification persists until falling
temperatures in autumn breaks down the density
difference between the two layers allowing them to mix
together again.
The significance of thermal stratification to anglers is
that the lower layer of water, deprived of surface contact,
slowly loses its dissolved oxygen and become less able to
support aquatic life. In deep lakes and reservoirs, this has
the effect of confining Cold water species, like trout, to a
narrow zone below the high temperature surface water but
above the bottom layer of cold water lacking oxygen. A
good echo sounder will sometimes show this prominent
layer of fish, with nowhere to go and very little to eat, and
the angler who can accurately get a lure or bait into this
"fish zone" can be extremely successful.

4 Applied Ecology and Environmental Sciences
3.1. Basic Characteristics of Stratification
Thermal Stratification is a term meaning temperature
layering. Some of the important features of stratification
are:
• A layer of aerobic water, near the surface, known as
the epilimnium, that is relatively warm and high in
dissolved oxygen (commonly above 7 ppm). In this
zone, temperature and dissolved oxygen (D.O.)
levels tend to be maintained by the penetration of
sunlight and mixing created by wind [9].
• A layer of water, known as the hypolimnium, that is
commonly anaerobic, extends from the bottom of the
reservoir and is relatively cool and low in dissolved
oxygen (commonly below 3 ppm) [3].
• A very thin layer of water, known as the thermocline,
where a rapid change in temperature and dissolved
oxygen occurs in between the epilimnium and
hypolimnium.
• Stratification becomes more severe during warmer
months when the intensity and duration of sunlight
increases, and mixing from reservoir inflow
decreases due to reductions in stream flow [9]. As the
severity of stratification increases, the contrast in
water temperature and D.O. between the epilimnium
and hypolimnium tends to become more pronounced,
and the position of the thermocline tends to rise,
effectively increasing the proportion of the
hypolimnium.
Figure 5. Stratification during summer
3.2. Seasonal Cycle of Thermal Stratification
During the summer, the surface water of a lake is
heated both indirectly by contact with warm air and
directly by sunlight. Warm water, being less dense than
cool water, remains near the surface until mixed
downward by turbulence from wind, waves, boats or other
forces. Because this turbulence extends only a limited
distance, the result is an upper layer of well-mixed, warm
water floating on the lower water, which is poorly mixed
and cool. Because of good mixing the epilimion will be
aerobic. The hypolimnion will have a lower DO and may
become anaerobic. The boundary is called the thermocline
because of the sharp temperature change that occurs
within a relatively short distance. The depth of the
epilimnion is related to the size of the lake. It is as little as
one meter in small lakes and as much as 20 meters or
more in large lakes. The depth of the epilimnion is also
related to storm activity in the spring when stratified is
developing. A major at the right time will mix warmed
water to a substantial depth and thus create a deeper than
normal epilimnion. Once formed, stratification is very
stable. It can be broken only by exceedingly violent
storms. In fact, as the summer progresses, the stability
increase because the epilimnion continues to warm, while
the hypolimnion remains at a fairly constant temperature.
In the fall, as temperatures drop, the epilimnion cools
until it is denser than the hypolimnion. The surface water
then sinks, causing overturning. The water of the
hypolimnion rises to the surface where it cools and again
sinks. The lake thus completely mixed. If the lake is in a
cold climate, this process stops when the temperature
reaches 4°C, since this is the temperature at which water is
most dense. Further cooling or freezing of the surface
water results in winter stratification. As the water warms
in the spring, it again overturns and becomes completely
mixed.
Figure 6. Seasonal cycle and turnover of thermal stratification
3.3. Factors That Affects Turnover
Wind, Solar radiation input, Depth, Air temperature,
Lake or pond size, Lake or pond bottom topography,
Streams entering lake or pond etc. are the main factors
that affects turnover.
As Thermal stratification occurs in a seasonal cycle
with the thermocline becoming more severe in late
summer and late winter. Lakes and ponds in warmer
weather regions experience a shorter annual cycle
spending more time in late summer and early fall
conditions.
3.4. Problems related to stratification [4]
• Fish kills: The spatial distribution of fish within a
lake is often adversely affected by thermal
stratification and in some cases may indirectly cause
large die-offs of recreationally important fish.
• Foul odors: As result of the release of hydrogen –
sulphide.
• Low oxygen level: For most ponds or lakes, oxygen
production is limited to wind diffusion, wave action
and photosynthesis, which often leads to dangerously
low dissolved oxygen levels, thermal stratification
and stagnation.
• Weed growth: Excessive amounts of aquatic weeds
throughout the water column can affect the balance
of the lake or pond’s ecosystem and also interferes
with recreational activities such as swimming,
fishing, boating and irrigation. Unchecked, nuisance

Applied Ecology and Environmental Sciences 5
aquatic vegetation can also cause water quality issues,
fish die offs, foul odors and an unsightly appearance.
• Algae growth: Algae feeds on nutrients in the water
that are released from decaying matter such as fallen
leaves, grass clippings, fish waste, uneaten fish food,
dead bugs, dead aquatic vegetation, as well as
phosphorous and nitrogen that washes into the pond
from the surrounding land. Large growth of algae can
lowering the depth of the lake as well as the further
use if it.
3.5. Potential Effects of Stratification on
Urban Water Authorities
One of the common causes of problems to water
resource operations is that the thermocline rises above the
designated off take level and water of poor quality is
drawn from the reservoir. Some of the most common
problems that can be associated with reservoir
stratification [9] are outlined below:
• Water treatment processes can become difficult to
manage and the cost of treatment can increase
substantially.
• The ability to meet regulatory standards in regard to
drinking water quality can become compromised.
• Water odor can become a problem, particularly as a
result of the release of hydrogen – sulphide.
• Manganese and iron particles can become prevalent
in anaerobic water. If these particles are not detected
during treatment, clothing items can become stained
during washing as these particles react with the
oxygen particles I washing detergent.
• The likelihood of receiving customer complaints can
increase.
• Adverse effects on the ecosystem within the reservoir
can occur as a result of the formation of minerals due
to low D.O. levels, potential impacts on aquatic life
such as fish kills and the potential for algal blooms to
occur may also increase.
In extreme cases, a severely stratified reservoir may
potentially need to be isolated from a water supply system
due to poor water quality.
4. Introducing Stratification Control
Measures
A healthy pond is all about nutrient and oxygen balance
[7]. The level of oxygen and how it is distributed
throughout the water column is one of the keys to pond
health. Oxygen in a pond comes from two natural sources
- diffusion from the air and, most importantly, as a
byproduct of photosynthesis. An imbalance of oxygen
occurs during the night, overcast days, ice/snow cover
when no sunlight is present and photosynthesis cannot
occur. Temperature also has a major impact on a pond's
balance of oxygen. As the temperature increases, thermal
stratification - the difference in temperature at different
depths of water - starts to occur.
An aeration system allows you to supplement the
actions of nature by adding a supply of diffused oxygen.
The use of aeration as a control measure can be very
effective to reduce stratification. Aeration would normally
be accomplished for the following purpose [7]:
• Aeration to limit the growth of algae and minimize
algae concentrations in the pond.
• Increase the Dissolved Oxygen (D.O.) level in the
basins in order to allow fish propagation.
• Increase D.O. levels in the basin to eliminate odors
and gasses escaping from the benthal (sludge)
deposits.
For ponds or lakes which are very deep, there can be a
need for destartification.
4.1. Destartification Systems
Destratification is a type of artificial circulation that
completely mixes a stratified lake's waters from top to
bottom and thereby eliminates or prevents summer
stratification [6] (the division of a lake into water layers of
different temperatures).
It is possible to provide the required oxygen in situ by
artificially mixing the lake or water bodies. It has been
shown [11] that the primary mechanism of oxygen transfer
is at the water surface even if compressed air is used as
mixing device. Riddick [10] concluded that “an aerator
should be regarded as a cheap, uncomplicated and
relatively efficient device for pumping water”.
Two techniques [13] are most common: air injection
and mechanical mixing.
• Air Injection (Diffuser) systems are the most
common destartification method. A compressor on
shore delivers air through lines connected to a
perforated pipe(s) or other simple diffuser(s) placed
near the bottom, typically in the deep area of the lake.
The rising air bubbles cause water in the
hypolimnion to also rise, pulling this water into the
epilimnion. When the colder, hypolimnion water
reaches the lake surface; it flows across the surface
and eventually sinks, mixing with the warmer
epilimentic water. Evebtually, the entire lake
becomes of nearly equal temperature with oxygen
distributed throughout. This aeration technique is
sometimes referred to as the air-lift method of
circulation.
• Mechanical axial flow pumps use a “top-down”
approach to set up a circulation pattern. A floatation
platform and frame support an electric motor,
gearbox, drive shaft and large propeller (6-15 feet
diameter). Its rotation pushes water from the lake
surface downward, setting up a circulation pattern
that prevents thermal stratification.
Figure 7. Air injection system

6 Applied Ecology and Environmental Sciences
Figure 8. Mechanical axial flow pumps
Other systems
Reference [13] proposed other systems as follows:
• Surface spray units consist of a float supporting an
electric motor-driven impeller. The rapidly-turning
impeller pulls water up a vertical tube and throws it
out in an umbrella or fountain shaped spray a few to
many feet above the lake surface. Atmospheric re-
aeration occurs in the sprayed water and at the
agitated lake surface.
Figure 9. surface Spray
Figure 10. Impeller-Aspirator systems
• Impeller-Aspirator systems consist of an electric
motor-driven impeller at the bottom of a hollow shaft
extending at an angle down into the water. The
assembly floats on the lake surface. The rapidly
turning impeller draws air down the shaft and propels
water and air bubbles into the lake. Aeration takes
place through air bubble and at the agitated lake
surface.
• Pump and Cascade system consists of a large pump
that moves lake water to the top of a ramp like chute
containing numerous baffles. The water cascades
down the ramp and falls back into the lake at a point
located as far as possible from the water inlet.
Aeration occurs in the cascade chute and in the
plunge pool as the water flows away from the ramp.
4.2. Hypolimnetic System
This system was developed in an attempt to solve some
of the problems inherent in destratification systems,
particularly to maintain the cold water layer at the bottom
of a lake or reservoir [6]. The technique introduces oxygen
to the water at the bottom of the lake without disrupting
the thermocline.
There are a few disadvantages when compared with
destratification systems. The supply of oxygen to the
hypolimnion is relatively slow because of the small
surface area available in the aeration apparatus across
which the oxygen transfer is made. For water bodies with
large hypolimnia or large surface areas, several units must
be considered.
Hypolimentic aerators can be further distinguished into
two basic groups [6]: full lift and partial lift.
Full lift systems transfer water from the hypolimnium
to the surface consequently back to the hypolimnium. This
system tends to be less prone to causing nitrogen super
saturation in the hyplimnion than the partial lift system.
Figure 11. Hypolimentic aeration unit (full lift, [11])
Figure 12. Hypolimentic aeration unit (partial lift, [1])
4.3. Benefits of Aeration [7]
• Dissolved oxygen increases to healthy levels
• Stagnation is replaced by a convection current

Applied Ecology and Environmental Sciences 7
• Thermal and Oxygen Stratification are eliminated
• Biological processes start to reduce the muck layer
• Nutrient balance is achieved
• Reductions in algae and weed growth
• Odors are eliminated as an exchange of gasses occurs
• No longer a breeding ground for Mosquito's and
Midge Fly's
• Fish habitat is restored
5. Conclusion
Aeration systems have been used to improve water
quality of lakes and reservoirs. However in some cases no
net benefit and even worsening of conditions have
occurred because of inappropriate use of aeration systems.
Because of this, a detailed evaluation of prevailing
chemical and biological limnology and anticipated effects,
as well as an engineering evaluation of most appropriate
technology is absolutely necessary.
On the other hand, too much oxygen in the water can
cause a variety of problems resulting from the water
becoming supersaturated [9]. Supersaturated water can
cause corrosion (the gradual decomposition of metal
surfaces) and sedimentation problems. In addition, air
binding occurs when excess oxygen comes out of solution
in the filter, resulting in air bubbles which harm both the
filtration and backwash process.
Aeration can also cause other problems unrelated to the
supersaturated water. Aeration can be a very energy-
intensive treatment method which can result in overuse of
energy [9]. In addition, aeration of water can promote
algal growth in the water and can clog filters.
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