6 Recovery of Eutrophic Lakes
The best way to prolong the life of a lake is to control the nutrient inputs to the lake
before it progresses through the mesotrophic state to the eutrophic state. This is
sometimes difficult or even impossible. If upon study of a lake recovery is considered possible, numerous methods are available [18–20].
6.1 Aeration
Several variations of aeration are available to prevent the hypolimnion from becoming anaerobic. This will tie up the phosphorus in an insoluble form and keep the
surface of the bottom deposits aerobic to prevent resolubilization of the phosphorus.
Aeration is generally more applicable to small lakes. The pressure to pump air to the
bottom of a deep lake requires special equipment.
When air is used, the system is designed to create a circulation within the lake so
that anaerobic hypolimnetic water is brought to the surface where natural reaeration
occurs. Whereas some reaeration results from the addition of the air, the surface
aeration is responsible for most of the reaeration. More than one air system may need
to be placed in a lake depending upon the shape of the lake. A disadvantage of the
complete circulation system is that the thermocline is destroyed and the lake
becomes isothermal from top to bottom at a mean temperature. Air systems must
be turned on before the hypolimnion becomes anaerobic. These systems are relatively inexpensive.
A modification of the plain aeration system is a hypolimnetic aeration system.
This consists of two concentric vertical tubes normally placed entirely in the
hypolimnion. The top of the larger tube is sealed. Water from near the bottom of
the lake enters the smaller inner tube where an aerator both lifts the water and aerates
it at the same time. At the top of the inner tube, the water overflows into the larger
outer tube and is carried back downward. The aerated discharge from the larger tube
is generally above the intake to minimize short circuiting back to the inlet tube. Since
the entire device is placed in the cold hypolimnion, there is little impact on the
temperature in the hypolimnion. Judicious placement of the intake and the discharge
minimizes the impact on the lake bottom, and the system maintains the normal
thermal stratification of the lake.
Oxygen has also been used instead of air. In this case, the oxygen provides the
source of the reaeration. This usually requires on-site generation of the oxygen.
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L. K. Wang et al.
The best way to prolong the life of a lake is to control the nutrient inputs to the lake
before it progresses through the mesotrophic state to the eutrophic state. This is
sometimes difficult or even impossible. If upon study of a lake recovery is considered possible, numerous methods are available [18–20].
6.1 Aeration
Several variations of aeration are available to prevent the hypolimnion from becoming anaerobic. This will tie up the phosphorus in an insoluble form and keep the
surface of the bottom deposits aerobic to prevent resolubilization of the phosphorus.
Aeration is generally more applicable to small lakes. The pressure to pump air to the
bottom of a deep lake requires special equipment.
When air is used, the system is designed to create a circulation within the lake so
that anaerobic hypolimnetic water is brought to the surface where natural reaeration
occurs. Whereas some reaeration results from the addition of the air, the surface
aeration is responsible for most of the reaeration. More than one air system may need
to be placed in a lake depending upon the shape of the lake. A disadvantage of the
complete circulation system is that the thermocline is destroyed and the lake
becomes isothermal from top to bottom at a mean temperature. Air systems must
be turned on before the hypolimnion becomes anaerobic. These systems are relatively inexpensive.
A modification of the plain aeration system is a hypolimnetic aeration system.
This consists of two concentric vertical tubes normally placed entirely in the
hypolimnion. The top of the larger tube is sealed. Water from near the bottom of
the lake enters the smaller inner tube where an aerator both lifts the water and aerates
it at the same time. At the top of the inner tube, the water overflows into the larger
outer tube and is carried back downward. The aerated discharge from the larger tube
is generally above the intake to minimize short circuiting back to the inlet tube. Since
the entire device is placed in the cold hypolimnion, there is little impact on the
temperature in the hypolimnion. Judicious placement of the intake and the discharge
minimizes the impact on the lake bottom, and the system maintains the normal
thermal stratification of the lake.
Oxygen has also been used instead of air. In this case, the oxygen provides the
source of the reaeration. This usually requires on-site generation of the oxygen.
270
L. K. Wang et al.