Lakes with high hydraulic retention times and/or having received excessive
pollutant (note: mainly nutrient, acid, heavy metals, spills, etc.) loadings can experience significant internal loading of nutrients and pollutants from the sediments
during anoxic periods. In such cases effective treatment of lake water and/or lake
sediments may be warranted.
Among the lake water improvement technologies, physical and chemical processes are feasible for pollutant inactivation, precipitation, and biotic harvesting.
Lake water aeration is a physical process. There are many lake water aeration
systems available. They increase the oxygen content of the lake water through
mechanical mixing/agitations, air injection, or pure oxygen injection. These aeration
systems either aerate lake waters at all depths causing thermal destratification or they
preserve the thermal gradient and aerate the bottom lake waters only applying
hypolimnetic aeration. For lake restoration, thermal destratification is beneficial for
most warm water fisheries, and hypolimnetic aeration can create or greatly expand
the cold water fishery potential of a lake.
Phosphorus is generally the nutrient that can be significantly removed. Phosphate
removal is usually the primary objective of lake restoration. Using pollutant (mainly
nutrient) inactivation or precipitation for treatment of standing bodies of lake water
has been practiced since 1983 when 2 mg/L of ferric chloride was applied to a few
selected reservoirs and lakes in Berkshire County, MA, USA. The first lake-wide
application of aluminum sulfate for nutrient inactivation occurred at Langston,
Sweden, in 1968. Since 1970 most of the larger lake water treatments using
aluminum sulfate have occurred in the United States, such as Horseshoe Lake,
Wisconsin, Dollar and Twin Lakes in Ohio, and Liberty Lake and Medical Lake
in Washington, USA. In 1971 to 1974, the highly eutrophic Cline Pond near
Corvallis, Oregon, USA, was treated by sodium aluminate and zirconium tetrachloride. In all the aforementioned cases, chemicals were dosed to lake waters by barge
distribution and/or manifold injection. Most of the lakes treated by chemicals have
shown reduced phosphorus content and less nuisance algal growth as well as higher
hypolimnetic dissolved oxygen [45]. There are many ways to control the nutrients
entering a lake, to control the existing nutrients within a lake, and to reduce the
existing nutrients within a lake. Each lake must be studied individually to determine
the best method to control eutrophication.
Biotic separation and harvesting is an important technique of nutrient removal
that can lead to a reversal of eutrophic conditions in lakes. Algae and other aquatic
plants such as water hyacinths, Typha latifolia, etc., can produce more biomass
containing phosphorus and nitrogen. Harvesting of these aquatic plant species thus is
an efficient method for nutrient removal from lakes. However, even the greatest
potential harvest will not remove enough nutrients to offset moderate to heavy
nutrient loading to lakes. Biotic harvesting as a lake restoration technique may
work only where phosphorus loading has already been reduced to less than 1 g/
m
2 /year [45]. A wide variety of mechanical harvesters has been designed for aquatic
weed harvesting. The most efficient and cost-effective technique for water-algae
separation and algae harvesting appears to be dissolved air flotation [43–46].
7 Lake Restoration
305
pollutant (note: mainly nutrient, acid, heavy metals, spills, etc.) loadings can experience significant internal loading of nutrients and pollutants from the sediments
during anoxic periods. In such cases effective treatment of lake water and/or lake
sediments may be warranted.
Among the lake water improvement technologies, physical and chemical processes are feasible for pollutant inactivation, precipitation, and biotic harvesting.
Lake water aeration is a physical process. There are many lake water aeration
systems available. They increase the oxygen content of the lake water through
mechanical mixing/agitations, air injection, or pure oxygen injection. These aeration
systems either aerate lake waters at all depths causing thermal destratification or they
preserve the thermal gradient and aerate the bottom lake waters only applying
hypolimnetic aeration. For lake restoration, thermal destratification is beneficial for
most warm water fisheries, and hypolimnetic aeration can create or greatly expand
the cold water fishery potential of a lake.
Phosphorus is generally the nutrient that can be significantly removed. Phosphate
removal is usually the primary objective of lake restoration. Using pollutant (mainly
nutrient) inactivation or precipitation for treatment of standing bodies of lake water
has been practiced since 1983 when 2 mg/L of ferric chloride was applied to a few
selected reservoirs and lakes in Berkshire County, MA, USA. The first lake-wide
application of aluminum sulfate for nutrient inactivation occurred at Langston,
Sweden, in 1968. Since 1970 most of the larger lake water treatments using
aluminum sulfate have occurred in the United States, such as Horseshoe Lake,
Wisconsin, Dollar and Twin Lakes in Ohio, and Liberty Lake and Medical Lake
in Washington, USA. In 1971 to 1974, the highly eutrophic Cline Pond near
Corvallis, Oregon, USA, was treated by sodium aluminate and zirconium tetrachloride. In all the aforementioned cases, chemicals were dosed to lake waters by barge
distribution and/or manifold injection. Most of the lakes treated by chemicals have
shown reduced phosphorus content and less nuisance algal growth as well as higher
hypolimnetic dissolved oxygen [45]. There are many ways to control the nutrients
entering a lake, to control the existing nutrients within a lake, and to reduce the
existing nutrients within a lake. Each lake must be studied individually to determine
the best method to control eutrophication.
Biotic separation and harvesting is an important technique of nutrient removal
that can lead to a reversal of eutrophic conditions in lakes. Algae and other aquatic
plants such as water hyacinths, Typha latifolia, etc., can produce more biomass
containing phosphorus and nitrogen. Harvesting of these aquatic plant species thus is
an efficient method for nutrient removal from lakes. However, even the greatest
potential harvest will not remove enough nutrients to offset moderate to heavy
nutrient loading to lakes. Biotic harvesting as a lake restoration technique may
work only where phosphorus loading has already been reduced to less than 1 g/
m
2 /year [45]. A wide variety of mechanical harvesters has been designed for aquatic
weed harvesting. The most efficient and cost-effective technique for water-algae
separation and algae harvesting appears to be dissolved air flotation [43–46].
7 Lake Restoration
305