dissolved air flotation [46]. It appears that DAF can adequately treat the dredged
materials for removal of nutrients, heavy metals, and other conventional and volatile
pollutants.
6.4 Sediment Fixation
Eutrophic lakes are synonymous with significant organic bottom deposits. When
these become anaerobic, they release their nutrients, specifically phosphorus. As the
lake overturns, these nutrients are distributed throughout the lake, enabling more
biological growth, which ultimately dies and settles to the bottom. Instead of trying
to remove these sediments, chemicals may be added to more permanently precipitate
the phosphorus. Aluminum salts have been found to be most effective since the
aluminum phosphate remains insoluble so long as the surface of the sediments, in
contact with aerobic water, remains aerobic [24, 25]. Iron salts are effective in
precipitating phosphates, but in the deep anaerobic sediments, the iron combines
with reduced sulfur to form ferrous sulfide that is more insoluble than the iron
phosphate, thus releasing the phosphate back into solution. Calcium salts are also
capable of forming precipitates of calcium phosphate; however, their high alkalinity
may undesirably raise the pH of the water. This may be desirable in acid lakes. Thus,
aluminum salts have been found to be most effective in tying up the phosphate
permanently in the sediments. As more organic material settles to the bottom,
reapplication may be necessary in future years. This becomes extremely expensive
for large lakes.
One difficulty in binding the sediment phosphate is establishing adequate contact.
The alum must be spread fairly uniformly over the bottom to be effective. This is
usually achieved by the use of boats crisscrossing the lake. A novel system was set
up in a sewage oxidation pond in California [26]. A mechanical mixer was installed
in the middle of the pond, providing both mixing and aeration. Alum was applied at
the mixer, which was solar powered. This eliminated a long power cord. The alum
combined with both the sediment phosphorus and the soluble or suspended phosphorus in the pond, settling to the bottom. Excessive biological growth was eliminated, and the upper liquid layer met the phosphorus discharge limits to the receiving
water.
7 Hypolimnetic Phosphorus Removal by DAF
A different approach is to remove the excess phosphorus from the anaerobic
hypolimnion. Here the phosphorus level may be high enough to be removed by
conventional precipitation by aluminum, iron, or calcium salts. A flocculation/
filtration system located on the shore could accomplish this. Successful use of
such a program at three lakes in Germany has been reported [27]. Further, a DAF
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materials for removal of nutrients, heavy metals, and other conventional and volatile
pollutants.
6.4 Sediment Fixation
Eutrophic lakes are synonymous with significant organic bottom deposits. When
these become anaerobic, they release their nutrients, specifically phosphorus. As the
lake overturns, these nutrients are distributed throughout the lake, enabling more
biological growth, which ultimately dies and settles to the bottom. Instead of trying
to remove these sediments, chemicals may be added to more permanently precipitate
the phosphorus. Aluminum salts have been found to be most effective since the
aluminum phosphate remains insoluble so long as the surface of the sediments, in
contact with aerobic water, remains aerobic [24, 25]. Iron salts are effective in
precipitating phosphates, but in the deep anaerobic sediments, the iron combines
with reduced sulfur to form ferrous sulfide that is more insoluble than the iron
phosphate, thus releasing the phosphate back into solution. Calcium salts are also
capable of forming precipitates of calcium phosphate; however, their high alkalinity
may undesirably raise the pH of the water. This may be desirable in acid lakes. Thus,
aluminum salts have been found to be most effective in tying up the phosphate
permanently in the sediments. As more organic material settles to the bottom,
reapplication may be necessary in future years. This becomes extremely expensive
for large lakes.
One difficulty in binding the sediment phosphate is establishing adequate contact.
The alum must be spread fairly uniformly over the bottom to be effective. This is
usually achieved by the use of boats crisscrossing the lake. A novel system was set
up in a sewage oxidation pond in California [26]. A mechanical mixer was installed
in the middle of the pond, providing both mixing and aeration. Alum was applied at
the mixer, which was solar powered. This eliminated a long power cord. The alum
combined with both the sediment phosphorus and the soluble or suspended phosphorus in the pond, settling to the bottom. Excessive biological growth was eliminated, and the upper liquid layer met the phosphorus discharge limits to the receiving
water.
7 Hypolimnetic Phosphorus Removal by DAF
A different approach is to remove the excess phosphorus from the anaerobic
hypolimnion. Here the phosphorus level may be high enough to be removed by
conventional precipitation by aluminum, iron, or calcium salts. A flocculation/
filtration system located on the shore could accomplish this. Successful use of
such a program at three lakes in Germany has been reported [27]. Further, a DAF
272
L. K. Wang et al.