annual basis in many situations, but could be feasible during summer low flows
Fiala and Vasata (1982)
when symptoms of eutrophication are most extreme.
also note that maintenance of aerobic conditions in the siltation basin is
essential to phosphorus removal, presumably because iron and/or calcium
plexation of phosphorus may be involved and because anaerobic reservoir sedi-
ments may release phosphorus at high rates and thus reduce the efficiency of
In deep preimpoundment basins, maintenance of aerobic conditions
removal.
might require artificial circulation.
Dry dams
In some areas, dry dams have been constructed to aid in flood control.
These dams may be particularly effective in silt and nutrient control as well,
since they receive and store the "first-flush" runoff, a portion of the
graph that can be heavily loaded with pollutants.
Prereservoir phosphorus removal
Wahnbach Reservoir (Federal Republic of Germany) was impounded in 1957.
Within 10 years, treatment of the water for drinking purposes became very
expensive, and organic compounds excreted into the water by algae were forming
precursors for the development of trihalomethanes.
Phosphorus was shown to be
the limiting element, and nutrient budget studies showed that more than
50 percent of it came from diffuse or
sources on the watershed,
making sufficient diversion nearly impossible. A smaller reservoir
3
(500,000 m
to serve as a floodwater retention basin and a phosphorus elimi-
nation plant (PEP) were built at the upper end of the main reservoir.
After detention in the smaller reservoir, water is pumped into the PEP
and treated with F e
to precipitate phosphorus, followed by a
electrolyte to form large
The water is then filtered through layers of
activated carbon, hydroanthracite, and quartz sand. The plant can handle up
-1
to 5
, and about 95 to 99 percent of phosphorus-containing compounds
are eliminated.
Output concentration to the main reservoir averaged 4
P
over 2 years.
Also, the PEP has high removal (99 percent) of colifonn
bacteria, chlorophyll, and turbidity, and lesser removal of chemical oxygen
demand (77 percent) and dissolved organic carbon (58 percent).
Water dis-
charged to the reservoir approaches drinking water quality, and the
state of the reservoir is now nearly oligotrophic.
Detailed descriptions of
the PEP at Wahnbach Reservoir are provided by Bernhardt (1980, 1981).
Costs
have not been reported.
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