Environmental Engineering Reference
In-Depth Information
bacterial decomposition of organic matter, including
algal blooms. Several techniques have been applied to
inactivate sediment P. Oxidation of organic matter en-
hances P binding in the sediment complexes. 'Sealing'
or stripping inactivates the sediment P, using the salts
of Ca, Fe or Al, to precipitate all P species to sediment
as a fl ock (see Cooke
et al
. 2005 ). The process removes
up to 90-95% of P. In the Netherlands, Boers
et al
.
(1992) applied FeCl
3
solution (about 5 mg Fe
3+
l
− 1
d
− 1
)
for P stripping by mixing the solution with lake water
and surface sediments using a water jet. However, high
external loading of P nullifi ed the positive effects.
Although Al immobilizes P more effi ciently than Fe, it
is potentially toxic and its use for P reduction in public
water supplies is generally avoided. Recently some
researchers have attempted to treat lake water with
chemical compounds including Phoslock
®
, a commer-
cial bentonite product coated with lanthanum, to
reduce P concentration in lake water. For example,
Egemose
et al
. (2010) treated water from three Danish
lakes with Phoslock
®
and evaluated the effects on P
concentration. Although this compound took several
days to reduce P levels, the sediment stability increased
by 265%. Moreover, this compound improved sediment
consolidation and removed colouration from the water,
thereby improving light penetration, which in turn
improved conditions for macrophyte colonization.
Wind-induced resuspension of organic matter and
nutrients and mixing with the water column can also
be reduced by creating deeper pits in shallow lakes.
Penning
et al
. (2010) carried out a model study in Lake
Loosdrecht, a shallow (Z
mean
1.8 m) and peaty lake in
the Netherlands. They calculate the effect of creating
three deep pits (
c
. 12 m), covering 10% (120 ha) of the
lake's total surface area (∼ 1200 ha). The study predicts
a decrease in the concentration of organic detritus by
25%, and in the light attenuation coeffi cient (α ) from
2.5 m
− 1
to 2.2 m
− 1
. Together with the P load reduction
measures and decrease in chlorophyll-a level, α is
anticipated to decrease to 2.0 m
− 1
, and to 1.6 m
− 1
if
combined with the deepening of the lake. Such
improvements of the underwater light climate are the
fi rst steps for the recovery of a submerged macrophyte
community. Such studies are extremely useful in
shallow, eutrophic lakes where wind-induced mixing
(and mixing reinforced by benthivorous fi sh) and con-
stant resuspension of nutrient-rich sediments delay
the lake recovery processes (Gulati & van Donk 2002).
Sediment dredging leads to a reduction of the inter-
nal P loading. Field studies concerning sediment
removal by dredging are rare, however, because they
are relatively expensive. Moreover, the sediment dred-
ging eliminates the toxic and hazardous compounds
and rooted aquatic plants, in addition to P removal.
Nevertheless, sediment removal has three disadvan-
tages: (1) during dredging, P from the sediments is
released into the water column, (2) fi nding disposal
sites for the extracted sediments is a major dilemma for
the lake restorers and managers and (3) new underly-
ing sediment layers exposed to the water could even
enhance P release.
Dilution and fl ushing of the lake
with n utrient - p oor w ater
Lakes can be replenished with water from extraneous
sources or from lakes with lower nutrient levels but
preferably rich in Ca
2++
and HCO
3
−−
. Dilution as a res-
toration tool, therefore, implies reducing per se the
nutrient levels in lake water to limiting concentrations
(i.e. fl ushing rate with diluted water exceeds the algal
growth rate). Cooke
et al
. ( 2005 , 149 - 64) cite several
examples where this technique was successfully applied
in lakes in the United States and in Europe. The winter
period is the best for fl ushing since both the dilution
water and the lake to be diluted have less suspended
material and algae than in other seasons. Because
good-quality dilution water is generally scarce and
expensive, due to the logistics of its transport, the tech-
nique has not gained great popularity. Flushing may
also be done with bicarbonate-rich water. In two lakes
in the Netherlands, winter fl ushing with bicarbonate-
rich (and P-poor) dilution water since 1979 resulted
in a pronounced drop in P-release. The increased bicar-
bonate is supposed to cause a better binding of P to the
sediments through buffering and lowering of the pH
(Hosper & Meijer 1986 ; Hosper 1997 ).
Lake b iomanipulation
1.
The t heoretical a spects
Lake biomanipulation or food web manipulation has
become routine for improving water quality of shallow
lakes and reservoirs. It both has an ecological basis
(Reynolds 1994) and complements the restoration
methods involving nutrient reduction. In conjunction,
these two measures can speed up lake rehabilitation.
Two important hypotheses have enhanced our insights
into food web relationships: (1) the
size - effi ciency
