Environmental Engineering Reference
In-Depth Information
The lack of a clear response to different external P
reductions does not, however, preclude an improve-
ment in water quality (Cooke
et al.
1993). Most
studies apparently did not monitor changes in lakes
long enough for the P levels to record a decrease below
a certain threshold level. A lake may take longer than
a decade to respond, as observed during a long-term
study in Loosdrecht Lakes (see papers in van Liere &
Gulati 1992).
conditions in the upper mixed layers (see Visser
et al.
1996), leading to shifts in phytoplankton dominance
by non-cyanobacterial forms. It may be concluded
that mixing or circulation, unlike nutrient reduction,
produces instantaneous improvements in water qual-
ity, without decreasing the nutrient concentrations or
loading. Moreover, it is more effective for deeper
lakes and long-lasting, positive effects of artificial cir-
culation are virtually absent, implying the need for
its repeated application.
12.3.2 In-lake measures of P-reduction
Preventing the internal P loading from sediments
External P-load reductions do not guarantee im-
provements in water quality of lakes in the short term
(van Liere & Gulati 1992), primarily because of ac-
cumulated P in lake sediments. Apparently, recycling
of this P from the sediments becomes more important
if the P in inflows is reduced. Therefore, high P con-
centrations, often observed in lakes during summer
(Hansen
et al.
2003), and algal photosynthesis and pro-
duction can go on unabated as they did before the
external control measures were taken. Thus, increased
internal loading may keep lakes in a eutrophic state
for years after the reduction of external loading (Sas
et al.
1989). We briefly discuss below a few restora-
tion methods (Cooke
et al.
1993) used for reducing
internal loading.
Phosphorus cycling between the sediments and water
is a complex and a relatively poorly understood phe-
nomenon. The release of P from sediment depends on
both redox and pH and involves bacterial decom-
position of organic matter, including algal blooms
(Brunberg & Bostróm 1992). Oxygen of organic mat-
ter will enhance greater binding of P in the sediment
complexes. The sediment phosphates can thus be
inactivated by 'sealing' or stripping, using the salts of
Ca, Fe or Al, to precipitate both inorganic P and par-
ticulate P, which then sediment as a floc (see Cooke
et al.
1993). The process removes up to 90 -95% of
P. However, if not bound firmly in the sediment this
P will contribute to a later increase in internal load-
ing. In the Netherlands, Boers
et al.
(1992) applied FeCl
3
solution (100 g of Fe
3+
m
−2
d
−1
) for P stripping. The FeCl
3
solution was diluted 100-fold with lake water and
mixed with surface sediments using a water jet. High
external loading of P, short residence time of water
and loss of binding capacity of FeCl
3
nullified the
positive effects. Al immobilizes P more efficiently
than Fe (Hansen
et al.
2003): it has a higher sorptive
capacity (for details see also Lewandowski
et al.
2003). However, Al is potentially toxic and its use for
removal of P in public water supplies should be
avoided.
Mixing of the water column
Artificial mixing involves aerating or oxygenating lakes
using pumps and jets and bubbled air from perforated
pipes at the lake bottom. The technique has been used
in the states of Minnesota and Wisconsin in the USA
to prevent fish dying during ice cover (Cooke
et al.
1993). Aeration oxidizes substances in the water
column, with an increased complexation of Fe and
Mn in the sediments, and reduction in internal P load-
ing due to a release from sediment. In addition, algal
biomass is reduced due to expansion of the mixed
layers and increased light limitation. However, nutrient
availability for phytoplankton in the photic zone may
even increase by circulation, causing an increase in
phytoplankton biomass.
Although literature on the effects of artificial aera-
tion on water quality is scarce, the mixing seems to
prevent cyanobacteria from exploiting the optimal light
Sediment removal by dredging
A drastic but more expensive technique to reduce
internal nutrient loading is to remove the P-rich sedi-
ment. Dredging simultaneously eliminates the toxic
and hazardous compounds and rooted aquatic plants
(Peterson 1981), and has the advantage that it does
not introduce alien substances into the waterbody.
