Environmental Engineering Reference
In-Depth Information
Figure 14.3.
Diagrammatic representation of a Helixor mixing system.
a marked reduction in pollutants since isolation and demonstrated that the
mixing system prevented bottom water anoxia and eradicated stratification.
The Helixors created a uniformly high DO (
90% saturation) throughout the
water column (Hendry et al.
1993
); a notable achievement compared to the
episodic bottom water anoxia experienced before and immediately following
isolation (
Fig. 14.4
). Maintenance of the aerobic layer at the sediment-water
interface is also demonstrated by the elimination of sulphide in the sediments
(Walker et al.
1993
).
The well-oxidised sediment water interface reduced the release of phos-
phorus from the sediments and, over time, eliminated release altogether (e.g.,
as observed by Sas
1989
). A general reduction in orthophosphate levels was
observed after isolation, which continued following the installation of the
Helixor mixing system. Concentrations generally remained below 0.05mg L
1
(Walker et al.
1993
), whilst over the same period 1.67mg L
1
were present in the
MSC. Recent 5-year average figures (2003 2007) for the Quays and the MSC are
0.023mg L
1
and 0.453mg L
1
, respectively. Ammonia concentrations were
reduced to 0.12mg L
1
in 1988 following isolation (Walker et al.
1993
) and
rarely approached 1mg L
1
(White et al.
1993
), whilst levels prior to impound-
occurred, as illustrated in the most recent 5-year average (2003 2007) concen-
tration of 0.035mg L
1
(APEM unpublished data).
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