Environmental Engineering Reference
In-Depth Information
3.4.4.1 Precipitation
Precipitation is the formation of insoluble products
from the combination of soluble reactants. The
removal of phosphorus is ultimately achieved
through physical storage of the phosphorus precipi-
tates within the substrate media. Naturally occur-
ring soluble iron and aluminium or calcium can
result in significant removal of phosphorus by pre-
cipitation (Ayoub et al.
2001
; de-Bashan and
Bashan
2004
). To enhance the phosphorus precipi-
tation within a wetland, at times, chemical dosing
is required, although this chemical dosing can be a
great disadvantage as the operational simplicity
inherent in the wetland system is lost and the poten-
tial for clogging increases (Brady and Weil
1999
).
Alum [Al(SO
4
)
2
.12H
2
0] and ferric chloride
(FeCl
3
) are most commonly used in conventional
wastewater treatment facilities to precipitate
phosphorus, with subsequent physical removal
from the system by co-settling with the organic
suspended solids (Lind
1998
).
be the most significant phosphorus removal
mechanism (Moshiri
1993
). Sorption is a general
term that encompasses both adsorption and
absorption mechanisms and is a time-limited pro-
cess based on the available surface area. Adsorption
is the process of accumulating substances on the
media surface, while absorption refers to the accu-
mulation within the pore structure of the media
(Stumm and Morgan
1996
). Sorption reactions are
based on an equilibrium condition where the rates
of reaction are fast in relation to the physical pro-
cesses of advection and dispersion (Kueper
2003
).
The process occurs on two different levels, viz.,
molecular and filter bed scales.
4
Removal of Heavy Metals
A heavy metal is a member of a loosely defined
subset of elements that exhibit metallic proper-
ties. It mainly includes the transition metals,
some metalloids, lanthanides and actinides.
When heavy metals enter a CW, they are distrib-
uted among its compartments with the main
pools being the substrate, the water column and
the vegetation (Dunbabin and Bowmer
1992
;
Sheoran and Sheoran
2006
). Kadlec and Knight
(
1996
) described the following major processes
responsible for the removal of heavy metals in
CWs: (i) binding to the substrate, particulates and
soluble organics; (ii) precipitation as insoluble
salts, mainly sulphides and (oxy)hydroxides; and
(iii) uptake by plants and microorganisms. There
are only minor gaseous removal pathways for
some metals including Hg, Se and As. Sheoran
and Sheoran (
2006
) have subdivided the mecha-
nisms into physical, chemical and biological pro-
cesses although the authors emphasised that the
processes are dependent on each other.
(
)
+
(
)
Al SO
2
Na PO
→
2
AlPO
↓+
3
Na SO
2
4
3
3
4
4
2
4
(
)
+ →
(
)
↓+
Al SO
6
HO Al OH
2
3
HSO
2
4
3
2
2
4
3
FeCl Na PO
+ →↓+
FePO
3
NaCl
3
3
4
4
+ →
(
)
↓+
FeCl HO Fe OH
3
3
HCl
3
2
3
Lime (CaO) can also be used for phosphorus
removal as (Lind
1998
):
→
(
)
↓+
3
CaO aPOHO aPO
+
2
+
3
6
NaOH
3
4
2
3
4
2
+ →
(
)
↓
CaOHO aOH
2
2
CaOCO aCO
+ → ↓
2
3
Chemical dosing is usually done prior to the
wetland. The operation and implementation of
chemical dosing is simple but results in increased
sludge and associated disposal costs and
increased operation and maintenance costs. The
quality of effluent achieved can be as low as
0.1 mg/L of phosphorus using chemical dosing.
4.1
Physico-chemical Removal
Mechanisms of Heavy Metals
Major physico-chemical removal processes of
heavy metals occurring in CWs include (1) sedi-
mentation and filtration, (2) sorption and (3) pre-
cipitation and co-precipitation.
3.4.4.2 Sorption
Sorption is the process of a substance adhering to
the surface of a wetland media and is considered to
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