(TSS), CWs when effectively designed can provide reliable removal without any
clogging problems for more than 20 years [74].
Plants stabilize the surface of the beds, provide good conditions for physical
filtration, prevent vertical flow systems from clogging, insulate the surface against
frost during winter, and provide a huge surface area for attached microbial growth
[75]. Plants metabolism affects the treatment processes to different extents
depending on the CW design. The most active reaction zone of constructed wetlands
is the rhizosphere (the root zone). This is where physicochemical and biological
processes take place that are induced by the interaction of plants, microorganisms,
the substrate and pollutants [76], including pharmaceuticals degradation [60]. Plant
uptake of nutrients is only of quantitative importance in low-loaded systems –
surface flow system (SF) [75]. However, for organic contaminants, plant uptake
and phytovolatilization, as well as contaminant accumulation and metabolic transformation, can be relevant for different plants and compounds [77], as well as
different CW designs.
The relative importance of a particular removal process can vary significantly,
depending on the organic contaminant being treated, the wetland type (e.g. SF, SSF,
VF) and operational design (e.g. retention time), the environmental conditions, the
type of vegetation within the system, as well as the substrate [29]. Anaerobic
processes predominate in HSSF (apart from in the proximity of the macrophyte
roots), facultative or aerobic processes usually prevail in SF, while VF and aerated
systems are dominated by aerobic processes. Hybrid systems are an effective way to
sequence these processes by including different designs/beds in a single treatment
system. CWs support a large spectrum of biogeochemical reactions and various
environmental conditions within a single bed. In fact, CWs are complex bioreactors
characterized by considerable fluxes of material and energy governing chemical
reactions over spatial and temporal gradients [73]. From an engineering perspective,
they are also attractive biofilm reactors in which the biofilm can be manipulated,
depending on the CW design and operation. Thus, pollutant removal depends on two
processes simultaneously occurring at different scales: (1) the various and coexisting
redox processes within a bed, and (2) the processes driven by the plant, namely,
those occurring at the rhizosphere scale. Two parameters can be helpful to understand the status of a given reactor/CW, the dissolved oxygen and the oxidation
reduction potential (ORP), including for pharmaceuticals removal [27, 28,
50]. Moreover, one should bear in mind that CWs are not a homogenous reactor,
but in fact zonation occurs in terms of pollutant concentration (inflow area > outflow
area) and with bed depth. The hydraulic retention time (HRT), including the length
of time the water is in contact with the substrate, biofilm and the plant roots, affects
the extent to which the removal or biotransformation of pollutants can occur. Typical
HRT is in the order 7–10 days in SF, 2–5 days in HSSF and minutes to hours in
VF. There is the general idea that longer retention times result in better performance,
which is in general true for SF and HSSF [29]. However, such comparison should
only be performed for the same design type, thus, CWs sharing the same removal
processes. A clear example is the higher nitrification capacity of VF versus HSSF,
including a minor area requirement for the same pollutant load, but indeed with a
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P. N. Carvalho
clogging problems for more than 20 years [74].
Plants stabilize the surface of the beds, provide good conditions for physical
filtration, prevent vertical flow systems from clogging, insulate the surface against
frost during winter, and provide a huge surface area for attached microbial growth
[75]. Plants metabolism affects the treatment processes to different extents
depending on the CW design. The most active reaction zone of constructed wetlands
is the rhizosphere (the root zone). This is where physicochemical and biological
processes take place that are induced by the interaction of plants, microorganisms,
the substrate and pollutants [76], including pharmaceuticals degradation [60]. Plant
uptake of nutrients is only of quantitative importance in low-loaded systems –
surface flow system (SF) [75]. However, for organic contaminants, plant uptake
and phytovolatilization, as well as contaminant accumulation and metabolic transformation, can be relevant for different plants and compounds [77], as well as
different CW designs.
The relative importance of a particular removal process can vary significantly,
depending on the organic contaminant being treated, the wetland type (e.g. SF, SSF,
VF) and operational design (e.g. retention time), the environmental conditions, the
type of vegetation within the system, as well as the substrate [29]. Anaerobic
processes predominate in HSSF (apart from in the proximity of the macrophyte
roots), facultative or aerobic processes usually prevail in SF, while VF and aerated
systems are dominated by aerobic processes. Hybrid systems are an effective way to
sequence these processes by including different designs/beds in a single treatment
system. CWs support a large spectrum of biogeochemical reactions and various
environmental conditions within a single bed. In fact, CWs are complex bioreactors
characterized by considerable fluxes of material and energy governing chemical
reactions over spatial and temporal gradients [73]. From an engineering perspective,
they are also attractive biofilm reactors in which the biofilm can be manipulated,
depending on the CW design and operation. Thus, pollutant removal depends on two
processes simultaneously occurring at different scales: (1) the various and coexisting
redox processes within a bed, and (2) the processes driven by the plant, namely,
those occurring at the rhizosphere scale. Two parameters can be helpful to understand the status of a given reactor/CW, the dissolved oxygen and the oxidation
reduction potential (ORP), including for pharmaceuticals removal [27, 28,
50]. Moreover, one should bear in mind that CWs are not a homogenous reactor,
but in fact zonation occurs in terms of pollutant concentration (inflow area > outflow
area) and with bed depth. The hydraulic retention time (HRT), including the length
of time the water is in contact with the substrate, biofilm and the plant roots, affects
the extent to which the removal or biotransformation of pollutants can occur. Typical
HRT is in the order 7–10 days in SF, 2–5 days in HSSF and minutes to hours in
VF. There is the general idea that longer retention times result in better performance,
which is in general true for SF and HSSF [29]. However, such comparison should
only be performed for the same design type, thus, CWs sharing the same removal
processes. A clear example is the higher nitrification capacity of VF versus HSSF,
including a minor area requirement for the same pollutant load, but indeed with a
396
P. N. Carvalho