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Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
the Watershed
- Phytotechnology and Ecohydrology -
7. MANAGEMENT OF FLOWS IN LARGE RIVERS FOR
WATER QUALITY IMPROVEMENT
A. Water quality improvement in constructed and natural wetlands - general
information
Fig. 7.1. Digital Terrain Model (DTM) of a potential
location above a lowland, drinking water reservoir for a constructed wetland
for water quality improvement
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Wetland systems have been successfully used
for water quality improvement. They may act as reservoirs, maintaining the
natural biogeochemical links between land, water and biota, or as a final step
in conventional wastewater treatment technologies. There are three basic types
of wetland systems: natural wetlands, constructed surface flow (SF) wetlands,
and constructed sub-surface flow (SSF) wetlands.
Water levels in the SSF
wetlands are usually less than 0.4 m. The purification processes are enhanced
by the variety of plant species. SSF are usually vegetated with the common reed
(Phragmites australis), cattail (Typha sp.), and/or bulrush
(Scirpus sp.). Water treatment occurs as a
result of microbiological processes taking place in a substrate that forms the
bed of the treatment wetland and is aided by uptake through the roots of the
vegetation.
In SF wetlands, treatment is primarily mediated through suspended matter
sedimentation and the vegetative uptake of contaminants. SF wetlands and
natural wetlands have variable water depths ranging from periodic inundation in
the case of wet meadows to a meter or more of water in deep water marshes.
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Wetlands have been used for water
purification in the following cases:
- Treatment of municipal, industrial, and agricultural wastewaters;
- Treatment of acidic mine drainage waters;
- Reduction of nutrient and pollutant
loads in runoff and stormwaters;
- Reduction of nutrient load in rivers
supplying downstream reservoirs.
The advantage of using wetlands as a
treatment measure is the variety of processes that occur with the wetlands that
combine to contribute to water quality improvement. It is this combination of
biogeochemical processes that make wetlands efficient treatment systems. These
processes include:
- Sedimentation, filtration, and
sorption of particulate matter within the wetland due to long water retention
times and large sediment surface areas;
- Assimilation and retention of
dissolved nutrients within the biomass present in the wetland;
- Oxidation and microbial transformation
of organic matter in wetland sediments;
- Denitrification of nitrogenous
compounds by microbial action within the wetland system.
There are the several advantages and some
limitations to wetland systems used in wastewater treatment applications. These
appear below and need to be taken into account before such wetlands are created
or designated for use in wastewater treatment processes (based on Moshiri 1993).
B. Dynamics of nutrient loads during flood events
The timing of nutrient loading transported to wetlands by rivers is determined by
catchment characteristics, climate (especially precipitation), and rivers
hydrology. Usually, in a case of highly degraded catchments having a
considerable contribution of non-point source pollutants, surface runoff resulting
from intensive rain events
can transfer large amounts of nutrients from the catchment, via the river, to
the wetland. These nutrients can be derived from materials adsorbed on to
eroded soils or from dissolved nutrient leaching (Figure 7.2).
Advantages:
- Low costs of construction and maintenance;
- Low energy requirements;
- Limited training requirements for operators ("low-technology"
system);
- Ease of creation using locally-available skills and materials;
- Greater flexibility and less susceptibility to loading variations than
conventional systems;
- Multiple purpose:
- Increased habitat and biodiversity;
- Flood control;
- Increased aesthetic appeal and value;
- Energy
and marketable plant production for sustainable development
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- Large land area requirements (thus recommended for small- to
medium-sized distributed communities);
- Reduced
efficiency during winter (especially in temperate regions)
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| Fig. 7.2. General relationships between increased
discharge resulting from
precipitation in a catchment area and nutrient concentrations in rivers.
Depending upon the pollution sources within the catchment area, the highest
concentrations appear during low discharge (1, 2)
or high/medium water events (3) |
In catchments where point sources of
pollution are present, their reduction is the first and necessary step to
improving riverine water quality. In such situations, wetlands may be used as a
final step in the wastewater treatment process, to reduce pollutant
concentrations to a level safe for the receiving environment.
In rivers where non-point sources of
pollution prevail, wetland systems may be applied both to purify polluted river
water as well as reduce the movement of nutrients and suspended solids from the
watershed to downstream reservoirs. In the latter case, the wetlands should be
located in the river valley upstream of the reservoir.
There are some general principles related to the pattern of nutrient transport by
rivers within degraded catchments. These are summarised below.
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The highest nutrient concentrations
and loads are observed during the first phase of a flood, while the flood
waters continue to rise (nutrient-condensing stage);
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Before river discharge reaches its
maximum, nutrient concentrations and loads start to decrease and continue to
decrease during the period following the flood peak (nutrient-dilution stage);
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During high/moderate floods of short
duration but with an high amplitude, the nutrients loads transported are
greater than during lower amplitude and longer duration events within a given
river;
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Flash
floods may result with dilution of concentrations of the transported
contaminants, however the loads are still high due to high hydraulic load. This
may periodically lower the efficiency of wetlands in water quality improvement.
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| Fig. 7.3. Observed
general trend of change of maximum discharge in temperate climate zone. The
intensity of flood determines the alternative role of impoundment in water
management - water quality or flood control |
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