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Constructed
Wetlands
Natural
wetlands, permanently waterlogged areas populated by hydrophytic plants
such as reeds, comprise a variety of sub-surface micro-habitats of
differing oxygenation and redox potential, and support a diversity of
microorganisms – bacteria, fungi, actinomycetes, protozoa
– which
degrade organic and inorganic substrates entering the system.
Constructed wetland systems are increasingly being employed for
treatment of wastewaters, sludges and industrial effluents as a
cost-effective, low energy and robust alternative to traditional
engineered biological treatment such as the activated sludge process.
Our
constructed wetland (vertical
flow) in Shenyang (China)
Constructed
wetlands (CWs) are classified according to their mode of operation as
surface flow, horizontal flow, vertical downflow or vertical upflow
type. They have been used successfully in the treatment of domestic
sewage, urban, highway and stormwater runoff, acid mine drainage,
agricultural waste, industrial effluents, landfill leachate and meat
processing effluent. BOD and solids reduction occurs through microbial
activity and removal of nitrogen and phosphorus through the processes
of denitrification, plant uptake and sorption.
 Artificial wetlands for
wastewater treatment were pioneered in Germany
and this natural treatment technology is now used
all over the world, not
only
for domestic waste waters but also for industrial effluents.
A lot of research
has been done in the last twenty years and adequate
design guidelines for the different
kinds of constructed wetlands are
available.
The natural ability of soils to
filter suspended solids mechanically and
chemical reactions (precipitation) with sewage constituents
are other forms
of cleaning
mechanisms, which interact. So pollutants are metabolized,
settled and absorbed in sequential anaerobic and
aerobic processes.
Reed
bed treatment system
The
reed bed treatment system combines aerobic and anaerobic decomposition
processes in a 1.0 m thick soil or substratum layer. The polyethylene
lined and refilled basins are planted with helophytes like Phragmites
communis, Typha latifolia, Typha angustifolia or other aquatic
macrophytes.
The wastewater percolates the filter
substrate vertically to the bottom drains.
Besides
the microbial and fungal decomposition of organic matter and pollutants
in the rooted soil or substrate matrix, chemical and physical
precipitation, adsorption and filter processes occur due to soil
constituents like clay minerals and humus particles. This is most
important for phosphate and ammonia binding. Some of the wastewater
nitrogen is released out of the artificial ecosystem to the atmosphere
as nitrogenous gases (denitrification).
Through
intermittent
loading of the reed beds a radical change of oxygen regime is achieved.
After water saturation by feeding with the distribution system a
drainage network at the base collects the purified water. The pore
space of the substrate is refilled with air thus enabling aerobic
decomposition processes.
Another part of oxygen
transfer into
the rhizosphere happens through a special helophyte tissue in the plant
stems and roots (aerenchym) from the air.
Clogging
effects of
the filter substrates (soil, sand, gravel) are prevented by the
continuous growth and decay of roots and rhizomes of the aquatic
macrophytes and the thereby remaining soil macropores. In this manner,
long-term water transport into the soil matrix is guaranteed.
The
substrate, which is filled in the sealed earth basins, is a
sitespecific mixture of selected components determined by aspects of
hydraulic conductivity and physico- chemical properties. The soil and
substratum mixture is a single case decision of planning further
depending on the composition of sewage, whether municipal or
industrial.
By means of high evapotranspiration of
the marsh
plants the wastewater tends to increase pollutant concentrations, thus
improving the efficiency of the microbial degradation process. These
artificial wetlands reduce both pollutant concentrations and the water
volume. Their performance has to be described by - pollutant load
parameters (kg BOD / [m² x d]), not just comparing
(pollutant)concentrations (mg/I) between inlet and outlet.
constructed
wetlands
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