Environmental Engineering Reference
In-Depth Information
increasing its porosity which may allow more
effective fluid movement in the rhizosphere.
When roots decay, they leave behind ports and
channels known as macropores which are effec-
tive in channelling water through the soil. Higher
plants in wetland systems may be viewed as tran-
sient nutrient storage compartments absorbing
nutrients during the growing season and releas-
ing large amounts at senescence. Aquatic vegeta-
tion may play an important role in phosphorus
removal and, if harvested, extend the life of a sys-
tem by postponing phosphorus saturation of the
sediments. Different species of aquatic plants
have different rates of heavy metal uptake; a con-
sideration for plant selection in a constructed
wetland used for water treatment and plants such
as cattails ( Typha sp.), sedges, water hyacinth
( Eichhornia crassipes ) and Pontederia sp. are
used worldwide; however, for subarctic regions,
buckbeans ( Menyanthes trifoliata ) and pendant
grass ( Arctophila fulva ) are also useful for metal
uptake.
Bacteria are of the greatest numerical impor-
tance in the constructed wetland system. The pre-
ponderance of bacteria living is facultative, and
they are able to live in either the presence or
absence of oxygen (Spellman 1997 ; Absar 2005 ).
Although both heterotrophic and autotrophic
bacteria are found in constructed wetland but, the
former predominate and obtain energy from the
carbonaceous organic matter in the influent
wastewater for the synthesis of new cells and also
release energy via the conversion of organic mat-
ter compounds such as CO 2 and H 2 O. Important
heterotrophic bacteria in activated sludge are
Achromobacter , Alcaligenes , Arthrobacter ,
Citromonas , Flavobacterium , Pseudomonas ,
Zoogloea and Acinetobacter (Oehmen et al.
2007 ). Many of these bacteria form floc particles
or clusters of bacteria that break down waste.
When filamentous bacteria are present in exces-
sive numbers or length, they often cause solid/
liquid separation or settleability problems (Gray
2002 ; Paillard et al. 2005 ).
A small number of fungi are capable of oxidis-
ing ammonia to nitrite and fewer still to nitrate.
The most common sewage fungus organisms are
Sphaerotilus natans and Zoogloea sp (Painter
3
Removal Mechanism
in Constructed Wetland
A constructed wetlands treatment system
(CWTS) is a bed composed of the substrate,
water-tolerant plants, the water column and a
microbial population. The substrate can be sand,
gravel or soil in which wetland plants are grow-
ing. Natural wetlands act as biofilter, removing
sediments and pollutants such as heavy metals
from the water. Vegetation in a wetland provides
a substrate (roots, stems and leaves) upon which
microorganisms can grow as they break down
organic materials. There are more indirect ways
in which plants contribute to wastewater purifica-
tion. Plants create a unique environment at the
attachment surface of the biofilm.
Treatment of wastewater within a constructed
wetland occurs as it passes through the wetland
medium and the plant rhizosphere. Decomposition
of organic matter is facilitated by aerobic and
anaerobic microorganisms present. Microbial
nitrification and subsequent denitrification
release nitrogen as gas to the atmosphere. The
removal of nitrogen from wastewater is important
because of toxicity of ammonia to fish if dis-
charged into water courses.
3.1
Role of Plants
and Microorganisms
in Constructed Wetland
Plants play an important role in CO 2 mitigation in
the atmosphere, settlement of suspended particu-
late matter (SPM) on their leaf and minimisation
of the flow of toxic metals in sewage/contami-
nated water flowed in the river stream. Heavy
metals such as cadmium, zinc, nickel, lead, chro-
mium and copper are concentrated in the river
water and the sediments. Besides, nitrate and
phosphates are also found in toxic levels. Certain
plants transport oxygen which is released at the
biofilm/root interface perhaps adding oxygen to
the wetland system (Pride et al. 1990 ). Plants also
increase soil or other root-bed medium hydraulic
conductivity. As roots and rhizomes grow, they
 
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