Environmental Engineering Reference
In-Depth Information
In a constructed wetland system receiving
secondarily treated domestic wastes contained
40.5 % of the total phosphorus influent. The
remaining 59.0 % was found to be stored in the
gravel substratum. Phosphorus removal in a sur-
face flow wetland treatment system planted with
one of Scirpus sp., Phragmites sp. or Typha sp.
was investigated by Finlayson and Chick ( 1983 ).
Most phosphorus transformations mediated by
microorganisms involve the mineralisation of
organic to inorganic phosphates (a process also
referred to as “decomposition”) or the conversion
of insoluble, immobilised forms of tertiary phos-
phate into soluble, mobile primary phosphates
that are more readily used by organisms.
Mineralisation of organic to inorganic phosphate
involves processes catalysed by phosphatase
enzymes, which are specifically involved in this
conversion. Many microorganisms produce these
enzymes. Phosphatase enzymes are present in all
organisms, but only bacteria, fungi and some
algae are able to secrete them outside of their
cells. As exoenzymes, they participate in the dis-
solution and mineralisation of organic phosphate
compounds in the environment. Without phos-
phatase enzymes, the presence of inorganic phos-
phorus would be limited to external sources, such
as fertilisers, and primary productivity would be
limited and dependent on these external sources.
Phosphate would remain sequestered in cell mat-
ter and unavailable for producers. The enzymatic
activity of microbial communities is critical for
the proper cycling of phosphorus within an
ecosystem.
Rock phosphate cannot be directly utilised to
replenish phosphorus requirement of plants but
instead utilised in the manufacture of superphos-
phate fertilisers by employing very costly and
tedious processes. Phosphorus promotes devel-
opment of deeper roots. To increase the availabil-
ity of phosphorus for plants, large amounts of
fertiliser is used on a regular basis. But after
application, a large proportion of fertiliser phos-
phorus is quickly transferred to the insoluble
forms.
Phosphorus has an important role to play in
plant growth and nutrition. It increases biological
activities like nodulation, nitrogen fixation and
nutrient uptake in soil and rhizosphere environ-
ment resulting in higher yield of crops. Its defi-
ciency results in stunted growth, lack of
chlorophyll, deepening of green and red colour of
leaves, and abnormal/arrested root development
and maturity delay of plant.
In this regard, some microorganisms play an
important function of solubilising phosphatic
compounds and making them available for plant
assimilation. Phosphorus solubilisation is carried
out by a large number of saprophytic bacteria and
fungi acting on sparingly soluble soil phosphates,
mainly by chelation-mediated mechanisms
(Whitelaw 2000 ). Some fungal species have min-
eralisation and solubilisation potential for organic
and inorganic phosphorus, respectively (Hilda
and Fraga 2000 ). Phosphorus-solubilising activ-
ity is determined by the ability of microbes to
release metabolites such as organic acids, which,
through their hydroxyl and carboxyl groups, che-
late the cation bound to phosphate, the latter
being converted to soluble forms (Sagoe et al.
1998 ). Phosphate solubilisation takes place
through various microbial processes/mechanisms
including organic acid production and proton
extrusion (Dutton and Evans 1996 ; Nahas 1996 ).
3.4.3 Phosphorus Solubilisation
Plant phosphorus nutrition has always been of
great concern as it is the most limiting nutrient.
About 96 % of phosphorus is present in the soil
as insoluble phosphate; inorganic phosphorus is
locked into crystal lattices of clay particles and is
the main reason behind its low solubility. Thus,
soils worldwide are deficient in assimilable inor-
ganic phosphate (H 2 PO 4 ) form. This low solubil-
ity of phosphatic compounds which is a
characteristic feature of soil phosphorus is
responsible for its limiting concentration in soil
solution and hence its unavailability to plants.
3.4.4 Chemical Removal Mechanisms
Phosphorus removal through chemical processes
is the most important means of accumulating
phosphorus in constructed wetlands (Kadlec and
Knight 1996 ). Chemical removal of phosphorus
occurs through precipitation and sorption
processes.
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