food and beverage sector.
A much simpler method of removing
phosphorus can be effected by
dosing a metal based coagulant into
the wastewater. The metal targets
phosphates via two routes: 1) When
a metal is added to wastewater
it reacts directly with phosphates
present in the wastewater, forming
the metal phosphate, which is
insoluble. 2) The metal ions hydrolyse
in water, forming a dense, gel-like
precipitate, (metal hydroxide), which
binds with phosphorus to form the
metal phosphate.
Once the metal combines with
phosphorus to form the metal
phosphate as discrete flocs, the
flocs can be removed from the water
phase.
Separation of the metal phosphate
is a fairly simple process and can be
achieved by filtration or clarification,
the latter using settling or flotation
methods. Filtration is rarely used
as a sole method of treatment as
the suspended solids level after
the chemical treatment stage is
usually too high, although filtration
is commonly used downstream of
a clarification stage as a means of
protection and polishing.
When it comes to flotation (more
specially a dissolved air flotation
system or DAF) versus clarifier, either
process will work. A small dose of
polymer upstream of the separation
device will enable a clear virtually
solids free stream to be discharged.
The choice of which process to
select can depend on available
space, cost, ongoing costs or can
simply be put down to a personal
preference.
Phosphorus is an important plant nutrient that
drives algal blooms in both freshwater and marine
ecosystems, and is a key factor for triggering
extensive blue-green algae (cyanobacteria) blooms
that can be toxic to humans and wildlife. High
concentrations of phosphates from human inputs,
such as industrial wastewater discharges, can cause
eutrophication, resulting in the death of fish, shellfish
and other species. As a result, environmental
agencies are focusing more attention on phosphorus
in wastewater discharges and imposing limits on
how much phosphorus can be discharged with
industrial wastewater. To ensure certainty in the
continuous and efficient removal of phosphorus
to comply with given discharge limits, a physical
chemical system wins hands down.
THE WRAP
Chemical treatment combined with
physical removal in a dissolved
air flotation system provides an
effective solution for removing
phosphate in industrial wastewater,
allowing the clean treated effluent
to be safely discharged.
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