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SECONDARY TREATMENT The
secondary
treatment
allows
to
separate,
from
wastewaters,
the
organic
substances
that
have
escaped
to
the
primary
sedimentation.
It
generally
consists
of
a
biological
treatment
and
a
sedimentation
step
for
the
separation
of
solids
from
the
clarified
liquid.
The
operation
principle
is
the
transformation
of
the
organic
matter
in
sludge
flocks
that
are
then
separated
in
the
secondary
sedimentation
tanks.
The
activated
sludge
process
and
the
trickling
filters
are
the
most
diffused
aerobic
process.
In
the
activated
sludge
process,
the
dispersed-growth
mass
reactor
is
an
aeration
tank,
containing
a
suspension
of
wastewater
and
microorganisms
(the
mixed
liquor).
The
trickling
filters
consist
of
tanks
or
towers
filled
with
support
media
such
as
stones
or
plastic
elements.
Wastewater
can
be
applied
intermittently
or
continuously,
over
the
media.
Microorganisms
become
attached
to
the
media
and
form
a
biological
film
which
metabolise
the
organic
matter.
Trickling
filters
have
been
largely
used
in
the
50’s
and
60’s
because
of
their
easy
management
and
low
power
consumption.
However,
with
the
improvement
of
technology,
trickling
filters
have
been
replaced
with
the
activated
sludge
process
that
allows
a
better
depurative
efficiency
even
if
with
higher
management
and
energy
costs.
The
removal
percentages
of
these
processes
of
treatment
are
presented
in
Table
1
and
Table
2.
Other common secondary treatment systems include the Rotating Biological Contactors (RBC) and the Waste Stabilization Ponds (WSP). The RBC are similar to the trickling filters in the way the biomass is attached on a support media, consisting of slowly rotating disks partially submerged in the wastewater flow. WSP require a large surface so they are mostly used in the rural areas and in the developing country mainly (with warm climate) where the land is available to reasonable costs. They are realized with a series of anaerobic, facultative (aerobic/anaerobic) and maturation pond with a retention time of several days (10 to 50 days), depending on the temperature and the effluent quality goals. The
organic
matter
removal
principally
occurs
in
the
anaerobic
and
facultative
ponds
while
the
pathogen
microorganisms
removal
mainly
occurs
in
the
maturation
ponds,
due
to
the
biological
oxidation.
The
typical
effluent
characteristics
of
well
designed
stabilization
ponds
are
listed
in
the
following
Table
3
and
Table
4. Table 3. Typical effluent characteristics from stabilization ponds (Source: USEPA, 1992).
The advantages of WSP systems can be summarized as it follows: - Simplicity: WSP are simple to construct: earth moving is the main task; civil works are minimal-preliminary treatment, inlets and outlets, pond embankment protection. They are also simple to operate and maintain: routine task comprise cutting the embankment grass, removing scum and any floating vegetation from the pond surface, keeping the inlet and outlets clear. - Low costs: because of their simplicity, WSP are much cheaper than other wastewater treatment process. There is no need for expansive electro-mechanical equipment, nor for high annual consumption of electrical energy. - High efficiency: BOD removal >90% are readily obtained in a series of well designed ponds. The removal of suspended solids is less, due to the presence of algae in the final effluents. They are particularly efficient in removing pathogenic microorganisms. Due to the specific characteristics, the WSP can be advantageously used for the greatest part of the wastewater agricultural reuse applications, respecting the common standards.
Finally, the secondary treatment can be acceptable for applications in which the risk of exposure of the human to reclaimed wastewater is low, as irrigation of non edible cultivations and landscape irrigation in areas of restricted access. CONVENTIONAL SECONDARY TREATMENT COSTS |