كيفية توظيف المجلطات والفلوكات مع معدات معالجة الصرف الصناعى

Settling Lagoon

The presence of a pretreatment settling
lagoon will allow the water plant to
maintain, on a year round basis, a
consistent and much better quality of raw
water.

A settling lagoon will enable the
water plant to let any temporary poor
quality surface water by-pass the plant
in-take, while concurrently permitting all
the settleable solids to be naturally
precipitated from the raw water prior to
treatment.

The choice of chemical
treatment should therefore be consistent
on a year round basis, as should be the
overall treatment regime.

A chemical
programme can be tailored to meet
specific needs and can be far less
forgiving than programmes that have to
satisfy fluctuating raw water quality
situations.

Direct Filtration

A direct filtration plant will require a
coagulant/flocculant combination that will
readily generate a robust yet filterable floc
with the raw water contaminants.

The
first inclination is to believe that the larger
the floc the better, but this is often not the
case.

A large floc will be easily filtered
from the water but all the filtering action
will take place on the top of the filter bed,
leaving the majority of the media unused.
A smaller, denser, and more robust floc
will also filter well but will use a greater
percentage of the filter bed resulting in an
improved quality of finished water and
longer run lengths before breakthrough.

Those coagulants that hydrolyze quickly
to form a sweep floc will show up well in a
jar test, however the sweep floc is very
fragile and may easily break up into fines
that will be difficult to filter.

The
prepolymerized inorganic metal salts are
the best option in direct filtration units.
There will be a temptation to consider the
use of organic polyelectrolytes as filter
aids, especially the pDADMAC's if there
is colour to remove.

The use of these
products can be beneficial, but only as an
aid to the primary coagulant.

Should the
dosage of these materials be above 4.0
mgL-1 there will be a very real tendency to
cause filter blinding and large "mud balls
".
Sedimentation and Filtration

The configuration of the settling basin
process will have to be considered,
especially the chemical feed point and the
intensity of the flocculators.

If there are
good flocculators, or mixing chambers,
with several minutes detention time (>5
minutes) then the best choice is a non
catalyzed prepolymerized metal salt.

Such a chemical regime will generate adense floc that will expel water naturally.
This floc will settle well, and depending
on the retention time in the main settling
basin may or may not require a
flocculant.

If the mixing energy or
detention time is insufficient, the best
option is to consider a sulphated alumina
salt since the catalyzing characteristic of
the sulphate will allow for reasonable
results. Since a sweep floc will surface
adsorb the raw water contaminants
without any real coagulation there will
always be a need for an organic polymer
flocculant and carryover may become a
concern.

Again there may be a temptation to
consider the use of either a pDADMAC
or a polyepiamine, they will show well in
a jar test.

As with the direct filtration
process care has to be taken since if any
unreacted short chain polymer is carried
over to the filter plant, mud balls and filter
blinding could result.

There is also the
possibility that the cationic solution
polymers may cause an overdose of
positive charge that may result in poor
settling and extra loading on the filters.

Solids Contact Clarifier
A well designed and operated solids
contact clarifier followed by a properly
sized filter plant will present few
challenges to any one chemical
treatment regime that it will not present
to another.

There is typically good
mixing time and intensity in the
centrewell, where there is also the
opportunity for floc collisions and growth.
A "good" sludge bed will provide a
straining and filtering medium, which can
be complimented by the inclined baffleplates that many clarifiers are now
equipped with.

If there is any
unacceptable carryover, the filter plant
will usually separate the pin floc from the
water.

There are several process variables that
can enhance or detract from the
performance of a chemical treatment
programme.

Raw water temperature,
chemical feed point(s), turbine speed,
sludge bed height, and clarifier blowdown
can all be adjusted so as to optimize the
performance.

All chemical treatment
programmes, if it is the correct choice for
the raw water and the treatment goals,
should be able to be optimized in a solids
contact clarifier.

Dissolved Air Flotation

Dissolved Air Flotation (DAF) Units are
becoming increasingly common in water
treatment.

In DAF units coagulant is
added to the raw water or wastewater just
before the flocculators.

A robust floc has
to be formed quickly since upon entering
the DAF unit the raw water is mixed with
an air pressurized stream of effluent
water.

The drop in pressure allows the
air to be released and the bubbles float to
the surface of the flocculator/clarifier.
The coagulated particles
absorb/adsorb/entrap the air bubbles and
also float to the top of the vessel.

A
thickened sludge forms at the top which
is periodically removed.

The coagulant has to be able to form a
very robust sludge that will resist fracture
in the flocculating chamber.

A limitation
of the DAF unit is that any sludge
particles that fracture will form a verysmall, almost colloidal, particle that will
pass through the filters and reduce the
quality of the finished water.

For this
reason it has been found that the
prepolymerized coagulants, that are held
together with chemical bonds, provide
the best sludge particle and give the best
finished water quality.

The alum
sludges, that rely on floc collisions to
build a settleable sludge tend to fracture
easily. For a DAF unit, a sludge stability
test is the best indicator of performance.
Do not be fooled by large fluffy flocs that
look good in a jar test.

Prepolymerized
salts are often the best choice and
should be screened first.

Mixing Intensity

The mixing intensity or available mixing
time will have a greater effect on the
performance of the chemical rather than
on the choice.

However, if there is
insufficient mixing to optimize the
coagulation process it is recommended
that a sulphated aluminum or
polyaluminum salt be considered.

A
sweep floc of aluminum hydroxide will be
generated that will allow for chemical
enmeshment of colloidal particles.

While
a true coagulation process will not have
taken place, with the aid of a good
flocculant, the solids should be
separated from the water.

If the mixing
regime is satisfactory it is better to move
away from the catalyzed materials and
rely on hydrolysis to generate the
cationic species required for coagulation
and particle collisions for flocculation.

Chemical efficiency will be much
improved and there may not be a need
for a flocculant.

Sludge Disposal

Sludge disposal is becoming a very
important operating parameter for many
utilities.

In some jurisdictions it is still
possible to return aluminum sludges to a
receiving water, including those that have
under gone pH adjustment with Na2CO3
or Ca(OH)2. However, lime softening
sludges that contain increased Ca(OH)2
concentrations and all iron based sludges
have to be dewatered and sent to a land
fill site.

Settling lagoons and dredging or
else the transfer to a municipal
wastewater plant are also options for
these more hazardous sludges.

The type
of equipment available to the utility
operator, or else the capital monies
allocated to plant improvement will have a
significant impact on the coagulant
choice.

Treatment Objective

The treatment objective will obviously
have an impact on the type of treatment
regime that is considered.

While the
processes available to all water plant
owners are the same there is a difference
in operating philosophy between a
municipal supplier and an industrial
facility.

In the municipal segment the
water plant serves the need to produce
quality water as an end in itself.

Due to
public health concerns the municipal
water plant operator will always strive to
supply the best quality water possible.

An industrial facility will often treat water
for use in the process of the plant.
Quality is determined by the needs of the
process only and not by public health
concerns.

Economics play a very
significant role in the selection of a
treatment regime since water treatment
is considered a cost of business.

To
confuse the issue, an industrial facility
that only has to meet process quality
guidelines may not be able to live with
poor quality water for long due to the
potential to manufacture off-specification
product.

In summary, a municipal water plant is
often run to meet regulations and
community standards while an industrial
process facility is run by economics (an
industrial potable water plant has to meet
all the government regulations).

If a water treatment plant operator is
planning to optimize the coagulation
and flocculation functions within the
plant the first option is to consider all
the fixed parameters, as shown above.

It is quite possible that by
understanding the process and the
chemicals available that many options
will be eliminated and that a few will
stand out as being good selections.

A
relatively easy process to select the
best from all the good options can then
be initiated.

consideration. In most cases there are
several parameters that have to be
considered.

Following the same process
outlined to select the most likely
coagulant will also give a good indication
as to the analytical requirements.

Raw
water quality, process equipment and
treatment objective will determine the
important operating parameters and also
identify the parameters that need to be
considered in the laboratory.

While each situation is different, there are
a number of consistencies throughout the
industry.

The following list of analytical
procedures should always be considered
before beginning an evaluation
procedure.

Without familiarity with the
necessary procedures and equipment
required in order to obtain accurate and
meaningful results, time spent on jar
testing would be worthless.

مجموعة تكنولاب البهاء جروب
عقيد دكتور
بهاء بدر الدين محمود
استشارى كيميائى

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