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coagulants and flocculants are used in waste treatment to
separate unwanted components from water and sludge. Not only
do these materials make the task easier, in some cases the
separation would be impossible without them. This page is
divided up into three sections. The first section deals with
inorganic coagulants and treatment chemicals. It presents
the products that are available, how they are typically used
and any pertinent factors relating to their use.
second section covers water soluble wastewater treatment
polymers. Here, too, we will look at the various products
that are used, how they are applied and the various factors
which determine their success in waste treatment.
third section will look at some of the typical waste treatment
applications that you are likely to encounter in the field.
In this section you will see how the inorganics and polymers
discussed in the first section are put to use.
we get started, let's look at some of the terms and concepts
that we will encounter when we are doing waste treatment:
- Coagulation refers to the destabilization of suspended
colloidal materials. Because these particles all carry the
same surface charge (usually negative), their mutual repulsion
is enough to prevent them from settling. Note that this
is different from precipitation. Also, note that this is
technically different from flocculation although the two
terms are often used synonymously.
- Flocculation refers to the bridging between particles
by a polymer chain, causing them to form flocs or larger
aggregates. These flocs float (flotation) or sink (sedimentation),
making them easier to remove from the system.
- Precipitation is the insolubilization of dissolved materials.
An example is the precipitation of iron by raising the pH
to 8 with sodium hydroxide to form an insoluble iron hydroxide
Dewatering - Sludge dewatering is the removal of water
from sludge to reduce its volume, lowering hauling costs.
This process, typically accomplished with belt filter or
screw presses or other equipment, also makes the sludge
easier to burn, reducing fuel costs.
- Clarifiers are large cylindrical or rectangular basins
which allow the separated solids to settle from the water,
permitting clear water to pass over a weir.
Air Flotation (DAF) - DAF is the process of passing
dissolved air through flocculated wastewater. The tiny bubbles
attach to the flocs, floating them to the surface. The resulting
raft is then skimmed from the top of the water, allowing
the clear water to exit the system.
- Electrolytes are atoms and molecules that, when dissolved
in water, ionize and carry a positive (cationic) or negative
(anionic) charge. Ferric chloride and aluminum sulfate are
- Polyelectrolytes are electrolytes which have more than
one charge-bearing atom in the molecule. Examples are acrylamide
copolymers and polyaluminum chloride.
Testing - Jar testing is the process of evaluating various
treatments in the laboratory to determine the best method.
Often a four or six paddle "gang-stirrer" is used
to view several treatments simultaneously.
INORGANIC COAGULANTS and TREATMENT CHEMICALS
coagulants can be electrolytes or polyelectrolytes and are
typically based on iron (ferric or ferrous), aluminum, calcium,
or magnesium. These coagulants all have one thing in common:
when they are dissolved in water they generate a highly
charged cation useful for destabilizing dispersed solids.
wastewaters contain finely divided solids or emulsified
liquids that are dispersed due to the mutual repulsion of
their surface negative charges. When a highly cationic ion
is introduced into this system, it interferes with this
repulsive stabilization and allows the particles to come
into close contact. This starts the coagulation process.
Van der Waals attraction and/or the use of polymeric coagulants
completes the process forming larger aggregates which can
be further flocculated or separated as is from the waste
following is a list of the most common inorganic coagulants
in use today and a brief overview of some of their features.
The best choice for a particular application depends on
the system and is usually determined only after jar testing
in the laboratory.
Chloride [FeCI3] - Typically sold in solution
form. Applications include phosphate removal, sludge conditioning
and dewatering, trace metals removal, and odor control.
Solutions are very acidic and corrosive. Available as a
solid and in solution (27-43% FeCl3) form.
Chloride [FeCI2] - Applications include phosphate
removal, odor control, heavy metals removal, controls toxic
sulfide generation in anaerobic digesters, oil & grease
removal, and sludge conditioning. Available in solution
form only (8-14% iron). Very acidic and corrosive.
Sulfate [Fe2(SO4)3] - Applications
include water clarification, decolorization of surface water,
sludge conditioning and dewatering, trace metals removal,
organics removal (including trihalomethanes), sulfide control,
phosphate removal, oil &grease separation and DAF. Available
as a solid and in solution (10-13% iron) form. Very acidic
Ferric Sulfate [Fe5(SO4)7(OH)]
- The newest of the iron salts. Billed as a replacement
for alum. Sold in solution form. Very acidic and corrosive.
Sulfate [FeSO4] - Applications include phosphate
removal, trace metals removal, and odor control. Available
in solid and solution (5-12% iron) form. Very acidic and
Chloride [AlCl3] - Applications include metals
removal, oil & grease separation and water clarification.
Available as a solid and in solution form. Acidic and corrosive.
Sulfate [alum, Al2(SO4)3]
- Perhaps the most widely used inorganic coagulant. Uses
similar to aluminum chloride. Available as a solid and in
solution form. Acid and corrosive.
Chloride [CaCl2] - Infrequently used for metals
removal, organics reduction and water clarification. CaCl2
also has great utility for. phosphate removal. Available
as a solid and in solution form. Nonhazardous.
Hydroxide [Mg(OH)2] - Used for pH control (maximum
pH of 9.0 helps prevent overshooting pH target). Helps reduce
sludge levels when used as a precipitant. Safe to handle.
Contains no heavy metals. Available as a 50% slurry or in
solution form. Can have problems with stability of slurry
and is slow to dissolve.
Chloride [PAC] - This describes a wide variety of materials
containing more than one aluminum atom in the molecule up
to about 13. These materials are typically described by
their Al2O3 content and basicity.
Al2O3 ranges from about 8% to 25%
and basicity is usually between 50 and 70% for most commercial
products. Aluminum chlorohydrate is an example of a PAC.
Some manufacturers replace part of the chloride content
with silicate or sulfate. Some materials are corrosive.
Available in solution form only.
Aluminate - Applications include color removal,, phosphorus
removal, lime softening, pH control, and many papermaking
applications. Very alkaline and very corrosive. Available
in solution form.
and Other Inorganic Treatment Chemicals
are numerous chemicals which are used in waste treatment
for precipitation and to aid in removal of unwanted constituents.
The following is a short list of products used mainly for
pH adjustment and buffering:
Carbonate (Soda Ash)
Hydroxide (Caustic Soda)
Hydroxide (Caustic Potash)
are several products used for metals precipitation: sodium
sulfide, sodium polysulfide, dimethyl dithiocarbamate (DTC,
also used as a microbiocide), and trimercapto-s-triazine
(TMT). Of these, DTC and TMT are the most widely used. These
materials form insoluble complexes with dissolved metals.
The insoluble complexes can then be settled as is or further
treated with coagulants or flocculants prior to removal.
The chief advantages of using these materials over precipitation
of metal/hydroxide complexes with caustic soda is that they
work even on chelated metals. Also they often allow even
lower heavy metals residuals in the final effluent.
WATER SOLUBLE WASTE TREATMENT POLYMERS
polymers used for water treatment began to be widely used
in the 1960's and have become a vital tool in the treatment
of wastewater and potable water. Although the basic chemistry
of these chemicals has not changed radically, new uses for
them are being discovered all the time.
word polymer stems from two Latin terms: mer, meaning unit
and poly, meaning many. That is, a polymer is "many
units". More specifically, a polymer is a chain or
network of single units (monomers) strung together. These
chains can be linear, branched, or crosslinked. In water
treatment applications, linear and branched polymers are
most frequently encountered. Crosslinked polymers are usually
only partially soluble in water and are, therefore, not
way of categorizing polymers is by their molecular weight.
This is simply a measurement of how large or long the chain
is. Molecular weights for water treatment polymers range
from a few hundred thousand to tens of millions. High molecular
weight polymers, those whose molecular weight is above 1
million, are shear sensitive meaning that their chains can
be broken into smaller fragments by excessive mixing. This
can be detrimental to their usefulness as we will see later.
and Charge Density
final way of identifying polymers is by their charge, both
the sign of the charge and its magnitude. Nonionics,as you
would expect, contain no charge-bearing groups (they are
not polyelectrolytes). These polymers are typically homopolymers
when dissolved in water, are negatively charged. Anionic
polymers are usually copolymers (polymers containing two
types of monomer units) of acrylamide and acrylic acid,
sodium acrylate or another anionic monomer. The charge is
located on a pendant group sticking off from the polymer
chain backbone. The charge on these polymers is pH sensitive;
they function best at a pH above 6.
cationics become positively charged when dissolved. Cationics
can be copolymers of acrylamide with a cationic monomer,
cationically modified acrylamide or a polyamine. The cationic
charge in these polymers is derived from nitrogen in the
form of a secondary, tertiary or quaternary amine group.
Those containing secondary or tertiary amines are sensitive
to pH. The charge on these polymers drops off quickly as
the pH rises above 6. In addition, they are susceptible
to attack by chlorine. Polyquaternary amines are pH insensitive
and function well over a broad pH range. They are also chlorine
resistant. In these polymers, the charge can be located
on a pendant group or may be in the backbone of the polymer
the measure of the resistance to flow of a liquid, is directly
related to several properties of polymers. The first of
these is the concentration of the polymer in solution. For
a given charge density and molecular weight, the higher
the concentration, the higher the viscosity.
property which affects viscosity is the molecular weight.
For a given charge and concentration, the higher the molecular
weight, the higher the viscosity.
property is charge density. For a given molecular weight
and concentration, higher charge densities give higher viscosities.
the structure of the polymer chains has an effect on the
viscosity. Branched polymers often give a creamy or syrupy
viscosity. Linear polymers will give a stringy or "leggy"
type of viscosity. By measuring this "stringiness"
(known as pituity or elongational viscosity), a relative
measure of the polymer's linearity can be obtained.
polymers are made, they are in coiled chains. This is particularly
true of high molecular weight polymers. When they are put
into solution, the charged areas on the chain repel each
other and force the chain to uncoil. As this occurs, the
viscosity of the solution increases. It is very important
to recognize that this process takes time; more time is
required for high molecular weight polymers. Since the charge
affects the speed at which the chain uncoils, higher charged
polymers will uncoil faster than low charged products. Indeed,
nonionic polymers may never fully uncoil since they carry
no charge. The ability of the polymer to do its job hinges
on it being completely uncoiled. Therefore, it is important
to allow an ageing period before polymers are used.
are several factors that impact on the dissolution of polymers,
especially high molecular weight polymers. Dissolved solids,
hardness, and other impurities can inhibit complete dissolution
since they shield the polymer's charged groups from repelling
each other. Because of this, softened or deionized (distilled)
water is preferred.
Types of Polymers
Powdered or Granular Polymers
majority of these polymers are made overseas in Europe or
Japan although there are now some domestic manufacturers
as well. These polymers benefit from the fact that they
are essentially 100% active so you are not paying for the
cost of shipping water. However, they suffer from the drawback
of being difficult to put into solution and they require
special feed equipment for making up large amounts of diluted
polymer. Typically, these polymers are put into solution
by the use of eductors or automated dilution systems. Improper
dilution of dry polymers can result in the formation of
"fisheyes". This is a very descriptive term for
globs of polymers that are wetted on the exterior of the
particle but dry on the inside. The gelatinous coating slows
down the dissolution process considerably and can result
in the plugging of other auxiliary equipment.
drawback to dry polymers is that they will pick up moisture
in the air and will solidify if they become too moist. Care
must be taken so they will be stored in a very dry location
and kept free from moisture.
final concern with dry polymers is due to their dustiness.
Nuisance dust and the hazard of residual (toxic) acrylamide
monomer in the dust are both OSHA hazards and employees
working with these materials should be made aware of them.
polymers can be nonionic, anionic or cationic and can have
a wide range of charge densities. They are typically high
molecular weight materials (MW > 1 million).
dilutions of dry polymers are limited by viscosity so the
upper range is about 1 to 2%. Usually it is under 1% to
permit adequate mixing of the solution.
or Solution Polymers
polymers are solutions of water soluble polymers in water.
They benefit from the advantage of being relatively easy
to put into dilute solution, requiring no sophisticated
equipment. However, since they are solutions, you are shipping
water with them which increases the cost. Additionally,
high molecular weight polymers are limited by viscosity
so they are frequently very dilute and very viscous. However,
some polymers such as polyamines and Mannich polymers are
available only in this form.
polymers are typically cationic and can have a wide variety
of charge densities and molecular weights. The concentration
of these products can range from 2% to 70% depending upon
the nature of the polymer.
dilutions of solution polymers are generally less than 10%
to permit adequate control over feedrates. However, for
the lower molecular weight materials, prior dilution is
not absolutely necessary if adequate mixing conditions exist.
are liquids comprised of oil droplets dispersed in water
or water droplets dispersed in oil. Water soluble emulsion
polymers belong to the latter, water-in-oil, group. The
polymer is concentrated in the water phase. Emulsion polymers
benefit from the fact that they are very easy to put into
solution and are quite concentrated (25% to 50%, typically)
even though they usually have very high molecular weights.
Also, their low bulk viscosity and liquid form makes them
very easy to handle, especially in automated systems. They
can be diluted by a variety of methods ranging from simply
pouring them into the vortex of mixing water to sophisticated
dilution systems which require very little manpower to operate.
suffer from the drawbacks that they are not 100% active
and, because they are emulsions, they will separate to some
extent over time. However, they are easily reconstituted
by brief mixing. Some products on the market are considered
to be microemulsions. Microemulsions are inherently stable
and this minor problem is overcome. On the other hand, some
of the microemulsions currently on the market require post-dilution
pH adjustment and they are quite expensive so there may
be a trade-off to achieve this added stability.
polymers can be nonionic, cationic or anionic. They can
have a wide variety of charge densities and are usually
medium to high molecular weight.
levels of these products are limited by viscosity so the
upper limit is usually 2% to 3%. In practice, however, it
is usually better to dilute to 0.5% to 1.0%. This permits
the full dissolution of the polymer. If lower dilutions
are to be used, they should be diluted from this stock solution.
Types of Polymers
following section describes some specific types of polymers
that are available for applications.
products are available in dry, emulsion and solution forms.
They are copolymers of acrylamide with a cationic monomer.
As mentioned above, they are relatively insensitive to pH.
However, pHs above 10 should be avoided if possible since
this pH will allow slow degradation of the polymer through
Polymers and Quaternized Mannich Polymers
(pronounced "manic") polymers are produced by
performing the Mannich reaction on the homopolymer of acrylamide.
The process results in a highly charged, high molecular
weight cationic polymer. They are quite inexpensive and
in some applications extremely cost-effective. However,
they have a short shelf-life (typically only a few months),
are usually extremely viscous (which make them hard to pump,
dilute and feed), are very bad smelling (rotten fish comes
to mind), and are prone to gelation. Also, they are tertiary
amine-based so they are not chlorine resistant or pH insensitive.
Finally, Mannich polymers contain various amounts of residual
formaldehyde, a known carcinogen. These drawbacks have limited
the use of Mannich polymers in many applications.
polymers are sometimes quaternized to make them more useful.
However, this adds significantly to their cost and makes
them even more viscous than their unquaternized precursors.
cationic solution polymers are often referred to as polyquaternary
amines or simply polyamines. However, it should be noted
that the term polyamine is used to refer to any chemical
containing more than one amine group, including those which
are not quaternized. These products are very versatile.
They are typically low to medium molecular weight, can be
linear or branched and are usually >20% active. Polyamines
are used in a variety of applications from oil emulsion
breaking to paint detackification. They go easily into solution,
have quite long shelf lives, do not have a repulsive odor
and are chlorine resistant and pH insensitive.
(Diallyl Dimethyl Ammonium Chloride) Polymers
polymers are usually referred to as DADMAC or DMDAAC polymers.
They are similar to polyamines in their characteristics
with the additional advantage that they can be copolymerized
with other monomers such as acrylamide.
are a variety of other polymers which are available. However,
this list is composed of products which are less common
and usually find niche uses. It is included here merely
tannins and gums
WATER TREATMENT MECHANISMS AND APPLICATIONS
to the large variety of applications for water soluble polymers,
it is nearly impossible to list all of them. In this section,
you will find an overview of some of the more common uses.
we have already discussed, contaminants in water carry a
slight surface negative charge which stabilizes them due
to electrostatic repulsion. This includes both oil and grease
emulsified in wastewater and finely divided solids suspensions.
Inorganic coagulants and polymeric flocculants neutralize
this charge which allows the particles to come closer together
and which destabilizes the suspension.