Steps of clarification

Inorganic coagulants
Polyelectrolytes
Color Reduction
Conventional clarification equipment
In-line clarification
Suspended matter in raw water supplies is removed by various methods to provide a water
suitable for domestic purposes and most industrial requirements. The suspended matter can
consist of large solids, settable by gravity alone without any external aids, and nonsettleable
material, often colloidal in nature. Removal is generally accomplished by coagulation,
flocculation, and sedimentation. The combination of these three processes is referred to as
conventional clarification.
Coagulation is the process of destabilization by charge neutralization. Once neutralized,
particles no longer repel each other and can be brought together. Coagulation is necessary
for the removal of the colloidal-sized suspended matter.
Flocculation is the process of bringing together the destabilized, or "coagulated," particles
to form a larger agglomeration, or "floc."
Sedimentation refers to the physical removal from suspension, or settling, that occurs once
the particles have been coagulated and flocculated. Sedimentation or subsidence alone,
without prior coagulation, results in the removal of only relatively coarse suspended solids.
Steps of Clarification
Finely divided particles suspended in surface water repel each other because most of the
surfaces are negatively charged. The following steps in clarification are necessary for
particle agglomeration:

Coagulation. Coagulation can be accomplished through the addition of inorganic

salts of aluminum or iron. These inorganic salts neutralize the charge on the
particles causing raw water turbidity, and also hydrolyze to form insoluble
precipitates, which entrap particles. Coagulation can also be effected by the addition
of water-soluble organic polymers with numerous ionized sites for particle charge
neutralization.
Flocculation. Flocculation, the agglomeration of destabilized particles into large
particles, can be enhanced by the addition of high-molecular-weight, water-soluble
organic polymers. These polymers increase floc size by charged site binding and by
molecular bridging.

Therefore, coagulation involves neutralizing charged particles to destabilize suspended

solids. In most clarification processes, a flocculation step then follows. Flocculation starts
when neutralized or entrapped particles begin to collide and fuse to form larger particles.
This process can occur naturally or can be enhanced by the addition of polymeric flocculant

aids.
Inorganic Coagulants
Table 5-1 lists a number of common inorganic coagulants. Typical iron and aluminum
coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on
initial raw water alkalinity and pH, an alkali such as lime or caustic must be added to
counteract the pH depression of the primary coagulant. Iron and aluminum hydrolysis
products play a significant role in the coagulation process, especially in cases where lowturbidity influent waters benefit from the presence of additional collision surface areas.
Table 5-1. Common inorganic coagulants

Name

Typical
Formula

Aluminum Al2(SO4)3
sulfate

14 to 18
H2O

Typical
Strength

Typical Forms
Used in Water
Treatment

Density

Typical Uses

60-70 lb/ft3 primary coagulant

17% Al2O3

lump, granular,
or powder

8.25%
Al2O3

liquid

11.1 lb/gal

AlCl3
6H2O

35% AlCl3

liquid

12.5 lb/gal primary coagulant

Fe2(SO4)3
9H2O

68%
Fe2(SO4)3

granular

70-72 lb/ft3 primary coagulant

Ferric-floc Fe2(SO4)3
9H2O

41%
Fe2(SO4)3

solution

12.3 lb/gal primary coagulant

Alum
Aluminum
chloride
Ferric
sulfate

Ferric
chloride
Sodium
aluminate

FeCl3

60% FeCl3,
35-45%
FeCl3

Na2Al2O4

38-46%
Na2Al2O4

crystal, solution 60-64 lb/ft3 primary coagulant

11.2-12.4
lb/gal
liquid

12.3-12.9
lb/gal

primary coagulant;
cold/hot
precipitation
softening

Variation in pH affects particle surface charge and floc precipitation during coagulation.
Iron and aluminum hydroxide flocs are best precipitated at pH levels that minimize the
coagulant solubility. However, the best clarification performance may not always coincide
with the optimum pH for hydroxide floc formation. Also, the iron and aluminum hydroxide
flocs increase volume requirements for the disposal of settled sludge.
With aluminum sulfate, optimum coagulation efficiency and minimum floc solubility
normally occur at pH 6.0 to 7.0. Iron coagulants can be used successfully over the much
broader pH range of 5.0 to 11.0. If ferrous compounds are used, oxidation to ferric iron is
needed for complete precipitation. This may require either chlorine addition or pH

adjustment. The chemical reactions between the water's alkalinity (natural or

supplemented) and aluminum or iron result in the formation of the hydroxide coagulant as
in the following:
Al2(SO4)3 +

6NaHCO3 = 2Al(OH)3- + 3Na2SO4 + 6CO2

aluminum
sulfate

sodium
bicarbonate

aluminum
hydroxide

sodium
sulfate

carbon
dioxide

Fe2(SO4)3 + 6NaHCO3 = 2Fe(OH)3- + 3Na2SO4 + 6CO2

ferric
sulfate
Na2Al2O4

sodium
bicarbonate
+

sodium
aluminate

ferric
hydroxide

4H2O = 2Al(OH)3water

aluminum
hydroxide

sodium
sulfate
+

carbon
dioxide

2NaOH
sodium
hydroxide

Polyelectrolytes
The term polyelectrolytes refers to all water-soluble organic polymers used for clarification,
whether they function as coagulants or flocculants.
Water-soluble polymers may be classified as follows:

anionic-ionize in water solution to form negatively charged sites along the polymer
chain
cationic-ionize in water solution to form positively charged sites along the polymer
chain
nonionic-ionize in water solution to form very slight negatively charged sites along
the polymer chain

Polymeric primary coagulants are cationic materials with relatively low molecular weights
(under 500,000). The cationic charge density (available positively charged sites) is very
high. Polymeric flocculants or coagulant aids may be anionic, cationic, or nonionic. Their
molecular weights may be as high as 50,000,000. Table 5-2 describes some typical organic
polyelectrolytes.
For any given particle there is an ideal molecular weight and an ideal charge density for
optimum coagulation. There is also an optimum charge density and molecular weight for
the most efficient flocculant.
Because suspensions are normally nonuniform, specific testing is necessary to find the
coagulants and flocculants with the broadest range of performance.
Primary Coagulant Polyelectrolytes
The cationic polyelectrolytes commonly used as primary coagulants are polyamines and

poly-(DADMACS). They exhibit strong cationic ionization and typically have molecular
weights of less than 500,000. When used as primary coagulants, they adsorb on particle
surfaces, reducing the repelling negative charges. These polymers may also bridge, to some
extent, from one particle to another but are not particularly effective flocculants. The use of
polyelectrolytes permits water clarification without the precipitation of additional
hydroxide solids formed by inorganic coagulants. The pH of the treated water is unaffected.
The efficiency of primary coagulant poly-electrolytes depends greatly on the nature of the
turbidity particles to be coagulated, the amount of turbidity present, and the mixing or
reaction energies available during coagulation. With lower influent turbidities, more
turbulence or mixing is required to achieve maximum charge neutralization.
Raw waters of less than 10 NTU (Nephelometric Turbidity Units) usually cannot be
clarified with a cationic polymer alone. Best results are obtained by a combination of an
inorganic salt and cationic polymer. In-line clarification should be considered for raw
waters with low turbidities.
Generally, waters containing 10 to 60 NTU are most effectively treated with an inorganic
coagulant and cationic polymer. In most cases, a significant portion of the inorganic
coagulant demand can be met with the cationic polyelectrolyte. With turbidity greater than
60 NTU, a polymeric primary coagulant alone is normally sufficient.
In low-turbidity waters where it is desirable to avoid using an inorganic coagulant, artificial
turbidity can be added to build floc. Bentonite clay is used to increase surface area for
adsorption and entrapment of finely divided turbidity. A polymeric coagulant is then added
to complete the coagulation process.
The use of organic polymers offers several advantages over the use of inorganic coagulants:

The amount of sludge produced during clarification can be reduced by 50-90%. The
approximate dry weight of solids removed per pound of dry alum and ferric sulfate
are approximately 0.25 and 0.5 lb, respectively.
The resulting sludge contains less chemically bound water and can be more easily
dewatered.
Polymeric coagulants do not affect pH. Therefore, the need for supplemental
alkalinity, such as lime, caustic, or soda ash, is reduced or eliminated.
Polymeric coagulants do not add to the total dissolved solids concentration. For
example, 1 ppm of alum adds 0.45 ppm of sulfate ion (expressed as CaCO3). The
reduction in sulfate can significantly extend the capacity of anion exchange systems.
Soluble iron or aluminum carryover in the clarifier effluent may result from
inorganic coagulant use. Therefore, elimination of the inorganic coagulant can
minimize the deposition of these metals in filters, ion exchange units, and cooling
systems.

Coagulant Aids (Flocculants)

In certain instances, an excess of primary coagulant (whether inorganic, polymeric, or a
combination of both) may be fed to promote large floc size and to increase settling rate.
However, in some waters, even high doses of primary coagulant will not produce the

desired effluent clarity. A polymeric coagulant aid added after the primary coagulant may,
by developing a larger floc at low treatment levels, reduce the amount of primary coagulant
required.
Generally, very high-molecular-weight, anionic polyacrylamides are the most effective
coagulant aids. Nonionic or cationic types have proven successful in some clarifier systems.
Essentially, the polymer bridges the small floc particles and causes them to agglomerate
rapidly into larger, more cohesive flocs that settle quickly. The higher-molecular-weight
polymers bridge suspended solids most effectively.
Coagulant aids have proven quite successful in precipitation softening and clarification to
achieve improved settling rates of precipitates and finished water clarity.
Color Reduction
Frequently, the objective of clarification is the re-duction of color. Swamps and wetlands
introduce color into surface waters, particularly after heavy rainfalls. Color-causing
materials can cause various problems, such as objectionable taste, increased
microbiological content, fouling of anion exchange resins, and interference with
coagulation and stabilization of silt, soluble iron, and manganese.
Most organic color in surface waters is colloidal and negatively charged. Chemically, colorproducing compounds are classified as humic and fulvic acids. Color can be removed by
chlorination and coagulation with aluminum or iron salts or organic polyelectrolytes.
Chlorine oxidizes color compounds, while the inorganic coagulants can physically remove
many types of organic color by neutralization of surface charges. The use of chlorine to
oxidize organic color bodies may be limited due to the production of chlorinated organic
by-products, such as trihalomethanes. Additional color removal is achieved by chemical
interaction with aluminum or iron hydrolysis products. Highly charged cationic organic
polyelectrolytes can also be used to coagulate some types of color particles.
Coagulation for color reduction is normally carried out at pH 4.5 to 5.5. Optimum pH for
turbidity removal is usually much higher than that for color reduction. The presence of
sulfate ions can interfere with coagulation for color reduction, whereas calcium and
magnesium ions can improve the process and broaden the pH range in which color may be
reduced effectively.
Conventional Clarification Equipment
The coagulation/flocculation and sedimentation process requires three distinct unit
processes:

high shear, rapid mix for coagulation

low shear, high retention time, moderate mixing for flocculation
liquid and solids separation

Horizontal Flow Clarifiers

Originally, conventional clarification units consisted of large, rectangular, concrete basins

divided into two or three sections. Each stage of the clarification process occurred in a
single section of the basin. Water movement was horizontal with plug flow through these
systems.
Because the design is suited to large-capacity basins, horizontal flow units are still used in
some large industrial plants and for clarifying municipal water. The retention time is
normally long (up to 4-6 hr), and is chiefly devoted to settling. Rapid mix is typically
designed for 3-5 min and slow mix for 15-30 min. This design affords great flexibility in
establishing proper chemical addition points. Also, such units are relatively insensitive to
sudden changes in water throughput.
The long retention also allows sufficient reaction time to make necessary adjustments in
chemical and polymer feed if raw water conditions suddenly change. However, for all but
very large treated water demands, horizontal units require high construction costs and more
land space per unit of water capacity.
Upflow Clarifiers
Compact and relatively economical, upflow clarifiers provide coagulation, flocculation, and
sedimentation in a single (usually circular) steel or concrete tank. These clarifiers are
termed "upflow" because the water flows up toward the effluent launders as the suspended
solids settle. They are characterized by increased solids contact through internal sludge
recirculation. This is a key feature in maintaining a high-clarity effluent and a major
difference from horizontal clarifiers.
Because retention time in an upflow unit is approximately 1-2 hr, upflow basins can be
much smaller in size than horizontal basins of equal throughput capacity. A rise rate of
0.70-1.25 gpm/ft of surface area is normal for clarification. Combination softeningclarification units may operate at up to 1.5 gpm/ft of surface area due to particle size and
densities of precipitated hardness.
In order to achieve high throughput efficiency, upflow units are designed to maximize the
linear overflow weir length while minimizing the opportunity for short-circuiting through
the settling zone. In addition, the two mixing stages for coagulation and flocculation take
place within the same clarification tank.
Although upflow units may provide more efficient sedimentation than horizontal designs,
many upflow clarifiers compromise on the rapid and slow mix sequences. Some types
provide rapid, mechanical mixing and rely on flow turbulence for flocculation; others
eliminate the rapid mix stage and provide only moderate turbulence for flocculation.
However, in most cases, users can overcome rapid mix deficiencies by adding the primary
coagulant further upstream of the clarifier. Figure 5-1 shows the rapid mix, slow mix, and
settling zones of a typical upflow, solids-contact clarifier.
Sludge Blanket and Solids-Contact Clarification
Most upflow designs are called either "sludge blanket" or "solids-contact" clarifiers. After
coagulation and/or flocculation in the sludge blanket units, the incoming water passes
through the suspended layer of previously formed floc. Figure 5-2 shows an upflow sludge

blanket clarifier.
Because the centerwell in these units is often shaped like an inverted cone, the rise rate of
the water decreases as it rises through the steadily enlarging cross section. When the rise
rate decreases enough to equal the settling rate of the suspended floc exactly, a distinct
sludge/liquid interface forms.
Sludge blanket efficiency depends on the filtering action as the freshly coagulated or
flocculated water passes through the suspended floc. Higher sludge levels increase the
filtration efficiency. In practice, the top sludge interface is carried at the highest safe level
to prevent upsets that might result in large amounts of floc carryover into the overflow.
Excessive sludge withdrawal or blowdown should also be avoided. The sludge blanket
level is often highly sensitive to changes in throughput, coagulant addition, and changes in
raw water chemistry and temperature.
"Solids-contact" refers to units in which large volumes of sludge are circulated internally.
The term also describes the sludge blanket unit and simply means that prior to and during
sedimentation the chemically treated water contacts previously coagulated solids. Solidscontact, slurry pool units do not rely on filtration as in sludge blanket designs.
Solids-contact units often combine clarification and precipitation softening. Bringing the
incoming raw water into contact with recirculated sludge improves the efficiency of the
softening reactions and increases the size and density of the floc particles. Figure 5-3
illustrates a typical solids-contact unit.
In-Line Clarification
In-line clarification is the process of removing raw water turbidity through the addition of
coagulant just prior to filtration. In-line clarification is generally limited to raw waters with
typical turbidities of less than 20 NTU, although upflow filters may tolerate higher loading.
Polyelectrolytes and/or inorganic coagulants are used to improve filtration efficiency and
run length. Polymers are favored because they do not create additional suspended solids
loading, which can shorten filter run length.
Filter design may be downflow or upflow, depending on raw water turbidity and particle
size. The downflow dual-media unit generally consists of layers of various grades of
anthracite and sand supported on a gravel bed. After backwashing, the larger anthracite
particles separate to the top of the bed, while the more dense, smaller sand particles are at
the bottom. The purpose is to allow bed penetration of the floc, which reduces the potential
for excessive pressure drops due to blinding off the top portion of filter media. Thus, higher
filtration rates are realized without a significant loss in effluent quality. Normal filtration
rates are 5-6 gpm/ft.
Coagulant Selection and Feeding for In-Line Clarification
The choice of a polymer coagulant and feed rate depends on equipment design and influent
water turbidity. Initially, in-line clarification was used in the treatment of low-turbidity
waters, but it is now being used on many types of surface waters. For most waters, the use
of a polymeric cationic coagulant alone is satisfactory. However, the addition of a high-

molecular-weight, anionic polymer may improve filtration efficiency.

Polymer feed rates are usually lower than those used in conventional clarification, given the
same raw water characteristics. Complete charge neutralization and bridging are not
necessary and should be avoided, because total coagulation or flocculation may promote
excessive entrapment of suspended solids in the first portion of the filter media. This can
cause blinding of the media, high pressure drops, and short operating runs.
Sufficient polymer is applied only to initiate neutralization, which allows attraction and
adsorption of particles through the entire bed. Often, polymer feed rates are regulated by
trial and error on the actual units to minimize effluent turbidity and maximize service run
length.
Because optimum flocculation is undesirable, polymers are injected just upstream of the
units. Normally, a short mixing period is required to achieve the degree of reaction most
suitable for unit operation. Dilution water may be recommended to disperse the polymer
properly throughout the incoming water. However, it may be necessary to move the
polymer injection point several times to improve turbidity removal. Due to the nature of
operation, a change of polymer feed rate will typically show a change in effluent turbidity
in a relatively short period of time.
Coagulation Testing
Raw water analyses alone are not very useful in predicting coagulation conditions.
Coagulation chemicals and appropriate feed rates must be selected according to operating
experience with a given raw water or by simulation of the clarification step on a laboratory
scale.
Jar testing is the most effective way to simulate clarification chemistry and operation. A
multiple-paddle, beaker arrangement (Figure 5-4) permits the comparison of various
chemical combinations, all of which are subjected to identical hydraulic conditions. The
effects of rapid and slow mix intensity and duration may also be observed.
In addition to determining the optimum chemical program, it is possible to establish the
correct order of addition. The most critical measurements in the jar test are coagulant
and/or flocculant dosages, pH, floc size and settling characteristics, floc-forming time, and
finished water clarity. To simulate sludge circulation, sludge formed in one series of jar
tests (or a sludge sample from an operating clarifier) may be added to the next jar test.
Results of jar tests are only relative, and frequent adjustments are necessary in full-scale
plant operation. Monitoring and control units, such as a streaming current detector, can be
used for on-line feedback control.
Zeta potential measurements have been used experimentally to predict coagulant
requirements and optimum pH levels. Because the measurement technique requires special
apparatus and a skilled technician, zeta potential has never become practical for controlling
industrial water clarification plants. Also, because zeta potential measures only one aspect
of the entire process, it may not reflect all conditions leading to coagulation efficiency.

Chemical Additions
The most efficient method for adding coagulation chemicals varies according to the type of
water and system used, and must be checked by means of jar testing. However, there is a
usual sequence:
1.
2.
3.
4.
5.

chlorine
bentonite (for low-turbidity waters)
primary inorganic and/or polymer coagulant
pH-adjusting chemicals
coagulant aid

Waters with a high organic content exhibit an increased primary coagulant demand.
Chlorine may be used to assist coagulation by oxidizing organic contaminants which have
dispersing properties. Chlorination prior to primary coagulant feed also reduces the
coagulant dosage. When an inorganic coagulant is used, the addition of pH-adjusting
chemicals prior to the coagulant establishes the proper pH environment for the primary
coagulant.
All treatment chemicals, with the exception of coagulant aids, should be added during very
turbulent mixing of the influent water. Rapid mixing while the aluminum and iron
coagulants are added ensures uniform cation adsorption onto the suspended matter.
High shear mixing is especially important when cationic polymers are used as primary
coagulants. In general, it is advisable to feed them as far ahead of the clarifier as possible.
However, when a coagulant aid is added, high shear mixing must be avoided to prevent
interference with the polymer's bridging function. Only moderate turbulence is needed to
generate floc growth.

Figure 5-1. Clarifier and zones.

Figure 5-2. Upflow sludge blanket clarifier. (Courtesy of the Permutit Company, Inc.)

Figure 5-3. Solids-contact clarifier. (Courtesy of Infilco Defremont, Inc.)