Journal of Environmental Protection, 2011, 2, 1250-1256
doi:10.4236/jep.2011.29144 Published Online October 2011 (
Copyright © 2011 SciRes. JEP
Enhancement of Quality of Secondary Industrial
Wastewater Effluent by Coagulation Process: A
Case Study
Hossam Altaher, Ahmed Alghamdi
Department of Chemical Engineering Technology, Yanbu Industrial College, Yanbu Industrial City, Kingdom of Saudi Arabia.
Received Augst 17th, 2011; revised September 23rd, 2011; accepted October 24th, 2011.
The local wastewater treatment facility in Yanbu Industrial City receives 24,000 m3/day of industrial wastewater. This
wastewater, mostly from refineries and petrochemical industries, go es through physica l, biological and chemical stages
of treatment. However, the treated water still fails to pass some of the permissible levels set by governmental agencies.
This research paper investigated the enhancement of the treatment processes to reduce the turbidity of the effluent
treated water. Ferric chloride, ferrous sulfate, alum and commercial synthetic cationic polymer were tried as coagu-
lants. Different conditions (i.e., pH, temperature, dose, stirring rate) were searched. Ferrous sulfate and polymer re-
duced the final turbidity to acceptable values with very low doses compared with other coagu lant.
Keywords: Coagulation, Turbidity, Industrial, Wastewater, Treatment, Polymer
1. Introduction
Turbidity consists of suspended material in water, caus-
ing cloudy appearance. This cloudy appearance caused
by scattering and absorption of light by these particles.
The suspended matter may be inorganic or organic. Ge-
nerally the small size of particles prevents rapid settling
of the material and water must be treated to reduce its
turbidity. Turbidity can provide food and shelter for pa-
thogens. If not removed, turbidity can promote regrowth
of pathogens in the distribution system leading to water-
borne disease outbreaks. Although turbidity is not a di-
rect indicator of health risk, numerous studies show a
strong relationship between removal of turbidity and re-
moval of protozoa [1].
Treatment for reducing turbidity involves filtration [2],
coagulation and flocculation [3]. Effective treatment of
turbid water remains a challenge. This is because micro-
bial reduction is decreased or prevented by turbidity par-
ticles that reduce access to target microbes or otherwise
protect them from inactivation by other mechanisms. Su-
spended matter in water reduces the microbiocidal effi-
cacy of chlorine and other chemical disinfectants, and it
physically shields microbes from the UV radiation, that
is responsible for much of its disinfection activity. There
is a need to investigate, characterize and implement
appropriate physical and physical-chemical technologies
for practical and low cost pre-treatment and treatment of
water taking into consideration turbid waters of different
quality with respect to particle characteristics and their
removal efficiencies prior to chlorination [4].
Solids are present in water in three main forms: su-
spended particles, colloids and dissolved molecules. Su-
spended particles such as sand, vegetable matter and silt
range in size from very large particles down to particles
with a typical dimension of 10 µm. Colloids are very fine
particles, typically ranging from 10 nm to 10 µm. Dis-
solved molecules are present as individual molecules or
as ions. Dissolved molecules cannot be removed by con-
ventional physical treatment. Thus, the removal of co-
lloids is the main objective and the most difficult aspect
in conventional water treatment. Most suspended solids
smaller than 0.1 mm found in water carry negative elec-
trostatic charges. Since, the particles have similar nega-
tive electrical charges and electrical forces to keep the in-
dividual particles separate, the colloids stay in suspen-
sion as small particles. To remove colloids, small parti-
cles have to be destabilized first and then they will form
larger and heavier flocks which can be removed by con-
ventional physical treatment. This process can be des-
cribed by clarification mechanism, which includes: coa-
Enhancement of Quality of Secondary Industrial Wastewater Effluent by Coagulation Process: A Case Study1251
gulation, flocculation and sedimentation [5].
Alum is the most widely used coagulant in water treat-
ment, because of its proven performance and cost effe-
ctiveness. The use of alum as a coagulant increases the
aluminum concentration in finished water [6]. The high
concentration of aluminum is also of concern because of
potential adverse health effects. Aluminum intake into
the body has been linked with several possible neuro-
pathological diseases including Alzheimer’s disease [7].
An approach to reduce the concentration of residual
aluminum in finished water is to use a synthetic polymer
or a naturally occurring polyelectrolyte as a coagulant
aid with primary coagulant alum. A polyelectrolyte in
conjunction with a metal coagulant improves coagulation
by accelerating the process of coagulation. The coagulant
aid reduces the requirement of alum and improves the
physical characteristic of flocs, which results in better
quality of treated water [8]. Other coagulants can also be
used. These coagulants include ferric salts and synthetic
polymers [9]. There has been considerable interest in the
development of natural coagulants. By using natural coa-
gulants considerable saving in chemicals and sludge
handling cost may be achieved. Chitosan is inexpensive,
biodegradable and nontoxic for mammals. Moreover, its
molecules have the ability to interact with bacterial sur-
face and are adsorbed on the surface of the cell and stack
on the microbial cell surface and forming impervious
layer around the cell, leading to the block of the cell [10].
During recent decades research on electricity applied
directly in water treatment has progressed well, making it
an attractive method for coagulation or clarification of
water, usually known as the electro-coagulation/electro-
chemical method. In this method direct current is passed
through aluminum/iron plates suspended in water. This
system causes sacrificial electrode ions to move into an
electrolyte. Undesirable contaminants are removed either
by chemical reaction and precipitation or by causing
colloidal materials to coalesce. They are then removed
by electrolytic flotation, or sedimentation and filtration.
Disinfection is also accomplished by anodic oxidation.
The mechanisms of coagulation were similar for electro-
coagulation and aluminum salts treatment. The diffe-
rence is mainly in the way aluminum ions are delivered
Polyelectrolytes are one of the most widely used che-
micals serving as coagulants/flocculents in modern water/
wastewater treatment. Their primary advantages are their
very low dosing requirement and their applicability over
a wide range of pH compared with alum or other inor-
ganic coagulants/flocculants. Polyelectrolytes are catego-
rized based on their product origin. Natural polyelec-
trolytes include polymers of biological origin derived
from starch, cellulose, and alginates. Synthetic poly-
electrolytes consist of single monomers polymerized into
a high-molecular-weight substance. The action of poly-
electrolytes changes according to their type. Cationic po-
lymers, in which the cations (positive charges) form the
polymer, reduce or reverse the negative charges of the
precipitate and therefore act as a primary coagulant.
Anionic polymers, based on carboxylate ions and poly-
ampholytes, carry primarily negative charges and help in
interparticle bridging along the length of the polymer, re-
sulting in three-dimensional particle growth and thereby
easy settlement. A third type of polymer, developed from
cationic polyelectrolytes of extremely high molecular
weight, is capable of offering both coagulation and bri-
dging [12].
The local wastewater treatment facility in Yanbu In-
dustrial City is responsible for the treatment of industrial
wastewater collected from the local industrial facilities.
Most of these are petroleum refineries and petrochemical
facilities. The capacity of treatment facility is 24,000
m3/day. However, it receives 32,000 m3/day. This may
be a reason of the turbidity of the secondary treated water
exceeding the limits set by the governmental agents.
Taken into consideration that the treated water after the
biological process is not disinfected, there is an urgent
need to decrease the turbidity to the lowest possible level
to avoid dangerous pollution of the Red Sea. So, a cri-
tical need for correction action was necessary. The ob-
jective of this research was to enhance the quality of the
secondary treated wastewater and reduce the turbidity of
the wastewater to the required limit.
2. Experimental
2.1. Materials
Raw water samples were obtained from the local waste-
water treatment facility at Yanbu Industrial City. The
samples were collected from the effluent of the industrial
wastewater treatment unit. The samples were well agi-
tated before each use. The coagulants used were of ana-
lytical grade. The alum was obtained from Panreac Qui-
mica SA (Spain), the ferric chloride from Loba Chemie
(India), the ferrous sulfate from TechnoPharmChem (In-
dia), and the polymer used was powder cationic polymer
M-969 Non-Hazard, Polyelectrolyte dry polymer. This
polymer was obtained from Metito Chemical Industrial
Ltd. 1% solutions of these coagulants were prepared by
dissolving 10 grams of each coagulant in 1 liter of dis-
tilled water. Sodium hydroxide and sulfuric acid were
used for adjusting pHs. These two solutions was pre-
pared by dissolving the gram equivalent weight of each
in one litre of distilled water.
2.2. Equipment
The pH was measured using pH501, EuTech Instruments.
Copyright © 2011 SciRes. JEP
Enhancement of Quality of Secondary Industrial Wastewater Effluent by Coagulation Process: A Case Study
The pH meter was calibrated before use all over the
range of pHs that was utilized. The turbidity was mea-
sured using Hach 2100AN Turbidimeter. TDS was moni-
tored using SevenEasy conductivitimeter from Metler
2.3. Coagulation/Flocculation Test
A conventional jar test apparatus (SLOTEQ flocculation
test unit, Model TR10) was employed for the test. It had
six stirrers. Every stirrer was controlled individually. All
tests were carried out with 500 ml of wastewater in
1-liter beaker. All tests were performed at room tem-
perature unless otherwise mentioned.
Optimum pH was determined by adjusting of 500 ml
aliquot of wastewater at different pHs using sulfuric acid
and sodium hydroxide. To each wastewater sample the
same dose of coagulant was added. A fast stirring of 250
rpm was applied for 2 minutes. The stirring speed was
then reduced to 50 rpm for 15 minutes. At the end of the
slow speed period, the mixing was stopped and the
formed flocs were left to settle down for 15 minutes. The
samples to be tested were taken 2 cm from the top of the
water level in the beaker. Turbidity, total dissolved solids
and pH were determined.
The effect of coagulant dose, rate of rapid and slow
stir-ing were determined similarly with change of appro-
priate conditions. To examine the effect of temperature a
sample was warmed over the hot plate to reach the re-
quired temperature and another sample was placed inside
the incubator to reach the required cold reduced tem-
To test the settling rate, 1-liter sample was treated with
the coagulant, rapidly-stirred for 2 minutes, slow-stirred
for 15 minutes and placed in 1-liter cylinder. The level of
the flocs in the cylinder was monitored with time for 30
3. Results and Discussion
3.1. Effect of Coagulant Dose
Figure 1 indicates the relation between the used coagu-
lant dose and its effect on the turbidity removal effici-
ency which was calculated according to the following
Turbidity removal efficiency % = [(initial turbidity –
final turbidity)/initial turbidity]*100
The figure indicates different effects and behaviors of
coagulants. The increase of ferric chloride dose from 100
mg/l to 1200 mg/l resulted in an increase of turbidity
removal efficiency from 86% to 97%. However, only
200 mg/l were enough to reduce the turbidity to the per-
missible turbidity level set by Royal Commission (15
NTU). Considerable decrease in the final pH value was
also observed. The final pH was function in the coagu-
Figure 1. Effect of coagulant dose on turbidity removal
efficiency. The pH of wastewater was 7.9, initial turbidity
was 138 NTU, and initial TDS was 11.15 mg/l.
lant dose. With a lowest pH value of 5.3 obtained when
applied 1200 mg/l of the coagulant. A relation of dose
and final TDS was also noticed. Increasing the dose in-
creased the TDS of the sample. The reason may be at-
tributed to the coagulant itself which represents an added
salt to the original sample. The effect of ferrous sulfate
was quite different. The preliminary experiments indi-
cated that high doses of this coagulant ( 400mg/l) re-
sulted in increase in turbidity of the final sample. The
dose range applied for this coagulant was reduced to be
between 1 - 6 mg/l. In this range the turbidity removal
efficiency was between 95% and 97.5%. The lowest dose
that was used (20 mg/l) was enough to drop the final
turbidity to 6.9 NTU, a value well below the permissible
level. The final TDS of the samples increased to an av-
erage value of 16.4 mg/l compared to an initial TDS of
11.31 mg/l. The final pH of all doses was 11.2. This
value is high compared with that set by governmental
agencies for treated wastewater (pH 6 - 9) which requires
more treatment to reduce this pH value to accepted dis-
charge limit. The polymer showed comparable turbidity
removal efficiency to that was obtained by ferrous sul-
fate. 1 mg/l of the polymer dropped the turbidity to 7.7
NTU. The polymer had other important effects. It did not
change the pH of the treated water nor the final TDS. All
the flocs gathered to form 4 big flocs during the slow
stirring period. That means no need for settling period.
The alum showed the weakest treating ability only at a
very high dose (1200 mg/l) the turbidity dropped to a
value lower than permissible level.
3.2. Effect of Wastewater pH
Figure 2 indicates the effect of pH on turbidity removal
opyright © 2011 SciRes. JEP
Enhancement of Quality of Secondary Industrial Wastewater Effluent by Coagulation Process: A Case Study1253
Figure 2. Effect of initial pH of wastewater on turbidity re-
moval efficiency. Initial turbidity was 138 NTU, and initial
TDS was 11.15 mg/l.
efficiency. The two acidic pH values (2.08 and 5.52) that
were tested for ferric chloride resulted in very high in-
crease in turbidity. The turbidity increased by factors of
17.4 and 32%, respectively, instead of decreasing at
these acidic values. The explanation may be the need for
alkalinity for precipitation of ferric hydroxide to take
place. The possible reactions responsible for coagula-
tions are as follows:
33 3
 
43 4
  (2)
24 3
 (3)
In acidic medium, the bicarbonate species required for
such reaction are not available. Consequently, no preci-
pitation would occur. Except for polymer, all other co-
agulants showed weak to mild coagulation efficiency at
acidic values. This can be interpreted using the previous
reactions. It is clear from the stoichiometry of the rea-
ctions that there is a different need for alkalinity for pre-
cipitation of hydroxides to take place, with alum having
the highest need (6 moles of alkalinity for every mole of
In the basic region, the ferric chloride indicated excel-
lent removal efficiency. At pH 7.95 (original pH of the
wastewater), ferric chloride showed the highest effi-
ciency (96% equivalent to final turbidity of 5.8). An im-
portant point must be taken into consideration which is
the dose. The ferric chloride dose that was used to reach
this value was 800 mg/l, compared to a dose of 100 mg/l
for ferrous sulfate and a dose of 20 mg/l that produced
97% and 92% removal efficiency, respectively. It can be
also noted all coagulant showed the same removal effi-
ciency at very high pH value. (nearly at pH value of 12).
The reason was not just the coagulant in this case. It was
noticed that adjusting the pH at this value resulted in
precipitation, even without adding any coagulant. The
enhancement of removal efficiency at that high pH may
be attributed to the formed precipitate that might have a
sweeping action to the suspended solids present in waste-
water, thus decreasing turbidity.
There is no single pH region that has been agreed
upon by researcher as the optimum pH region for coagu-
lation. Wang et al. [13] suggested the pH region of 3 - 8.
Other researchers [14-16] suggested pH region (4 - 5.4)
if using ferric salts as coagulants and water contains or-
ganic colloids.
The pH value was found to have a pronounced effect
on the final TDS of the sample. Figure 3 represents such
relation. The general trend for the relation between pH
and TDS is the decrease of TDS with increasing the pH
up to approximate pH value of 7.95 (initial pH of waste-
water), followed by increase of TDS with further in-
crease in pH. The reason is attributed to the sodium hy-
droxide and sulfuric acids that were added to increase or
decrease the pH. Both of them increase the TDS of the
wastewater sample.
3.3. Effect of Wastewater Temperature
Figure 4 represents the effect of temperature on removal
efficiency. The temperature range investigated (15˚C -
48˚C) represents the temperature range in Yanbu Indus-
trial City between Winter and Summer. The figure indi-
cates negligible effect on turbidity removal in case of
ferric chloride, ferrous sulfate and polymer. The effect of
temperature is considerable using alum.
The lowest turbidity removal using this coagulant was
achieved at 25˚C (70.5%). The efficiency increased to
Figure 3. Effect of initial pH of wastewater on final TDS of
wastewater. Initial turbidity was 138 NTU, and initial TDS
was 11.15 mg/l at room temperature.
Copyright © 2011 SciRes. JEP
Enhancement of Quality of Secondary Industrial Wastewater Effluent by Coagulation Process: A Case Study
Figure 4. Effect of wastewater temperature on turbidity
removal efficiency.
73.5% at 15˚C and to 79.4% at 48˚C. Fitzpatrick and et
al. [17] studied the effect of temperature on flocculation
using different coagulant. However, their results are dif-
ferent. The coagulation using alum was better at higher
temperature. They attributed that to breakage of flocs at
low temperatures. Bratby [1] related the effect of tem-
perature to the mechanism of coagulation. He mentioned
that this effect is pronounced in case of enmeshment
mechanism and less severe in case of adsorption-type
mechanism. Guan et al. [18] reported that the turbidity
removal efficiency increased gradually by increasing
3.4. Effect of Stirring Rate
The effect of stirring rate on the turbidity removal pro-
cess for both rapid and slow stages are represented by
Figures 5 and 6, respectively. It is evident from figures
that the stirring rate does not have any effect on the effi-
ciency of the removal process. Hanhui et al. [5] had si-
milar results. They proposed a relation between effici-
ency of flocculation with type of flocculants. In case of
organic compounds neither stirring time nor stirring in-
tensity affects coagulation-flocculation excessively. The
results of this experiment suggest combining the coagu-
lation and flocculation in one process with stirring for
reasonable time (15 minutes). This procedure will save a
lot of power.
3.5. Settling Test
Figure 7 presents the settling rate of flocs formed by
different coagulants. The plot representing the sedimen-
tation of flocs formed by polymer is not represented here.
As was mentioned before, all the flocs that were formed
consolidated to form 4 big flocs that floated at the sur-
face of the sample. That had taken place within three
minutes of the slow stirring process. Figure 7 indicates
Figure 5. Effect of slow stirring rate on turbidity removal
efficiency. Initial turbidity was 138 NTU, initial pH was 7.9
and initial TDS was 11.15 mg/l.
Figure 6. Effect of rapid stirring rate on turbidity removal
efficiency. Initial turbidity was 138 NTU, initial pH was 7.9
and initial TDS was 11.15 mg/l.
Figure 7. Sludge settling rate. Initial turbidity was 138 NTU,
initial pH was 7.9 and initial TDS was 11.15 mg/l.
opyright © 2011 SciRes. JEP
Enhancement of Quality of Secondary Industrial Wastewater Effluent by Coagulation Process: A Case Study1255
that ferrous sulfate-flocs have the better settlabilty among
other flocs. Within 15 minutes the sludge reached its
lowest volume compared to 22 and 28 minutes for alum
and ferric chloride, respectively. The final sludge volume
is another indication about the excellent characteristics of
ferrous sulfate as a coagulant. This volume was 50 ml
compared to 100 and 110 ml for alum and ferric chloride
respectively. The sludge volume index (SVI) for the
three coagulants are 25, 55 and 50 ml/l, respectively in-
dicate good settling properties.
4. Conclusions and Recommendations
The removal efficiency of turbidity from industrial waste-
water was experimentally investigated using coagulation
technique. Four different coagulants were tested. The
temperature did not have a preannounced effect on the
coagulation process. The pH of the wastewater had an
important effect on turbidity removal efficiency. The
highest removal efficiency was found at higher pH. Even
without the addition of coagulant a considerable part of
the turbidity would precipitate at elevated pH. The Stir-
ring rate also does not have an effect either. Consequ-
ently, the slow and fast steps of stirring can be consoli-
dated in just one step with moderate velocity to save
money and time. Ferrous sulfate and polymer were found
to be the best polymers. Even at very low doses (in the
range of 1 - 5 mg/lit) the turbidity was reduced to much
lower than that required by the governmental agencies.
The settling rate for ferrous sulfate was 15 minutes com-
pared to 4 minutes for the polymer. The jar test must be
applied regularly to keep up with the continuous changes
of the properties of the received industrial wastewater
which ensures good quality of the treated water.
5. Acknowledgements
The authors are grateful to the local wastewater treat-
ment facility for supplying the industrial wastewater, and
polymer used in this work. The authors would like to
thank Mr. AbdelRahman Mohanna and Mr. Samy Jaber
for their help through this work.
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