Pretreatment of Wastewater Streams from Petroleum/Petrochemical Industries Using Coagulation

doi:10.4236/aces.2011.14035

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Advances in Chemical Engi neering and Science , 2011, 1, 245-251

doi:10.4236/aces.2011.14035 Published Online October 2011 (http://www.SciRP.org/journal/aces)

Pretreatment of Wa stewater Streams from

Petroleum/Petrochemical Industries Using Coagulation

Hossam Altaher*, Emad ElQada, Waid Omar

Chemical Engi neeri n g Techn ol o gy Dep art m e nt, Yanbu Industrial College, Saudi Arabia

E-mail: *[email protected] (or [email protected])

Received June 27, 2011; revised July 19, 2011; accepted August 12, 2011

Abstract

Coagulation-flocculation processes using different types of conventional coagulants, namely, ferric chloride

(FeCl3), aluminum sulfate (AL2(SO4)3·18H2O), lime and ferrous sulfate (FeSO4) were investigated using the

Jar-test technique. A further aim is to determine the optimum conditions for the treatment of industrial

wastewater effluents i.e. coagulant dosage, mixing rate, temperature and pH control. Under optimal condi-

tion of process parameters, coagulation/flocculation process was able to lower the turbidity well below the

permissible level (1.8 NTU). The results indicate that ferric chloride had superior efficiency compared with

other coagulants with efficient dose of 800 mg/l. The optimal initial pH of the effluents that enhanced the

turbidity removal was 8.6. The temperature showed no significant effect on the turbidity removal.

Keywords: Ferric Chloride, Coagulation, Wastewater, Petrochemicals, Pretreatment

1. Introduction

This template, Yanbu Industrial City, at the Red Sea

Coast of Saudi Arabia, is considered as one of the major

industrial cities in the Kingdom of Saudi Arabia. The

city accommodates several large refining and petro-

chemical plants as well as a broad range of other manu-

facturing and support enterprises [1]. This inexorable

growth in the scale of the petrochemical industries and

oil refinery was largely responsible for the remarkable

ecological problems at Yanbu Industrial City .

This forces the Saudi government to issue strict legis-

lation concerning the quality of industrial wastewater

effluents and the industries are not allowed to discharge

any treated or untreated effluent in open channels and

even after treatment, the reclaimed water must have to

comply with direct discharge standards before discharge

to the sea [2]. So the entire industrial sectors send their

wastewater effluents to a local wastewater treatment

plant to treat their waste effluents to an increasingly high

standard. Actually, the treatment system consisting of

physical, chemical, and biological units is not enough in

its current state to reach the permissible levels of dis-

charge especially for turbidity. However, the focus of

this paper is the enhancement of coagulation process in

an attempt to comply with turbidity standards for obvi-

ous health issues.

Coagulation, adsorption on activated carbon, precipi-

tation, evaporation, ion-exchange, oxidation, and bio-

degradation and membrane filtration are known as an

industrial pollution prevention technology and used for

the decontamination of contaminated water and waste-

water [3]. According to Renault et al. [3], complete

treatment will clearly require several steps and it is o ften

appropriate to combine several methods of purification

before maximal efficiency is obtained.

Coagulation/flocculation is a widely-used process in

the primary purification of water and in industrial

wastewater treatment [3-5]. This method has a prefer-

ence in the primary purification processes mainly due to

the ease of operation, high efficiency, cost effective Also,

it uses less energy than alternative treatment [5-7].

Coagulants, both inorganic and organic such as alu-

minum sulfate (alum), ferrous sulfate, ferric chloride and

ferric chloro-sulfate are widely used as coagulants in

water and wastewater treatment for removing a broad

range of impurities from effluent, including organic mat-

ter, turbidity, color, microorganism, colloidal particles

and dissolved organic substances [4,5,8-10]

Wang et al [11] demonstrated that many factors can

influence the efficiency of coagulation-flocculation pro-

cess such as the type and dosage of coagulant/ floccu-

lant, pH, mixing speed and time, temperature and reten-

tion time. An appropriate combination of these factors is

246 H. ALTAHER ET AL.

desirable to obtain a high efficiency of treatment.

Dosta et al [9] and Franceschi et al [12] illustrated the

role of the pH in determining the electrical charge of

organic and inorganic colloids and considered it as a

major factor in the hydrolysis of aluminum salts.

The process usually consists of the rapid dispersal of a

coagulant into the wastewater containing solid particles

followed by an intense agitation commonly defined as

rapid mixing [13]. The coagulant aggregates the particles

into small flocs that slowly settle by charge neutraliza-

tion in negatively charged colloids by cationic hydrolysis

products and incorporation of impurities in an amor-

phous hydroxide precipitate (sweep flocculation), there-

by facilitating their removal in subsequent sedimentation,

floatation and filtration stages [5,6]. Zheng et al. [10]

reported that the basic prerequisites for an effect- tive

coagulant are the charge neutralization capacity and the

bridge-aggregation ability.

The aim of this systematic study was to optimize the

coagulation-flocculation process and investigate the ef-

fect of wastewater initial pH, affects the type of coagu-

lant and coagulant dosage, Temperature and mixing con-

ditions in order to enhance the efficiency of the coagula-

tion—flocculation process especially focusing on the

optimal turbidity removal.

2. Materials and Methods

2.1. Wastewater Samples

Wastewater was obtained from the wastewater treatment

facility in Yanbu Industrial City. This wastewater is a

mixture of industrial wastewater from all the industrial

facilities in Yanbu Industrial City. However, most of

these industrial facilities are petrochemical companies

and refineries. The wastewater was collected from the

influent collection well and sent directly to our lab.

Wastewater was stored at 4˚C and was equilibrated to

room temperature before use. The main characteristics of

the wastewater are presented in Table 1.

2.2. Preparation of Coagulants

Ferric chloride (FeCl3) was obtained from Loba Chemie

(India), Aluminum sulfate (Al2(SO4)3·18H2O) was ob-

tained from Panreac Quimica SA (Spain), Ferrous sulfate

(FeSO4) was obtained from TechnoPharmChem (india).

The solutions were prepared by dissolving 10g of each

substance in distilled water and the solution volumes

were increased to 1 liter. Each 1 ml of these stock solu-

tions was equivalent to 20 mg/l when added to 500 ml of

wastewater to be tested.

Table 1. Main characteristics of wastewater.

BOD51

mg/lit TSS2

mg/lit TDS3

mg/lit TS4

mg/lit Total hardness

mg/lit as CaCO3

57.5 22172323 4545 190

1After 1/10 dilution; 2TSS: Total suspended solids; 3TDS: Total dissolved

solids; 4TS: total solids.

2.3. Comparison of Different Coagulants

Coagulation and flocculation tests were performed in a

standard jar test apparatus (SOLTEQ flocculation test

unit, Mode TR 10) consisting of six paddles and

equipped with 6 beakers of 1 liter volume. Each beaker

was filled with 500 ml of wastewater sample. The per-

formance of the coagulants was compared by adding 50

ml of each one to 500 ml of wastewater without adjust-

ing pHs of the samples. The samples were agitated at a

flash mixing speed of 300 rpm for 3 minutes followed by

slow mixing speed of 50 rpm for 15 minutes. At the end

of the stirring period the flocs were allowed to settle

down for 20 minutes. The samples were taken by im-

mersing a pipette 2 cm below the surface of the water.

Turbidity, total dissolved solids, and pH were measured

using Hach 2100 AN turbidimeter, SevenEasy conduc-

tivity meter from Metler Toledo, and pH501 EuTech

Instrument, respectively. All the equipment were cali-

brated before use.

2.4. Effect of pH

To measure the effect of initial pH 7, 1-liter beakers were

filled with 500 ml of the wastewater. The pH of each of

sample was adjusted at different value using 1 N of sul-

furic acid or sodium hydroxide solutions. The pHs tested

covered the range from 3 up to 11.5. To each beaker 50

ml of FeCl3 were added. The mixture were rapidly mixed

at 300 rpm for 3 minutes, slow mixed for 15 minutes at

50 rpm and allowed to settle for 20 minutes. Samples to

be analyzed were taken from the supernatant 2 cm below

the level of the liquid.

2.5. Effect of Coagulant Dose

The optimum dose of the selected coagulant was deter-

mined by placing different volumes of coagulant (5, 10,

20, 30, 40, 50, 70 ml) in conical flasks. The volume was

adjusted to 70 ml in each flask by adding distilled water

(65, 60, 50. 40, 30, 20, 0 ml, respectively). This proce-

dure was applied to keep the total volume of the treated

samples at the same value. The rapid stirring speed was

300 rpm for 3 minutes and the slow stirring speed was 50

rpm for 15 minutes, followed by 20 minutes of settling.

The effect of coagulant dose on pH of solution was

H. ALTAHER ET AL.

247

measure by adding different doses of coagulant to 500 ml

aliquot of the wastewater, stirring for 2 minutes and then

measuring the pH of the different solutions. The effects

of rapid and slow mixing speed were studied similarly.

The rapid mixing speed was varied from 100 to 350 rpm

keeping the slow mixing at a constant value of 50 rpm.

The slow mixing effect was studied by varying the speed

from 45 rpm to 100 rpm keeping the rapid mixing at a

value of 30 0 rpm.

2.6. 30-Minutes Settling Tests

This experiment as usual with a jar test protocol. Two

beakers were filled with 500 ml aliquot of wastewater.

40 ml of FeCl3 were added to each beaker. The mixtures

were rapidly stirred at 300 rpm for 3 minutes, followed

by slow stirring at 50 rpm for 15 minutes. The two sam-

ples were placed in 1-liter cylinder and allowed to settle

down without disturbance. The volume of the sludge in

the cylinder was observed with time. All the above men-

tioned tests were performed at room temperature.

2.7. Effect of Temperature

The effect of temperature was tested by adjusting the

temperatures of three wastewater samples (500ml each)

at three different temperatures (13˚C, 22˚C and 43˚C).

To each sampl e 40 ml of FeCl3 were added. The mixture

were stirred rapidly for 3 minutes at 300 rpm, slow-

stirred at 50 rpm for 15 minutes, and allowed to settle for

20 minutes. The final temperature was measured to take

into account the effect of any change that may have oc-

curred.

3. Results and Discussion

3.1. Comparison of Different Coagulants

The objective of this experiment was to determine the

best coagulan t that can be used to reduce tu rbidity to the

permissible level for such wastewater. It is well known

that the behavior of coagulant may change from waste-

water to another according to many factors including

alkalinity, pH, and different constituents of wastewater

[11, 12]. Figure 1 depicts the removal efficiencies for

different coagulants. It was found that ferric chloride had

superior efficiency in removing turbidity compared with

other coagulants at the sp ecified conditions.

Final turbidity of 1.8 NTU is well below the permissi-

ble level set by governmental agencies in Yanbu Indus-

trial City (15 NTU) for such parameter. Ferrous sulfate at

these conditions increased the turbidity of the sample,

while lime has negligible effect on turbidity removal.

Figure 1. Effect of different coagulants on turbidity removal.

Alum was found to have comparable removal efficiency

to FeCl3, but the later was chosen to avoid the hazardous

effect of alum .

3.2. Effect of pH on Treatment Process

The pH value is a very important factor in the coagula-

tion process. The optimum value of pH depends essen-

tially on the properties of the water treated, type of the

coagulant used and its concentration. Abdulaziz et al. [14]

attributed the effect of pH on coagulation process as a

balance of two competitive forces; (1) forces between H+

and metal hydrolysis products for interaction with or-

ganic ligands that may be present in water, and (2) forces

between hydroxide ions and organic anions for interact-

tion with metal hydrolysis products. The effect of pH can

be explained by the study of the reactions involved with

the coagulants as depicted below [15]:

332 32

2FeCl3Ca(HCO )2Fe(OH) ()3CaCl6CO

32 3

2FeCl3Ca(OH)2Fe(OH) ()3CaCl

It is clear that the increase of the concentration of al-

kalinity will shift the reaction to the right direction

(product side), i.e. enhancing the coagulation/ floccula-

tion process. It can be depicted from Figure 2 that the

optimum initial pH for turbidity removal is 7 and 8.6

giving removal efficiencies of 95.9% and 95.2%, re-

spectively. The later pH was selected for further tests for

two reasons. First, it was the initial pH of the raw

wastewater. So, no need for adding chemicals to adjust

the pH. Second, as illustrated by Figure 3, the coagulan t

dose will lead to a decrease in pH of the solution until it

reaches an acidic value, which is not required during

coagulation. By the addition of iron chloride (FeCl36H2O)

as coagulant, the suspended negatively charged solid

particles are destabilized.

The removal of solids in water by settling or filtration

of the solid particles must be incorporated as flocks and

248 H. ALTAHER ET AL.

Figure 2. Effect of initial pH on turbidity removal.

Figure 3. Effect of coagulant dose on solution pH.

these flocks are formed after dosing of the coagulant.

The addition of FeCl36H2O, as coagulant, to water

will result in formation of Fe(OH)3 which dissociates to

form different positively charged ions and negatively

charged ions which are produced due to the following

hydrolysis reactions [16]:

Fe(OH)Fe3OH





Fe2HOFe(OH)H O



 



232

Fe(OH)H OFe(OH)H O

 

 

2Fe4H OFe(OH)2H O





Fe(OH) OHFe(OH)





As the above reactions indicate, in addition to iron hy-

droxide, the following hydrolyses products of Fe3+ are

also formed: Fe(OH)2+, Fe(OH)2+, Fe(OH)–4. The con-

centration of each ion depends on the pH, dose concen-

tration and equilibrium constant for each reaction which

is temperature dependant. The calculated dependence of

the concentration of each hydrolysis product over wide

pH range and different dose concentrations is depicted in

Figure 4.

The results obtained indicated that the best turbidity

removal was achieved at dose concentration of 800 mg/l

and pH range between 7 and 8.6 as indicated in Figures

Figure 4. Effect of initial pH on turbidity removal.

2 and 5, respectively. By referring to Figure 4, these

operating conditions are located in the above dark rec-

tangle. In this region, large amounts of precipitated

Fe(OH)3 are formed with the absence of positively

charged particles. This is evidence that at these optimum

conditions a precipitation coagulation, or sweep coagula-

tion, is the dominant mechanism where colloids are in-

corporated into neutral (iron) hydroxide flocs.

The measurements presented in Figure 2 showed that

the pH value of water decreases by increasing the dose

concentration of FeCl36H2O.

As a result of the dosing of iron chloride, OH- ions are

removed and the pH will decrease. The magnitude of the

pH drop depends on the buffering capacity of the water.

The higher the buffering capacity, the smaller the pH

drop is. When the pH drop is too large, pH will be in-

creased by dosing a base, such as caustic soda.

It is worthy to notice that, by examining Figure 4, the

pH value influences the solid particles stabilization in

two counter currently directions. At low dosage concen-

tration, increasing the pH leads to an increase in the

negatively charged ions (stabilization effect) and a de-

crease in the concentration of the positively charged ions

(destabilization effect). The achieved maximum turbidity

removal at high dose concentration over the optimum pH

range between 7 and 9 can be explained through the en-

hancement of precipitation of Fe(OH)3. This forces the

sweep coagulation process. This process cannot occur in

strongly basic medium or strongly acidic medium.

The observed drop in turbidity removal in strongly ba-

sic medium is due to the increased concentration of the

negatively charged ions which contributes negatively in

the coagulation process by its stabilization effects.

3.3. Effect of Coagulant Dose

As illustrated by Figure 5, the highest efficiency of tur-

bidity removal to such wastewater was achieved using

800 mg/l of ferric chloride. This dose resulted in turbid-

ity removal efficiency of 97.5% equivalent to a final tur-

bidity of 2.2 NTU. This high dose can be decreased to

half its value (400 mg/l) to produce turbidity removal

H. ALTAHER ET AL.

249

Figure 5. Effect of Coagulant Dose on turbidity removal.

efficiency of 93% (equivalent to final turbidity of 5.9

NTU). This later value can be accepted from a pretreat-

ment process for such wastewater taking into considera-

tion it would go through more treatment processes (two

biological treatment processes followed by sand filtra-

tion).

3.4. Effect of Mixing and Settling Rates

The effect of agitation rate on the turbidity removal effi-

ciency is illustrated by Figures 6 and 7. The highest tur-

bidity removal (98.1%) was achieved at 200 rpm of rapid

mixing. However, the other rates produced comparable

results. The lowest removal efficiency was obtained at a

stirring rate of 350 rpm (95.6%). The next one was at

100 rpm (96.55%). Comparing the later value obtained at

the lowest speed with the rate that produced the highest

removal efficiency (98.1% at a rate of 200 rpm), it is

clear that the negligible difference in removal efficiency

suggests the use of the lowest velocity that will save en-

ergy. Similar results were obtained for the slow mixing

rates. All the tested rates gave comparable removal effi-

ciencies ranging from 98.2% up to 98.7%. These re-

moval efficiencies are equivalent to final turbidities less

than 2 NTU. So, the lowest velocity (45 rpm) can be

used for a slow mixing step. Figure 8 illustrates the set-

tling process after flocculation/ coagulation. The solid

volume reached a value of 13% of its initial value after

90 minutes. The sludge volume index for the produced

sludge was calculated to be 190 which is an acceptable

value.

3.5. Effect of Temperature

The temperature of Saudi Arabia may reach 50 Degrees

Celsius. This high temperature in Saudi Arabia is very

common. The average temperature during the winter

season is roughly around 8 to 20 degrees Celsius.

However, even in the summers, the nights are really

chilly as the desert tends to become cold once the sun

Figure 6. Effect of Rate of Rabid Mixing on turbidity re-

moval.

Figure 7. Effect of Rate of slow Mixing on turbidity removal.

Figure 8. Settling rate of the sludge.

sets down. The effect of temperature on the coagulation

process was studied at three different temperatures as

illustrated by Figure 9. It is clear that the temperature

does not have a considerable effect within the studied

temperature range (13 up to 43˚C). Slight differences

were noticed due to this temperature effect. Removal

efficient obtained at room temperature 22˚C (96.3%) was

the highest compared to 95.4% and 95.8% achieved at 13

and 43˚C, respectively.

250 H. ALTAHER ET AL.

Figure 9. Effect of Temperature on turbidity removal.

4. Conclusions

The focus of this paper was to investigate the potential

use of coagulation-flocculation process for the removal

of turbidity from industrial wastewater influents using

aluminum sulfate and iron salts. The experiments con-

ducted confirm the significant effect of pH on coagula-

tion process.

Increasing pH form acidic range to alkaline range

promotes turbidity removal indicating the significant role

played by pH in imparting surface charge of organic and

inorganic colloids. The optimum pH for the removal of

turbidity from industrial effluents under the experimental

conditions used in this work was = 8.6.

Under optimal conditions of process parameters, a

coagulant dose of 400 mg/l was efficient to remove 93%

of the effluents’ turbidity.

Rate of mixing range used in this work showed negli-

gible differences in the turbidity removal efficiencies and

this suggests that the lowest mixing rate can be used to

save energy.

Coagulation-flocculation process has proved an effi-

cient process to remove turbidity from industrial waste-

water effluents.

5. Acknowledgement

The authors gratefully acknowledge the financial support

provided through the Planning and Development De-

partment of Yanbu I ndustrial College.

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