G.A. Rassoul and D.R. Rzaige 

 

IJCPE Vol.9 No.3 (2008) 

 

 

 
Iraqi Journal of Chemical and Petroleum Engineering 

 Vol.9 No.3 (December 2007) 37-41 
ISSN: 1997-4884 

 

Optimization of Biochemical Treatment of Tannery Wastewater 

NERAN K. IBRAHIM , ALI H. ALWAN
*
and MAHMOOD M. BARBOOTI

 * *
 

*
 Department of Chemical Engineering, University of Technology, Baghdad, Iraq. 

** 
 Environmental Analysis Department, Ministry of Environment, Baghdad, Iraq 

Abstract 

The present work is concerned with the finding of the optimum conditions for biochemical wastewater treatment for a 

local tannery. The water samples were taken from outline areas (the wastewater of the chrome and vegetable tannery) in 

equal volumes and subjected to sedimentation, biological treatment, and chemical and natural sedimentation treatment. 

The Box-Wilson method of experimental design was adopted to find useful relationships between three operating 

variables that affect the treatment processes (temperature, aeration period and phosphate concentration) on the 

Biochemical Oxygen Demand (BOD5). 

 

The experimental data collected by this method were successfully fitted to a second order polynomial mathematical 

model. The most favorable operating conditions for the treatment are; 

Temperature    32.5oC,  

Aeration period    10 hours and 

phosphate concentration  16.8 mg/L.  

 

On using the optimum conditions a mathematical model simulating the operation for the treatment was obtained. 

                                                                                                                               

 

 

Introduction 

In leather industry (tanning) and for practical reasons, 

almost all tanneries are located on rivers to provide them 

with process water (30 to 80 m3 for 1 ton of processed 

raw skins) [1] and for the disposal of their effluents. The 

effluent from this industry is mainly waterborne [2]. The 

components of the effluent arise from purification of raw 

hides and skins before processing as well as from residual 

chemicals from the production processes. Hence, the 

tanning industry had been recognized as a major 

contributor to water pollution problems. 

 

Biodegradable organic matter consumes oxygen and 

nutrients in complex biochemical reactions until rendered 

inert [3]. This exerts a biochemical oxygen demand as one 

of the fundamental parameters used to regulate the quality 

of the effluent [4]. The contaminants in industrial 

wastewater are removed by physical, chemical and 

biological means [5]. Facilities for handling wastewater 

are usually considered to have three major parts: 

collection, treatment and disposal [6]. Pre-aeration is used 

to “freshen” the wastewater and to assist the removal of 

oil and grease [7]. Secondary treatment processes 

commonly consist of biological processes. This means 

that living organisms which control the environment of 

the process are used to partially stabilize (oxidize) organic 

matter not previously removed by treatment processes and 

to convert it into a form which is easier to remove from 

the wastewater [8]. Sedimentation or primary treatment 

makes the wastewater much clearer. Two clarifiers are 

used9 to provide detention time (3 h) where, almost 60% 

of the suspended solids (SS) will either settle to the 

bottom or float to the surface and be removed. Removal of 

University of Baghdad 

College of Engineering 
Iraqi Journal of Chemical 

and Petroleum Engineering 

 



EFFECT OF TEMPERATURE ON CORROSION OF CARBON STEEL BOILER TUBES IN DILUTE SODUIM 

CHLORIDE SOLUTION 

 

2 
IJCPE Vol.9 No.3 (2008) 

 

these solids will usually reduce the BOD5 of the waste 

approximately 35%. 

 

The next step is the biological treatment which can 

typically be divided into aerobic and anaerobic. Anaerobic 

biological treatment is an oxygen-devoid process. Aerobic 

biological treatment is done in the presence of oxygen. It 

is applicable to wastewater containing bio-degradable 

organic constituents and some non-metallic inorganic 

constituents [2]. The activated sludge process (ASP) is the 

currently used biological treatment process for wastewater 

in Al-Za’afaraniya tanning factory, southern Baghdad. 

The system consists of an equalization basin, a settling 

tank, an aeration basin, a clarifier, and a sludge line. The 

recirculation of the biomass, which is an integral part of 

the process, allow microorganisms to adapt changes in 

wastewater composition with relatively short acclimation 

time and also allow a greater degree of control over 

acclaimed bacterial population [10]. 

 

For a proper control of the ASP, the growth of the micro-

organisms should be controlled. Bacteria make up about 

95% of the activated sludge biomass. These single celled 

organisms grow in the wastewater by consuming (eating) 

biodegradable materials such as proteins, carbohydrates, 

fats and many other compounds. Some important factors 

acting on growth and activity of bacteria in biochemical 

wastewater treatment are: food-to-microorganism ratio 

(F/M); use of oxygen [11]; formation of Floc [12]; mixing 

[12]; ; pH [13]; temperature [6] and the effect of nutrients 

[14]. Biological oxygen demand (BOD) is a measure of 

the oxygen used by microorganisms to decompose this 

waste. A large quantity of organic waste in the water 

requires large amount of bacteria to decompose it. Thus, 

the demand for oxygen will be high (high level of BOD). 

As the waste is consumed or dispersed through the water, 

BOD levels will begin to decline [15]. 

 

Nitrates and phosphates in a body of water can contribute 

to high BOD levels. Nitrates and phosphates are plant 

nutrients and can cause plant life and algae to grow 

quickly. When plants grow quickly, they also die quickly. 

This contributes to the organic waste in the water, which 

is then decomposed by bacteria. This results in a high 

BOD level [15]. When BOD levels are high, dissolved 

oxygen (DO) levels decrease because the oxygen that is 

available in the water is being consumed by the bacteria. 

Since less dissolved oxygen is available in the water, fish 

and other aquatic organisms may not survive [16]. The 

present work is an attempt to shed more light on the 

pollution potential of “Al-Za’afaraniya” tanning factory. 

The BOD5 is taken as a parameter to indicate the 

optimum condition of biological treatment in ASP. 

 

EXPERIMENTAL PROCEDURE 

Materials 
Phosphoric acid (70 wt%) was supplied by a local 

company. The chemicals used for the determination of 

BOD5 and phosphate were of analytical grade. 

An agar was used to grow bacteria, and consists of: beef 

extract 3 g; peptone, 5 g; agar, 15 g and distilled water to 

make a volume of one liter. The culture media is pH = 7.0. 

Malt extract agar was used to grow fungi. it consists of: 

malt extract, 20 g ; peptone, 5 g, agar, 15 g and distilled 

water to make a volume of one liter. Culture media: pH = 

3.5-4.0 

 

Laboratory Treatment Units 
 

Fig. 1 shows a schematic diagram of the experimental 

unit used for the biochemical wastewater treatment. The 

unit consists of: A temperature regulated water bath of (60 

x 40 x 16cm) and 1.2 kW power was equipped with a 

stainless steel electrical stirrer (1.2 kW) and coupled with 

voltage regulator. The clarifier was a 3- liter Pyrex flask 

with a discharge valve at the bottom and a tap near the 

top, for the collection of treated wastewater. Air was 

introduced via a pre-calibrated Rotameter with and an air 

Sparger to ensure even distribution. A Pyrex burette was 

used for the delivery of nutrient (phosphoric acid). 

Some supplementary equipments were used including an 

incubator (Memmert, Germany); a microscope 

(Olympus); an autoclave for sterilization (Express, 

England); a Digital Grating Spectrophotometer (Pye 

unicam, England); and a Universal Pocket Meter Multiline 

P4 including: I. pH combined electrode with integrated 

temperature probe Sen tix 41; and II, Dissolved oxygen 

(DO) probe, Cell Ox 325. 

 

Procedure 
The samples were taken from outline area from the 

wastewater of the vegetable and chrome tannery, in equal 

volumes. The samples were first screened to remove hair 

and skins pieces and then neutralized by adding sulfuric 

acid (50 wt%) to a final pH of (7-9). Sedimentation 

(settling) process was then carried out to reduce the solid 

content to about 65% within 2-3 h. A specified volume of 

the sample (30 Lit) was placed in the bath. The 

microorganism (activated sludge) where added to the 

water sample in the bath, and aeration was started.  

 

Following the Box-Wilson method of experimental design 

the operating parameters used, were in the range; T=20 - 

45o C  

t=5-10 hour  

PO4 concentration=5-20 mg/L.  

 



G.A. Rassoul and D.R. Rzaige 

 

IJCPE Vol.9 No.3 (2008) 

 

After the biochemical treatment, a polyelectrolyte at a rate 

of 15 mg/L was added to the mixture in the bath for a 

period of one hour to improve the sedimentation process 

of. In the natural sedimentation stage, the sample was 

taken from the bath and clarified for about 3 h, and then a 

specified volume (1.0 L) of the clarified water was taken 

for the BOD5 measurement. 

 

 

 
 

 

 

Fig. 1: The Experimental Apparatus: 

 1, Water bath; 2, Burette, 3, Compressor; 4, Pipes; 5, 

Rotameter; 6, Air sparger; 7, Voltage variae box; 8, 

Stirrer; 9, Clarifier  

 

Methods of Examination: 
The TSS and volatile suspended solids (VSS) tests were 

done according to the WHO methods for pollution 

control, 1982. The (DO) concentration was measured 

using a (Cell DX 325) type device, which consists of a 

gold-metal electrode (Wissen Schuftliche Tech. Werk., 

Germany). The accuracy of this method is (0.1 mg/Lit). 

The readings were checked versus titration method with 

standard 0.025 N sodium thiosulfate using starch as an 

indicator. 

 

The dilution, the BOD5 measurement and the 

determination of the phosphate concentration were carried 

out in accordance with standard methods [17]. The 

preparation of agar plate and the isolation of discrete 

colonies from a mixed culture were carried out according 

to the published methods [18]. 

 

 

RESULTS AND DISCUSSION 
 

 

This work deals with effect of three variables 

(temperature, aeration period and phosphate 

concentration) on BOD5.  

 

Postulating the Mathematical Model 

A second order polynomial equation was employed in the 

range of the independent variables. Three variables were 

considered. The general form of a second polynomial 

equation can be given as follows: 

 

Y = B10 + B11X11 + B12X12 + B13X13 + B14X12 X13 

+ B15X2 12 + B16X213 + B17X11X12 + B18X11X13 + 

B19X12X13                                                                (1) 

 

The coded and real variables as well as the real values of 

BOD5 are given in Table 1. The data of Table 1 were 

fitted to equation (1) so that the regression analysis of 

central composite design to the approximating model to 

obtain the optimum conditions of the process. The 

coefficients of polynomial equation were evaluated. Thus, 

the best form of equation 1 is: 

 

Y = 288.498 + 34.846 X11 + 18.289 X12 + 9.892 X13 + 

36.896 X211 + 9.151X212 + 7.317X213 + 5X11X12 + 

3.75X11X13 + 3.749X12X13                                      (2) 

 

Correlation coefficient ® = 0.977, 

Percentage square error (S) = 1.8%. 

 

To test the significance of each term in equation (2), the 

F-distribution test was used employing the variance of 

each term in multivariable correlation, according to Table 

1 [19]. The calculations indicated insignificant interaction 



EFFECT OF TEMPERATURE ON CORROSION OF CARBON STEEL BOILER TUBES IN DILUTE SODUIM 

CHLORIDE SOLUTION 

 

4 
IJCPE Vol.9 No.3 (2008) 

 

between the variables (X1X3, X1X2, X2X3) are. Thus the 

best form of the relationship is 

 

Y = 288.498 + 34.846X11 + 18.289X12 + 9.892X13 + 

36.896X211 + 9.151X212 + 7.317X213                     (3) 

 

The optimization process [20] was applied to equation (2) 

to find the optimum operating conditions and the results 

indicate the following conditions to attain the minimum 

BOD5 value (256 mg/L): 

 

X11 = Temperature = 32.5o C, 

X12 = Aeration period = 10 h, 

X13 = Phosphate concentration = 16.8 mg/L. 

 

 

Effect of Different Operation Variables on 

the Values of BOD5: 

 

Effect of Temperature: 
Fig. 2 shows the influence of temperature on BOD5 at 

different aeration periods at fixed phosphate concentration 

(16.8 mg/L). It is clear that, the BOD5 decreases with 

increasing temperature down to a value of 250 mg/Lit at 

32.5o C and aeration period of 10 h. Beyond this 

temperature, the BOD5 values increase. Fig. 3 shows the 

effects of temperature on BOD5 at various phosphate 

concentrations and constant aeration period of 10 h. The 

BOD5 reaches 250 mg/Lit at temperature of 32.5o C and 

phosphate concentration of 16.8 mg/L. 

 

Fig. 4 shows the response surface function developed by 

the model considering temperature and aeration period. 

The response obtained is minimum BOD5 at temperature 

of 32.5o C and aeration period of 10 h. It is clear that 

increasing in temperature from 20o C to 32.5o C, leads to 

an increase in the activity of microorganisms to degrade 

the organic material. Beyond 32.5o C, the activity of 

microorganism decreases [12, 21].  

 

Effect of Aeration Period: 
The effects of the aeration period on the BOD5 at 

different temperatures and phosphate concentration are 

shown in Figs. 5 and 6, respectively. The BOD5 values 

decrease with increasing the aeration period and reaches 

250 mg/L at a temperature of 32.5o C at a fixed phosphate 

concentration of 16.8 mg/L (Fig. 5). Fig. 6, indicates that 

the BOD5 reached a value of 250 mg/L at aeration period 

of 10 h and phosphate concentration of 16.8 mg/Lit at 

constant temperature of 32.5o C. The increase in the 

aeration period to a certain limit lowers both the BOD5 

and the MLSS, since there will be longer time for the 

microorganism to decompose the organic matter into 

simpler materials [4]. However, the increase in the oxygen 

supply above this limit, will improve the growth and 

reproduction of the microorganism and needed high cost 

for aeration basin [2]. The competition between these 

microorganism for nutrient, may lead to starvation and 

reduce the number of microorganism [22]. 

 

Effect of Phosphate: 

 
Fig. 7 shows the influence of phosphate concentration on 

BOD5 at different temperatures using constant aeration 

period of 10 h. The BOD5 reaches a value of 250 mg/L at 

phosphate concentration of 16.8 mg/L and at a 

temperature of 32.5o C. Fig. 8 shows the influence of 

phosphate concentration on BOD5 at various aeration 

periods and constant temperature. The BOD5 decreases 

when phosphate concentration increases up to a level of 

16.8 mg/L. Beyond this level, the BOD5 increases slowly 

since more phosphate concentration helps algae growth. 

When these algae die and decay they give organic 

material which increases the BOD5. Thus it may be 

assumed that, the input of 1 mg phosphorus lead to the 

growth of about 100 mg of algae dry matter [22]. Fig. 9 

shows isometric relationship between aeration period, 

phosphate concentration with the BOD5. The response 

obtained is minimum (BOD5) at aeration period 10 h and 

phosphate concentration of 16.8 mg/L. Microorganisms 

require certain nutrients for growth. The basic nutrients of 

abundance in normal raw sewage are carbon (C), nitrogen 

(N) and phosphate (P) with the ratio of C:N:P of 

approximately 100:10:1 [23]. Phosphorus is an essential 

element, it is part of the structure of DNA and RNA, and 

is an important intermediate in metabolism [24]. 

 

Bacteriological tests indicated the presence of bacteria and 

protozoa in the ASP. The types of bacteria found on the 

surface after incubation were: 

I. Staphylo cocci spp. and II. Bacilli. Meanwhile, fungi 

did not appear on culture, since they favour pH lower than 

the pH of ASP. 

 

Applications of Optimum Condition for Wastewater 

Treatment Plant Plant: 

After carrying out the preliminary processes, the 

wastewater samples were taken from the effluent of two 

tanneries, with their BOD5 values reduced from 760 mg/L 

to 610 mg/L. The optimum conditions that have been 

obtained above 

 

(Temperature = 32.5o C; Aeration period = 10 h; and 

Phosphate concentration = 16.8 mg/L)  

were used for the biochemical treatment. The treatment 

resulted in a decrease in the BOD5 form 436 mg/Lit to 75 

mg/L in laboratory. However, the actual BOD5 value of 

the effluent from the first aeration basin in plant was 100 

mg/L. 



G.A. Rassoul and D.R. Rzaige 

 

IJCPE Vol.9 No.3 (2008) 

 

Fig.2: Effect of temperature on BOD5  at constant 

phosphate level Concentration  (16.8mg/L) 

 

 

 

Fig.3: Effect of temperature on BOD5 at constant 
aeration period  (10 h). 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig.4: Effect of Temperature and aeration  period on 

BOD5.  

 

 

 

Fig.5: Effect of aeration period on BOD5 at 

constant phosphate concentration (16.8 mg/L) 

 

 

 

 

 

 

 



EFFECT OF TEMPERATURE ON CORROSION OF CARBON STEEL BOILER TUBES IN DILUTE SODUIM 

CHLORIDE SOLUTION 

 

6 
IJCPE Vol.9 No.3 (2008) 

 

 

 

 

 

 

 

Fig. 6: Effect of aeration period on BOD5 at 

constant temperature (32.5 °C) 

 

 

 

 

 

 

 

Fig.7: Effect of phosphate concentration on 

BOD5 at constant aeration period  (10 h) 

 

 

 

 

 

 

 

Fig. 8: Effect of phosphate concentration  on  

BOD5  at constant temperature  (32.5 °C) 

 

 

 

 

 

 

 

 

 

CONCLUSIONS 

1.         The optimum conditions for biochemical 
wastewater treatment the case under study were: 

Temperature, 32.5
o 

C; Aeration period 10 hr and 

Phosphate concentration, 16.8 mg/L. 

2. The biochemical oxygen demand value could be 
reduced from 436 mg/L down to a value of 30 mg/L 

after biochemical treatment at the application of 

optimum conditions. 

3. Only bacteria and protozoa were found as 
microorganisms in activated sludge, because the 

alkaline condition and unaeration are not suitable for 

the living of fungi. 

 

REFERENCE 

 
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Organization”, J. No. 46, 1999. 

2. Pictel, J.,”Waste Management Practices, 
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AIRATION  PERIOD ( hr)

B
 O

 D
5
  

(m
g
/L

it
)

220

260

300

340

380

420

460

4.5 5.5 6.5 7.5 8.5 9.5 10.5

PO
4

= 5 m g/Lit

P0
4

= 8.2 m g/Lit

PO
4

= 12.5 m g/Lit

PO
4

= 16.8 m g/Lit

PO
4

=  20 m g/Lit

PO
4

   (mg/Lit)

B
 O

 D
5
  

( 
m

g
/L

it
 )

220

280

340

400

460

520

2 6 10 14 18 22

T= 20 
0
C

T= 25.3
0
C

T= 32.5 
0
C

T= 39.7 
0
C

T= 45 
0
C

PO
4

   ( mg/Lit)

B
 O

 D
  

5
  

( 
m

g
/L

it
 )

220

260

300

340

380

420

460

2 6 10 14 18 22

t = 5 hr

t = 6 hr

t = 7.5 hr

t = 9 hr

t = 10 hr



G.A. Rassoul and D.R. Rzaige 

 

IJCPE Vol.9 No.3 (2008) 

 

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