Al-khwarizmi 
Engineering   

Journal 

Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37- 45 (2008) 

 

 

pH-Control Problems of Wastewater  

Treatment Plants 
 

Dr. G. M. Alwan  
Chemical Engineering Departement/University of Technology 

 

(Received 6 May 2007; accepted 21 May 2008) 

 

 

Abstract 

Experimental investigations have been carried out to investigate the pH-control problems of industrial electroplating 

wastewater treatment plants. The accurate and sensitive PID control system could treat most problem and disturbances 
in the normal operation of the water treatment. However, conventional treatment was replaced by proprietary treatment 

agent called a QUASIL which was found to be more effective for a wide range of pH. 

  

Key words: Wastewater, Heavy metals, Precipitation, pH-control. 

 

 

Introduction  
The wastewater from metal plating facilities 

contains contaminants such as heavy metals, oil 

and grease, and suspended solid at levels which 

are considered hazardous to the environment and 
could pose risk to public health. Because of the 

high toxicity and corrosivity of metal plating 

waste streams, they require to pre-treatment of 
their wastewater prior to releasing them to 

municipal sewers or receiving waters.  

Conventionally, precipitation is the method of 
choice for removal of heavy metals. Of the few 

precipitation methods, hydroxide and sulfide are 

the main two methods currently in use and 

hydroxide precipitation is by far the most widely 
used method, Durhamd and Patria, (2006). 

The control of water-PH plays an important 

role in these treatments, since each step takes 
place in a separate tank and the entire treatment 

requires several control loops to adjust the PH and 

to get perfect separation of toxic metals. 

 

Experiment Set-Up 
The present work studied several industrial 

electroplating wastewater treatment plants located 

in Iraq with capacities ranging from (5-20) 

m
3
/day. 
The streams of wastewater contain heavy 

metals such as (Cu, Fe, Cr, Cd and Ni), oil and 

grease, suspended solids and chelating or 

complexing agents.  

 

The experimental data were obtained on-site 
and the water samples were analyized at the 

laboratories of the ministry of Industry in 

Baghdad. The results of the analysis are given in 
Tables (1&2) showing average concentrations 

taken from several wastewater stations. 

 

pH-Control 
The acidity or the alkalinity of any solution 

can be indicated by the term of PH which stands 
for the power of the hydrogen ions H

+
.PH is 

related to the concentration (or more strictly the 

activity) of hydrogen ions by the following 

equation Pirece et al, (1970): 
pH = - Log10 [ H

+
 ]                      …(1) 

Also, alkalinity indication relates with 

hydroxide ion as: 
POH = - Log10 [ OH

-
 ]                       …(2) 

For neutralization process, the measurement 

equation Kwok et al., (2003) is: 

  502 4log
1 0

.

w
kGGpH  2

10
log     …(3) 

The value of G is zero at neutral point (PH =7). 

The PH process control is one of great 

importance in the chemical industry especially 
wastewater treatment plants. PH process control is 

widely used as the end point control in the 

wastewater treatment plants Shinsky, (1973). 

 
 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 
 

38 
 

The control loop mainly includes; sensor, 
controller and final control element which is 

almost a control valve. The PH-sensor (electrode) 

is a primary measuring element and like other 
primary element its dynamic response 

characteristics can play a significant role in 

control loop accuracy and stability. PH electrode 

dynamically behaves as a first order lag system 
Giusti and Hougen, (1961). 

 

Dynamic Modeling of Treatment process 
It is difficult to formulate and identify a 

mathematical model for the PH process as small 

amounts of polluting elements will change the 
process dynamics considerably Shinsky, (1973). 

A general dynamic model of the PH process 

had been discussed earlier by McAvoy, et al., 
(1972). They have derived a mathematical model 

from the first principle, i. e., mass balances for 

species and by combining their balances with 

equilibrium and electro-neutrality conditions. 
Honson and Dale, (1994) have proposed the 

dynamic model of the PH system using 

conservation equations and equilibrium relations. 
Modeling assumptions include perfect mixing, 

constant density, and complete solubility of the 

ions involved. 
However, in the present work, the dynamic 

model of the single stage PH process is proposed 

as a first order lag system, i. e., of the type: 

 
PHkU

dt

PHd
                       …(4) 

depended upon the conservation equations of the 
process and using modeling assumptions which 

include; perfect mixing, isothermal conditions and 

complete solubility of the ions. 

From the control engineer's point of view, a 
linearized model where both the time constant (τ) 

and process gain (k) vary with PH and with the 

buffer concentrations in the system, and U 
indicate the input feed rate of acid and base 

solutions. 

So, the transfer function of the single 
neutralization stage becomes: 

 
 
  1


s

k

sU

sPH
sG

p


                     …(5) 

Since the wastewater treatment contains 

several stages of neutralization processes, the 
overall dynamics model will be: 

 
    111 21

21




s......ss

k.....kk
sG

n

n
o


               …(6) 

where n indicates the number of neutralization 

stages. 

In the present work, the values of the time lag 
(τ) were determined between (3-4) hours and the 

process gain (k) between (1.5-2.0) PH/(m
3
/hr) for 

each stage, which always varies depend upon the 
size of tanks and the operating conditions. 

 

Dynamic of pH Electrode 
In a continuous flow cell, the PH electrodes 

responses can be generally, represented by two 

time constants Gusiti and Hougen, (1961), one 
arising from the rate of diffusion of ions into and 

out of the glass electrode surface, the other from 

solution mixing within the flow cell. The cells 

performance function, or transfer function can be 
written as: 

 
  11

1

21 


ss
sGe

                     …(7) 

where: 

τ1 is the mixing time constant 
τ2 is the diffusion time constant. 

The diffusion time constant is primarily a 

function of dynamic response of the measuring 

electrode itself. The mixing time constant is a 
function of the continuous flow cell in which the 

electrode measures the PH of the solution. 

In the present work and by analyzing the 
experimental responses of the electrodes (process 

reaction curve method, Stephanopouls,1984), the 

PH-electrode could be represented by the first 

order lag system as follows: 

 
1

1




s
sG

e


                                   …(8) 

where the time constant (τ) between (1-2) 

seconds. 
However, since the time constant of PH-

electrode was very small when compared to that 

of the process, it does not affect the response 

speed of the PH process. 
The dynamic model of the process has direct 

effect on the design of closed-loop control system. 

Then the tuning of selected PID control which 
depends upon the values of dynamic 

characteristics of the process (process gain and 

time constant) gives agreement results with the 
operating of the wastewater treatment system. To 

increase the efficiency of the processes, it is better 

to use computer control system to detect and 

control directly any disturbances in the process 
variables in the system (Figures 3 & 4). 

 

 

 

 

 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 

 

 39 

pH Control Problems  
Several problems occur as a result of using the 

PH process control in wastewater treatment 
plants, these are: 

1. The precipitation of heavy metals from the 
wastewater needed adjusting the alkalinity 
and acidity of water depend upon the nature 

of ions while another group of ions is 

precipitated  in both conditions as shown in 

Figure (1). For example, Fe ion is precipitated 
in the acidity condition at PH of (3.0 to 4.0), 

while ions of Cu and Cr are precipitated in 

both conditions at PH of (5.0 to 8.5). Other 
ions of heavy metals such as (Zn, Cd, and Ni) 

are precipitated in alkalinity condition. These 

behaviors indicate that the process is required 
in the multistage process to neutralize and 

separate the heavy metals and also needing to 

use automatic control system for each stage. 

Additionally many apparent PH problems are 
really measurement problems rather than 

process problems due to the presence of 

complex agents or chelating, oil and grease 
and dissolved solid which contaminat the 

surface of sensor and could affect response 

and sensivity of PH-detectors. So the sensors 

are always cleaned and calibrated. 
2. The PH process is highly non-linear Richter et 

al, (1974), for any small change in PH of the 

wastewater means a big change in the bulk 
concentration. For example, at PH 7 there is 

10 times acidity as much as PH 8, and that at 

PH 7 there is 100 times as much acidity as at 
PH 9. Then, the adjustment of PH with high 

accuracy is now recommended. 

The above problem could grow to   a complex 

situation when the wastewater has more than 
three types of heavy metals Bakkaloglu et al, 

(1998). 

Advanced and accurate control system now is 
needed to adjust the dosing flow of acid / base 

solution. 

The experimental results have proved that the 
regulating PID controller is shown in Figure 

(2),  more effective than on-off type, since 

PID action has operate in regulated 

characteristics which give it enough time to 
correct the error. 

3. Since the wastewater streams have various 
values of PH regarding the conditions of 
industrial process, the acidity of water must 

be monitored directly before any treatment. 

The PH of a mixed solution (wastewater and 

acid / base solution) is not just an average of 
the two solutions of PH values, but is also 

determined by the power of the solutions, and 

to a lesser extent by more esoteric factors 

Bailey, (1980). For example, when a solution 

of caustic soda with PH of 8.3 is added to the 

wastewater of PH 8, the mixed solution would  
actually be below PH 8. 

In the present work, the optimum PH of 

hydroxide precipitation was between 8.5 to 
9.5 while for sulfide was between 3.0 to 4.5. 

These values were limited experimentally 

through several sets of experimental runs. It is  
recommended to use sensitive electrode to 

detect any small change in measurements. 

Also perfect noise-filter is implemented with 

the sensor to reject any undesirable signals to 
the controller. 

4. It is difficult to formulate and identify a 
mathematical model for the PH process as 
small amounts of polluting elements in the 

wastewater would change the process 

dynamics considerably. 

Consequently, interpreting PH problems and 
solution requires knowledge of more than just 

the PH of the solutions involved. 

The entire treatment involves other processes 
such as, flocculation with a polymeric 

material, settling…etc, to get acceptable 

concentration limit of the heavy metals as 
shown in Table (2). This technique is time 

consuming and requires extensive set-up. 

Several private communications were made 

with international companies to get better 
design. Finally the result was, new technique 

called "Aquasil" treatment. This technology is 

commercially introduced in 1995 as an 
efficient alternative to conventional practices. 

 

Aquasil Wastewater Treatment  
Aquasil technology is an innovative treatment 

process that takes up and removes contaminants 

from wastewater. In the present work, several 
experiments were made by using Aquasil solution 

which proved good efficiency of separation as 

shown in Table (2). The aquasil agent is a single 
product, simultaneous process that utilizes the 

principles of fast kinetics and synergy for the 

removal of the most contaminants from the waste 

stream by using only single separator (Figure5). 
So the PH-control problems could minimize as a 

result of reduced process stages because the 

Aquasil agent was effective in the wide range of 
PH from 2 to 12. Also it lowers the level of COD 

and TDS.  

 

 

 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 
 

40 
 

Conclusions  
1) The precipitation of heavy metals from 

the wastewater is achieved through 

several values of PH-solution depending 
upon the nature of ions. 

2) It is difficult to formulate and identify a 
mathematical model of wastewater 
treatment process. It behaves dynamically 

as               a multicapacity system and the 

approach dynamics model of each stage 
taken to be a first order lag system. 

3) Automatic control of water-PH plays an 
important role in the waste treatment 

process. 
4) Advanced on-line PID control has better 

efficiency than on-off type for the 

nonlinear PH process which can measure 
and treat directly any undesirable process 

signals. 

5) PH control problems great with the 
conventional treatment of multistage 
process compared the modern and 

efficient single stage aquisil treatment.  

 
Table 1 

Analysis of heavy metals in the raw wastewater. 

Constituen

t 
Concentration (mg/liter) 

Cd 60.0 

Cr 50.0 

Cu 15.0 

Fe 20.0 

Ni 50.0 

Zn 40.0 

 
Table 2  

Analysis of heavy metals after treatment by 

(conventional and Aquasil technique). 

Constit
uent 

Concentration 

(mg/liter) 

Conventi

onal  

aqu

asil 

Cd 1.0 0.6 

Cr 0.7 0.3 

Cu 0.3 0.1 

Fe 0.9 0.6 

Ni 0.4 0.2 

Zn 0.3 0.1 

 

 

Nomenclature 
COD: Chemical oxygen demand. 
G:      Distance from neutrality given by  

          [ H
+
 ] – [ OH

-
 ]. 

G(S):  Transfer function. 

Ge(S): Transfer function of PH – electrode. 
Go(S): Overall Transfer function of process. 

Gp(S): Transfer function of PH – process tank. 

k:        Process gain(PH/(m
3
/hr)). 

kw:      Dissociation constant of water. (mole/ liter) 

H
+
:      Concentration of hydrogen ion (mg / liter). 

OH
+
:   Concentration of hydroxide ion (mg / liter). 

PH:     Acidity indication. 
PHI:    Acidity indicator (sensor). 

PID:     Proportional, integral and derivative  

            controller. 
POH:   Alkalinity indication. 

s:          Laplace transform. 

TDS:    Total dissolved solids. 

t:          Time (hours). 
τ:          Time constant. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 

 

 41 

 

 

 

 

Fig. 1. Precipitation of Heavy Metals. 

Fig. 2. Flow Diagram of PID Control System. 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 
 

42 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3. Hardware Components of On-Line Control System. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig.4. Block Diagram of the Computer Control Function. 

Operational 
Amplifier 

Noise  

Filter 

A / D 

Converter  

 

Transmitter  
D / A 

Converter  

 

D
ig

it
a
l 

C
o

m
p

u
te

r
 

W
a
st

e
w

a
te

r 
 

tr
e
a
tm

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n
t 

p
la

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G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 

 

43 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

F
ig

. 
5
. 

F
lo

w
 D

ia
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G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 
 

44 
 

 

References  

[1] Durham, B. and Patria, L., "Wastewater a 
reliable water resource", Chem. Engng, 
October, 50, (2006). 

[2] Pierce, W.C., Haenisch, E.L., and Sawyer, 
D.T., "Quantitative Analysis", John-Wiley, 
(1970). 

[3] Kwok, D.P., Deng, Z.D., and Leung, T.P., 
"Fuzzy neural control of systems with 
unknown dynamics using Q-learning 

strategies", proceeding of the 12
th
 IEEE, 

International Conference on Fuzzy Systems, 

Vol. 1, 25-28, May 2003. 
[4] Shinsky, G., "PH and control in process and 

Waste Stream", John Wiley, N.Y, (1973). 

[5] McAvoy, T. J., Hsu, E., and Lowenthal, S., 
"Dynamics of PH in a controlled stirred tank 

reactor", Ind. Eng. Chem. proc. Des Dev., 

Vol. 11, P. 68, (1972). 

[6] Henson, M. A. and Dale, E. S., "Adaptive 
nonlinear control of a PH neutralization 

process", IEEE Tran. Cont. sys. Tech., Vol. 2, 

No. 3, Aug. (1994). 

[7] Giusti, A.L., and Hougen, J.V., "Dynamic of 
PH electrodes", Control Engineering, April, 

(1961). 

[8] Stephanopouls, G."Chemical process control, 
an introduction to theory and practice" , 
Prentis-Hall, 2

nd
 edition, N.G., (1984). 

[9] Richter, J.D., Fournier, C.D., ash, R.H., and 
Marcikic, S., "Waste neutralization control-
digital simulation spots nonlinearities", 

Instrumentation Technology, April, (1974). 

[10] BAkkaloglu, T.J. et al, "Screening of 
various types biomass for removal and 
recovery of heavy metals (Zn, Cu, Ni) by 

biosorption, Sedimentation and desorption", 

Water Science & Technology, 38, 6, P.269-
277, (1998). 

[11] Bailey, P.L., "Analysis with Ion-Sensitive 
Electrodes", John-Wiley, (1980). 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



G. M. Alwan                                           Al-Khwarizmi Engineering Journal, Vol. 4, No. 2, PP 37-45 (2008) 

 

45 

 

 مشاكل السيطرة على الحامضيت في محطاث معالجت المياه الصناعيت
 

غانم مقبول علوان 
قسى انهُذست انكيًياويت / انجايعت انتكُىنىجيت 

 

 
 الخالصت

نقذ تبيٍ بأٌ .  يشًم انبحث دراست انًشاكم انخاصت بانسيطرة عهً حايضيت يياِ انتصريف انصُاعيت نًصاَع انطالء انكهربائي

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. االسهىب انًتبع انقذيى وانسائذ بطريقت أفضم عٍ طريق انًعانجت بًادة األكىاسم وانتي وجذث بأَها تعًم بًذي كبير نهحايضيت