Most Urban growth is taking place in informal settlements where government is unwilling or unable to provide wastewater services. Effective wastewater treatment is so expensive that is rarely achieved in practice, particularly in the fast urban centers of developing countries. The sewerage system must be effective and adequate enough to receive different types of wastewater discharges. Onsite systems must be designed for pollution control and recovery of resources. Improved design, construction, operation and maintenance of our onsite systems are essential, especially the wastewater treatment plants. One of the most important operations in wastewater treatment and also very effective in treatment system is aeration of sludge which occurred in aeration basins. Although the aeration basins are reinforced concrete structures, the corrosion has attacked them. The main objective of this study is to analyze and defeat the cause of corrosion in one of the largest Wastewater Treatment Plant (WTP) in Egypt. A mathematical model to simulate the problem based on governing equations of vortex motion is achieved and a repairing technology to correct the corrosion is proposed. Moreover, the study assists in choosing the suitable design to avoid this problem in further projects.

1. Corrosion Phenomena at Aeration Basins
in Waste Water Treatment Plant
M. Abdelkhalek Ibrahim*, A. Kram Shehata**, M. Adel EL-Gammal***
*Assistant Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
**Assistant Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
***Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
Fenómenos de corrosión en las balsas de aireamiento en una planta de tratamiento de aguas residuales
Fenòmens de corrosió a les basses d’aireig en una planta de tractament d’aigües residuals
Recibido: 22 de mayo de 2006; revisado: 10 de noviembre de 2006; aceptado: 13 de noviembre de 2006
AFINIDAD
REVISTA DE QUÍMICA TEÓRICA Y APLICADA
EDITADA POR LA ASOCIACIÓN DE QUÍMICOS E INGENIEROS
DEL INSTITUTO QUÍMICO DE SARRIÁ
Afinidad (2007), 64 (527), 71-76

2. 71
Corrosion Phenomena at Aeration Basins
in Waste Water Treatment Plant
M. Abdelkhalek Ibrahim*, A. Kram Shehata**, M. Adel EL-Gammal***
*Assistant Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
**Assistant Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
***Professor, Civil Engineering Dept., National Research Center, Cairo, Egypt.
Fenómenos de corrosión en las balsas de aireamiento en una planta de tratamiento de aguas residuales
Fenòmens de corrosió a les basses d’aireig en una planta de tractament d’aigües residuals
Recibido: 22 de mayo de 2006; revisado: 10 de noviembre de 2006; aceptado: 13 de noviembre de 2006
RESUMEN
La mayor parte del crecimiento urbano se está produ-
ciendo en núcleos de población informales en los que
los gobiernos están poco dispuestos o son incapaces
de proporcionar servicios para tratar las aguas resi-
duales. El tratamiento efectivo de las aguas residuales
es tan caro que raramente se realiza en la práctica, par-
ticularmente en los centros urbanos de los países en
vías de desarrollo. El sistema de alcantarillado ha de ser
suficientemente efectivo y adecuado para recibir dife-
rentes tipos de descargas de aguas residuales. Los sis-
temas locales se han de diseñar para el control de la
contaminación y la recuperación de recursos. La mejo-
ra del diseño, construcción, operación y mantenimien-
to de nuestros sistemas locales es esencial, especial-
mente en lo concerniente a las plantas de tratamiento
de aguas residuales. Una de las operaciones más impor-
tantes en el tratamiento de aguas residuales, y muy efec-
tiva, es el aireamiento de los fangos que se realiza en
las balsas de aireamiento. Aunque las balsas de airea-
miento son estructuras de hormigón armado, la corro-
sión las ataca. El objetivo principal de este estudio es
analizar i batir la causa de la corrosión en una de las
plantas de tratamiento de aguas residuales (Wastewater
Treatment Plant, WTP) más grandes de Egipto. Se con-
sigue un modelo matemático para simular el problema
basado en las ecuaciones que gobiernan el movimien-
to del vórtice, y se propone una metodología reparado-
ra para corregir la corrosión. Además, este estudio ayu-
dará a escoger el diseño apropiado para evitar este
problema en futuros proyectos.
Palabras clave: Medio ambiente. Planta de tratamiento
de aguas residuales. Balsas de aireamiento. Corrosión.
Movimiento del vórtice.
SUMMARY
Most Urban growth is taking place in informal settlements
where government is unwilling or unable to provide was-
tewater services. Effective wastewater treatment is so
expensive that is rarely achieved in practice, particularly
in the fast urban centers of developing countries. The
sewerage system must be effective and adequate enough
to receive different types of wastewater discharges. Onsite
systems must be designed for pollution control and reco-
very of resources. Improved design, construction, ope-
ration and maintenance of our onsite systems are essen-
tial, especially the wastewater treatment plants. One of
the most important operations in wastewater treatment
and also very effective in treatment system is aeration of
sludge which occurred in aeration basins. Although the
aeration basins are reinforced concrete structures, the
corrosion has attacked them. The main objective of this
study is to analyze and defeat the cause of corrosion in
one of the largest Wastewater Treatment Plant (WTP) in
Egypt. A mathematical model to simulate the problem
based on governing equations of vortex motion is achie-
ved and a repairing technology to correct the corrosion
is proposed. Moreover, the study assists in choosing the
suitable design to avoid this problem in further projects.
Key words: Environment. Wastewater treatment plant.
Aeration basins. Corrosion. Vortex motion.
RESUM
La major part del creixement urbà s’està produint en
nuclis de població informals en els que els governs són
poc disposats o incapaços de proporcionar serveis per
tractar les aigües residuals. El tractament efectiu de les

3. aigües residuals és tan car que rarament es realitza a la
pràctica, particularment en els centres urbans dels paï-
sos en vies de desenvolupament. El sistema de clave-
gueram ha d’ésser prou efectiu i adequat per rebre dife-
rents tipus de descàrregues d’aigües residuals. Els
sistemes locals s’han de dissenyar per al control de la
contaminació i la recuperació de recursos. La millora
del disseny, construcció, operació i manteniment dels
nostres sistemes locals és essencial, especialment pel
que fa a les plantes de tractament d’aigües residuals.
Una de les operacions més importants en el tractament
d’aigües residuals, així com molt efectiva, és l’aireig dels
fangs que es realitza a les basses d’aireig. Tot i que les
basses d’aireig són estructures de formigó armat, la
corrosió les ataca. L’objectiu principal d’aquest estudi
és analitzar i batre la causa de la corrosió en una de les
plantes de tractament d’aigües residuals (Wastewater
Treatment Plant, WTP) més grans d’Egipte.
S’aconsegueix un model matemàtic per simular el pro-
blema basat en les equacions que governen el moviment
del vòrtex, i es proposa una metodologia reparadora per
corregir la corrosió. A més, aquest estudi ajudarà a esco-
llir el disseny apropiat per evitar aquest problema en
futurs projectes.
Mots clau: Medi ambient. Planta de tractament d’aigües
residuals. Basses d’aireig. Corrosió. Moviment del vòrtex.
1. INTRODUCTION
Maintaining social care for every one in Egypt is one of
the most important duties of the government, and in this
aspect maintaining health and well being for individuals
has been considered in Egypt Basic Legislation. Wastewater
is one of the greatest environmental problems in Egypt,
where more than 3000 local villages and sub-villages with
a total population of about 25 millions have no facilities of
wastewater treatment and/or wastewater collection
(3)
. An
increasing proportion of surface and ground water resour-
ces in Egypt is polluted mainly due to inappropriate dis-
posal of municipal wastewater, infiltration from onsite sani-
tation facilities and excessive use of fertilizers and
pesticides in agriculture. Due to the shortages in agricul-
tural water, the use of wastewater has become unavoida-
ble. Community Wastewater Management Services in Egypt
evolved in three phases. During the first phase, wastewa-
ter services were provided to large cities and urban cen-
ters. In The second phase, currently underway, wastewa-
ter collections services and some treatment works were
provided to secondary towns. The third phase, now being
considered, will provide wastewater services to smaller
towns and communities. On the other hand, domestic water
use in increasing due to general rise in the standard of
living. Accordingly, wastewater generation increased as
shown in table I [AMBRIC Report -1].
72
TABLE I
Water Consumption and Wastewater generation
(Billion-m3/day)
1980 1985 2000
Water Consumption 2.25 2.42 4.62
Wastewater generation 1.15 1.34 2.82
Figure 1. The operations of wastewater treatment in Tanta WTP.

4. Really, there is need for re-evaluation of the environmen-
tal scenarios, water resources and wastewater drainage
systems. Commonly used onsite wastewater systems fail
to protect the water resources and the environment becau-
se of their poor design, lack of maintenance and increa-
sed loading and population densities. The plants are essen-
tial part of our wastewater treatment systems. Therefore,
the authors are interested in the plants and their mainte-
nance problems as a logic step to increase the efficiency
of the wastewater system in Egypt. Figure (1) shows the
operation of the wastewater treatment in one of the WTP
in Egypt.
In this study one of the common problems in Egypt WTP
is under scrutiny. The problem is corrosion in the aeration
basins. The answers to questions such as What? Why?
How? about the problem are studied. A mathematical model
to simulate the problem based on governing equations of
vortex motion is presented. A repairing technology is pro-
posed to combat the corrosion phenomenon. Moreover,
the study assists in choosing the suitable design to avoid
this problem in further construction of aeration basins. The
corrosion problem in aeration basin of Tanta WTP (one of
the largest Wastewater Treatment Plant in Egypt) will be
chosen as a case of study.
2. WHAT ABOUT THE PROBLEM?
In WTP a lot of operations involved in the treatment the
wastewater are made. Metcalf and Eddy
(5)
summarized the
steps of the wastewater treatment operation. One of the
most important steps is the aeration of sludge which needs
large basins. The aerations basins are usually reinforce-
ment concrete structures and have large dimensions.
The aeration operation of the WTP is accomplished by
using mechanical motors as shown in figure (2) which are
distributed at equal distance not exceeding 8.00m. The full
aeration operation is accomplished in 4 to 6 hours.
Table II shows the dimensions of WTP which are under
investigation by authors of this study.
Figure (3) obtained clearly the aeration motors and its fane
diameter (about 2.00m). The aeration motors are used to
activate the sludge for 4 to 6 hours with a rotation speed
of more 120 rpm .
While preparing the maintenance report for the WTP in
Tanta, the authors found that there were deep holes per-
haps more than 150mm in depth and 1000mm in diameter
which caused the corrosion of steel bars in the base of the
aeration basin. The holes were located exactly under the
aeration motor as shown in figure (4).
73
TABLE II
Aerations basins dimensions for WTP in Egypt
City Length (m) Width (m) High (m)
Zagazik 60.00 40.00 4.00
Tanta 64.00 16.00 4.70
Dommiate 80.00 16.00 6.00
Figure 2. The distribution of the mechanical aeration motors
in Tanta WTP.
Figure 3. The mechanical aeration motor in Tanta WTP
aeration basins.
Figure 4. The significant holes which occurred in Tanta
WTP aeration basins.
2. WHAT IS THE PROBLEM?
The analysis of the problem was done as follows (see figu-
re 5):
1. Firstly, High rotation speed causes strong and rapid vor-
tex motion as shown in figure (6).
2. Damage of concrete cover due to vortex motion force.
3. Erosion of the concrete base causes deep holes after
long time.
4. Corrosion of the reinforcement steel bars.

5. 5. Finally, more and more stops of WTP for maintenance
works which decreases the efficiency of the plant and
causes reduction in working life
(2)
.
3. HOW TO SIMULATE THE PROBLEM?
A mathematical model to simulate the problem based on
governing equations of vortex motion is presented. The
problem of corrosions was analyzed in section 2 and con-
cluded that the main cause of corrosion was the vortex
motion. It is due to high rotation speed of aeration motors.
The vortex motion force generates stresses on the base of
the aeration basin. To evaluate the value and direction of
this force, the effective forces on the base under the action
of vortex motion should be calculated. Figure (5) shows
the main effective forces through the vortex motion and
the combined force. The main effective forces are res-
pectively:
1. The weight of the wastewater (W).
2. The Centrifugal Force (F) which was calculated by Kurmi
(4)
as following:
ω
2
x
F = W
(–––––––
) (1)
2g
Where:
ω is the velocity of vortex motion and depends on a
number of aeration motor revaluation,
g is the gravity acceleration,
x is the radius of the stressed area by vortex motion,
74
Figure 5. The affecting parameters on the aeration basin
base.
Figure 7. The main effective forces through the vortex
motion and their resultant.
Figure 6. Strongly and rapidly vortex motion in Tanta WTP
aeration basins due to high rotation speed of aeration
motors.
The vortex motion force (the resultant force) value, direc-
tion and its stressed area are calculated as:
1. The value of vortex motion force is:
P = F sin ψ + W cos ψ (2)
Where:
ψ is the slope of the vortex motion angle on the verti-
cal direction.
2. The slope of the vortex motion angle on the vertical direc-
tion is:
ω
2
x
tan ψ = F/ W = ––––––– (3)
g
3. The radius of the stressed area by vortex motion is:
x = √r
2
– 2gh /ω
2
(4)
Where:
r is the radius of the aeration motor fan.
h is the height of aeration motor fan above the aera-
tion basin base.
By substituting in equations 1 to 4 with the values of the
parameters of our problem, the vortex motion value, its
direction and its stressed area are computed. The mathe-
matical model has the ability of computing the vortex motion
for numerous cases with different boundary conditions.

6. 4. THE PROPOSED TECHNOLOGY FOR
REPAIRING:
The repairing technology which was proposed for defea-
ting he vortex motion in our field problem can be stated as
mentioned below (see figure 8):
1. Firstly, using the model to simulate the problem and
compute the vortex motion value, direction and its stres-
sed area (P, ψ, r)
2. Removing the old concrete.
3. Cleaning the steel bars and painting them with Epoxy
Primer
(6, 7)
.
4. Covering the old concrete by Epoxy Bonding Agent
5. Fixing a Circular plate made of iron to resist the aggres-
sive media, with a radius = 1.2 r (more than the stres-
sed area of vortex motion) as well as designing the pla-
te thickness to bear the vortex force.
6. CONCLUSIONS
The main conclusions of the present study can be sum-
marized in the following points:
1. The main cause of the corrosion problem in the WTP
aeration basin is the vortex motion and it is generated
due to high rotation speed of aeration motors.
2. The accurate values of the vortex motion forces and
their direction as well as the stressed area due to their
effect can be computed easily by using the mathema-
tical model based on the governing equations of the
vortex motion.
3. The repairing technology using an iron circular plate
succeeded in resisting the vortex motion and protec-
ting the base from the effect of the aggressive media.
4. Two methods of design are recommended for the aera-
tion basins to avoid the corrosion problem in further
projects. Firstly, the absorbance method which is based
on absorbing the vortex motion by changing the shape
of the base. Secondly, the disturbance method which
depends on disturbing the vortex motion before attac-
king the base of the aeration basin.
75
Figure 8. The proposed technology for repairing the base
of the WTP aeration basin in Tanta city.
Figure 9. The absorbance method for avoiding the corro-
sion problem in the base of the WTP aeration basin in Tanta
city.
Figure 10. The disturbance method for avoiding the corro-
sion problem in the base of the WTP aeration basin in Tanta
city.
5. THE SUITABLE DESIGN TO AVOID
THE PROBLEM:
The present section gives a clear view of the suitable design
for the aeration basins to avoid the corrosion problem which
appears due to vortex motion. Two methods of design are
recommended for this purpose:
5.1. THE ABSORBANCE METHOD:
The absorbance method based on absorbing the vortex
motion by changing the shape of the base. The following
steps are done to achieve that:
1. Accurate evaluation of the vortex motion value and its
shape using the mathematical model.
2. Finding the level of safe stresses surface (A-A).
3. Formatting the base as a trapezoidal section to adb-
sorb the stresses of the vortex motion (see figure 9).
5.2. THE DISTURBANCE METHOD:
The disturbance method depends on disturbing the vor-
tex motion before attacking the base of the aeration basin
as follows:
1. Accurate evaluation of the vortex motion shape using
the mathematical model.
2. Addition of concrete blocks to increase the surface area
which automatically decrease the vortex motion stres-
ses and disturb their actions.

7. 7. BIBLIOGRAPHY
(1)
. AMBRIC: «Greater Cairo Wastewater Project-Sludge
Management Study», First Report, Ministry of
Reconstruction, New Communities, Housing and Utilities
(1989).
(2)
. Evans A.R. and Smith D.G.: «Deterioration and Life
Expectancy of Concrete Structures in a Middle East Town»,
Journal of Concrete, January (1987).
(3)
. Halim S. A. and Abdel-Halim W: «Selection and Evaluation
of Appropriate Sanitation Systems in Rural Egypt», the
International Conference on Water Technology and
Researches (AQUA Tec 2006).
(4)
. Kurmi R. S.: «Hydraulics, Fluid Mechanics and Hydraulic
Machines», 12th Edition, S. Chand & Company Ltd., Ram
Nagar, New Delhi, (1982).
(5)
. Metcalf and Eddy: «Wastewater Engineering, Treatment,
Disposal and Reuse», 3rd Edition, McGraw-Hill, Inc., New
York (1990).
(6)
. Mussa, O. A.: «The problems of concrete structures and
How to extend their efficiency», The International
Conference on Building and Construction (INTER BUILD
1996).
(7)
. SMPH: «Buildings Deterioration in the Arab world and
Methods of Repair», The Saudi Ministry of Public works
and Housing, Riyadh, (1992).
76