WATER TREATMENT
Aeration and
WATER TREATMENT
gas stripping
Qw, cw,0 Qa, ca,e
Qa,ca,0 Qw, cw,e
1
k1
0.8
0.6 k2
K (-)
k3
k4 k5
0.4
0.2
0
0.001 0.01 0.1 1 10
RQ
k1
k2
k3
k2t = 1.61
k4 kD = 0.039
k5 T = 10oC
,aeration and gas transfer water treatment
Framework
This module explains aeration and gas stripping.
Contents
This module has the following contents:
1. Introduction
2. Theory of gas transfer
2.1 Equilibrium
2.2 Kinetics
2.3 Mass balance
2.4 Solutions for the basic equations
3. Practice
3.1 Cascade
3.2 Tower aerator
3.3 Plate aerator
3.4 Spray aerator
3.5 Alternative aeration systems
136
, water treatment aeration and gas transfer
1 Introduction
Aeration (gas addition) and gas stripping (gas re-
moval) are normally the first treatment steps dur-
ing the production of drinking water from ground-
water or riverbank water. This artificially induced
gas transfer aims at the addition of oxygen (O2)
and the removal of carbon dioxide (CO2), meth-
ane (CH4), hydrogen sulfide (H2S), and other
volatile organic compounds (for example 1.2 Di- Figure 1 - Cascade aeration
chloropropane (1.2 DCP), Trichloroethene (TRI),
Tetrachloroethene (PER) and Trichloromethane the water falls over a weir into a lower placed
(chloroform)). trough. When the falling stream enters the water
body, air is entrapped in the form of bubbles, pro-
Gas transfer is seldom applied in the treatment of viding for a mixture of water and air in which gas
surface water because surface water has been transfer will occur.
in contact with air for a prolonged period. Conse-
quently, surface water contains sufficient oxygen, The tower aerator (Figure 2) consists of a cylin-
and other gases, like methane and hydrogen sul- drical vessel of steel or synthetic material that is
fide, are absent. filled with packing material, usually consisting of
The addition of oxygen is required for the oxida- elements of synthetic material. Water falls down
tion of bivalent iron (Fe2+), manganese (Mn2+) and air is blown in a co-current or counter-current
and ammonium (NH4+). These substances are direction.
present in dissolved form in groundwater. Due to
chemical and biological oxidation, the substan- A plate aerator (Figure 3) is a horizontal perfo-
ces can be removed by following a filtration step. rated plate. Water flows over the plate and air is
This will be discussed in the chapter on granular blown through the orifices, creating a bubble bed
filtration. of air and water above the plate.
Reducing the carbon dioxide concentration leads Sprayers (Figure 4) are typically used because
to a rise in pH and a reduction of aggressive car- of their simple implementation in existing treat-
bon dioxide that is able to disintegrate (concrete) ment plants. By spraying, a contact surface be-
pipes. tween the air and water is created for the gas
Methane should be removed because its pres- exchange.
ence has negative influences on the filtration pro- 2 Theory of gas transfer
cesses.
Hydrogen sulfide has an annoying odor (rotting
eggs) and therefore needs to be removed from
the water.
Volatile organic compounds are usually toxic;
some of them are even carcinogenic. Obviously,
these compounds are not allowed in drinking wa-
ter.
To achieve gas transfer a number of systems
have been developed over the years.
One of the oldest systems is the cascade (Figure
1). The water falls in several steps. In each step, Figure 2 - Tower aeration
137
Aeration and
WATER TREATMENT
gas stripping
Qw, cw,0 Qa, ca,e
Qa,ca,0 Qw, cw,e
1
k1
0.8
0.6 k2
K (-)
k3
k4 k5
0.4
0.2
0
0.001 0.01 0.1 1 10
RQ
k1
k2
k3
k2t = 1.61
k4 kD = 0.039
k5 T = 10oC
,aeration and gas transfer water treatment
Framework
This module explains aeration and gas stripping.
Contents
This module has the following contents:
1. Introduction
2. Theory of gas transfer
2.1 Equilibrium
2.2 Kinetics
2.3 Mass balance
2.4 Solutions for the basic equations
3. Practice
3.1 Cascade
3.2 Tower aerator
3.3 Plate aerator
3.4 Spray aerator
3.5 Alternative aeration systems
136
, water treatment aeration and gas transfer
1 Introduction
Aeration (gas addition) and gas stripping (gas re-
moval) are normally the first treatment steps dur-
ing the production of drinking water from ground-
water or riverbank water. This artificially induced
gas transfer aims at the addition of oxygen (O2)
and the removal of carbon dioxide (CO2), meth-
ane (CH4), hydrogen sulfide (H2S), and other
volatile organic compounds (for example 1.2 Di- Figure 1 - Cascade aeration
chloropropane (1.2 DCP), Trichloroethene (TRI),
Tetrachloroethene (PER) and Trichloromethane the water falls over a weir into a lower placed
(chloroform)). trough. When the falling stream enters the water
body, air is entrapped in the form of bubbles, pro-
Gas transfer is seldom applied in the treatment of viding for a mixture of water and air in which gas
surface water because surface water has been transfer will occur.
in contact with air for a prolonged period. Conse-
quently, surface water contains sufficient oxygen, The tower aerator (Figure 2) consists of a cylin-
and other gases, like methane and hydrogen sul- drical vessel of steel or synthetic material that is
fide, are absent. filled with packing material, usually consisting of
The addition of oxygen is required for the oxida- elements of synthetic material. Water falls down
tion of bivalent iron (Fe2+), manganese (Mn2+) and air is blown in a co-current or counter-current
and ammonium (NH4+). These substances are direction.
present in dissolved form in groundwater. Due to
chemical and biological oxidation, the substan- A plate aerator (Figure 3) is a horizontal perfo-
ces can be removed by following a filtration step. rated plate. Water flows over the plate and air is
This will be discussed in the chapter on granular blown through the orifices, creating a bubble bed
filtration. of air and water above the plate.
Reducing the carbon dioxide concentration leads Sprayers (Figure 4) are typically used because
to a rise in pH and a reduction of aggressive car- of their simple implementation in existing treat-
bon dioxide that is able to disintegrate (concrete) ment plants. By spraying, a contact surface be-
pipes. tween the air and water is created for the gas
Methane should be removed because its pres- exchange.
ence has negative influences on the filtration pro- 2 Theory of gas transfer
cesses.
Hydrogen sulfide has an annoying odor (rotting
eggs) and therefore needs to be removed from
the water.
Volatile organic compounds are usually toxic;
some of them are even carcinogenic. Obviously,
these compounds are not allowed in drinking wa-
ter.
To achieve gas transfer a number of systems
have been developed over the years.
One of the oldest systems is the cascade (Figure
1). The water falls in several steps. In each step, Figure 2 - Tower aeration
137
