WATER TREATMENT
Softening
WATER TREATMENT
HCO3- raw water (mmol/l) 6
Ca(OH)2 NaOH
diameter 0.5 - 4 m 5 NaOH Na2CO3
Na2CO3
4
E 3
Na2CO3
2
1
in progress 0
0 1 2 3 4 5
height +/- 6 meters
hardness reduction [Ca2+] (mmol/l)
groundwater NaOH groundwater Ca(OH)2
surface water NaOH surface water Na2CO3
in rest
D
C
B
A
F
A supply of hard water
B supply of lye
C periodic dosing of sand grains (0.1-0.4 mm)
D forming pellets
E outlet for softened water
F periodic outlet of pellets (2 mm)
,softening water treatment
Framework
This module explains softening.
Contents
This module has the following contents:
1 Introduction
2 Principle
2.1 Why softening?
2.2 Water quality and softening
2.3 Softening processes
2.4 Pellet reactor
2.6 Softening in a treatment plant
3 Theory
3.1 Equilibrium
3.2 Kinetics
3.3 Mass balance
3.4 Hydraulics
3.6 Influence of parameters
4 Practice
4.1 Split treatment
4.2 Choice of chemicals
4.3 Construction alternative for reactors
4.4 Seeding material
4.5 Pellet storage
154
, water treatment softening
Table 1- Reasons why softening is applied
1 Introduction
Public health
Groundwater normally remains in the subsoil for - decreased release of heavy metals from dis-
many years before it is pumped up or flows out into tribution network
- no use of household softening devices
the surface water. Due to the long residence time in
the subsoil, groundwater is in chemical equilibrium
Ethics
(i.e., calcium carbonate equilibrium).
- prevention of stains
Groundwater comes in contact with the atmos-
- user’s comfort
phere when it is pumped up or discharged into
surface water. When carbon dioxide disappears Environment
from the water, it is not in calcium carbonate equi-
- reduction of heavy metals in sludge WWTP
librium anymore. - reduction in use of detergent and decreased
Also, when water is heated the equilibrium is phosphate content in wastewater
changing, the Ca2+ and HCO3- - ions will precipi- - reduction of concentrate discharge of
tate in the form of calcium carbonate (CaCO3). household softening devices
Especially high concentrations of Ca2+ and HCO3-
- ions will lead to inconveniences for the customers Economy
because of the calcium carbonate scaling (e.g., - reduction in usage of detergent
- reduction of scaling and corrosion of house-
deposits in water boilers).
hold equipment
To prevent precipitation of calcium carbonate at - reduction of energy consumption of heating
the customers’ taps, calcium ions are partially devices
removed from the water by drinking water com- - reduction in damage to clothes
panies. This is called softening.
reduced release of heavy metals. Other reasons
2 Principle why softening is used are given in Table 1.
2.1 Why softening?
The financial benefits of softening are greater than 2.2 Water quality and softening
the costs. The hardness of water is classified from very soft
Amsterdam Water Supply calculated that the to very hard (Table 2).
benefits of softened water for one single household
comes to a saving of approximately 45 euros a A number of water quality parameters are influ-
year (mainly as a result of their decreased use of enced as a result of the softening process. For
detergent, less maintenance on washing machines these parameters, standards are included in
and boilers, and lower energy costs), whereas the the Dutch National Drinking Water Standards
softening costs for a household are approximately (Waterleidingbesluit). In addition to these standards,
10 euros a year. guideline values were developed by VEWIN.
In addition to decreasing the hardness of water,
another important reason for softening is the
Table 2 - Classification of hardness
unit very soft soft fairly soft fairly hard hard very hard
mmol/l <0.5 0.5 - 1.0 1.0 - 1.8 1.8 - 2.5 2.5 - 5.0 > 5.0
eq/m3 <1 1-2 2 - 3.5 3.5 - 5 5 - 10 > 10
*D <3 3-6 6 -10 10 -15 15 - 25 > 25
155
Softening
WATER TREATMENT
HCO3- raw water (mmol/l) 6
Ca(OH)2 NaOH
diameter 0.5 - 4 m 5 NaOH Na2CO3
Na2CO3
4
E 3
Na2CO3
2
1
in progress 0
0 1 2 3 4 5
height +/- 6 meters
hardness reduction [Ca2+] (mmol/l)
groundwater NaOH groundwater Ca(OH)2
surface water NaOH surface water Na2CO3
in rest
D
C
B
A
F
A supply of hard water
B supply of lye
C periodic dosing of sand grains (0.1-0.4 mm)
D forming pellets
E outlet for softened water
F periodic outlet of pellets (2 mm)
,softening water treatment
Framework
This module explains softening.
Contents
This module has the following contents:
1 Introduction
2 Principle
2.1 Why softening?
2.2 Water quality and softening
2.3 Softening processes
2.4 Pellet reactor
2.6 Softening in a treatment plant
3 Theory
3.1 Equilibrium
3.2 Kinetics
3.3 Mass balance
3.4 Hydraulics
3.6 Influence of parameters
4 Practice
4.1 Split treatment
4.2 Choice of chemicals
4.3 Construction alternative for reactors
4.4 Seeding material
4.5 Pellet storage
154
, water treatment softening
Table 1- Reasons why softening is applied
1 Introduction
Public health
Groundwater normally remains in the subsoil for - decreased release of heavy metals from dis-
many years before it is pumped up or flows out into tribution network
- no use of household softening devices
the surface water. Due to the long residence time in
the subsoil, groundwater is in chemical equilibrium
Ethics
(i.e., calcium carbonate equilibrium).
- prevention of stains
Groundwater comes in contact with the atmos-
- user’s comfort
phere when it is pumped up or discharged into
surface water. When carbon dioxide disappears Environment
from the water, it is not in calcium carbonate equi-
- reduction of heavy metals in sludge WWTP
librium anymore. - reduction in use of detergent and decreased
Also, when water is heated the equilibrium is phosphate content in wastewater
changing, the Ca2+ and HCO3- - ions will precipi- - reduction of concentrate discharge of
tate in the form of calcium carbonate (CaCO3). household softening devices
Especially high concentrations of Ca2+ and HCO3-
- ions will lead to inconveniences for the customers Economy
because of the calcium carbonate scaling (e.g., - reduction in usage of detergent
- reduction of scaling and corrosion of house-
deposits in water boilers).
hold equipment
To prevent precipitation of calcium carbonate at - reduction of energy consumption of heating
the customers’ taps, calcium ions are partially devices
removed from the water by drinking water com- - reduction in damage to clothes
panies. This is called softening.
reduced release of heavy metals. Other reasons
2 Principle why softening is used are given in Table 1.
2.1 Why softening?
The financial benefits of softening are greater than 2.2 Water quality and softening
the costs. The hardness of water is classified from very soft
Amsterdam Water Supply calculated that the to very hard (Table 2).
benefits of softened water for one single household
comes to a saving of approximately 45 euros a A number of water quality parameters are influ-
year (mainly as a result of their decreased use of enced as a result of the softening process. For
detergent, less maintenance on washing machines these parameters, standards are included in
and boilers, and lower energy costs), whereas the the Dutch National Drinking Water Standards
softening costs for a household are approximately (Waterleidingbesluit). In addition to these standards,
10 euros a year. guideline values were developed by VEWIN.
In addition to decreasing the hardness of water,
another important reason for softening is the
Table 2 - Classification of hardness
unit very soft soft fairly soft fairly hard hard very hard
mmol/l <0.5 0.5 - 1.0 1.0 - 1.8 1.8 - 2.5 2.5 - 5.0 > 5.0
eq/m3 <1 1-2 2 - 3.5 3.5 - 5 5 - 10 > 10
*D <3 3-6 6 -10 10 -15 15 - 25 > 25
155
