Treatment Plants for Phosphorus Removal from Wastewater
Settleable floes
Primary particles
Colloids
Dissolved material :
dp > 100f..lm
1 J.lm < dp < 100 f..lm
10- 3 f..lm < dp < 1 f..lm
"dp" < 10- 3 f..lm
Colloidal particles can remain stable in this finely dispersed condition. One of the
reasons is that the particles are so small that the Brownian movements, caused by
the collision of the water molecules with the colloids, dominate over the influence
of the gravity on the particles. Furthermore, electric repulsive forces, caused by
surface charges on the colloids, prevent the coagulation of the particles and the
subsequent sedimentation.
The aggregation of particles which occurs in the coagulation process is the result of
a destabilization of the colloids, usually produced by adding chemicals. The destabilization may take place through several different mechanisms depending on the
chemical composition of the colloids and the type of the coagulation chemical.
However, the understanding of the destabilization mechanisms is as such very
incomplete.
Below three concepts are discussed which are important for the coagulation - not
just in connection with the precipitation of phosphates from wastewater, but for the
coagulation processes in general. These are:
- hydrophillic/hydrophobic properties
- zeta potential
- isoelectric point (isoionic pH)
Hydrophillic/hydrophobic properties
The wastewater contains hydrophillic particles - that is, colloids having a strong
tendency to bind or absorb water, and hydrophobic particles - that is, water repellent colloids. Roughly speaking, the organic colloids are hydrophillic and the
inorganic colloids are hydrophobic. The reason for the hydrophillic property of the
organic colloids is that on their surface there are water absorbing or binding groups
such as amino groups (-NH2), hydroxyl groups (-OH) and organic acid groups
(-COOH), see Fig 10.5. The binding or absorption of water occurs through the
production of so-called hydrogen bindings. The result is that the hydrophillic
particles remain enclosed by a water jacket which follows the particles in their
movements. This water jacket is termed bound water.
The hydrophobic particles do not have such a water jacket.
Zeta potential
The colloids in the wastewater mainly have a negative charge. This phenomenon
can be verified by placing the particles in an electric field. From the field potential,
the rate of movement, the viscosity etc., a potential can be calculated, termed the
zeta potential, the importance of which will be further detailed below.
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Settleable floes
Primary particles
Colloids
Dissolved material :
dp > 100f..lm
1 J.lm < dp < 100 f..lm
10- 3 f..lm < dp < 1 f..lm
"dp" < 10- 3 f..lm
Colloidal particles can remain stable in this finely dispersed condition. One of the
reasons is that the particles are so small that the Brownian movements, caused by
the collision of the water molecules with the colloids, dominate over the influence
of the gravity on the particles. Furthermore, electric repulsive forces, caused by
surface charges on the colloids, prevent the coagulation of the particles and the
subsequent sedimentation.
The aggregation of particles which occurs in the coagulation process is the result of
a destabilization of the colloids, usually produced by adding chemicals. The destabilization may take place through several different mechanisms depending on the
chemical composition of the colloids and the type of the coagulation chemical.
However, the understanding of the destabilization mechanisms is as such very
incomplete.
Below three concepts are discussed which are important for the coagulation - not
just in connection with the precipitation of phosphates from wastewater, but for the
coagulation processes in general. These are:
- hydrophillic/hydrophobic properties
- zeta potential
- isoelectric point (isoionic pH)
Hydrophillic/hydrophobic properties
The wastewater contains hydrophillic particles - that is, colloids having a strong
tendency to bind or absorb water, and hydrophobic particles - that is, water repellent colloids. Roughly speaking, the organic colloids are hydrophillic and the
inorganic colloids are hydrophobic. The reason for the hydrophillic property of the
organic colloids is that on their surface there are water absorbing or binding groups
such as amino groups (-NH2), hydroxyl groups (-OH) and organic acid groups
(-COOH), see Fig 10.5. The binding or absorption of water occurs through the
production of so-called hydrogen bindings. The result is that the hydrophillic
particles remain enclosed by a water jacket which follows the particles in their
movements. This water jacket is termed bound water.
The hydrophobic particles do not have such a water jacket.
Zeta potential
The colloids in the wastewater mainly have a negative charge. This phenomenon
can be verified by placing the particles in an electric field. From the field potential,
the rate of movement, the viscosity etc., a potential can be calculated, termed the
zeta potential, the importance of which will be further detailed below.
319