Treabnent Plants for Phosphorus Removal from Wastewater
a.
Bridging between the colloids via organic or inorganic polymers.
b.
Entrapment, or enmeshment, of the colloids.
c.
Reduction of the thickness of the electric double layer of the particles resulting
in a reduction of the particle repulsion.
as to a As the colloids usually have a negative charge it could be expected that only
positively charged (cationic) polymers are able to produce bridging. However,
negatively charged (anionic) and nonionic polymers can also be very effective.
The effect of the anionic polymers can be explained in that a multivalent cation, for
example calcium ions, is the link between polymer and colloid.
The effect of nonionic polymers may be due to the polar groups which are present
in the polymer chain where a positive and negative charge is found around certain
atoms. These charged areas may interact with the charged colloids.
It should be stressed that the polymers cannot just function through the bridging
mechanism, but they also partly neutralize the surface charge of the colloids and
hence reduce the particle repulsion. This is further discussed under item c.
as to b It is a precondition for the entrapment, or enmeshment, mechanism that
apart from the colloids there are floes in the liquid, for example of precipitated metal
hydroxide, which catch the colloids.
as to c Reduction of the particle repulsion can only take place in that cations, for
example Ca ++, Fe+++ or AI+++ ions, accumulate close to the negative colloid surface.
This means a displacement of the isoelectric point for the particle. Acids and bases
can exert a similar effect.
10.2.3 Flocculation
During the flocculation process, the primary particles are brought together to larger
particles, floes. The purpose of this sub-process is to increase the particle size of the
precipitated material so that it can be separated which typically takes place by
sedimentation or flotation.
The flocculation is often carried out in a system as shown in Fig 10.13.
In each of the flocculation chambers, the water is subjected to a gentle stirring by
means of mechanical flocculators, also called paddle flocculators. This causes the
primary particles to collide and aggregate to floes.
The sketch shows three flocculation chambers. In practice, normally 1 to 4 chambers
are used.
In some cases the aeration tank is used in activated sludge plants as precipitation
and flocculation tank (simultaneous precipitation). Here the aeration system agitates the water. It may result in the problem that the turbulence caused by the
aeration is so big that the chemical floes are partly broken up so that the effluent
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a.
Bridging between the colloids via organic or inorganic polymers.
b.
Entrapment, or enmeshment, of the colloids.
c.
Reduction of the thickness of the electric double layer of the particles resulting
in a reduction of the particle repulsion.
as to a As the colloids usually have a negative charge it could be expected that only
positively charged (cationic) polymers are able to produce bridging. However,
negatively charged (anionic) and nonionic polymers can also be very effective.
The effect of the anionic polymers can be explained in that a multivalent cation, for
example calcium ions, is the link between polymer and colloid.
The effect of nonionic polymers may be due to the polar groups which are present
in the polymer chain where a positive and negative charge is found around certain
atoms. These charged areas may interact with the charged colloids.
It should be stressed that the polymers cannot just function through the bridging
mechanism, but they also partly neutralize the surface charge of the colloids and
hence reduce the particle repulsion. This is further discussed under item c.
as to b It is a precondition for the entrapment, or enmeshment, mechanism that
apart from the colloids there are floes in the liquid, for example of precipitated metal
hydroxide, which catch the colloids.
as to c Reduction of the particle repulsion can only take place in that cations, for
example Ca ++, Fe+++ or AI+++ ions, accumulate close to the negative colloid surface.
This means a displacement of the isoelectric point for the particle. Acids and bases
can exert a similar effect.
10.2.3 Flocculation
During the flocculation process, the primary particles are brought together to larger
particles, floes. The purpose of this sub-process is to increase the particle size of the
precipitated material so that it can be separated which typically takes place by
sedimentation or flotation.
The flocculation is often carried out in a system as shown in Fig 10.13.
In each of the flocculation chambers, the water is subjected to a gentle stirring by
means of mechanical flocculators, also called paddle flocculators. This causes the
primary particles to collide and aggregate to floes.
The sketch shows three flocculation chambers. In practice, normally 1 to 4 chambers
are used.
In some cases the aeration tank is used in activated sludge plants as precipitation
and flocculation tank (simultaneous precipitation). Here the aeration system agitates the water. It may result in the problem that the turbulence caused by the
aeration is so big that the chemical floes are partly broken up so that the effluent
324