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ISSN 0378-4738 = Water SA Vol. 33 No. 2 April 2007
ISSN 1816-7950 = Water SA (on-line)
249
Differentiation between different kinds of mixing in
water purication – Back to basics
P Polasek
Water & Wastewater Treatment Consultant, PO Box 61965, Marshalltown 2107, South Africa
Abstract
The term mi xing is conf using because it is used to describe tra nspor t mecha nisms for both ash mixing (reagent dispersion
and homogenisation with water mixing) and agitation (occulation mixing) because each of these mechanisms requires dif-
ferent ow characteristics in order to take place with maxi mum efciency. Flash mixing should take place under condit ions
of mixing on macro-scale with macro-turbulent eddies being formed and agitation u nder conditions of mixi ng under micro-
scale with micro-turbulent eddies being formed. Agitation ta kes place under high- or low-intensity agitation. Only the condi-
tions of agit ation can be characterised by velocity grad ient. Differentiation bet ween ash m ixing and agitation is discussed.
Keywords: mixing, agitation, mixing intensity, homogenisation, occulation
Introduction
Mixing is an important operation in any water purication
process. It facilitates dispersion and homogenisation of added
reagents with water and contacts between the particles leading
to their combining into readily separable ocs. The efciency
of the water purication process is, therefore, dependent on the
mixing conditions under which the for mation of occulent sus-
pension takes place.
When designing a occulation system, the mixing condi-
tions, which include uniform distribution of a velocity eld in
the agitated volume of water, are not optimised in respect of the
most effective utilisation of the added destabilisation reagent
and the formation of ocs, the properties of which should be
most suitable for the method that is selected for their separation.
The reasons are that the importance of the mixing intensity, its
duration and ow characteristics on the properties of formed
ocs such as their shape, size and compactness (density), are not
yet fully appreciated and/or understood in engineering practice.
In waterworks design practice, which prevails to date
(Schutte, 2006), the processes of the reagent dispersion and
homogenisation with water and the oc formation take place in
two separate chambers under the conditions of rapid and slow
mixing:
• Rapid mixing is intended for dispersion and homogenisation
of added reagent with water and, therefore, is considered a
suspension not forming mixing. It takes place with a mean-
root-square velocity gradient G = 80 to 100 s-1 over a period
of T = 10 to 60 s (Amirtharajah and Trusler, 1986; Claus,
1967; Fair and Geyer, 1958; Hudson, 1965).
• Slow mixing is intended for the formation of occulent sus-
pension. It takes place with a velocity gradient = 20 to 60
s-1 for a period of T = 15 to 30 min and even longer (Fair and
Geyer, 1958; Hudson, 1965). Generally, the resultant ocs
formed are of a wide range of sizes, densities and settling
velocities. These slow mixing conditions are referred to in
this paper as the customary occulation conditions.
It follows from the above that the difference between these two
mixing conditions is only in the mixing intensity, characterised
by velocity gradient G.
The rapid and slow mixing conditions as described above are
suitable for jar tests, i.e. a batch process, but not for the water-
works through-ow process. In a through-ow system the char-
acter of mixing applied to the dispersion and homogenisation of
added reagent with water is different to that of the oc formation,
should these two processes take place most efciently. Theory
assumes that the nal products of homogenisation of hydrolys-
ing destabilisation reagent with water, which takes place in a
rapid mixing chamber, are destabilised particles of impurities,
and that these destabilised particles are transformed in the
subsequent oc-formation process, which takes place in slow
mixing occulation chamber, into readily separable ocs. How-
ever, these theoretical assumptions are seldom obtained under
current design practice and operational conditions. The likely
causes for deviations from theory have their origin in defects
of a hydrodynamic nature. These include inability to complete
dispersion and homogenisation of the added reagent with water
in the rapid mixing chamber and formation of suspension within
the occulation chamber. This results in a functional shifting of
the individual processes into subsequent unit operations, where
the optimum conditions for such processes no longer exist. For
instance, the dispersion of reagent and its homogenisation with
water continue in the occulation chamber and the oc forma-
tion process in the sedimentation tan ks/clariers and lters and
sometimes even into the reticulation system. Obviously, this
adversely affects the performance efciency of the works in its
entirety as well as the quality of the puried water. Therefore, in
through-ow system, the rapid and slow mixing cannot be dif-
ferentiated by mixing intensity only but by different transport
mechanisms applicable for each of these processes. Regrettably,
this is not always respected in water works design.
The lack of differentiation between different characteristics
of ow required for rapid and slow mixing often results in both
G
+27 082 833 4330; fax: +27 12 347-4969;
e-mail: polasek@mweb.co.za
Received 4 April 2006; accepted in revised form 26 January 2007.
250 Available on website http://www.wrc.org.za
ISSN 0378-4738 = Water SA Vol. 33 No. 2 April 2007
ISSN 1816-7950 = Water SA (on-line)
these processes taking place in the same unit operation (the rapid
mixing stage is totally omitted), where the optimised conditions
do not exist for either of them.
Certain misunderstandings and misconceptions with respect
to the importance of the different character and intensity of mix-
ing and its duration that are applicable to these two processes
have their roots in the term mixing. The ter m mixing is indis-
criminately used to describe the different characters of mixing
(transport mechanisms) required for both of these processes. In
order to avoid confusion as to which transport mechanism is
referred to, there is a need to differentiate between the two kinds
of mixing and to identify them under different names, namely
(Polasek, 1980a; 1981):
• Flash mixing (dispersion/homogenisation mixing)
• Agitation (occulation mixing).
The character of mixing that suits best the requirements of these
processes can be dened as follows:
• Flash mixing (dispersion/homogenisation) – mixing on
macro-scale, in which partial volumes of water are trans-
ferred over long distances and macro-turbulent eddies are
formed.
• Agitation (oc formation) – mixing on micro-scale, in which
partial volumes of water are transfer red over short distances
and micro-turbulent eddies are formed, and uniform dist ri-
bution of a velocity eld throughout the volume of agitated
water is produced.
It should be emphasised that ash mixing inuences the ef-
ciency of the destabilisation process, which determines the qual-
ity to which the water is pur iable by the works under the reac-
tion conditions applied, whilst the conditions of agitation under
which the occulation process takes place profoundly inuence
the separability of formed ocs in general, and the attainable
settling velocity in particular, as well as the processing of the
produced sludge.
Flash mixing
Effective ash mixing is required in terms of the chemical reac-
tions point of view because homogenisation of the hydrolysing
destabilisation reagent with water is accompanied by many
chemical reactions such as hydrolysis, polymerisation of the
products of hydrolysis and the diffusion of polymers to the sur-
face of particles of impurities. Some of these reactions are irre-
versible. The most important reaction is adsorption. The hydro-
lysing reagent particles have a tendency to mutually bind to one
another when there is no free particle surface area in their vicin-
ity. This leads to the formation of precipitates or to the binding
of the particles of the hydrolysing reagent onto the surfaces of
particles of impurities already occupied by the hydrolysing rea-
gent, thereby causing restabilisation of these particles. In both
instances, these particles become inactive resulting in ineffec-
tive utilisation of the destabilisation reagent dosage applied.
The principle task of the dispersion and homogenisation
processes is the maximum utilisation of the added reagent in the
chemical reactions. Therefore, achieving the highest uniform-
ity in the concentration of the added reagent and the pH in the
water being puried in the shortest period of time is essential.
The mixing conditions required are attained in a ash mixer that
provides for the transfer of small quantities of destabilisation
reagent over long distance and their dispersion inside the turbu-
lent eddies.
Ineffective dispersion / homogenisation results in the forma-
tion of local under- and over-dosed volumes of water. In the
under-dosed volumes of water an insufcient amount of reagent
prevents sufcient destabilisation of impur ities. In the over-
dosed volumes of water excessive amounts of the reagent result
in the restabilisation of the just-destabilised par ticles of impuri-
ties. This results in lowering the efciency of the purication
process, or in the need for increasing the reagent dosage. Con-
sequently, the water contains a mixt ure of particles in differ-
ent stages of destabilisation, i.e. destabilised, non-destabilised,
partly destabilised and restabilised particles. Homogenisation
then continues under sub-optimal conditions in the subsequent
unit operations, which are intended for the formation of ocs
and their separation and not for the homogenisation of the desta-
bilisation reagent with the water. This is the reason why a sepa-
rate, adequately sized ash mixer should always form a separate
unit operation of any waterworks. Fur ther more, with respect to
homogenisation efciency, it is equally important that the addi-
tion of the reagent is continuous, steady and free of pulsation.
The ash mixing of organic polymers is a specic problem.
Their stock solutions are usually highly viscous and cannot be
easily dispersed and homogenised with water. In addition to this
problem organic polymers also have a tendency to mutually bind
to one another or onto surfaces already occupied. This neces-
sitates intensive ash mixing over a longer period. On the other
hand, the mixing must be carefully controlled in order to avoid
breakage of the polymer chains through prolonged ash mixing.
This problem can be overcome by dosing a solution of low con-
centration that is more readily dispersible.
When pretreatment of water with alkali or acid is required,
the limiting requirement for the homogenisation mixing is not
the velocity of homogenisation but the completion of the reac-
tions of added reagent(s) with water together with the adjustment
of water pH prior to the addition of destabilisation reagent.
When pretreatment of water by oxidation is required, an
adequate contact period under the conditions of turbulent ow is
required for the completion of oxidation reactions. Its efciency
is dependent on the oxidizing agent as well as on the type and
arrangement of the oxidation chamber.
Agitation
The agitation of water is brought about by hydraulic or mechani-
cal means. Since non-uniform distribution of a velocity eld
exists in the agitated volume of water, the magnitude of tangen-
tial forces varies throughout the occulating system. This results
in considerable differences in the magnitude of tangential forces
that are affecting the ocs being for med. Consequently, ocs of
different sizes and structures and of a broad range of sedimen-
tation velocities are formed, which unfavourably inuence the
sizing of the sedimentation plant. In case of the mechanical agi-
tation these variations are inuenced by the type, size and speed
of rotation of the stirrer, geometr y of the basin and character of
the ow induced by the stirrer. In the case of the hydraulic agita-
tion, these variations are inuenced by water velocity in ume,
geometry of the ume and changes in the direction of ow.
Under the customary occulation conditions, slow mixing
is considered to be instrumental for the formation of readily
settleable suspension. Its purpose is to facilitate the formation
of large, kinetically unstable ocs. The idea that the for mation
of large ocs is benecial is associated with the belief that the
intensity of agitation should not exceed a cer tain limit beyond
which oc breakage occurs. Therefore, slow mixing is often
designed such that its intensity decreases as the process of oc-
culation progresses (tapered occulation).
Available on website http://www.wrc.org.za
ISSN 0378-4738 = Water SA Vol. 33 No. 2 April 2007
ISSN 1816-7950 = Water SA (on-line)
251
The basic changes taking place during the occulation proc-
ess include the changes in the number of the destabilised parti-
cles of impurities, the number of ocs being formed from these
particles and in the size, shape and density of the formed ocs.
The ocs formed under the accustomed occulation conditions
are large, voluminous and of a geometrically loose, widely
branched, spatially extended lattice structures containing large
volumes of voids lled with water. They are of low density and
very fragile with a tendency to fragment. Such ocs are grossly
non-homogenous in size as well as in density (Tesarik, 1967;
Hereit et al., 1983). Since sedimentation tanks must be sized for
the nest suspension, such ocs are not particularly suitable for
rapid sedimentation.
In contrast to the customary occulation conditions, the
inline high-density suspension (IHDS) occulation process
takes place with high agitation intensity over the entire occula-
tion process until occulation optimum is reached. As a result
the compaction of the formed ocs and thereby their density are
very favourably inuenced. The oc densication is a result of
reduced volume of voids in the micro-ocs lled with entrapped
water (Polasek, 1980b; Polasek and Mutl, 2003; 2005a; 2005b;
Polasek and Van Duuren, 1981). Depending on the resultant size
of ocs required, their formation can take place under two dif-
ferent agitation intensities:
• High intensity (high energy) agitation with GH > 50 s-1
• Low intensity (low energy) agitation with GL < 50 s-1
The high and low agitation intensities involve the same trans-
port mechanism and differ only by the agitation intensity, i.e. the
magnitude of the G-value.
When the micro-ocs are required these are formed with a
high agitation intensity preferably with a GH = 100 – 500 s-1 in
the rst occulation phase. When rapidly settleable macro-ocs
are required these are formed in the second occulation phase
with a low agitation intensity, preferably with a
G
L
= 5 – 20 s
-1
, from
the micro-ocs formed with high GH in the preceding rst oc-
culation phase (Polasek, 1980b; Polasek and Mutl, 2003; 2005a;
2005b). The resultant macro-ocs are much denser than those
formed under the customar y occulation conditions because
they are formed from much denser micro-ocs.
The agitation intensity together with its duration should be
optimised with respect to the properties of ocs that are required
in view of the separation method selected. The character of ow
induced by agitation must be designed such that the most uni-
form distribution of the velocity eld throughout the occulation
chamber is achieved, if maximum separation efciency at the
highest settling velocity of the formed ocs is to be attained.
Conclusions
The term mixing as currently used does not differentiate between
the transport mechanism required for the dispersion and homog-
enisation of added reagent with water and that for occulation of
destabilised particles into larger ocs, even though their charac-
ters are entirely different. Therefore, it is necessary to differenti-
ate between:
• Flash mixing (reagent dispersion and homogenisation mix-
ing) – the mixing on macro-scale, in which partial volumes
of water are transfer red over long distances and macro-tur-
bulent eddies are formed inside which the added reagent is
dispersed
• Agitation (occulation mixing) – the mixing on micro-scale,
in which partial volumes of water are transferred over short
distances and micro-turbulent eddies are formed which
facilitate formation of readily separable ocs. Depending
on the ultimate size of the ocs required, which is deter-
mined by the selected method of separation, agitation takes
place in one or two consecutive phases differentiated by dif-
ferent intensities of agitation (IHDS occulation process),
namely:
- High-intensity (high energy) agitation aimed at the
formation of micro-ocs, GH > 50 s-1 applied over the
entire occulation process until occulation optimum is
reached
-
Low-intensity (low energy) agitation aimed at the forma-
tion of large and rapidly settleable macro-ocs,
G
L
<50 s
-1
.
The high- and low-agitation intensities involve the same trans-
port mechanism and differ only by the agitation intensity, i.e.
magnitude of the G-value. Irrespective of the agitation intensity,
the conditions of agitation must be designed to create uniform
distribution of a velocity eld throughout the volume of agitated
water.
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252 Available on website http://www.wrc.org.za
ISSN 0378-4738 = Water SA Vol. 33 No. 2 April 2007
ISSN 1816-7950 = Water SA (on-line)
