Aeration of a wastewater lagoon environment to accelerate
BNR (Biological Nutrient Removal) is well known and
acknowledged technology. The delivery mechanism
typically consists of submerged, specialized aerators.
Current technology tends to be ineffective and require
significant investments in equipment over time.
The combined mixing and aerating action of a horizontal
directional aeration system may yield improved oxygen
transfer rates over that of vertical aeration technology. A
horizontal aerator combines conventional aspirator
technology with blower assisted aeration which produces
fine bubbles that significantly improve oxygen transfer. It
also induces horizontal flow that keep solids from settling.
CFD simulation of lagoon configurations with vertical and
horizontal aerators was conducted to provide the qualitative
difference between directional and vertical mixing and
quantitative data for Residence Time Distribution (referring
to a perception that directional aerators/mixers may cause
short circuiting between flow in and out).

A horizontal or directional aerator differs from standard
aeration equipment and is particularly suited for use in
lagoons. In horizontal aerators air is injected in front of an
impeller. The spinning impeller provides energy that is
needed for air dispersion and also provides a far reaching
flow of oxygen saturated fluid. Dispersion relies on
turbulence produced by the impeller. Horizontal aerators
also offer benefits in having a compact size, easy placement
at any location in a lagoon, low pressure drop for oxygen
delivery, efficient gas dispersing head, and produce very
high horizontal flow.
Vertical aspirators or high speed floaters are a common
technology used in lagoon configurations. They use
pumping to aerate and the pumping direction may be up or
down. In down pumping air is entrained from a vortex
created by mixer action and goes through an impeller
where it is dispersed into small bubbles. In an up pumping
a stream of water is thrown into the air where it is saturated
with oxygen then mixes with the body of water. All mass
transfer relies on fine spray that develops a large surface
area.
The challenge in selection of equipment for lagoon aeration
is how to select optimum placement of aerators.
Calculation of oxygen transfer requirements is very straight
forward, however the placement requires a great deal of
experience and the support of CFD. A lagoon that is fitted
with vertical aerators only will experience solids deposit at
the bottom. Solids are lifted only in small areas adjacent to
aerators. Get easy pay through payday advance today.

High shear rotor-stator mixers are widely used in process industries including the
manufacture of many food, cosmetic, health care products, fine chemicals and
pharmaceuticals. Rotor-stator devices provide a focussed delivery of energy, power & shear
to accelerate physical processes such as mixing, dissolution, emulsification and deagglomeration.
To reliably scale-up these devices we need to understand the relationship
between rotor speed and flow rate and the energy dissipated by these devices. In-line rotor
stator mixers differ from in-tank versions because the flow is usually controlled
independently of the rotor speed. For in-tank devices the turbulent power can usually be
adequately described by single impeller type power number1. For an in-line rotor-stator
mixer it is found that the power transmitted by the rotor drops in proportion to decreased
flow rate and a single power number is not adequate. Kowalski2 and Baldyga et al.3
proposed that the power draw of a rotor-stator mixer can be described by the expression;
P= POZ ρ N3 D5 + k1 M N2 D2 (1)
This expression consists of two main elements. Firstly a term reflecting the power required
to rotate the shaft in response to the resistance of the liquid in the process chamber where ρ
is density (kg/m3), N is rotor speed (rps) and D is rotor diameter (m). Secondly a term for the
centrifugal energy given to the fluid which is then convected away by the mass flow rate, M
(kg/s). The two constants POZ and k1 are normally obtained from a multi-linear regression a
large matrix of experiments covering a wide range of flow rates and rotor speeds 4.
In the experiments described herein, it has been found that good estimates of the constants
can be obtained using a simplified set of trials. In Method 1 we note that there are two sets
of conditions under which constant turbulent power numbers are obtained as follows.
• When the outlet valve is closed, so we have zero flow, then the power is a minimum
given by P = POZ ρ N3 D5 with the characteristic power number POZ
• When the valve is fully open and the rotor-stator device acts as the sole pumping agent
then the flow rate is proportional to the rotor speed (figure 1) with M = ρ k2ND3. Power
draw is a maximum given by Pmax = POU ρ N3 D5 which can also be expressed as:
POZ ρ N3 D5 + (POU – POZ)ρN3 D5 with the characteristic power number POU
Substituting for M in eq 1 & rearranging gives Pmax = (POZ + k1 k2) ρ N3 D5 = POU ρ N3 D5 (2)
which in turn yields k1 = (POU – POZ) / k2 (3)
Figure 2 presents an example of the values of PO determined for these two extreme cases
and illustrates that the values are constant for all rotor speed and figure 3 presents the
comparison of measured and predicted power draw for a range of conditions.
In Method 2 we consider the example of a fixed speed device where flow rate can be varied
by means of a backpressure valve. In this case the power is a linear function of the flow rate
(fig 4) where the intercept is given by POZ ρ N3 D5 and the gradient by k1 N2 D2 and thus POZ
and k1 can be determined. This method also has the advantage of being suitable with power

determined from a heat balance because it does not explicitly require measurement at zero
flow rate (this is possible with the torque measurement used above). However at low rotor
speeds and high flow rates the temperature rise is small the heat balance is subject to
significant errors although the accuracy was improved by lagging the equipment and careful
calibration of the temperature probes. Table 1 presents the values of the constants
determined by the two approaches for an example arrangement of rotor and stator. In the
presentation we will present results for other arrangements complete with statistical analysis. easy pay through payday loan

Mixing in agitated tanks represents a fundamental part of
the chemical, petrochemical, pharmaceutical, food and
paper industries. This unit operation is commonly carried
out under steady flow conditions with the impeller
centered with respect to the vessel and rotating at constant
speed in one direction only. Mixing has been a popular
subject for the last years. Most studies have been focused
on power consumption, measurements of mixing times
and visualization of flow patterns. A number of techniques
have been developed for visualizing flow patterns in
stirred vessels. Mavros (2001) reviewed the use of very
sophisticated techniques for observing the flow behaviour
in stirred vessels including particles image velocimetry
(PIV), planar laser induced fluorescence (pLIF) and
thermal pulses, among others.
Mixing operations are usually carried out in some
applications at low to moderate Reynolds number, in
which the turbulent regime cannot be achieved. The
presence of important segregated regions and dead zone
generated with open impellers at low Reynolds number
has been extensively reported in the literature (Solomon et
al., 1981; Lamberto et al., 1996). Such pathologies persist
even if baffles and mixed flow impellers are used resulting
in very long mixing times, high energy consumption and
undesirable by-products due to poor reaction selectivity
can be generated. The most common way to vanish and
eliminate these regions consists of increasing the
rotational speed, however, this leads to an energy
consuming process, long mixing times, and in some cases
it may be detrimental to the final product especially when
mixing shear sensitive media.
It has been demonstrated that the flow structures generated
under steady flow conditions can be destroyed if the flow
is continuously perturbed if the impeller is lightly
displaced from tank centreline in the radial direction as
well as by using time-dependent revolutions. The works
reporting the use of spatial and temporal conditions
promoting chaotic flows in stirred vessels are reviewed in
this paper.
STEADY-STATE FLOW
Mixing operations are usually carried out at low to
moderate Reynolds number. The presence of important
segregated regions and dead zone generated with open
impellers at low Reynolds number has been extensively
reported in the literature (Solomon et al., 1981; Lamberto
et al., 1996). Those pathologies persist even if baffles and
mixed flow impellers are used resulting in very long
mixing times, high energy consumption and undesirable
by-products due to poor reaction selectivity can be
generated. The most common way to enhance mixing by
minimizing the effect of such non-homogeneities consists
of using wide impeller or increasing the rotational speed.
However, the latter option leads to high-energy levels and
in some cases it could be detrimental for materials
sensitive to shear.
Solomon et al (1981) reported for the first time the
observations of well-mixed regions around the impellers
surrounded by stagnant fluid, which are known as caverns
in the case of yield stress fluids. Considering a cavern as a
sphere with center on a Rushton turbine they proposed
empirical expressions for determining the geometrical
characteristics of such flow structures. The observations of
these flow structures were later confirmed by Alvarez et al
(2002) and Ascanio et al (2002).
UNSTEADY-STATE FLOW
The most common way to enhance mixing by minimizing
the effect of flow structures such caverns (well-mixed
regions) and isolated regions (static or quasi-static zones)
consists of using wide impellers or by increasing the
rotational speed. However, the latter option leads to highenergy
levels and in some cases it could be detrimental for
materials sensitive to shear. Therefore, other alternatives
based on the theory of chaos principles have been
developed and reported in the literature. They consist
basically on the use of spatial and temporal scenarios as
described below.

Eccentric impellers
Different alternatives based on pioneering studies on
chaotic laminar mixing have been reported in the
literature, in which it was demonstrated that
homogenization could be quickly achieved by using
eccentric cylinders rotating alternatively in both directions
during short times (Ottino et al 1988; Swanson and Ottino
1990; Muzzio et al 1991; Muzzio et al 1992). One of the
main reasons of using eccentric impellers is that they
provide the best mixing action where a vortex is not
required or not desirable.
Alvarez et al (2002) proposed the use of spatial conditions
based on the radial position of the impeller with respect to
the tank centreline. They showed that a minimal radial
displacement of the impeller can enhance the mixing
performance in terms of the mixing times. Similar results
were found by Ascanio et al (2002) by using different one
or two off-centered impellers in the stirred vessel.
Dynamic perturbations
Lamberto et al 1996 proposed the use of time-dependent
stirring speed to destroy the caverns generated under
steady mixing conditions. They demonstrated that the
mixing of Newtonian fluids could be clearly enhanced if
the fluid dynamics was continuously perturbed. This
method was shown to prevent the formation of segregated
regions close to the impeller, confirming the theoretical
results of Aref (1984). Using time-dependent rotation
directions changes conditions, Ascanio et al (2002)
demonstrated that segregated regions can be gradually
vanished and finally destroyed if the flow is continuously
perturbed.
It is clear from the above contributions based on the
fundamental concepts of chaos theory that nonconventional
mixing strategies appear to be promising to
enhance mixing in stirred tanks in laminar regime. During
the symposium a general overview of the works reporting
scenarios based spatial and temporal conditions will be
given for preventing the formation of coherent segregated
regions in the vicinity of the impeller resulting in energyefficient
processes.