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Courtesy of Vivendi Water
- Oxidation Contactors
- Ultraviolet Reactors
- Coagulation-Flocculation Reactors
The efficiency of water treatment is linked to the performance of chemical
and biological reactors that are used throughout the treatment process.
The hydrodynamics of these reactors govern their performance. In the past,
process design was based solely on experience and costly chemical and
biological measurements. Today, CFD is used to assess the hydrodynamics
of the reactors as well, and thereby improve reactor performance for both
drinking and wastewater treatment processes. Vivendi Water, which services
over 110 million customers throughout 100 countries, has used FLUENT since
1992 for this purpose. Using CFD, engineers can make improvements to many
aspects of reactor performance, including the optimization of hydraulic
efficiency and prevention of scale-up problems.
Oxidation Contactors
In drinking water treatment plants, oxidation contactors are baffled
reactors in which water and an oxidant, either ozone or chlorine, are
in contact for a period of time to eliminate viruses and bacteria. To
quantify the disinfection efficiency of working contactors, a residence
time distribution (RTD) is measured. To quantify the efficiency of proposed
contactor designs or operating conditions, CFD is used. Both single phase
(water only) and multiphase (water and ozone) systems can be studied in
this manner. For example, oxidation contactors at several drinking water
treatment plants in France (Annet Sur Marne, Nice, and Neuilly Sur Seine)
have been simulated to diagnose the disinfection efficiency. The FLUENT
results were used to suggest changes in the contactor geometry to bring
about performance improvements during the rehabilitation phase of the
process.
Velocity field and path lines inside an oxidation contactor provide useful
information such as the localization of dead zones, recirculation loops,
and short cuts
Ultraviolet Reactors
Ultraviolet light is absorbed by the proteins RNA and DNA in micro-organisms.
Absorption of high doses of UV by proteins in cell membranes ultimately
leads to the death of the cell. However, at much lower doses of UV, absorption
by DNA merely disrupts the ability of the micro-organism to reproduce.
A cell that cannot reproduce cannot infect other cells. UV reactors are
designed to deliver a dose of UV radiation to micro-rganisms in the water
as it flows around a lamp. The disinfection efficiency of these reactors
depends on the lamp power and residence time of the water around the lamp.
Simulations of UV reactors are targeted toward resolving the hydrodynamics
of the reactor while including the radiation field. The radiation models
provided by FLUENT can be used for this purpose. Many particles (representing
micro-organisms) are introduced to the flow field, and the received UV
dose is determined for each one. Engineers can then ascertain the percentage
of micro-organisms that receives a sufficient dose to be inactivated and
the percentage that receives too high a dose.
Particle tracks colored by incident radiation represent trajectories of
microorganisms in a UV reactor
Coagulation-Flocculation Reactors
The step of coagulation-flocculation plays a dominant role in the drinking
water treatment process. The goal of this step is to agglomerate suspended
particles and make easily settling composites called "flocs".
The first step, coagulation, consists of the destabilization of colloidal
particles upon the addition of certain chemicals, such as aluminum or
iron salt, which react with the water in stirred tanks. This critical
mixing stage is strongly influenced by the turbulence intensity. The particles
are then flocculated in more slowly stirred tanks. The rate of floc formation
is directly proportional to the velocity gradients in the stirred tanks.
Floc aggregation and breakup are determined by the local shear stresses.
FLUENT has been useful in simulations of the stirred tanks, providing
results for the flow field, impeller performance, and RTD. The results
allow researchers to determine the optimum conditions for the coagulation-flocculation
step through the testing of various configurations and operating conditions.
Flow path lines in a flocculation tank
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