Page 93 - Industrial Wastewater Treatment, Recycling and Reuse

68 Industrial Wastewater Treatment, Recycling, and Reuse

• Process models aim to simulate the overall process including the waste-
water treatment plant and try to minimize water requirements as well as
optimize water treatment plant operation.
• Equipment models aim to use computational fluid dynamics (CFD)
models to optimize and intensify individual equipment used in the
treatment plant.
Both these types of model are widely discussed in the published literature.
The use of process models for optimizing water usage and treatment is dis-
cussed in detail in Chapter 12 of this book. These are therefore not dis-
cussed here. CFD models in the last two decades have evolved into an
important tool to optimize water treatment, recycling, and reuse applica-
tions. CFD models essentially use mass, momentum, and energy conserva-
tion equations to represent real-life processes (see Ranade, 2002,for a
comprehensive discussion on computational flow models and their appli-
cations). Recent advances in understanding the physics of flows, numerical
methods to solve complex partial differential equations and computing
power, and the CFD simulations (numerical solutions of the model equa-
tions using computers) allow users to make apriori predictions of the flow
fieldandthereforehaveopenedupseveral new opportunities to optimize a
variety of processes. There are several possible applications in the area of
water treatment, recycling, and reuse. Some of the possible applications
are the following:
1. Designing of water handling systems and pipe networks; flow distribution
2. Flocculation/coagulation/clarifiers/settling tanks
3. Cavitation devices for water treatment
4. Chlorine, oxygen, ozone, or hydrogen peroxide contacting/reactors
including surface aerators
5. Anaerobic and aerobic digesters.
In each of the applications mentioned above, there are several opportunities
to use CFD models for performance enhancement. For example, in settling
tanks or clarifiers, it is essential to design the configuration in such a way that
settled particles are not re-entrained. In a conventional clarifier, the incom-
ing wastewater stream at the top creates turbulence in the clarification zone.
The hydraulic energy of the incoming flow needs to be dissipated in order to
avoid turbulence in the clarifier (which may cause re-entrainment). CFD
models can be used to devise optimal configuration to avoid re-entrainment
as well as to realize additional advantages. For example, Pophali (2007) has
used CFD models to develop an improved clarifier. A sample of his
simulated results is shown in Figure 1.21.