Models

This chapter examines some of the approaches for attacking problems. The concept of modeling is a theme common to all. Modeling is an engineering method (Box 3.1).

About 10-15 unit processes comprise the field of water treatment, depending on how they are categorized. Perhaps there are 80-100 technologies developed from them. Table 3.1 lists 13 unit processes and associated technologies. Fundamental principles operative include

. Sieving of particles by screens (ranging from bacteria by membranes to large objects by bar screens)

. Creating conditions for application of a ‘‘passive’’ force on particles (e.g., gravity), or an ‘‘active’’ force (e.g., centrifugal) to cause transport

. Turbulence and diffusion for the transport of particles to cause contacts between reactants

. Charge neutralization

. van der Waals attraction between molecules and a surface (such as activated carbon), or charge attraction (such as between ions and an ion-exchanger material)

. Various chemical reactions such as . Redox, acid-base . Precipitation . Complexation . Biochemical . Cell synthesis

. Membrane processes involving retention of ions and molecules, i.e., reverse osmosis=nanofiltration

Generally, the unit processes listed in Table 3.1 are the results from a heritage of only since about 1900, albeit the earliest technology was slow sand, with the first installation in 1829 for London. Proprietary innovations have expanded the array of technologies, but most are variations of the unit processes listed in Table 3.1.