Fundamentals of Water Treatment Unit Processes
Water treatment is accomplished by a "treatment-train", which is a sequence of "unit-processes". Some 10-15 unit processes have been identified, depending on how they are classified; those in this book include: screening, settling, mixing, coagulation,flocculation, filtration, adsorption, ion-exchange, membrane filtration, gas-transfer, disinfection, oxidation, precipitation, biological treatment. Stated in broad terms, the objectives of the unit processes are to effect separations (including particles, organic molecules, and ions); and/or to cause "state" changes (inactivation of organisms, oxidation ions or molecules, precipitation of selected cations, conversions of various organic molecules to bacteria and other products).
Content Table
Unifying Strands of Practice
The idea of a "unit-process" comes from chemical engineering and was introduced into the water treatment field in the early 1960's. The traditional approach was to consider treatment of drinking water, sewage, industrial wastes, and more recently hazardous wastes as independent since their practices in design and operation and their legal mandates have followed different historical tracks. A common thread that unifies these strands is that the theory for a particular unit process is the same for its different applications, albeit each has its own variations. Also, engineers, managers, operators, scientists, and other professionals work "across-the-board", sometimes specializing in one kind of water, but as a rule, are not constrained by education, practice within an organization, or job description. The foregoing may apply to municipalities, consulting engineering firms, state health/environment agencies, federal governments, universities, industries, manufacturer's and their representatives. Roles may include teaching, research, consulting, regulation, operation, design, planning, management, equipment design and manufacture, etc.
Technologies
For each of the 10-15 unit-processes, a range of "technologies" have been developed. To illustrate, settling includes traditional large basins, tube settlers, and plate settlers. Mixing may be accomplished by impellers, submerged jets, flat plates, bubbles, multiple coagulant inlets, or any means to cause advection and create turbulence. In other words, the 10-15 unit processes include many generic technologies and a host of proprietary variations.
Practice Approaches and History
Approaches to design, operation, research, innovation in equipment, regulation, and other modes of practice have evolved empirically complemented by rationales from science. All of these modes may be aided by "rules-of-thumb", lore, pilot plants, theory, and computer models. A knowledge of process history helps to understand current design approaches and how many have been supplanted gradually by science, mostly since the 1950's. Although practices are subject to change, principles remain constant.
Sample Figures
Treatment train unit processes for drinking water (pdf)
Particles hitting plate, accumulating, exceeding shear resistance and sliding off (pdf)
Reduction in net potential of a colloid as affected by coagulation (pdf)
Rapid filter in filtration mode (pdf)
Wave-fronts (pdf)
Radiation spectrum and wave-lengths for disinfection (pdf)
Breakpoint chlorination (pdf)
Resources
The issues in this article are addressed fully in The book, Fundamentals of Water Treatment Unit Processes - Physical, Chemical, and Biological, CRC Press, Boca Raton, FL and IWA Publishing, London, UK, 2010. Derived from Water Treatment Unit Processes - Physical and Chemical, CRC Press, Boca Raton, FL, 2006, the book has been revised and shortened to focus more on principles and the essentials of practice, with two chapters added on biological treatment. As with the 2006 book, the 2010 revision includes: history, practice, theory, sidebars that amplify on issues and persons pertinent to the topic at hand but slightly external to the flow of logic within the chapter. Many side-headings outline the chapter contents and permit rapid review and scanning to a point of particular interest. In some cases the detail may not be of interest and may be skipped without loss of continuity. Example problems illustrate applications of principles and methods of calculation. Many of the tables are spreadsheets, down-loadable from the CRC web-site but abstracted in the text, that illustrate calculations based on scenarios, i.e., assumed conditions or uncertainties in variables that generate families of solutions, as opposed to a single value. Such tables are prefixed by the letter "CD" to distinguish from text tables. In some cases, the "CD" tables have embedded figures, linked to calculation results. Problems are included in the text, and a solutions manual is available from the CRC web-site. Finally, each chapter has a glossary, intended to expand the perspective of a topic and to include common terms.
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