TABLE OF CONTENTS Water has been called the closest thing to a universal solvent because it will dissolve, at least to some extent, most compounds. Raw waters, surface waters in particular, also contain suspended solids including colloidal particles, microorganisms, and organic complexes. These compounds and suspended solids will cause problems in high-purity treatment systems, and must be removed or reduced before final purification. While an in-depth examination of pretreatment methods is outside the scope of this book, an overview of these techniques reveals their capabilities.
Figure 5-1 shows a flow schematic of a common pretreatment process. The methods outlined in this scheme are:
Microbiocide feed
Clarification/softening
Media filtration
Activated carbon filtration
Flow Schematic of a Common Pretreatment Process
Each provides a very important function.
Chlorine has been the principal microbiological control agent for many years, and has proven to be the most economical biocide. However, use of chlorine gas has been curtailed due to safety and environmental concerns. Popular replacement feed chemicals now include sodium hypochlorite (liquid chlorine), bromine, chlorine dioxide, and in some cases ozone. (UV light is also a possibility. Supplement 5-1 outlines a UV light arrangement at a manufacturing facility). The mechanisms by which chlorine and other oxidizing agents disinfect water are described in chapter 6. The oxidizing biocide should be added in great enough strength to maintain a slight residual (0.1 to 0.5 ppm) throughout the pretreatment process. This helps prevent the growth of microbes in equipment downstream of the injection point. Chlorine and...
