TABLE OF CONTENTS Prior to the water passing to the main treatment processes there is normally some form of preliminary or pre-treatment. Processes classed as pre-treatment include raw-water storage, screening, aeration, straining, preliminary settling, and pre-ozonation. Any of these processes might be found at a particular plant but it is improbable that all would be needed. Each performs a particular function and unless the problem they are designed to eliminate is present in the raw water they can be omitted.
It used to be common to chlorinate water prior to treatment, to reduce problems of attached growth within the main treatment process but this is now uncommon due to the standards for THMs. Now it is increasingly common to pre-ozonate water, which disinfects, oxidizes some of the complex organic chemicals, and, for some waters, also improves the performance of the subsequent clarification process. Pre-ozonation is covered in Chapter 11.
Coarse screens are provided at river intakes to prevent floating material of fairly large size entering the works. The steel bars forming the screen are normally quite substantial (typically about 25 mm dia. or equivalent) and are spaced about 100 mm apart. They are normally slightly angled from the vertical to facilitate raking. Sometimes the bars are mounted in frames which are duplicated so that one frame can be lifted for cleaning or repair without unscreened water entering into the plant. The velocity of water through the screen openings should not exceed 0.5 m/s. It is almost universal to provide...
