Introduction

The main applications of organic polyelectrolytes in potable water production are in coagulation and flocculation, and in the dewatering of treatment plant sludges. The water production processes are usually followed by sedimentation and filtration, although with only slightly contaminated waters the sedimentation step may be omitted. Flotation is an option instead of sedimentation, especially for algae laden waters. The sludges obtained from the various separation processes have very high water contents and must be further concentrated to minimise transportation costs; polymers have a role in this sludge conditioning.

The impurities present in the source water can be in the form of suspended material, such as clay, silica, microbial cells or algae, and as dissolved and colloidal natural organic matter, and as dissolved salts. Raw water processing normally involves physicochemical procedures, based on coagulation and flocculation of suspended solids and colloids, and the adsorption of soluble material on solid substrates such as metal hydroxide floes. The focus in this Chapter is on the use of soluble polymers in coagulation and flocculation processes.

Polymers have been utilized in coagulation/flocculation processes for water purification for at least four decades [1]. In comparison with alum, some of the advantages flowing from the use of polymers in water treatment are:

• lower coagulant dose requirements

• a smaller volume of sludge

• a reduction in the ionic load of the treated water

• avoidance of the presence of aluminium ions

Polymers are especially beneficial in coping with the problems of slow settling floes in low-temperature coagulation or in treating soft coloured waters, where they improve settleability and increase the toughness of floes [4]. The capacity of a treatment facility may be more than doubled with the formation of larger and stronger floes, the rate of solid and water phase separation can be significantly increased, and the dosage of other chemicals lowered. Also, the range of waters that can be treated is wider.

There are disadvantages of course, with higher costs in particular situations and environmental factors being the main concern. There is a greater sensitivity to incorrect dosage, with turbidity and natural organics removal less efficient in some instances [3], Degradation in the presence of chlorine or other disinfectants covered in a recent report [5] is reviewed here.

With a few notable exceptions [6] there is not a great deal of published information on the relationship between polymer structure and treatment performance in drinking water production; that is, on the influence of molecular structure on coagulation/flocculation, on the rates of both precipitation and sedimentation, on product water quality and on the solids content of the final sludge.

The types of impurities present in poor quality water supplies are first briefly outlined.

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