TABLE OF CONTENTS This chapter looks in some detail at what is often a vital treatment process: coagulation and flocculation. This chapter focuses on the theory and practice of destabilization of fine solids present in water; Chapter 6 considers the types of coagulants and their chemistry.
Coagulation and flocculation is an area of water treatment that developed greatly over the last 20 30 years of the twentieth century. Prior to this, plants often had very simple arrangements for dosing and mixing chemicals. These generally worked well with lightly loaded plants, at the cost of high chemical use. With the development of advanced flat-bottomed clarifiers and DAF much more attention has been paid to the destabilization of water containing fine material and the conditions necessary for the formation of easily removed flocs. Also economic pressures have meant that savings in chemical costs have become more important.
It is convenient to think of solids as being present in water in three main forms: suspended particles, colloids, and dissolved solids (DS). Suspended particles may be coarse or fine particles of, for example, sand, rocks, or vegetable matter. They range in size from very large particles down to particles with a typical dimension of 10 ?m. Suspended particles will under quiescent conditions either settle or float. Colloids are very fine particles, typically between 10nm and 10 ?m. Finally there are DS that are present as individual molecules or as ions. Figure 5.1 shows the size ranges of materials present in water.
