INTRODUCTION

Following coagulation and flocculation, water containing floc normally passes to a settlement/clarification phase. In the past the only process used was some form of settlement basin in which the flocs settled out. Nowadays it is common to use either DAF or some form of sludge-blanket clarifier; neither of these can be described as settlement basins; both are better described as clarifiers. This chapter considers both traditional settlement basins and the more modern clarification processes. It considers some of the practical aspects of clarifier selection and design and some of the various types of clarifier in more detail. There are two basic types of settling basins: horizontal flow and vertical flow; and two other types of clarifiers: vertical-flow solids contact tanks; and DAF tanks. First horizontal- and vertical-flow tanks are considered, followed by vertical-flow solids contact tanks; and DAF.

Horizontal- and vertical-flow tanks predominantly remove previously flocculated particles; there is some additional flocculation but this is not a major factor. Horizontal-flow tanks can be either rectangular or circular. Vertical-flow tanks are normally square hopper-bottomed tanks.

Vertical-flow solids contact tanks achieve much higher efficiencies partly by passing flow through a layer of sludge/floc. Flocculation and entrapment of fine particles as the flow passes through the sludge/floc layer means that these particles, which would otherwise rise and pass onwards, are retained within the floc layer. They have been developed from hopper-bottomed vertical-flow tanks, which are very deep, and have evolved into flat-bottomed upward-flow clarifiers that have a relatively shallow depth of construction.