INTRODUCTION

In a conventional water-treatment plant, the filtration stage was often considered the core of the process. The incoming flow to the filters was clearly non-potable, but after disinfection the filtered water was virtually potable water. As water-quality standards have developed this perception is no longer true in many cases, but filtration is still arguably the most important process in most treatment plants, and the process has been developed over the years to be more able to produce water able to meet higher water-quality standards. In the past filtration referred only to granular filtration, using sand or other granular material. Nowadays, where their use is appropriate, non-granular filters are becoming increasingly popular. This chapter mainly considers granular filtration in its various forms.

Basically the process of filtration consists of passing water through a granular bed, of sand or other suitable medium, at low speed. The media retains most solid matter while permitting the water to pass, and the filtrate from a filter performing well will be crystal clear with a turbidity of less than 0.2 NTU. Except for treatment of high-quality groundwaters, filtration almost always follows a clarification stage. Thus there is a trade-off between the performance of the clarification and the performance of the filters; the more effective the clarification the less the filters have to do and vice versa.

In order to achieve the required quality of filtered water, filtration has to remove particles far smaller than the sizes of the openings between the filtering media. Table 9.1...