scope:

Introduction

The definition of filtration contains the word medium. This medium can be granular (e.g., sand or coal) or it can be composed of synthetic material (membranes). This book focuses on the evaluation of granular filter media.

By definition, filtration does not seem complicated. Water (i.e., the matrix) simply passes through media, and the suspended particles are removed. However, the design, operation, and maintenance of filters are not necessarily simple. Granular filter media come in many different forms, including silica sand, garnet sand, diatomaceous earth, and anthracite coal. They all serve to accomplish the same goal: the removal of solid particles from a water matrix. Granular filters are available in many configurations, including slow sand, rapid sand, declining rate, dual media, multimedia, constant pressure, increasing pressure, mixed media, deep bed, diatomaceous earth, and high rate.

Filters are an integral component of the multiple-barrier approach to water treatment. Filtration is the second-most important treatment process, next to disinfection, because filters are responsible for the removal of protozoans that are not inactivated by disinfection. Filters are engineered systems and should be treated as such. Filter design is a science with its own set of criteria based on desired performance standards. The media type and specifications are developed based on the quality of water that is being treated and the upstream processes that will condition the water for the filtration process.

Too often, filters are not treated like mechanical equipment, which is subject to a preventive maintenance program. This means that filters often are not checked and maintained regularly. This can lead to the degradation of the filter media and the quality of the water that is produced by a water treatment plant.

Filters are not just sieves that remove suspended matter; they are miniature water treatment plants. Filters are systems that consist of the filter media, the media support, the backwash system, secondary wash systems (if present), piping, metering, and controls. These are integral to the proper operation of the filter.