Chapter 12 - Slow filtration

Abstract

This chapter focuses on the process of slow sand filtration. A slow sand filter is essentially a bed of sand through which the pre-treated water passes (downwards), together with the necessary structures to control the flow, and to remove the water after filtration. As filtration progresses, the suspended and colloidal matter from the raw water are retained in the very top of the filter bed. The clogging material may be removed and the filter restored to its original capacity by scraping off this top layer of dirty sand to a depth varying from one to a few centimeters. A slow sand filter needs a ripening period, during which positively charged particles are adsorbed. Positive charges may accumulate on some of the filter grains, so that oversaturation occurs, by which the charge on these coated particles reverses and becomes positive. A major part of the changes in water quality brought about by slow sand filtration are produced by algae, which are first carried along by the raw water, and then multiply actively during the often long intervals between successive cleanings. The most important feature of algae is their ability to build cell material from simple minerals with the aid of solar energy. The installation of a slow sand filter is favorable when the raw water is moderately polluted, but has a low turbidity, or where chemical supplies are unreliable, or when there is a shortage of skilled labor to operate rapid sand filters.