scope:

Introduction

Early water treatment was performed to improve appearance or taste. With the discovery that diseases such as cholera and typhoid fever were transmitted by ingestion of water contaminated with particulate germs, people came to recognize that taste and smell alone are not accurate indicators of the acceptability of water. As a result, additional treatment technologies beyond the commonly used sedimentation basins were needed. Experimental work in Louisville, Ky., Cincinnati, Ohio, and Pittsburgh, Pa., brought about the development of the rapid sand filters, which were shown to significantly reduce both turbidity and bacteria in water. These filters required addition of a coagulant to allow the coarse sand to remove colloidal materials. The coagulant was usually added to the raw water as it entered the sedimentation basin. After a few hours of contact in the basin, the water was sent to rapid sand filters. To control the coagulation process, treatment performance was often "eyeballed" by observing the floc formation in the sedimentation basin and adjusting the chemical dose to maintain a "beautiful-looking" basin.

Another milestone in drinking water treatment was the use of chlorine as a disinfectant. Introduced in the United States at the beginning of the 20th century, this practice led to a dramatic reduction in the occurrence of waterborne diseases. This combination of coagulation, sedimentation, filtration, and disinfection constituted an early multiple-barrier approach to microbial control in drinking water technology