scope:

Introduction

The first successful practice of water filtration in the United States involved use of slow sand filters in which raw water was applied directly to large sand beds, but these filters were not suitable for treatment of muddy river waters like those found in the Ohio, Mississippi, and Missouri River valleys and their tributaries. In the 1890s and very early 1900s, George Fuller's filtration tests in Louisville and Cincinnati and Alan Hazen's testing program in Pittsburgh showed that turbid waters could be treated successfully by addition of coagulant chemical, clarification, and rapid sand filtration. The capability of a process train consisting of coagulation, mixing, flocculation, sedimentation, and rapid sand filtration to treat raw water having a wide range of turbidity resulted in widespread acceptance of this process train, which came to be called conventional treatment in the United States. Adoption of conventional treatment by a large number of water systems and of chlorination by even more water systems resulted in a very large decrease in the number of cases and number of deaths caused by typhoid fever in the early decades of the twentieth century.

Prior to World War II the focus on water treatment was on disinfecting water and providing clear water to drink. Coagulation and filtration had been shown to remove a substantial fraction of bacteria from water, and combined with chlorination, conventional treatment provided a double barrier against passage of pathogenic bacteria into drinking water. With the realization that viruses also could be transmitted by drinking water, the microbiological challenge broadened. Conventional treatment was found to be capable of removal of polioviruses in the 1960s, and in the 1980s and 1990s removal of protozoan cysts was shown to be within the capabilities of coagulation and filtration when these processes are managed properly. Results of studies on removal of asbestos fibers by coagulation and filtration proved that this process could remove both microbes and inorganic particles in a very wide range of sizes, from considerably less than 1 µm to tens of µm.