Retention of microbial pathogens by filtration

Microbial contamination is the biggest concern for drinking water suppliers. A wide variety of microbial pathogens including enteric viruses, E. coli 0157:H7 and Cryptosporidium parvum are spread out through water sources even in developed countries such as United States. Hence, the primary goal of water treatment process is to ensure that the drinking water is free of pathogenic viruses, bacteria and protozoa. Because no single treatment process can be expected to remove all of the different types of pathogens, the international health authorities has promulgated a multiple barrier approach. The idea is to use at least one physical and one chemical process to ensure pathogen attenuation to a safe level. One of the most employed physical water treatment processes is filtration. Filtration is a common name for a process of physical retention of pathogenic microorganisms by size rejection or by accumulation of the pathogens within its media. Water treatment facilities employ either granular media filters or membrane filters. Four of most known filtration processes are slow sand, rapid granular, microfiltration and ultrafiltration. Degree of microbial retention by a filter depends on pathogen's transport, accumulation and inactivation. Each process has many variables/characteristics and for the same application a retention level may differ by orders of magnitude. Some of the variables are macroscopic and others are microscopic, their interplay and relative influence on the degree of retention are discussed, and useful conclusions are drawn.