Physical Removal of Pathogens

Pathogens can be reduced through physical removal processes. This involves a sequence of treatment steps called clarification steps: coagulation, flocculation, and sedimentation. These steps are followed by filtration. However, physical removal alone will not reduce or eliminate all pathogens, so filtration is usually followed by inactivation processes such as disinfection.

Viruses are roughly one hundred times smaller than most bacteria and are only visible under an electron microscope. Given their extremely small size and variations in surface charge, viruses are much more difficult to remove using physical methods compared to bacteria and protozoa. Partial viral reduction can be achieved using the following filtration systems: conventional, direct, slow sand and diatomaceous earth (see table below). In addition to filtration, using disinfection technologies, individually or in combination, is a critically important barrier in achieving the appropriate viral pathogen log reduction. Filtration is extremely important in reducing turbidity, which can inhibit disinfectants.

Cysts and oocysts of the protozoans Giardia and Cryptosporidium, respectively, are much larger than viruses and can be partially removed by membrane filtration in addition to the filtration processes discussed above concerning the physical reduction of viruses.

The relative effectiveness of different filtration systems is shown below in Table A: Maximum Pathogen Log Reduction Credit Assignment for Filtration Systems. Pathogen log reduction credit assignment is based on filtration systems meeting operational and design criteria and consistently meeting filter effluent turbidity objectives.