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Chapter 15
FIELD WATER DISINFECTION
Recommendations are considered Category 1B by the WMS Panel of Expert Reviewers
I. GENERAL INFORMATION
All surface water carries a risk of enteric illness due to ingestion of waterborne pathogens
that include bacteria, viruses, protozoan cysts, and some parasitic eggs or larvae. Risk
varies with geographic location. In North America, Giardia lamblia is the most common
microbial contaminant, but Campylobacter jejuni, enterotoxigenic E. coli, enteric viruses,
and Cryptosporidium have caused outbreaks of illness. In developing countries, surface
and tap water must be considered contaminated. Potential microorganisms include
protozoa (Entamoeba histolytica, Giardia lamblia, Cryptosporidium), bacteria (E. coli,
Shigella, Vibrio cholerae, Salmonella, C. jejuni, V. parahaemolyticus), viruses (hepatitis
A and other enteric viruses), or helminths. Potable drinking water can be prepared by
disinfecting water using one of several means.
II. METHODS OF WATER DISINFECTION
A. Heat. As in pasteurization, temperatures above 160° F (70° C) kill all enteric
pathogens within 30 minutes, and 185° F (85° C) is effective within a few minutes. Thus,
disinfection occurs during the time required to heat water from 140° F (60° C) to boiling
temperature, so any water brought to a boil, even at high altitudes, is safe.
B. Filtration may be used for Giardia and other protozoal cysts, enteric bacteria, and
parasitic eggs. However, for field use, filtration alone does not adequately remove
viruses, although many are removed by adhering to larger particles. The maximum
effective filter pore size for Giardia and amoeba cysts is 5 microns. For enteric bacteria,
it is 0.2 to 0.5 microns, depending on filter design. Cryptosporidium oocysts require less
than 3 microns pore size, while other parasitic eggs and larvae are removed with a 20 to
30 micron filter. Most commercial filters claim removal of Giardia only or Giardia and
bacteria. Claims for viral removal should be discounted because they are not well
substantiated. Some filters incorporate activated charcoal or an iodine resin. (See
Techniques below.) Newer filtration systems take advantage of microtubules that provide
an absolute 0.2 micron filtration size, with a rapid flow rate achieved by their massive
total surface area due to the number of tubules wrapped into the filter.
C. Clarification techniques remove suspended particulate matter and many
microorganisms. They are not adequate to disinfect water reliably, but may be used to
improve clarity and remove organic matter prior to filtration or halogenation.
1. Sedimentation is the separation of large particles by gravity. Simply allow water to
stand in any container for at least one hour and then decant.
2. Coagulation, flocculation (C-F) removes smaller, suspended particles (colloids) that
will not settle with simple gravity. This method works best on "cloudy" water, such as
