Water Filtration

INTRODUCTION

Water fi lters are used to remove microorganisms. Th ere are some fi lters that have a chemical disinfectant matrix that are eff ective against some viruses; however, fi lters are not always eff ective in removing viruses. A combination of fi ltration and purifi cation of contaminated water are recommended to ensure water safety (CDC, 2009).

Filters usually have a hand pump that draws water into the fi lter, and they work by pumping water through an intake hose or by slow gravity fl ow and through a microscopic fi lter that strains out harmful microorganisms. Th ere are diff erent sizes of fi lters depending on what organism is to be fi ltered out (Backer, 2011).

Most portable water fi lters are made of ceramic material, fi ber or compressed granulated activated carbon (GAC); all of these materials create irregular and complex passages to catch organisms. Depth fi lters have a large capacity for holding particles, so they don’t clog as fast as single-layer membrane fi lters do. When a fi lter is clogged, the water pressure increases and forces water through the fi lter with the risk of possibly forcing microorganisms through it and back into the water.

Most portable fi lters have a prefi lter on the intake tubing to protect the main fi lter. If the fi lter does not come with a prefi lter, a fi ne mesh cloth or coff ee fi lter may be used instead. Flow can be improved in a clogged fi lter by back-fl ushing or cleaning the surface of a ceramic fi lter to remove large particles trapped close to the surface (Backer, 2002).

Advantages: Water fi lters are easy to operate, require no holding time for treatment, may improve the taste and appearance of water, and remove all microorganisms with the combination of chemical disinfections (Dvorak et al., 2009).

mICROORGANISm SIZE AND FILTRATION (Backer, 2011)

Average Size (Micrometer)Organism Maximum Recommended Filter Rating (Micromet Absolute)1

Viruses Enteric Bacteria (E. coli) Cryptosporidium oocyst Giardia cyst 0.03 0.5 X 3.0 - 8.0 4.0 - 6.0 6.0 - 10.0 X 8.0 - 15.0 Not Specifi ed (Optimally 0.01) 0.2 - 0.4 1 3.0 - 5.0

Disadvantages: Water filters may not be solely reliable in removing viruses, are bulky and add weight to baggage, will eventually become clogged, and they require cleaning or replacement (Dvorak et al., 2009).

Reverse Osmosis

Reverse osmosis units reverse the flow of water with osmosis, forcing water to pass from a more concentrated solution to a more diluted solution through a semi-permeable membrane. A reverse osmosis unit has a pore size of about 0.0001 and is very effective in removing protozoa, bacteria, viruses, common chemical contaminants, and desalinating water. Because reverse osmosis units are expensive and have a slow output, they are mostly recommended as survival aids for ocean voyagers (Backer, 2002).

Advantages: Effective in removing viruses and the taste of chlorine and iodine (Dvorak et al., 2009).

Disadvantages: Filters can be pricey and time consuming with the use of hand pump filters (Dvorak et al., 2009).

Coagulation - flocculation (CF)

This process removes particles that won’t settle by gravity, particles that taste badly, and particles that make water cloudy. CF is capable of removing many but not all microorganisms. Coagulants such as aluminum sulfate, lime, ferrous sulfate, etc., are added, stirred well, allowed to settle, and then poured through a coffee filter to remove particles (Backer, 2002).

NSF International is a nonprofit, nongovernmental organization that tests water filters and develops standards and product certification for public health and safety. They work in partnership with the World Health Organization (WHO) for Food and Water Safety and Indoor Environment. They are qualified by the American National Standards Institute (ANSI), the International Accreditation Service (IAS), and the Standards Council of Canada (SCC) for third-party certification (Backer, 2002).

Because NSF testing is expensive and voluntary, some filters may be effective but have not been NSF tested. Units that are not NSF certified but are more likely to be effective are those that contain reverse osmosis or have an absolute pore size of 1 micron or smaller (Backer, 2010).

O Infor mation (Backer, 2011) (CDC, 2009)

• Membranes in some filters can be damaged by chlorine in the water. Always follow manufacturer’s instructions for use, care and replacement. • Filters that take a significant impact should be thrown away - filters that are cracked inside pose a risk of contaminated water flowing through the crack. • The intake hose should be treated as if it is contaminated because it is submerged in unfiltered water and it should also be stored seperately from the filter.