In the third part of our series on backcountry water treatment, we focus on microfiltration. This method is essential for ensuring safe drinking water in the wilderness, reducing pathogen loads to safer levels. In this article, we delve into the principles, effectiveness, and practical considerations of using microfilters in backcountry settings.
Introduction to Core Function of Microfilters
The primary purpose of a water filter is to make drinking water safe by reducing pathogens. While speed, cost, and weight are important factors, the core function is pathogen reduction. Many comparisons of water filters rate them based on flow rates, but filtration percentages are crucial for determining the filter’s effectiveness.
Filtration Percentages
Not all filters have the same removal percentages. The filtration capability of a filter is often labeled with a log reduction rate, which measures the percentage of pathogens removed. For example, a three-log reduction means removing 99.9% of pathogens, while a six-log reduction means 99.9999%.
Understanding Risk Tolerance
Water safety is a matter of reducing risk rather than completely eliminating it. The acceptable level of illness risk varies from person to person. Some may prefer the absolute smallest chance of infection, in which case boiling water is the most reliable method. Others may tolerate a higher level of risk, opting for filters that offer sufficient pathogen reduction for their needs.
Examples of Risk Tolerance
- High Risk Tolerance: Some individuals, like hikers who drink untreated water and risk illnesses like Giardia, may prefer minimal or no treatment. This approach, however, often leads to gastrointestinal issues.
- Moderate Risk Tolerance: Many prefer to use filters that reduce risk to a manageable level, balancing convenience and safety. These individuals might use microfilters with high log reduction rates.
- Low Risk Tolerance: Those with a low risk tolerance might combine filtration with other methods, such as boiling or chemical treatment, to ensure maximum pathogen reduction.
Pathogens in Backcountry Water
Different pathogens present varying levels of risk. The most common pathogens include Giardia, Cryptosporidium, and bacteria like E. coli.
Giardia
- Prevalence: Giardia is a common cause of parasitic gastrointestinal disease in the U.S., with an estimated 20,000 cases annually.
- Infective Dose: The minimum effective dose is about 10 cysts. Drinking water with low concentrations of Giardia cysts may still pose a risk over time, especially on long hikes where water is consumed in larger quantities.
Cryptosporidium
- Prevalence: Cryptosporidium can be found in surface water, with concentrations up to 300 oocysts per liter.
- Infective Dose: Unlike Giardia, a single oocyst can cause infection. This makes high log reduction rates critical for effective filtration.
Bacteria (E. coli)
- Prevalence: E. coli and other bacteria can be present in water sources contaminated by fecal matter. Concentrations can vary significantly depending on environmental conditions and land use.
- Infective Dose: For the O157:H7 strain of E. coli, the infective dose is 5-10 cells. High concentrations of pathogenic bacteria necessitate filters with high log reduction capabilities.
Filtration Rates and Standards
The EPA sets standards for microbiological water purifiers, requiring them to remove six logs of bacteria, four logs of viruses, and three logs of protozoa. Microfilters typically focus on protozoa and bacteria, with varying degrees of effectiveness.
Log Removal Rates
- Three-log Reduction: Removes 99.9% of pathogens. Suitable for general use but may not be sufficient for highly contaminated water sources.
- Four-log Reduction: Removes 99.99% of pathogens. Provides a higher level of safety.
- Five to Eight-log Reduction: Removes 99.999% to 99.999999% of pathogens. Offers the highest level of protection, suitable for the most challenging water sources.
Practical Use of Microfilters
- Routine Filtration: For most backcountry scenarios, using a filter with at least three-log reduction is sufficient. For higher safety, filters with five-log or more reduction are preferable.
- Filter Maintenance: Proper maintenance, including regular backflushing, is crucial for maintaining filter effectiveness. Some filters, like the Sawyer Mini, can be backflushed to restore flow rate and extend service life.
Case Study: Giardia and E. coli in Backcountry Water
Giardia
- Example Scenario: A canteen of water with 0.3 Giardia cysts per liter gives a 1 in 33 chance of infection. Using a three-log filter reduces this to 0.0003 cysts per liter, significantly lowering the risk.
E. coli
- Example Scenario: Water from a contaminated source with 2,000 bacteria per liter. A six-log filter reduces this to 0.002 bacteria per liter, reducing the risk of infection to a manageable level.
Conclusion
Microfiltration is a critical method for ensuring safe drinking water in the backcountry. Understanding log reduction rates and maintaining proper filter use can significantly reduce the risk of waterborne illnesses. While some may prefer the added security of combining filtration with other methods, high-quality microfilters provide a reliable and efficient solution for most backcountry water treatment needs.
Stay tuned for the next installment in this series, where we will explore virus protection and the use of purifiers in backcountry settings.