On the outside, most water purifiers and microfilters are indistinguishable and share many common features.
Most purifiers include a filter element to remove protozoa, and helminth eggs and larvae and occasionally bacteria. The big difference is that purifiers have an additional antimicrobial feature, usually in the form of an iodine impregnated resin, that is supposed to kill bacteria and viruses as they pass through it.
Some units use electrostatically-charged elements to attract microorganisms to the resin surfaces. The idea is that the device strains out large halogen-resistant pathogens, such as protozoa and helminth eggs and larvae, with perhaps a one micron filter element and then the purification element kills or inactivates bacteria and viruses as they pass through the impregnated resin.
There is great debate about how well iodine resins work, given the short time period that bacteria and viruses are in contact with them, and the known need for sufficient contact time.
In some purifiers, tiny amounts of iodine are released into the water. This residual halogen continues to disinfect water until it is drank and adds to the effectiveness of the purifier, particularly if the water sits for 30 minutes or more before being consumed.
Some devices incorporate a granular activated carbon filter after the iodine element to remove the iodine, other chemicals and offensive tastes from the treated water. The problem with this scenario is that the iodine is removed almost immediately after being added to the water, which reduces contact time to almost nothing.
To Purify or not to Purify?
The limited amount of independent test data on purifiers generally agrees that they are very effective treatment devices when used according to manufacturer guidelines.
The best recommendation for purchasing a purifier is to look for one that combines a filter element with pores one micron or smaller to strain out protozoa and helminth eggs and larvae and an iodine resin to kill bacteria and inactivate viruses.
Since it is impossible to determine the microbiological contamination levels in recreational water without a lab, after pumping, let water sit for 20–30 minutes to allow the iodine to do its job.
Granular activated carbon attachments or elements should not be used if they remove iodine from the water immediately after it is added. They can be reattached and the water run through a second time to remove the iodine after the water has sat for an appropriate amount of time.