Backflushing a water filter is the process of flushing trapped material from its filter media by simply reversing the flow of water through the filter. Today, it is most common–and most important to do–with filters that use hollow fiber technology to clean water. When done regularly, it keeps your filter at its maximum flow rate, ensuring fast, easy, and reliable filtration in the field. As a bonus, it will also ensure you get the maximum usable life from your filter.

Periodic backflushing is required for the MSR® AutoFlow™, HyperFlow™, TrailShot™ and Thru-Link™ water filters. While the process might sound intimidating, we’re here to debunk that myth and show you just how quick and easy backflushing can be in the field—no tools required. 

What Backflushing Does

The AutoFlow, HyperFlow, TrailShot, and Thru-Link microfilters all use hollow fiber technology to filter your water. These 'fibers' are essentially a bundle of tiny straws. As unfiltered water flows through their porous walls, contaminants are trapped, and clean, filtered water flows into the center of the straw and out of the filter. Over time, these contaminants build up and slow your filter down. Backflushing dislodges and rinses away the contaminants, allowing the water to flow freely again.

If you don't backflush regularly, all that particulate gets driven deeper into the fibers' walls, making it harder–and eventually impossible–to remove. That process is how filters die, so backflushing regularly is actually saving you money in the long run.

It is particularly important to backflush frequently (sometimes after every liter) when filtering from notoriously challenging sources such as glacial runoff with extremely fine silt or tannic (tea-colored) water rich in dissolved organic matter. Simply put, you can never over-backflush a filter. 

To keep your trip and your filter running smoothly, let's look at how to backflush your MSR hollow fiber microfilter as we walk through the simple steps for each filter and share video instructions for each.

How to Backflush the MSR AutoFlow Filter

• Start by filtering at least a half-liter of water into a reservoir, such as an MSR Dromedary bag.
• Remove the inlet hose from its barb on the male disconnect, then hold or hang the clean reservoir above the filter cartridge and allow the water to flow back through the cartridge and out the inlet tube. Allow at least a half-liter to flow through the filter cartridge to remove any debris collected in the fibers.

How to Backflush the MSR HyperFlow Filter

Backflushing the HyperFlow regularly is critical. The added force of the pump not only makes the HyperFlow incredibly fast, but it can also drive particulate deeper into the walls of the fibers, especially with aggressive pumping. You'll want to perform the backflushing process in a controlled area to avoid losing any small parts. (Pro Tip: Watch the video on this one!)

1. Start by removing the clean side cover cap, then attach the filter to a reservoir or bottle using the Quick Connect bottle adapter.
2. Filter at least a half-liter of water into the reservoir or water bottle.
3. Remove the inlet hose and unscrew the pump inlet from the pump cylinder. Make sure not to pump any air through the filter at this time.
4. Reverse the large check valve and reinstall it inside the pump inlet.
5. Pull the pump cylinder to the end of its stroke, then turn it clockwise until the piston locks.
6. Use the cylinder to unscrew the piston from the filter cartridge.
7. Pull the small check valve out, reverse it, and reinstall it inside the piston.
8. Carefully thread this pump cylinder onto the filter cartridge.
9. Thread the pump inlet back onto the pump cylinder. Again, don't pump any air through the filter.
10. Now you're going to pump water through the filter to flush the cartridge. Pump at least 10 full strokes of filtered water. Wait for the cylinder to fill with water during each stroke. **If you're using a hard bottle, such as a Nalgene, invert the container to keep air away from the outlet.
11. When you're done, disassemble the pump and reinstall the large and small check valves in their original forward-flow positions.
12. Dry off the pump and then reassemble it.

How to Clean the MSR TrailShot Filter

1. Start by simply pumping water until the bulb is half full.
2. Shake the filter up and down for 20 seconds.
3. Remove the inlet hose from the nozzle and squeeze out the dirty water.
4. Reconnect the hose.

If cleaning the filter fails to restore water flow, you should replace the cartridge.

How to Clean the MSR Thru-Link Inline Filter

1. With the filter installed, suck on the bite valve to pull water through the filter, filling the drink tube with filtered water.
2. Detach the filter from the reservoir.
3. Blow into the bite valve to push all the water back through the filter.
4. Reattach the filter to the reservoir.

With a little practice, backflushing your water filter will become second nature and just part of your routine, ensuring you've got all the water you need when you need it. 

Related Posts:

• MSR Water Products & Treatment Systems Guide
• Water Treatment 101: When & Why Should I Treat Backcountry Water?
• Water 101: Decoding Water Filter Testing Claims

Updated. Originally Published February 2, 2021.