Water Maker Prefilters

Water Maker Prefilters


We get questions all the time regarding the prefilters on a marine water maker.  What is their function, what micron size works best, can I buy them off the shelf from a place like Home Depot, and can they be washed and reused?


The purpose of the prefilters is to remove as much suspended solids from the sea water stream as possible to minimize clogging of the RO Membrane.  “Suspended Solids” is just a technical way to say dirt particles, Zooplankton (animal plankton), Phytoplankton (plant plankton), or any solid chunk of junk that could get stuck in the RO membrane and eventually reduce the fresh water production.


The industry standard size rating for the prefilters is 5 micron.  1 inch equals 25400 microns, so only the smallest of particles would make it through a 5 micron prefilter and those should be small enough to pass through the RO membrane and out with the brine reject without clogging the membrane.  Some low output water makers only have a single 5 micron prefilter, while our high output water makers come standard with two prefilters to provide progressive filtration.  First a 20 micron prefilter followed by a 5 micron.  It isn’t that this progressive filtration will remove more particles, because a 5 micron prefilter is a 5 micron prefilter after, even if it has a 20 micron prefilter in front of it.   The benefit of adding two-stage prefiltration is that it extends filter life and decreases pressure drop through the prefilters as they become loaded with suspended solids.


If your water maker is using an industry standard size prefilter, then you should be able to buy them anywhere, so beware the water makers with proprietary parts that will lock you into their high priced consumable supplies!  For example our 5 and 20 micron prefilters are made by Flow Max and you can buy them from us at $7ea.  There are a few water maker companies out there using proprietary prefilters that range in cost from $15 to $35…ouch!


When buying water maker prefilters, be sure to use pleated prefilters rather than string wound or blown sold.  In addition to having more filtration surface area, the pleated prefilters can be cleaned and re-used several times, while the string wound and solid blown prefilters may be a bit cheaper but can’t be cleaned and reused.  When we were cruising Mexico, we routinely were able to find the standard size prefilters in places like Home Depot and local hardware stores.  But rather than $7ea, expect to pay closer to $15 to $20ea, so it pays to stash away at least a year’s supply, which I think is 6-5 micron and 6-20 micron prefilters.


The benefit of pleated prefilters is that they can be washed and reused up to 3-4 times.  My standard operating procedure was to pull out my 20 and 5 micron prefilter once per month and wash them in a bucket of sea water followed by a fresh water rinse.  I would then hang them on the lifeline to dry in the sun for the UV to help kill any animals still living in the prefilter.  With two sets or prefilters rotating in and out, I was able to get 3-4 months of use from a single filter set, saving money and storage space.