I know it sounds like a simple thing to change out the 5-micron prefilter on a Schenker Watermaker.

• Unscrew the filter bowl
• Replace the old filter with a new one
• Screw back on the filter bowl

What’s so difficult about that to warrant a tech class topic?
Many things in life that seem easy and straightforward can in fact be easy. However, there is a somewhat obvious and often overlooked important detail to a prefilter change that needs to be addressed to save you from a potentially costly mistake. One like I was dealing with today…😳

The 5-micron prefilter is protecting the RO Membrane and Energy Recovery Pump from solids larger than 5-microns in size. Solids smaller than 5-microns just pass through the Energy Recovery Pump and flow out of the RO Membrane with the brine discharge. But solids larger than 5-microns can get lodged in the RO Membrane spiral wound matrix and literally plug up the RO Membrane, reducing product water production and actually increasing the PPM of the product water. There is something even worse than a plugged-up membrane and that is scratched up seals, cylinders, and internal working surfaces of the Energy Recovery Pump. If the internal seals and working surfaces of your Energy Recovery Pump get damaged, it’s time for a rebuild. Now of course that’s something I believe an average cruiser can do with good tech support and I’ve done it many times on WhatsApp over Starlink (like today). But do you carry all the spare seals and parts aboard that could be damaged? Maybe…maybe not. So an ounce of prevention here can go a long way.

Firstly, you never run water from any source into your Energy Recovery Pump without a 5-Micron prefilter in place. Not fresh water, not membrane cleaning reagents, not pickling reagents, nothing. Anytime water is flowing into your Energy Recovery Pump, you must have the protection of the 5-micron prefilter.

Now let’s address how a simple prefilter change can go horribly wrong. Your 5-micron prefilter is working great, doing what it should be doing and you are happily making water for the long showers Rich promised you if you had a watermaker aboard. Life is good, until…

Scenario 1:
You got into shallow waters and while you may not admit it at the cruiser bar, you may have gently bumped the soft sandy bottom a bit, but no harm, no foul. Your watermaker was running at the time and some of that sandy bottom was stirred up and was sucked into the watermaker through hull. No worry however, your 5-micron prefilter did its job and filtered out the sand.

Scenario 2:
You usually only like to run the watermaker in clean blue water, but because your non-boater friends came for a visit and used water in the shower like it was…well Water…you now need to run your watermaker in the less than clear water anchorage. The water looks pretty turbid actually and you can’t see the 20ft bottom, but oh well…you make water. Again your 5-micron prefilter does its job filtering out some decent size chunks and bits.

Scenario 3:
While making water you noticed a change in sound from your feed pump and upon investigation found that your raw water strainer was completely clogged with gunk of unbelievable funky smell and lots of small bits of shells. Who knows how those shells got in there, but you clean it all out and start making water again. But during the process of cleaning out the raw water filter screen in the galley sink (and getting in trouble with the Admiral for doing yet another smelly job in the galley) some of those solid bits get pushed through the screen and sit on the outlet flow side of the filter screen, so that when you restart the watermaker, down those bits go into the 5-micron prefilter.

Scenario 4:
It’s time to layup the boat for summer storage and pickle the watermaker. So out comes the SC1 and 5-gallon bucket from the aft deck to mix up the picking reagent. Well the bucket was last used to hold a few lobsters as you prepared the BBQ but you are in a hurry and don’t give the bucket a good fresh water rinse, besides you are low on water and are running late to catch that flight home. So the sand and lobster shell bits from the pickling bucket get sucked into the watermaker along with the pickling reagent. The watermakers pickled and off you go, returning 4 months later in the fall. Thankfully the 5-micron prefilter again saved you and removed the solid bits before they went into the Energy Recovery Pump and Membrane.

In all 4 scenarios, the 5–micron prefilter did exactly what it’s designed to do…then comes disaster.

t’s time to change the 5-micron prefilter. This time you promise yourself that you are not going to lose the filter canister o-ring overboard when you dump the filter bowl water overboard. You are also convince yourself that you are not going to spill seawater on the Victron inverter charger like 2yrs and $1500 ago. So you carefully unscrew the filter bowl, and without spilling and drop slide the old filter element up and out and slide the new filter element in. Screw on the bowl and boom….off you go to recommission the watermaker. Unfortunately some of the sand, shells, and lobster bits that were safety filtered out by the 5-micron prefilter fell to the bottom of the filter bowl during the filter element swap. What was once safety on the Outside of the 5-micron prefilter is now dangerously on the inside with a direct path to the seals, cylinders and working surfaces of your Energy Recovery Pump. 🤯

During the troubleshooting process for sporadic and asymmetrical pressure pulsations, Rich walks you through checking some internal parts and Holy Toledo, scratches and gouges are everywhere. What happened…how can this be…isn’t this covered under warranty (uh No).

I know Prefilter changes sound easy, they are. But as this story thumb-typed at 40,000ft over the Baja Mexico Peninsula points out, missing little details of simple things can cause big problem. Make sure your 5-micron filter bowl is clean and never run water into your watermaker without the 5-micron prefilter installed.