Product Support
Reverse Osmosis Membrane Care
The least understood but the most magical part of a reverse osmosis watermaker is the membrane but how do we take care of them? Membrane manufactures post all kinds of information on how to take care of membranes with the assumption they're being used in a commercial RO plant. It's worth finding this info online to get a better understanding of the process. What we as mariners do with them, the manufacturers really don't want to know about. We operate in an uncontrolled environment pulling feed water straight from wherever the boat might be with no idea of what's in the water that might affect the membrane. Fortunately, over the decades marine watermakers have been available, certain best practices have been developed. Here are a few basic rules.
1. Don't expose the membrane to any amount of chlorine
2. Don't let the membrane freeze or get over 113F (45C) It can handle getting hotter but some degradation can happen
3. Don't let the membrane dry out. Keep new membranes sealed in the packaging they came in.
4. Don't let oil get in the system
The fun fact is, membranes are happy to separate the salts from the water. This doesn' wear them out. There is no life limit on this process as far as I know. The things that affect membrane life mostly come from mineral and biological contaminants in the feed water. RO membranes work because there is a constant turbulent feed water flow over the membrane while the fresh water is getting pushed through. This keeps the salt and other contaminants from bunching up against the membrane and are sent back overboard as a concentrate or "Brine". Unfortunately, there are conditions where these contaminants can get stuck on the surface affecting the process. There are cleaners that can be used to attempt to remove them with mixed results. Minerals barely dissolved in the feed water can get stuck on the membrane which needs an acidic cleaner to remove them but this isn't very common in our case. More commonly, it's biological growth that forms on the membrane while it's sitting that causes a membrane to degrade. An alkaline cleaner is used to remove this type of fouling.
It used to be the rule of thumb that if you run your watermaker every day it will keep biofouling from forming. This is almost true. It can still happen just more slowly. Most watermakers today stress the need to flush the seawater from the system after use and many have automatic flush systems with a carbon filter in the flush water line to remove any chlorine that might be in the ships water tank. This works well in keeping biofouling from forming as what can grow in saltwater doesn't grow in freshwater and vise-versa. The down side is you are having to give back some of the water you just made to do this but it's well worth it in the long run. This is why the best practice is to run the watermaker long enough to fill the tanks, flush it, then run again when the tank is low, which might be a few days later. The membrane is happier running for fewer longer stints than many short ones if it's flushed after. Biofouling can happen in fresh water but happens much more slowly and if the system is re-flushed every few days almost stops it but not completely. This is why properly storing the system when not using it for a while is necessary.
So how do you know when a membrane is fouling? The manufacturers recommend cleaning when the output drops 10%. Letting it go after that will make it harder to get an adequate clean. All well and good if you are running a plant with 24/7 system monitoring and a known and treated feed flow. On a boat, normal differences in feed water temperature and salinity will cause membrane performance to vary well over 10%. This, unfortunately, is perfectly normal. But if you are operating in the same area of the ocean for a while you might see the changes in performance as the membrane gets fouled. In a conventional system, you would see a drop in product output. In a system with a hydraulic pressure intensifier energy recovery pump like Electromaax, Schenker, Spectra and others, you will see an increase in the feed pressure and amp draw but very little change in the product flow up to a point
.
So you feel your membrane may be fouled and you haven't sailed to Alaska or the Dead Sea since the last time you ran the system? Since we can't directly know what is fouling the membrane the recommended procedure is to first clean with the alkaline cleaner then the acid cleaner. Record what the system is doing beforehand. After the alkaline clean, run the system normally and record any changes in performance. Do the acid clean then record the changes. Most likely, one of the cleaners had an effect but not the other. If you are operating in the same waters and the fouling comes back, just using the one that worked before is OK. If neither cleaner helped or the performance dropped after both cleans then there is some other permanent damage to the membrane which may call for membrane replacement. Every cleaning is hard on the membrane so rarely does a cleaning bring back the performance 100%. It's important to follow the cleaning procedures outlined in your systems manual but here are things that can help. Warming the cleaning solution to 90 degrees F (32C) improves the cleaning. How to do that on a boat? It's usually not done. Letting the solution soak without circulation for an hour to two then circulate some more can sometimes help.
The best bet is to prevent fouling in the first place with regular flushing and pickling when not in use. Almost always, a membrane will last for years under regular use and proper storage but it's always possible that something in the feed water can take out a membrane even when we do our best. If planning a big trip to far off places, take a spare. They are not that expensive compared to the big picture.
For R.O. water purifiers over 750 GPD contact David in Support
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