Watermaker Pre-Filters

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pmyatt

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We are looking for an in-expensive(!) source of 5 micron 10" pre-filters for our new water maker. The agent wants (obviously) to sell us his special "branded" filters on the basis that they are the only reliable source! Being a natural cynic - I have my doubts as to the veracity of his statement. Would welcome advice on sources. Have been told that ordinary domestic water filters are not suitable as they cannot cope with salt water - don't know if this is really true.
 
Interesting!!! Am told by the salesman that the string/paper/plastic filters are for the photographic industry and that they fall apart very quickly if used in salt water. Additionally, have also been fed the usual standby of "Using any other filters will invalidate the warranty".
 
I suggest you buy a domestic filter for £2.50 ish and put it in a bucket of sea water for a few weeks to see what happens.

I'd have a look at the small print of the warranty to see whether it's true. Salesmen are not above telling porkies about things like that!

When we had our watermaker installed I asked where I could get replacement filters. The answer was 'they're standard domestic/industrial filters, you can buy them anywhere'.
 
We used string wound, pleated material, or solid plastic foam type on our watermakers, which make 75 tonnes per day fresh water at less than 350ppm TDS. They last about a month, so should be easily adequate for leisure use. There has been very little difference between the cheapest and most expensive!
We also use a multi layer sand/gravel pre-filter but at leisure use quantities you would be better advised using a net or bag filter - these can, despite manufacturers instructions, withstand a couple of washes to extend their life.
Please bear in mind that RO units do not make potable water, it still needs to be sterilised, we use standard domestic bleach to get about 3 -6 ppm.
 
Interested that you say that water makers do not make potable water. I was under the impression that the membrane would remove almost all bacteria and viruses. Nevertheless, as an added accessory we have a UV sterilization unit in the line before the tank so, hopefully, that takes care of any nasties that get through the membrane. Would not be keen to add chlorine to the product water as we pass it back through the membrane after we have finished making product water to help preserve the life of the membrane, and have been told by salesman that chlorine would definitely stuff it in very short order.
 
I was told that we should chlorinate product water to prevent it growing algae though I understood that to mean that as the water had no chemicals it wouldn't kill or inhibit the bugs already in the tank. I can't see how bacteria or algae could get through the membrane.

We tended to only make water we wanted to use straight away so didn't bother with chlorinating. I did once miscalculate the quantity of bleach to add and ended up with a tank of water that tasted like a swimming pool! It was also surprising how hard it was to find pure bleach - most of the bleach-based household cleaning products contain loads of other things you wouldn't want to drink.
 
Sorry, you are absolutely right, chlorine will 110% kill RO membranes (As will any oil in the feed), you need non return valves between pot water and the RO system. Totally fine to use product water for membrane flushing before it has chlorine added.
I have always assumed that biological contamination got through the membranes the same way as the 300 ppm salinity - in theory perfect RO product should have zero salinity but no seal or membrane is perfect.
On board we generally use Clorox for Pot water treatment, no adverse effects.....

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Looking at it from a theoretical point of view, the membranes pass H2O but (mostly) block Cl- and Na+. One would imagine they would block anything of larger molecular weight, e.g. a virus, not to mention unicellular organisms.

The likely cause of contamination is bugs already in the output pipework or tank which are not inhibited by untreated water.
 
Yes you're right but apparently it is possible (rare) to pass e coli or something nasty like it through the RO membrane. But let’s keep this in perspective, I think you would be hard pressed to find any water source that didn’t have these viruses, its a question of how many. If you keep your water system well flushed then it should be all right, and as you say, bugs in the pipe work will be a more serious threat than those that arrive with the water. Use as much water as possible (while you have a secure or assured supply) to keep the bug population down. This is as true on a boat as it is on land.

Remember the advice for legonella - run the tap before you shower, easy.
 
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