Watermaker back flushing - is she right?

[Renx]

Like me, you've probably spend countless hours wondering about carbon filters and back flushing your watermaker

I've been pondering a re-build of the basic pre-filter system on my 20 year old Dessalator Watermaker - see attached image - and working out the best way to add a carbon filter for back flushing.

Then in the middle of my complex circuit diagram my wife suggested just swapping out the string pre-filter for a carbon filter when doing the back flush from the water tank.

Please tell me this is a bad idea so I don't have to admit I've wasted countless hours and copious amounts of paper

Cheers..

Woody
@mothershipadrift

[Tellie]

It looks like the Admiral is right. It would work but it will become a pain after a while doing this say every other day, plus the water spillage. I'd be willing to bet you'll be adding a charcoal filter housing to the mix fairly soon. I assume the plumbing you pictured is all going away in your rebuild? If it is, as it should (especially the thin walled copper piping) I would just go ahead and plumb in the charcoal filter. Any standard clear housing should work fine, maybe an extra $40 in expense, well worth the investment.

[jamhass]

As Tellie suggests, but be sure to plumb the charcoal filter on the fresh water side before the 3-way valve so raw sea water does not go through it.

[Tellie]

PS: No string wound filters. Use quality pleated filters.

PSS: "is she right?"

You must be a newly wed.

[waterman46]

We have a carbon filter in the output of our fresh water tanks, just before the water goes to the fresh water pressure pump. So ALL fresh water in the boat is carbon filtered. There is no issue with flow rate (we have the galley, two showers, fresh water on deck and a washing machine, sometimes two or more drawing water at the same time). Its just a standard size filter. If you simply carbon filter all your water as it leaves the tanks then you can obviously make any connection to that system that is convenient, for the backflush. Or maybe I just didn't understand your question.

[Hobiehobie]

Could be wrong, but you are not actually back flushing with fresh water. You are flushing in same direction as the seawater but just getting rid of any seawater from the membranes for short term storage. And the filter is not there to remove particulate matter but to remove any trace of chlorine in the water. Thus a regular particle filter will not work. Chlorine will ruin the membranes fast.

That is why it’s ok to wash the membranes with RO water you have made. But not good to rinse with freshwater you may have put in your tanks from a chlorinated domestic supply.

Others are probably more able to assist, but I think a ordinary household filter may be a quick path to shorten your membrane life span.

[Tellie]

Originally Posted by Hobiehobie

Could be wrong, but you are not actually back flushing with fresh water. You are flushing in same direction as the seawater but just getting rid of any seawater from the membranes for short term storage. And the filter is not there to remove particulate matter but to remove any trace of chlorine in the water. Thus a regular particle filter will not work. Chlorine will ruin the membranes fast.

That is why it’s ok to wash the membranes with RO water you have made. But not good to rinse with freshwater you may have put in your tanks from a chlorinated domestic supply.

Others are probably more able to assist, but I think a ordinary household filter may be a quick path to shorten your membrane life span.

[mvmojo]

Waterman46 has the right idea. Carbon filter the output of your fresh water pump and plumb it to a 3 way valve on the input of your RO seawater pre-filter. Flip the valve and you're flushing both your pre-filter and your membrane with carbon filtered water from your tanks. We flush every time we run the watermaker for 2-3 minutes. I'd recommend you also install a tee so you can easily winterize the watermaker or do an acid flush. BTW, the pink antifreeze makes a good preservative for the membrane and keeps it from freezing. Sodium metabisulfite, the conventional preservative, does not appreciably lower the freezing point of water and therefore does not act as an antifreeze.

[waterman46]

As others note, it is important that the pre-filter for any fresh water from a shore supply that is used to flush the membrane must be the carbon type. I found some good info at

purewaterproducts.com/articles/carbon

regarding removal of chlorine and chloramines by carbon filtering.

[SV THIRD DAY]

Originally Posted by mvmojo

BTW, the pink antifreeze makes a good preservative for the membrane and keeps it from freezing. Sodium metabisulfite, the conventional preservative, does not appreciably lower the freezing point of water and therefore does not act as an antifreeze.

I live PG as a water maker pickling preservative!!!!

[David B]

Originally Posted by Renx

Like me, you've probably spend countless hours wondering about carbon filters and back flushing your watermaker

I've been pondering a re-build of the basic pre-filter system on my 20 year old Dessalator Watermaker - see attached image - and working out the best way to add a carbon filter for back flushing.

Then in the middle of my complex circuit diagram my wife suggested just swapping out the string pre-filter for a carbon filter when doing the back flush from the water tank.

Please tell me this is a bad idea so I don't have to admit I've wasted countless hours and copious amounts of paper

Cheers..

Woody
@mothershipadrift

Ideally an additional filter housing for a dedicated carbon filter, but if too hard/no room, then a combination carbon/sediment filter cartridge.

Another way, of course is to only have RO water in your tanks - or filter through a carbon filter when filling at the dock. However then you have the potential issue of no chlorine/chloramine disinfectant in the tanks.

For us (also a Desalator), I use a combi cartridge in place of the regular string sediment cartridge. At some stage, with a bit of re-arranging I may find room for a second housing, but that's a future project. For the tank-water safety aspect, we use a Seagull filter under the galley sink for our drinking water. It is certified microbially safe, and the one airlines use, so good enough for us.