Idiot's Guide to Watermakers, Please

[Tellie]

Originally Posted by anjou

Sadly i dont have a pic of the unit.
Im sure you may be right about it being a fresh water unit as it was found installed inside a large GRP tank that was cracked and therefore in the dumpster.

I will make some enquiries, is there a good way to check what type it is?

If you can get the numbers off the membranes themselves I can tell you what it is. You may get lucky but what you described sure sounds like a fresh water unit and was probably in the dumpster for good reason. Even if it were a salt water unit sometimes what it costs to repair may make it not worth the effort. But I'm keeping my fingers crossed for you. I sent you a PM as well.

[Tellie]

Originally Posted by TAREUA

What would be the difference in an R/O membrane for a salt water system or a fresh water system?

Price!
Other than that they can look alot alike. Salt water RO systems require a lot more pressure to begin the RO process than fresh water and generally produce more water in a given time than a fresh water unit. The membranes are not interchangable and a fresh water unit will not do what you are looking to get done on a boat.

[Strygaldwir]

Almost all the Salt water (SW) type Reverse Osmosis (RO) filters require upward of 500 Pounds per Square inch of pressure. There is no DC motor, of a reasonable size, that will generate sufficient pressure for salt water RO by themselves. There are AC pumps that can do it. There are mechanical pumps which can readily do it. All the other DC solutions use a form hydraulic intensifier to get sufficient pressure (flow requirements are there too). So, most likely, that's not a SW unit, or a significant piece is missing.

Couple of additional points. Desalination makes a lot of sense when you're moving, cruising. It makes far less sense when you're sitting at a dock and water is available from a hose nearby, even if you have to do extensions! Second, it may not be the water that is insufficient for 30 gallon showers, it may be the available hot water. 12 gallons of hot water is rather large for most mid size boats. 8 gallons is fairly normal and even 6 gallon units are very common. After you use about half the tank, you start to find that you need a much higher % of hot water to cold. This means you have even less water available for the shower. So, in most cases a 10-16 gallon hot shower is going to be at or around the max. When my wife washes her hair, that's about what she uses.