Reverse Osmoses.

[Alan Wheeler]

So what exactly is meant by the term "Reverse Osmosis" From what I have so far seen, a water maker simply pumps water through a series of ever decreasing particle size filters, till everything other than pure water is filtered out. Not kinda what I thought Osmosis was or reverse Osmoses for that matter.
Or are there two or more different types. If yes, then what type is better.
One thing I here in most all cases, is that the filters on water makers are expensive and don't last long in dirty water. But so far, this is the fastest means of making large quatities of water.

[Strygaldwir]

Osmosis is diffusion through a semi-permiable membrane across a concentration gradient. ie some types of membranes, like cell wall and such, will allow certain molecules to flow accross them until the ionic concentrations between the two sides of the membranes are equalized.

So if you stick a ballon that is made out of a semi permeable memberane and it is filled with fresh water in salt water. The fresh water will migrate accross the membrane until the concentration of salts are equal. I n this case until the ballon is empty.

Reverse osmosis is, well, the reverse. You apply enough pressure across the membrane and instead of fresh water leaching into the salt water, you end up with fresh water being pressured to the area of less concentration.

There are several types of filters, TFT and such, but they all work on similar principles. the membrane is spiral wrapped around a core, salt water is placed under pressure within the core. On the outer layers fresh water is the product. These membranes don't per se have small holes in them, unless you consider a hole to be the size that will let a water molecule through and not a salt molecule.

The membranes are sensitive to certain chemical polutants. Chlorines, chloromines, and petrochemicals. They will destroy the membrane. What is ususally done is the inclussion of pre filters to remove these contaminates. Usually you'll have something on the order of a 5 micron filter to remove most particulates then and additional filter of activated charcol to remove the chemical polutants. If you keep these two filters serviced and excercise proper care, the membrane can last a year or more.

I try never to use my when I can't see through the water. This helps. Oh, and by the way, the product water is VERY pure. Almost equivalent to distilled water in some instances it is more pure! So you may have to do things like take mineral supplements to get back some of the trace minerals you'll no longer get from your drinking water.

Keith

[Alan Wheeler]

Uh hu. so it is a little more complicated than just a fine filter. Thanks Keith

[capt lar]

Great explanation Keith. I worked with R.O. 15 years or more ago when building high end second homes on islands. The systems were expensive and maintenance was never ending. The technology was initially used in military and medical fields for creation of pure water, but when storing, we had to use ozone or other methods to keep bacteria from growing. The membranes would fail frequently and replacement cost was very high. I think Millipore was the primary leader in the field at that time.
I see boats for sale with water makers these days and they seem to be a matter of fact additional piece of equipment. Anyone wish to say who makes the best product these days and how much maintenance is required ? capt. lar

[Strygaldwir]

I did a bit of research before I purchased a RO filter for the boat, but I choose the Spectra 150. It is a very efficient water maker. It produces about 1 gallon of water for 1 amp of 12 volt power. It uses something called a Clark pump to perserve some of the energy that would be used to expell excess fluid to push the other side of a piston. This helps in maintaining pressure and results in a high degree of efficiency. It has an option that allows the pump to operate in almost a completely automated fashion. This optition , MPC, lets you tell the filter how much to run each day. It also controls backflushing the pump to keep the membrane free of debris and inhibit bacterial growth that can destroy the filter. Well worth looking at.