Reverse Osmosis

Osmosis is the process of a liquid flowing through a (semi-permeable) membrane. If you have a container with a membrane dividing it into two sections, and you put pure water on one side and salt water on the other, the osmotic pressure differential between the two sides will force water from the pure side into the salt water side. This is because osmotic pressure will try to equalize the salt concentration in the two sides, drawing pure water into the salt water in an attempt to dilute it.

Reverse Osmosis is the process of applying physical pressure to contaminated water in a container that has a semi-permeable membrane as part of its construction. This physical pressure will overcome the osmotic pressure and will force pure water through the membrane, but the contaminants (salt etc) will be filtered out by that same membrane. Depending on the pressure used and the construction of the membrane, reverse osmosis (RO) systems can clean water of everything down to the molecular level.

The Good

Membrane filtration has become more common in the last few years. There are several types of membrane filtration but the main kinds of pressurized membranes are microfiltration, ultrafiltration, nanofiltration and reverse osmosis. The following chart from Wikipedia gives a good idea of what each level of filtration is capable of.

"Cut-offs of different liquid filtration techniques" by Peter in s - Own work.
Licensed under Creative Commons Zero, Public Domain Dedication via Wikimedia Commons

Since some people aren't familiar with the metric system, I'll break down the micrometer scale on the top.

• From the right-hand side: 1000 micrometers is the same as 1 millimeter. 1 thousandth of a meter is about the size of a grain of sand.
• In the middle: 1 micrometer (often called a micron) is 1 millionth of a meter, slightly smaller than a red blood cell. You'd need a good microscope to see something this small.
• On the far left-hand side: 1 nanometer, or 0.001 micrometers. 1 billionth of a meter. Simple molecules and large atoms are found in this size range - break out the electron microscope and you might be able to see things at this level.

As you can see, RO is about the best form of filtration available. It is so good at filtering water that it will take the salt out of seawater. 

The Bad

There are downsides to increasing levels of filtration. however. The main problem is that the smaller the pore size in the filter, the higher the pressure needed to force the water through it. Here are some general examples:

• “Standard” filter: 0 to 1 bar (0 to 15 psi). The pull of gravity will exert this much flow pressure.
• Microfiltration: up to 2 bar (up to 30psi). Easily hand pumped; gravity if the filter is large.
• Ultrafiltration: 1 to 10 bar (15 to 150 psi). Hand pump pressure. 
• Nanofiltration:  3 to 20 bar (45 to 300 psi). Mechanically pumped, or by hand with slow/low output.
• Reverse Osmosis: 10 to 80 bar (150 to 1200 psi) Mechanically pumped, or by hand with very slow/low output.

Other downsides are cost and life:  The higher the level of filtration, the higher the cost of the filter and the shorter the life of the filter. Using coarse pre-filters to strain out the larger chunks will extend the life of your RO membrane.

The biggest downside to RO filters, though, is the simple fact that they are not something that you can make at home. The membranes have such tiny pores that there is no way to produce them (reliably) outside of a factory. If you do choose to get an RO system, purchase spare filter cartridges and store them carefully.

What to Buy

For about $200, you can get a filtration system that fits under your sink and will provide 50 to 75 GPD (gallons per day) of water cleaner than the stuff you buy in bottles. Here's an example:

This unit provides 5 different filters that will remove just about anything you may find in your water. The downside is that it requires water pressure of at least 45 psi, or a booster pump which will require electricity. 45 psi is possible from a city water supply, or you could use a water tower that is at least 105 feet higher than your filter (0.433 psi per foot of height).

If you need something more portable, this unit is designed to be used to desalinate sea water and is small enough to be carried in a lifeboat. It costs about $550.00 and will put out about 1.2 gallons per hour of hard hand-pumping. That's about enough for one person for a day, so you'd need to set aside an hour just to purify your water every day.

If you are providing water for a community or responding to a disaster and have the money, there are units that will put out clean water in large quantities. This one will put out 10,000 GPD (415 gallons per hour, or about 7 gallons per minute) but requires a 220V AC power source. It only costs $110,000.00...

The military uses similar units mounted on a flatbed trailer (with its own generator for power) to provide clean drinking water for troops in the field. A personal friend of mine operated one in Iraq during the second Gulf War. The main problem he had was the fact that the waste stream was so contaminated that they couldn't discharge it back into the river that they were drawing water from. They had to form lagoons in a field using a plastic lining and let the waste evaporate, then treat the sludge as toxic waste.

In conclusion, if you need clean water, a Reverse Osmosis system will give you the cleanest water available. but at the cost of money and the likely need for electricity.