RO Mystery-Need Help

From my experience conventional reverse osmosis systems are not as effective as they should be. As an example, my water has a TDS (total dissolved solids) of 700 ppm, which is basically the impurities in the water. After running a conventional reverse osmosis system, my drinking water was about 200 ppm. This obviously did a good job at filtering, but could be much better.

After looking at my reverse osmosis unit I discovered two weaknesses in the system. The first one I found was that the unit has a automatic valve that starts filtering water when the storage tank water pressure (the low pressure side of the membrane) is less than 60% of the incoming water pressure (the high pressure side). I noticed that if I would drain my storage tank empty and then let the reverse osmosis unit fill up the storage tank my water was initially at 40 ppm, but as time went on it would gradually raise to 200 ppm. This happens because when the reverse osmosis unit is turned off (when the tank pressure is 60% of the incoming water pressure) osmosis occurs in the membrane housing. Basically what happens is that you get about 2 cups of contaminated water on the clean side of the membrane. When I use my reverse osmosis unit I usually only use a few cups at a time (this water comes from my storage tank). When I do this it lowers the pressure in the tank and turns the reverse osmosis unit on, pumping the contaminated water into my storage tank. It just gets the contaminated water out of the membrane housing, and the tank is now filled back up. This happens over and over again until the storage tank water is not very clean.

The second problem with conventional reverse osmosis units is that the incoming water pressure is not great enough to produce ultra pure water. If your incoming water pressure is 35psi, your revers osmosis system will fill your storage tank to about 21 psi before shutting off. This only gives you 14 psi pressure difference across your membrane, which will just barley overcome the osmotic pressure, and will not produce pure water. With 30 psi across the membrane my water was getting very pure water (about 25ppm), but to get this pressure across the membrane you will need about 75psi in your incoming water.

The solution to these problems are not too difficult. To solve the first problem, I use a pressure switch to tell the reverse osmosis unit when to start filtering and when to stop. I also replaced the normal 60% shutoff valve with electric valves so the turn on when the pressure in the tank gets low, and shuts off when it gets full. This means it will not turn on after every cup of water you use, I also setup a valve to drain all the contaminated water in the membrane housing before it starts filling the storage tank. This way you will only get pure water in the tank.

The solution to the second problem was a little easier, I just added a booster pump. I am now boosting mine to 100 psi, and I fill my storage tank up to 30 psi. So I am getting 70 psi across my membrane, and producing water with less than 10ppm in my storage tank!!! This is from water with over 700ppm!!