Thursday, 30 January 2014

Nanofiltration: Soften Water Without The Salt | Membranes

Many of us have a conventional water softener in our homes easily identifiable by the plastic bin requiring the addition of salt from time to time. In fact conventional softeners using ion exchange resin have been with us for decades now.  Some of us also have a reverse osmosis drinking water system sometimes installed to lower the sodium content of our water that was caused in part by our water softener.  The technology we are discussing today is a relative of the reverse osmosis system called nanofiltration.  An example of what a nanofiltration system looks like can be seem at the following link.

Nanofiltration System

The following video describes how the membrane material is made.


If you watched the video you would have possibly noticed the membrane material is about 150 microns thick in total.  this is equivalent to three pollen grains stacked on top of one another.  This thickness is also about 1 and 1/2 human hairs thick.   Most of this material is support for the working layer of material that is only 0.2 micron thick.  This is just a little thinner than the largest known virus.

This material is processed to generate tiny pores through which water will pass leaving the dissolved salts behind.  The holes in the nanofiltration membrane are slightly larger than the ones for reverse osmosis or about 1/4 to 1 nanometer in diameter.  This would be equivalent to 1 to 4 water molecules wide.  This small size would exclude most contaminants in water by shear size alone but the holes also have charge properties that can reject molecules that would be small enough to fit through.

Now that w have the material to make the membrane we must find a form that allows us to utilize its unique properties. After many stages of development including flat plates, tubes, etc. one shape seams to dominate the current market.  The following video produced again by Dow Chemical's Filmtec shows how this format is made and how it works.


The spiral would configuration is now the most dominant and practical configuration known today.  One other difference between reverse osmosis membranes and their nano counterparts is the gap or separator in nanofiltration is usually larger to allow for greater fouling resistance.

The following picture shows us what a spiral  wound membrane looks.
Next week we will talk more about how membranes work and how we use them in nanofiltration systems.
In the mean time please visit us at our website purifiner.com


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