Reverse Osmosis and Membrane Filtration


What is commonly called reverse osmosis, or RO, is a catch all term for technologies in which contaminated water is forced through a membrane with very small holes. Contaminants remain on the inbound side of the membrane and pure, or purer, water is pushed to the reverse side.

Because contaminants come in a variety of sizes and because it takes more energy and finer membranes to get every contaminant, membranes of several sizes have been created. The less energy needed and the larger the holes in the membrane, the lower the cost. It makes no sense to use more energy or a more expensive membrane than is needed to do the job.

In a sense, the membrane selected is “right sized” for the contaminant sought to be addressed. For example, some particles are larger than others. A grain of sand needs a less fine membrane than does salt dissolved in the water. Likewise, bacteria are larger than viruses. So, if the goal is to rid the water of bacteria alone, a coarser membrane using less energy is possible. With vast amounts of water needing to be purified, the lower the energy load, the better for the environment and for budgets.

To be sure, at all levels of filtration, the holes in the membrane are incomprehensibly small, but the difference between the largest – called microfiltration, or MF – and the smallest – called reverse osmosis – is significant.

The four levels of filtration, the approximate pore size, and how each is most commonly used is:

  1. Microfiltration (MF) – for removal of particles, some bacteria and some protozoa [membrane pores are 1/254,000 of an inch]

  2. Ultrafiltration (UF) – for removal of all bacteria and protozoa, and some viruses [1/2,540,000 of an inch]

  3. Nanofiltration (NF) – for removal of all viruses and also to improve the taste profile of the drinking water [1/25,400,000 of an inch]

  4. Reverse Osmosis (RO) – for removal of all inorganic contaminants regardless of size [1/254,000,000 of an inch]

RO is most commonly used in desalination as the membrane can separate out even dissolved salt and minerals found in the water. In all of these systems, the removal of the microorganisms means that less chemical treatment is needed in the water to kill off bacteria, protozoa and viruses.

Seth Siegel