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Word of the Week: RODI Filtration


RO/DI filtration stands for reverse osmosis deionization filtration. At its most simple level, an RO/DI filter removes contaminants from water, resulting in pure H2o.  But how does it work?

Most RO/DI filters have four unique stages of filtration. Let’s start with stage 01, sediment filtration.

MD Advanced 4-Stage KleanWater RO/DI Filter

I) Sediment Filter

Sediment Filter

The first stage is a sediment filter, which removes visible particulate matter.  It is just like a sponge filter in any aquarium… it lets water pass through, but catches large suspended particles like food, waste, and debris. A standard 1.0 micron filter will remove particles greater than 1 micron, while a 0.5 micron filter is more dense and thus filters out even smaller particles.

II) Carbon Filter

Carbon Filter

The second stage is a carbon filter, which while also removing visible particulate matter, goes beyond to remove chlorine, volatile organic compounds (VOCs), odors, and unpleasant tastes from the water.

Activated carbon is charcoal that has been treated with oxygen to create a wide open pore structure between the carbon atoms. It uses a filtration process called chemical adsorption. Basically, as water passes through the highly porous charcoal, certain impurities become trapped/chemically bonded to the carbon. While carbon filtration is great at removing chlorine, VOCs, odors, and tastes from the water, it is not effective at removing dissolved compounds, salts, or minerals, hence the need for further filtration in an RO/DI system.

III) Reverse Osmosis

Reverse Osmosis Membrane

The third stage, reverse osmosis, is a bit more confusing to understand. First off, we need to understand what osmosis is.  Simply defined, osmosis is the process whereby water molecules travel through a semipermeable membrane from an area of lower to higher solute concentrations.  Huh?  The easiest way I understand this is to think about human sweat.  How does sweat pass through your skin?  Well, through osmosis.  A little bit of salt is deposited in your sweat glands, and through osmosis, water passes through your skin (a semipermeable membrane), and toward the area with a higher salt concentration.

Reverse Osmosis in the Human Body
© My First Fish Tank 2020

Why does water act this way?  We can think about it in two ways: physical size and ionic charge.  First, if we visualize the semipermeable membrane, the openings are only big enough for water molecules to pass through, so larger molecules are stuck on one side or the other. 

Physically speaking, if a solution only contained pure H2O, those water molecules would be zooming back and forth between the membrane in equal amounts.  But when you add a larger solute to one side of the semipermeable membrane (something like salt), those larger compounds could block the semipermeable membrane from time to time. Thus the smaller water molecules would not be able to travel through that membrane.  Not only that, but the water molecules will be bumping into the larger salt compounds, thus reducing their overall chance of passing through the membrane.  Both of these factors result in a net movement of water from the side without salt to the side with salt.

© My First Fish Tank 2020
© My First Fish Tank 2020

The second way to understand osmosis of water molecules is through ionic charges.  Salt, when dissolved in water, breaks down into a positively charged sodium ion, and a negatively charged chloride ion. Water is a polar molecule, meaning there is a partial negative charge on the oxygen atom a partial positive charge distributed between the two hydrogen atoms. So, the oxygen atom in a water molecule is attracted to the positively charged sodium ion, while the positively charged hydrogen atoms are attracted to the negatively charged chloride ion.  Thus, there is again a net movement of water molecules through the semipermeable membrane from the side without salt to the side with salt.

© My First Fish Tank 2020
© My First Fish Tank 2020

So if you think about this relating to purifying water, osmosis wouldn’t do a thing.  Because the natural tendency of water is to move toward larger ions, molecules, and particles.  So you would just be making “dirty” water.  

© My First Fish Tank 2020

Reverse osmosis applies external pressure to reverse this process, forcing the smaller water molecules through a semi-permeable barrier, thus separating water from the larger ions, molecules, and particles.  Reverse osmosis produces purified water, because only H2O can pass through the semi-permeable membrane. Reverse osmosis membranes can remove up to 99% of impurities in water, such as salt, particulate matter, lead, and many other chemicals.

© My First Fish Tank 2020

IV) Deionization Resin

Last up is deionization resin, which compared to reverse osmosis, is a bit easier to understand. By the time your water reaches the di resin, it is likely 99% clean, but not quite there yet. Whereas an RO membrane is a kind of mechanical filtration, di resin is a chemical filter. Deionization removes both organic and inorganic ionized minerals and salts through a process of ion exchange.  Basically, di resin removes total dissolved solids, or tds, from the water by use of ion exchange resins. So how does this work exactly?

© My First Fish Tank 2020

Di filtration contains both a positively charged cation resin and a negatively charged anion resin.  Let’s say there is still a little bit of dissolved salt (na+cl-) in your ro product water.  As that water passes over the positively charged cation resin, the positively charged sodium ion is exchanged for a hydrogen ion (H+). And when that same water passes over the negatively charged anion resin, the negatively charged chloride ion is exchanged for for a hydroxyl ion (OH-).  The hydrogen and hydroxyl ions join back together forming pure H2O.

For example, the positively charged cationic resin attracts the positively charged ions in water such as calcium, magnesium, and sodium, while the negatively charged anionic resin attracts those negatively charged ions such as fluoride, chloride, and sulfate.

Episode 04’s word of the week, RODI filtration