Reverse Osmosis Explainer
What they don’t tell you about RO solutions.
What is reverse osmosis?
Originally developed in California in collaboration with the State of Israel to turn saltwater into drinking water, reverse osmosis (RO) is absolutely useful in technical applications under controlled conditions. Its use in the home, however, raises questions.
How it works
A differential pressure between the inlet and clean water side forces the water through the RO membrane..
What they don’t tell you about RO solutions
The RO membrane is made of polyamide, which is polymerized on a porous support material in such a way to create a large surface area. The filter structure includes plastic supports, perforated pipe, and a curtain-like spacer that allows water to flow between the layers of the membrane.
That’s an awful lot of plastic offering bacteria-loving biofilm plenty of room to grow—even on the clean water side.
Moreover, defects in the polyamide layer and trapped dust particles in the membrane coating allow for pathogens and particles to pass through the RO membrane itself.
This means that while RO is effective at catching ions it is not a great solution for filtering dissolved organic matter. Depending on the type of pesticide or other dissolved organic matter, the filtration rate is only between 20% and 80%.
RO and acidic water
Another problematic effect of RO is a fundamental change in your tap water’s equilibrium.
Cold tap water is usually in chemical equilibrium, it is neither acidic nor caustic and does not attack surfaces, tissue or mucous membranes. When driving water through a Reverse Osmosis, water changes its pH and becomes aggressive and acidic. In large public treatment systems, this is corrected by correcting the filtered water’s pH value again. In domestic units, this step is missing.
RO and water wastage
RO’s dirty little secret is that it is incredibly inefficient in the residential setting. Conventional reverse osmosis systems create just over a gallon of waste water to produce a quarter gallon of filtered water. If each person consumes an average of one gallon of water a day, a four-person household would produce 17 gallons of waste water a day to meet their daily drinking requirement. That’s enough to almost fill a bathtub.
Does reverse osmosis remove pathogens?
RO was developed as an industrial application. It’s use in the home presents significant challenges, including the formation of biofilm on the RO membrane.
Bacteria smaller than 0.5 microns are smaller than the standard carbon filters used as pre filters in many compact, residential RO systems. This means this bacteria can pass directly to the membrane. Once they do, they find a concentrated supply of nutrients—an ideal environment for biofilm to develop.
The biofilm bacteria produces EPS substances (aka: slime), which then provides even more nutrients on which bacteria colonies can thrive and which, in turn, accelerate the transport of solutes and other pathogens through the membrane.
But even worse: The way that Reverse Osmosis membranes are produced and its filters are glued and welded together leaves a lot of micro-holes in the readily assembled filter. Enough that RO membranes are not rated as a disinfection device by drinking water legislators - this means that pathogens do pass through the filters!
Substances RO effectively address
- Hardness (calcium and magnesium)
- Salts (sodium chloride and other)
- Metal ions
Substances RO does not effectively address
- Endocrine disruptors
- Trace pharmaceuticals
- Viruses and bacteria
- Chlorine, which destroys the membrane