Reverse osmosis as “overkill" in home drinking water treatment?

There are water resources that are of such poor quality that even ultrafiltration cannot improve them satisfactorily. This is not usually the case in Germany, but it is in many (coastal) regions around the world. The water then contains a high proportion of dissolved solids, e.g. substances such as minerals, salts or metals. This is referred to as a high TDS value (TDS = Total Dissolved Solids). According to the very strict US EPA’s water quality recommendation, drinking water is safe to drink up to a TDS concentration of 500 mg/L. In contrast, water with higher dissolved solids concentrations can taste bitter or salty (seawater), is actually undrinkable and could potentially cause health effects like electrolyte disorders.

Reverse osmosis is a treatment process originally developed to desalinate seawater. It removes not only solids but also dissolved organic matter, salts (ions) and metals from drinking water. Under pressure, tap water is forced through a semipermeable membrane through which only the water molecules themselves pass. Reverse osmosis removes all pathogens and pollutants such as heavy metals, nitrates or pesticides from the water. It thus offers a very high degree of purification and is superior to other treatment processes.

However, the technology also has some disadvantages:

  • All valuable ingredients such as minerals and salts are also removed, leaving only the water molecules (pure H2O, “osmosis water”). The osmosis water is somewhat similar to distilled water and must be remineralized again, i.e. artificially enriched with minerals, because otherwise it not only tastes bland but is also harmful to health (electrolyte disorders) if only pure osmosis water is drunk.
  • Producing one liter of osmosis water usually takes much longer (5 to 10 minutes) than with ultrafiltration, for example.
  • RO systems are able to filter only a fraction of the input water as purified while rejecting the rest as it contains all dissolved solid removed from the purified parts due to the operating mode (crossflow). Residencial RO  requires to reject about 1 to 3 times the amount of purified water produced.
  • Due to a lowered pH value, not remineralized osmosis water is acidic and thus aggressive/corrosive.
  • For residential use RO can only be installed efficiently in one tap at a reasonable price. Centralized house RO system are expensive and demand a considerable amount of space to operate.

Tap water makes a relevant contribution to the mineral supply, especially with calcium and magnesium. These minerals are present in water as free ions and can therefore be absorbed by the body more easily than from food. There are now numerous studies confirming the negative health consequences of demineralized water, including a higher risk of cardiovascular disease, cancers, certain neurodegenerative diseases, and pregnancy complications.

Reverse osmosis as “overkill”

The high consumption of energy, tap water, and the time required, as well as the need for remineralization, really justify the use of reverse osmosis only when there is no alternative for treating local water for the reasons described above. For all other cases,  reverse osmosis makes  sense only for technical applications and under controlled conditions, for example in medical technology and industry where pure water (distilled or RO water) is needed, as it is  an overkill as a filtration method for drinking water at home. It involves an unnecessarily high technical effort and cost – takes a sledgehammer to crack nuts”.

Ultrafiltration is perfectly adequate for drinking water treatment in most cases. As a purely mechanical filtration process using a hollow fiber membrane, ultrafiltration reliably retains all suspended solid constituents and turbidity from the drinking water that are too large to pass through the particularly fine pores of the filter medium (ultrafiltration with a defined pore size of approx. 0.02 μm). More than 99.99999% of all bacteria, Legionella and parasites and 99.99% of all viruses are  removed, as are suspended solids, particles such as microplastics and turbidity. Substances dissolved in the water, such as pharmaceutical residues, pesticides and heavy metals, cannot be filtered out directly by ultrafiltration, but can be removed by in a prefiltration stage by a dedicated media filter (Seccua Biofilter).

In the most cases, but the ones were the water suffers from dissolved solid contamination, Ultrafiltration is a superior choice over reverse osmosis, offering  a similarly high degree of purification , but without removing valuable ingredients such as minerals, salts from the drinking water preserving the natural water taste and health benefits while being environmentally friendly requiring a fraction of the electricity and waste water (only 10 to 15 liters a day for flushing) an RO would require. Additionally, Ultrafiltration systems can be installed in a PoE of the house offering protection to the entire house and not only one tap without requiring complicated installation.