Reverse osmosis: what it is, how it works, and when it's really needed. Technical guide for a waste-free choice

Summary

Reverse osmosis is often cited as the ultimate water purification technology, capable of making almost any liquid potable. But is it always the right choice for the home, office or restaurant? In this technical article, we explain how the semi-permeable membrane works, analyse the pros (removal of heavy metals and pollutants) and cons (demineralisation, water wastage and acidity). DKR guides you in understanding this powerful technology, explaining when it is essential and when, instead, microfiltration may, in some cases, be a more logical and sustainable choice. In the vast landscape of water treatment, few technologies are as well-known and discussed as reverse osmosis. Often sold as a panacea for all water problems, it is sometimes commercially presented as the only way to have “pure water”. Unfortunately, the sector suffers from some retailers who adopt a non-neutral approach, seeking to sell the most expensive product instead of the most useful one; this harms the entire market and confuses the end customer, despite the efforts of trade associations such as AIAQ, Acqua Italia and Amitap to promote correct commercial ethics.

However, as we have often reiterated in our technical column, at DKR (which produces dispensing systems and not filtration systems) we do not believe in universal solutions. For this reason, we try to be neutral in the description and use of these technologies, availing ourselves of the opinion of external experts, such as Mr. Danilo Turola of the company Filtra, a business specialising in water treatment founded in 1999 with roots dating back to the 70s, now in its third generation and part of a group capable of covering the water treatment market 360°.

To respond, we must abandon the slogans and delve into fluid mechanics and chemistry.

What it is and how it works: physics beyond the filter

To understand reverse osmosis, one must first understand natural osmosis. In nature, if we bring two liquids with different saline concentrations into contact, separated by a semi-permeable membrane (which allows water to pass through but not salts), water tends to move from the less concentrated solution to the more concentrated one to dilute it and balance the salinity.

Reverse osmosis inverts this natural process by applying an external force.
Through a high-pressure pump, water with a high concentration of mineral salts is pushed against a synthetic membrane with very small pores (in the order of 0.0001 microns). The pressure overcomes the natural osmotic resistance and forces pure H₂O molecules to pass through to the other side, leaving behind almost all dissolved substances (salts, metals, bacteria, viruses).
The result is the separation of the flow into two paths:

1. Permeate: Purified (reverse osmosis) water with a very low fixed residue.
2. Concentrate (or brine): water loaded with pollutants and salts that is discharged down the drain.

The advantages: when the barrier is total

Reverse osmosis is, technically, the most advanced filtration method available at domestic and industrial levels. Its advantages are objective in critical situations:

• Removal of complex pollutants: it is extremely effective (removal rates ranging from 90% to 99%) against substances that standard activated carbon filtration struggles to retain, such as nitrates, arsenic, heavy metals (lead, mercury) and the much-feared PFAS (forever chemicals).
• Microbiological barrier: the pore size of the membrane is much smaller than that of any bacteria or virus, ensuring physical sterilisation of the permeate.
• Desalination: it is the only technology capable of making brackish water or water with excessively high conductivity drinkable.

Technically advanced solutions are available on the market today, such as the patented HDO and FLO reverse osmosis systems by Filtra, which are distinguished by their quick and simple maintenance, an aspect often underestimated when choosing a domestic system.

Limitations and critical issues: why “pure” doesn't always mean “better”

If osmosis is so powerful, why doesn't DKR recommend it indiscriminately? Because this technology has chemical and environmental “costs” that need to be carefully evaluated.

The paradox of demineralisation

Osmosis does not distinguish between “bad” and “good”. It removes arsenic, but it also removes calcium, magnesium, and potassium. The result is oligomineral water, poor in nutrients and with a tendency towards acidic pH. The World Health Organisation (WHO) has repeatedly warned that drinking only demineralised water for long periods can have contraindications, as it reduces micronutrient intake and can have an excessive diuretic effect. Furthermore, water without salts tastes “empty” or “flat”. For this reason, reverse osmosis systems must have a mixing system that allows the salinity of the permeate to be adjusted, or alternatively, a remineralisation system can be installed at the outlet of the osmosis unit to reintroduce only the ’good“ salts, such as calcium, magnesium, and potassium, into the water.

Water wastage

To produce one litre of osmotised water, reverse osmosis systems must discard water; traditional systems discard between 2 and 4 litres, although modern technologies now allow for recovery rates of up to 50% in new systems. In an era of water scarcity and a focus on sustainability, installing a system that discards 70% of the incoming water is a choice that must be justified by a genuine technical necessity (e.g. non-potable well water), but is ethically questionable when applied to mains water that is already potable and safe.

Chemical aggressiveness

As explained for softeners, water devoid of salts is chemically “starved” and aggressive. It tends to corrode the metals of pipes and tanks if not correctly remineralised post-treatment or by means of the salinity regulator.

Regular maintenance of filter cartridges is a crucial element in ensuring long-term performance: quality components, such as housings with nickel-plated brass heads manufactured by specialist companies, are now a standard adopted by the main operators in the sector.

The context of use: DKR's vision

So, when do we use reverse osmosis? The answer is: when the context demands it.

• Companies like Filter, Pioneers in the dissemination of under-sink reverse osmosis systems in Italy in the first decade of the 2000s, they helped make this technology accessible for both domestic and professional use.
• In the Horeca sector (café and washing): Here, osmosis is often fundamental, but for technical, not health, reasons. For professional dishwashers, osmosised water guarantees sparkling glasses without streaks (which are salt residues). In coffee machines, it allows precise control of the water profile for perfect extraction, even though “blending” systems are often used to reintroduce a small amount of minerals essential for the body of the coffee.
• In areas with problematic water: If water analysis detects the presence of nitrates above the norm (common in some agricultural areas), arsenic, or dangerous levels of sodium, osmosis is the essential safety solution to make the water potable.
• Zones with high salinity: There are areas where the public network does supply drinking water, but with high salinity that can sometimes exceed 500 mg/l of TDS. This high salinity makes the water unpleasant to the taste, therefore its reduction is recommended using a reverse osmosis system.
• Offices and homes with compliant aqueduct: theIn these cases, which represent the majority of Italian users, DKR usually recommends microfiltration or ultrafiltration. These technologies remove chlorine (which alters the taste), sediment, and bacteria, but leave the water's natural saline profile intact. This results in good, safe water that is rich in minerals beneficial to health and without water wastage (no wastewater).

Frequently Asked Questions (FAQ)

What exactly is reverse osmosis and how does it work?

It's a process that reverses natural osmosis by applying a high-pressure pump to force water through a semipermeable membrane (0.0001 micron). This separates purified water (permeate) from dissolved substances, such as salts, metals, bacteria, and viruses, which are discarded.

L'osmosi inversa è in grado di rimuovere una vasta gamma di contaminanti, tra cui: * **Sali disciolti:** Questo include il sodio, il cloruro, il solfato e il calcio, che sono i principali responsabili della durezza dell'acqua. * **Metalli pesanti:** Come piombo, mercurio, arsenico, cromo e cadmio. * **Batteri e virus:** Di dimensioni troppo grandi per passare attraverso la membrana semipermeabile. * **Pesticidi ed erbicidi:** Molti di questi composti chimici vengono efficacemente rimossi. * **Sedimenti e particelle:** Anche se spesso vengono utilizzati pre-filtri per rimuovere la maggior parte delle particelle più grandi. * **Alcuni contaminanti organici:** A seconda delle loro dimensioni molecolari. * **Fluoruro:** Spesso presente nell'acqua potabile. * **Nitriti e nitrati:** Comuni inquinanti agricoli. È importante notare che l'efficacia dell'osmosi inversa può variare a seconda della qualità della membrana utilizzata, delle condizioni operative (come pressione e temperatura) e della concentrazione iniziale dei contaminanti nell'acqua. Inoltre, l'osmosi inversa rimuove anche i minerali benefici dall'acqua, il che può essere un aspetto da considerare a seconda delle preferenze personali per la salute.

It is extremely effective (removing 90% to 99%) against complex pollutants that activated carbon filtration struggles to retain, including nitrates, arsenic, heavy metals (lead, mercury) and PFAS (forever chemicals). It also provides a complete microbiological barrier.

What are the limitations of water treated with reverse osmosis?

The main limitation is demineralisation: osmosis also removes useful minerals such as calcium and magnesium, resulting in nutrient-poor water with an acidic pH. The World Health Organisation has reported possible contraindications for exclusive long-term consumption.

Is it true that reverse osmosis causes “water waste”?

Yes. Traditional systems waste between 2 and 4 litres of water for every litre produced (although modern technologies allow for recovery rates of up to 50%). This water wastage must be justified by a genuine technical necessity.

When does DKR recommend microfiltration instead of reverse osmosis?

For the majority of users in Italy with compliant aqueducts, DKR recommends microfiltration or ultrafiltration. These remove chlorine and sediment, improving taste, but keep the natural saline profile intact, are more sustainable, and do not produce wastewater.

Personalised analysis and advice: contact the expert

Do you want to know if reverse osmosis is the right choice for your water?

At DKR, we don't sell universal solutions, but only the technology best suited to your real needs, supported by the advice of our experts.

Contact us today for a personalised consultation and discover the perfect dispensing system for you, without waste and without surprises.