Reverse Osmosis Water Filters: How the Membrane Works – and What to Look Out For

Although tap water in Germany is strictly controlled, it often contains substances that many would prefer not to have in their glass: limescale, nitrates, microplastics, or even PFAS. One of the most effective methods to reduce these substances is reverse osmosis – a technology originally from space travel and now used in modern water filters.

Summary:
In reverse osmosis (RO), tap water is forced under pressure through a semi-permeable membrane. Pure water (permeate) passes through the membrane, while dissolved substances such as ions, metals, PFAS, particles, and most germs are retained (concentrate).

 

How does reverse osmosis work – in 60 seconds?

• Osmosis: Normally, water flows from the side with few dissolved substances to the side with more dissolved substances – until an equilibrium is reached. The driving force for this is the so-called osmotic pressure.
• Reverse Osmosis: Here, we simply reverse the process. Tap water is forced against the membrane under pressure.
   
  • Pure water (permeate) passes through the fine pores.
    
    
  • Unwanted substances (concentrate) such as salts, nitrates, metals, or germs are retained and discharged.

This is how you get clean drinking water – directly from the tap.

 

 

The typical filter stages – logically structured

A modern reverse osmosis filter works in several steps. Each stage has a clear task to ensure that truly clean water ends up in your glass:

1. Pre-filtration (Sediment & Activated Carbon)
   Here, coarse particles like rust, sand, or limescale residues are captured. At the same time, the activated carbon binds chlorine and organic substances – this protects the membrane and already ensures better-tasting water.
   
   
2. RO Membrane (Heart of the System)
   The semi-permeable membrane allows almost only water molecules to pass through. Dissolved ions, metals, nitrates, PFAS, bacteria, and viruses are retained. This stage makes the real difference compared to simple activated carbon.
   
   
3. Post-filtration (Activated Carbon Polish)
   Small residual traces are filtered out here. This stage also improves taste and removes odors – for clear, fresh water.

💡 How NaDeex makes it easier:
The NaDeex Countertop Direct Connection Filter combines these stages in a smart system:

• 2-in-1 Composite Pre-filter (Sediment + Activated Carbon)
• RO Membrane
• Antibacterial Carbon Block as Final Stage

This means for you: fewer cartridge changes, longer membrane lifespan, and consistently fresh water – without a tank and without refilling.

 

What does reverse osmosis realistically filter?

Reverse osmosis is one of the most thorough methods for purifying tap water. The membrane acts like an extremely fine sieve that allows almost only water molecules to pass through.

• Dissolved Ions & Metals
  Substances like sodium, nitrate, or lead are significantly reduced because they cannot pass through the tiny pores of the membrane.
  
  
• PFAS ("Forever Chemicals")
  Studies show that reverse osmosis reliably removes both short and long PFAS chains. Activated carbon alone can vary depending on water chemistry – here, RO is significantly superior.
  
  
• Particles & Microorganisms
  Bacteria, viruses, and microplastics remain on the membrane. The post-carbon filter removes any remaining traces and simultaneously ensures better taste.
  
  

💡 Why this is particularly relevant now:
From January 12, 2026, a new limit will apply in the EU: a maximum of 0.1 µg/L for 20 PFAS combined. So, if you really want to ensure drinking water quality, you can hardly avoid RO.

 

TDS: The one number you should know about reverse osmosis

The TDS value ("Total Dissolved Solids") describes how many substances are dissolved in the water – measured in ppm. Simply put: the higher the TDS, the more "invisible" residues such as salts, nitrates, or metals are in the water.

In the lab, TDS is determined by evaporation. In everyday life, a small TDS meter is sufficient: It measures electrical conductivity, which directly depends on the dissolved ions.

💡 By the way: The NaDeex water filter already has an integrated TDS meter. You can always see on the display how pure your water is – without any additional device.

What is a good TDS value?

• Tap water: often 150–800 ppm depending on the region.
  
  
• Reverse osmosis water (permeate): significantly lower, usually two-digit.
  
  
• Goal: consistently low, not necessarily "0". A small amount of minerals can even make water taste more pleasant.
  
  

Why tankless is smarter

• Hygiene: No standing water = lower risk of germs.
  
  
• Convenience: No more refilling canisters, water comes directly from the tap.
  
  
• Consistency: Always fresh permeate, consistent taste and TDS value.

💡 This is exactly where NaDeex comes in:
The countertop direct connection filter combines a pre-filter, RO membrane, and antibacterial carbon block. This gives you consistently clean water – without a tank, without effort.

How do I check the quality?

• TDS meter: Measure before/after – if the value drops significantly, the membrane is working.
  
  
• Taste: Less "salty-mineral," coffee & tea unfold their aroma more clearly.
  
  
• Maintenance: Change pre-filter on time, replace membrane according to manufacturer's interval – then performance remains high.

 

 

FAQ – the most frequent questions about reverse osmosis

❓Does RO remove all minerals – is that unhealthy?

No. You get most minerals from food, not from water. A low TDS value is primarily relevant for taste and limescale protection. If desired, the water can be slightly remineralized – it's usually not necessary.

❓Does RO help against PFAS ("forever chemicals")?

Yes. Studies show that reverse osmosis removes PFAS significantly better than simple activated carbon filters. Membrane quality, maintenance, and water chemistry are crucial.

❓Which number do I need to keep an eye on?

The TDS value. Simple measuring devices immediately show you how thoroughly the water has been purified. Also important: filter changes according to the interval, so that performance remains constant.

Conclusion: Reverse Osmosis – clean water, made easy

Reverse osmosis is one of the most effective methods to free tap water from harmful substances, limescale, and even PFAS. Crucial factors are the right membrane, a well-designed filter system, and easy handling in everyday life.

This is exactly where NaDeex comes in: The countertop direct connection filter combines 2-in-1 Composite (Sediment + Carbon), RO membrane, and antibacterial carbon block in a tankless system. With an integrated TDS meter, you can see the water quality at a glance at any time.

👉 Simply set it up, connect it, and test it for 14 days risk-free – after that, you won't want to drink anything else.

To the water filter: NaDeex Water Filter