Membrane Filtration
Physical barrier for germs, particles and microplastics – without chemicals
Contents of this page
- 1. What is membrane filtration?
- 2. Filtration methods compared
- 3. How does a membrane work?
- 4. Pore sizes and retention rates
- 5. Membrane materials
- 6. Ideal application areas
- 7. Limitations and restrictions
- 8. Combination with other methods
1What is membrane filtration?
Membrane filtration is a purely physical separation process in which water is pressed through a semi-permeable membrane. The membrane acts like an extremely fine sieve: water molecules pass through while particles, microorganisms and other contaminants are retained depending on pore size.
Unlike chemical disinfection methods, membrane filtration does not change the chemical composition of the water. No substances are added – purification is achieved exclusively through mechanical separation.
Absolute vs. Nominal
With filter specifications, one distinguishes between:
Absolute filtration
100% retention of all particles above the stated size. Standard for sterile filtration.
Nominal filtration
Approx. 85-95% retention. More cost-effective, but not suitable for critical applications.
2Filtration methods compared
Depending on pore size, various filtration methods are distinguished. Each has its specific application area:
| Method | Pore size | Retention | Pressure |
|---|---|---|---|
| Microfiltration (MF) | 0,1 – 10 µm | Bacteria, yeasts, algae, sediment | 0,1 – 2 bar |
| Sterile filtration | 0,1 – 0,22 µm | All bacteria, protozoa, microplastics | 1 – 3 bar |
| Ultrafiltration (UF) | 0,01 – 0,1 µm | + Viruses, macromolecules | 1 – 10 bar |
| Nanofiltration (NF) | 0,001 – 0,01 µm | + Multivalent ions, pesticides | 5 – 20 bar |
| Reverse osmosis (RO) | < 0,001 µm | + Dissolved salts, minerals | 10 – 80 bar |
Size comparison for reference
Human hair
70 µm
Bacterium (E. coli)
1-3 µm
Virus
0,02-0,3 µm
Water molecule
0,0003 µm
3How does a membrane work?
A filtration membrane consists of a thin, porous layer with defined pore sizes. Water is pressed through the membrane (pressure filtration) or sucked through (vacuum filtration).
The separation effect is based on the sieve effect: particles larger than the pores are physically retained. This makes the process extremely reliable – it does not depend on chemical reactions or environmental conditions.
The retained particles accumulate on the membrane surface as a filter cake or are continuously removed in cross-flow filtration.
Filtration modes
Dead-end filtration
Water flows perpendicularly through the membrane. Simple, but filter cake must be regularly removed.
Cross-flow filtration
Water flows parallel to the membrane. Self-cleaning through flow, but more complex.
Absolute safety through physical barrier
Unlike chemical methods, membrane filtration does not depend on contact times, temperatures or concentrations. As long as the membrane is intact, all particles above pore size are retained 100%. This makes it particularly reliable for safety-critical applications.
4Pore sizes and retention rates
Sterile filtration with 0.1 µm pore size offers excellent retention rates for all relevant drinking water contaminants:
Retention rates at 0.1 µm
- Bacteria (E. coli, Legionella)>99,9999%
- Protozoa (Giardia, Cryptosporidium)100%
- Fungi and yeasts100%
- Microplastics (>0.1 µm)100%
- Sediment, rust, sand100%
Not retained
- VirusesPartially
- Dissolved chemicalsNo
- Pharmaceutical residuesNo
- HormonesNo
- Minerals, saltsNo
The 0.1 µm sterile filtration
The pore size of 0.1 µm (= 0.0001 mm) is the gold standard for bacteriological filtration. It was chosen because the smallest known bacteria (e.g. Mycoplasma) are approximately 0.2-0.3 µm in size. With 0.1 µm, there is an additional safety factor of 2-3 that guarantees reliable retention even with pore size variation.
5Membrane materials
The membrane material influences flow rate, service life, resistance and cost. The most important materials at a glance:
Polyethersulphone (PES)
Hydrophilic, high flow rate, chemically resistant. Standard for drinking water.
Polyvinylidene fluoride (PVDF)
Extremely chemically resistant, long-lasting. For demanding applications.
Ceramic
Highest temperature resistance, very long-lasting. More expensive but regenerable.
Hollow fibre
Very large filter area in a small space. Ideal for compact systems.
Performance indicators
0,1
µm pore size
= 0,0001 mm
99,9999%
bacterial retention
6-log reduction
1-3 bar
Operating pressure
normal household installation
6Ideal application areas
Sterile filtration is particularly suitable when reliable protection against microbial contamination without chemical additives is required:
Perfectly suited
- Point-of-use filtration – directly at the tap for drinking water
- Mobile applications – Motorhomes, boats, expeditions
- Legionella protection – especially with older installations
- Microplastic removal – increasingly relevant
- Immunocompromised persons – additional safety
Typical use scenarios
- Households with uncertain water quality
- Camping & motorhomes with changing water sources
- Boats and yachts with tank water
- Gastronomy and food processing
- Hospitals and care facilities
7Limitations and restrictions
Despite their excellent properties, membrane filtration has technical limitations:
No removal of dissolved substances
Membrane filtration only removes particles, not dissolved substances such as chemicals, pharmaceutical residues, hormones, pesticides or heavy metals. Activated carbon filtration is required for these.
Limited virus protection
Viruses (0.02-0.3 µm) are smaller than the 0.1 µm pores. For reliable virus protection, a combination with UV-C disinfection is recommended.
Blocking at high turbidity
Heavily contaminated water can quickly clog the membrane. Pre-filtration (e.g. 5 µm sediment filter) significantly extends service life.
No protection after the filter
Filtration only works at the point of application. Recontamination can occur in downstream pipes or tanks. For long-term protection, a combination with silver ion preservation is recommended.
8Combination with other methods
Optimal water treatment combines various methods to cover all types of contamination:
The WM Aquatec Multi-Barrier Principle
Activated carbon block
Removes dissolved pollutants: chlorine, pesticides, pharmaceutical residues, heavy metals
Sterile filtration
Removes bacteria, parasites, microplastics – absolute physical barrier
UV-C / Silver ions
Inactivates viruses, prevents recontamination in tanks and pipes
Conclusion: Membrane filtration as core building block
Sterile filtration with 0.1 µm is the centrepiece of any professional water treatment. It offers absolute safety against bacterial contamination with simple handling and without chemicals. In combination with activated carbon and UV-C/silver ions, comprehensive protection against all conceivable contaminants is achieved.
Discover more technologies
Learn more about the other technologies that make WM Aquatec products so effective.