The Role of Absolute Filters in Producing Ultrapure Water

In the world of laboratories and high-precision industries, ultrapure water (UPW) is a cornerstone for achieving reliable and reproducible results. Whether it’s in molecular biology, semiconductor manufacturing, or pharmaceuticals, the quality of ultrapure water must be uncompromised. This is where absolute filters come into play, acting as critical components in the water purification process.


What Are Absolute Filters?

Absolute filters are precision-engineered filtration media designed to remove particles, microorganisms, and impurities down to a specific, guaranteed pore size. Unlike nominal filters, which may allow small percentages of particles to pass, absolute filters guarantee retention of contaminants at or above their rated pore size, typically ranging from 0.2 microns to as small as 0.01 microns.

This performance is essential in ultrapure water systems, where even the smallest impurities can compromise results.


Key Purposes of Absolute Filters in Ultrapure Water Systems

  1. Removal of Fine Particles and Microorganisms
    Absolute filters are typically used as a final barrier in the water purification process. They efficiently remove fine particulates, bacteria, and even endotoxins, ensuring water meets the stringent purity requirements for laboratory and industrial applications.
    • Example: A 0.2-micron absolute filter can effectively eliminate bacteria such as Pseudomonas aeruginosa, a common contaminant in water systems.
  2. Safeguarding Downstream Processes
    In ultrapure water systems, components like deionization resins and reverse osmosis membranes require protection from fouling and clogging caused by particulates. Absolute filters act as guardians, prolonging the life of these critical components and ensuring consistent performance.
  3. Critical for Sterile Applications
    Absolute filters are indispensable in applications requiring sterility, such as pharmaceutical water systems or lab environments for cell culture. Their reliability ensures that no microorganisms are introduced into sensitive experiments or manufacturing processes.
  4. Achieving Consistent Water Quality
    The removal of sub-micron particles ensures that ultrapure water meets the strict standards set by organizations like ASTM International, CLSI, and ISO. This consistency is vital for industries where deviations in water quality can lead to product defects or failed experiments.
  5. Supporting Environmental Responsibility
    By enabling the removal of contaminants at such fine levels, absolute filters help reduce the reliance on chemicals for water purification, supporting more sustainable practices in laboratories and industries.

Advancements in Absolute Filter Technology

Modern absolute filters are designed with high-flow, low-pressure-drop characteristics, allowing for efficient water movement without compromising on filtration quality. Some are even integrated with anti-biofouling coatings to reduce microbial growth and maintain filter longevity.

For ultrapure water systems, these advancements translate into cost savings, higher reliability, and reduced maintenance.


The Bigger Picture: A Clean and Reliable Future

Absolute filters are more than just a filtration medium; they are a crucial enabler for technological and scientific advancements. By providing a reliable barrier against contaminants, they empower laboratories and industries to push boundaries with confidence.

Whether you’re running sensitive chemical analyses, producing pharmaceuticals, or manufacturing semiconductors, absolute filters are at the heart of ultrapure water systems, ensuring the purity and reliability your operations demand.


If you’re considering upgrading or implementing ultrapure water systems, understanding how absolute filters fit into your process can make all the difference. Ready to explore solutions tailored to your needs? Let’s connect!

Why is ultrapure water corrosive?

Ultrapure water is considered corrosive due to its extreme purity and lack of dissolved ions. Here’s why:

1. Ion Deficiency and Aggressiveness:

  • Deionization: Ultrapure water has been stripped of nearly all its dissolved ions and impurities, making it highly ion-deficient. This creates a strong chemical potential to absorb ions from any material it comes into contact with.
  • Aggressiveness: Because it lacks ions, ultrapure water is “hungry” for them. It will readily dissolve and absorb ions from surfaces, such as metals, plastics, and even glass, in an attempt to reach a more stable chemical state.

2. High Resistivity:

  • Electrical Properties: Ultrapure water has very high electrical resistivity (around 18.2 megohm-cm at 25°C). This means it does not conduct electricity well due to the absence of free ions. Materials that would normally resist corrosion in regular water can become vulnerable when exposed to ultrapure water because the water can more easily pull ions from the material.

3. Surface Reactions:

  • Surface Leaching: When ultrapure water comes into contact with a material, it can leach ions and molecules from the surface, leading to corrosion or degradation. For example, in metals, this can lead to pitting or general corrosion, and in plastics, it can lead to the leaching of additives or plasticizers.

4. Impact on Protective Layers:

  • Oxide Layers: Some metals, like stainless steel, rely on a thin oxide layer for corrosion resistance. Ultrapure water can dissolve or disrupt this protective layer, making the underlying metal more susceptible to corrosion.

5. Non-Buffering Nature:

  • Lack of Buffering Capacity: Ultrapure water has no buffering capacity, meaning it can easily become acidic or basic if exposed to contaminants or air. This shift in pH can further enhance its corrosive properties.

Conclusion:

Ultrapure water’s corrosive nature is not due to any chemical aggressiveness like that of acids or bases, but rather its extreme purity and strong tendency to equilibrate by absorbing ions and impurities from the materials it contacts. This makes it particularly challenging to handle and store without contamination or material degradation.

Is Ultrapure water corrosive?

Type 1 ultrapure water is so pure that it is actually corrosive to some materials. This is because ultrapure water has an extremely high resistivity (typically 18.2 megohm-cm at 25°C) and lacks any dissolved ions, which means it has a strong tendency to absorb ions and impurities from any material it comes into contact with. This can cause corrosion or degradation in materials that aren’t specifically designed to handle such high-purity water, such as certain metals and even some types of glass or plastics. Because of its aggressive nature, ultrapure water is often used in semiconductor manufacturing, pharmaceuticals, and other applications where even the slightest contamination can have significant consequences.

Some of the recommended materials that can be used for handling and processing ultrapure water include.

1. Polytetrafluoroethylene (PTFE)

  • Properties: Excellent chemical resistance, low extractables, high purity, and non-reactive.
  • Applications: Tubing, seals, gaskets, and fittings.

2. Perfluoroalkoxy Alkane (PFA)

  • Properties: High purity, excellent chemical resistance, and maintains clarity and flexibility.
  • Applications: Tubing, fittings, and valves.

3. Polyvinylidene Fluoride (PVDF)

  • Properties: Good chemical resistance, high purity, and mechanical strength.
  • Applications: Piping, fittings, and valve components.

4. Polypropylene (PP)

  • Properties: Good chemical resistance, low cost, and suitable for DI water at lower purity levels.
  • Applications: Piping, tanks, and valve bodies.

5. High-Purity Polyethylene (HDPE)

  • Properties: Good chemical resistance and high purity.
  • Applications: Tubing, containers, and fittings.

6. Polyetheretherketone (PEEK)

  • Properties: Excellent chemical resistance, high strength, and low extractables.
  • Applications: Tubing, fittings, and pump components.

7. Quartz (Silica)

  • Properties: Extremely high purity and inertness.
  • Applications: Piping and containers, often in semiconductor processing.

8. Stainless Steel (316)

  • Properties: High corrosion resistance, often electropolished for ultra-high purity applications.
  • Applications: Piping, valves, and fittings, usually for non-corrosive DI water applications.
  • Avoid using 304 grade stainless and ensure all welds have been polished and passivated.

9. Borosilicate Glass

  • Properties: High resistance to chemical leaching, but less durable than plastic options.
  • Applications: Laboratory containers and some piping systems

Read the next article on ‘Why is ultrapure water corrosive?‘ to find out more.

Is Type 1 or Type 2 water higher quality?

Short answer, type 1 water is purer than type 2, but there is a lot more to take into consideration before specifying one.

As the use of instruments requiring specific qualities of water increases, laboratories are moving away from the traditional Type 1, 2, and 3 specifications and toward a set of guidelines which are laid out in the Clinical and Laboratory Standards Institute (CLSI) G40-A4-AMD guidelines.

These include:

  • Clinical Laboratory Reagent Water (CLRW)
  • Special Reagent Water (SRW)

There are three main bodies who have had influence in establishing water standards over the years, these are CLSI with the G40-A4-AMD guidelines (mentioned above), ISO with 3696:1987, and ASTM which uses D1193-06.

All of these standards vary in some way and can cause confusion when a grade is specified without directing which organisation’s standard is being referenced. This makes it essential to ensure that your laboratories unique requirements are accounted for when specifying what level of purification you require.

Clinical Laboratory Reagent Water (CLRW)

For reference a standard Purific purification system would fall into the CLRW range with the following properties and is comparable to a traditional Type 2 in water quality. This grade water comes from the regular specifications for blood chemistry analysers requiring this high purity water.

  • ≥10 MΩ/cm at 25 °C Resistivity
  • <10 CFU/mL
  • TOC <500 mg/g (ppb)
  • 0.22 µm absolute final filtration near or at the final output stage of the purification system.

Special Reagent Water (SRW)

Special reagent water is used when different quality water to CLRW s required. In essence it is similar to CLRW but may have more or less stringent parameters added to meet the requirements of the laboratories procedures.

Some applications include

  • Trace organic analysis.
  • DNA and RNA testing.
  • Trace metal analysis.
  • Cell/tissue/organ culture and florescent antibody detection of microorganisms.
  • Low CO2 water.

Feel free to call one of our friendly team to ask further questions should your laboratory require a specific grade water on 1800 573 316 or send an email to service@purific.com

Is PP (Polypropylene) or HDPE (High-Density Polyethylene) better for processing DI water?


Both PP (Polypropylene) and HDPE (High-Density Polyethylene) are commonly used for processing DI (Deionized) water, but which one is better depends on the specific application and requirements.

PP is known for its excellent chemical resistance and is often used in applications that require high purity, such as laboratory and medical equipment, as well as in the semiconductor industry. PP is also a good choice for applications that require high-temperature resistance.

HDPE, on the other hand, is known for its toughness and durability. It is commonly used in water treatment applications, including processing DI water, because it is resistant to corrosion and does not react with water or other chemicals.

Overall, both PP and HDPE are good choices for processing DI water. The choice between the two materials will depend on the specific requirements of the application, such as the required level of purity, temperature resistance, and durability.

What is Reverse Osmosis?

Reverse Osmosis is a technology that is used to remove a large majority of contaminants from water by pushing the water under pressure through a semi-permeable membrane.

How does reverse osmosis work?

Reverse Osmosis works by using pressure on the feed side of the RO which forces the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.

The purified water that passed through the RO membrane, is called permeate water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject.

For more in depth reading click here to read what puretecwater.com says about reverse osmosis.

How tight is too tight?

A common problem labs have is that the filter housing (bowl) gets tightened too much when the Ion-Ex or 1 Micron filters are changed and the next time they need to be replaced it seems like it is impossible to loosen it.

When doing up the bowl make sure the O-ring is in place and has not fallen out, without the O-ring the filter housing can seize as there is little give in the head and it plastic on plastic.

Bowls should only be done up by hand. The seal is provided by the O-ring, so it should be replaced if it does not seal when don up hand tight. The black spanner is intended for for loosening the housings.

IMPORTANT: If the bowl is over tightened they can crack as the styrene material it is made from can be quite brittle.

What is TOC?

Any organic molecule you break down you will find carbon in. Total organic carbons (TOC) is the amount of carbon found in an organic compound and can be used a indicator of water purity.

Water also has inorganic carbon (IC) but IC is not included in the TOC measurement. (TOC = TC – IC)

How does does TOC affect me as a pathologist?

This is where the importance of consistently pure water becomes essential. In pathology impure water can give false readings of samples, incorrect analysis and diagnosis of medical conditions and contaminate your analyser.

As a guide the following levels of TOC are what are acceptable in type 1,2 and 3 water as according to the American society for Testing and Materials (ASTM)

Type 1 2: <50 ppb of TOCs

Type 3 : <200 ppb of TOCs

Water quality parameters for ISO grades.

PerameterGrade 1Grade 2Grade 3
pH value at 25oC — 5.0-7.0
Conductivity (μS/cm)  at 25oC, max0.11.05.0
Oxidisable matter Oxygen content (mg/l), max0.0080.4
Absorbance at 254 nm and 1 cm optical path length, absorbance units, max0.0010.01
Residue after evaporation on heating at 110oC (mg/kg), max12
Silica (SiO2) content (mg/l), max0.010.02

American Society for Testing and Materials (ASTM)
The ASTM uses D1193-06 and has four grades of water (see table below).

Water quality parameters for ASTM types.

PerameterType I*Type II**Type III***Type IV
Conductivity (μS/cm) at 25oC, max0.0561.00.255.0
Resistivity (MΩ-cm) at 25oC, max18.01.04.00.2
pH at 25oC5.0–8.0
TOC (μg/l), max5050200No limit
Sodium (μg/l), max151050
Sodium (μg/l), max33500No limit
Chloride (μg/l), max151050

*Requires use of 0.2 μm membrane filter; **Prepared by distillation; ***Requires the use of 0.45 μm membrane filter.