Test Data

Filtration

The act of filtration is best described as a method of separating or removing solid particles using a filter. The filter could be a simple sieve used to remove lumps from flour. Filtration methods work on the same basic principle – the material is passed though one or more apertures (holes) of a certain size to remove all particles greater than that aperture size.

Filtration Rating

When considering a water filter, you'll probably note that each water filter supplier generally advises a micron rating. The rating advises the minimum particle size that the filter will remove from the water and is usually quoted in micrometer/micron or by its abbreviation μm.

To give you some perspective on this, a micron is a unit of length that is one millionth of a meter or 10-6 m or 1/25,400 of a inch and particles of 1 micron in size need to be magnified to be seen by the human eye for example with a microscope.

With most filters the filtration rating purely relies upon size of the pores/apertures in the filter retaining particles larger than the pore/aperture size. This is known as surface filtration and if the aperture size is sub-micron the flow through the filter may be slower for a given surface area.

Size Guide Microns*

0.02 - 0.5 Viruses which can cause disease in humans

0.2 -1 0 Bacteria (range 0.2-1 micron width and 1-10 micron length

2 – 50 Protozoan Cysts – protective capsules formed by protozoa in preparation for a resting or reproductive phase

*as referenced by WQA Technical Guidance

https://wqa.org/resources/bacteria-virus/

How Important is Ceramic Depth Filtration?

Doulton & British Berkefeld ceramic filters not only use the carefully controlled pore size of their ceramic to remove particles from water, but also the wall thickness, this is known as depth filtration. Ceramic depth filtration of a specific pore size will filter out considerably smaller particles than an equivalent pore size surface filter for the following reasons: -

1. A surface filter acts as a single barrier to the passage of water.

2. The water passing through a ceramic filter has to travel much further than the distance of the ceramic wall thickness, because it has to follow the path of pores in the ceramic. The path through the filter twists and turns through sharp angles due to the complicated ceramic structure, so particles that may have penetrated the surface of the ceramic become trapped within the structure.

3. The build up of particles trapped by the ceramic cause a pre-coat on the ceramic resulting in increased filtration efficiency of the ceramic.

4. Weak Van der Waals forces attract the small particles to the ceramic, causing them to be adsorbed onto the wall of the ceramic.

To appreciate the distance and how difficult a path the water has to follow, consider that the wall thickness of the ceramic is 1000 – 2000 times greater than the pore size of the ceramic filter. Also, the pores are sharp and jagged which increases the capability of the ceramic filter to trap particles.

What Else is Important to Know About Doulton & British Berkefeld Ceramic Depth Filtration?

Unlike many filters when the Doulton & British Berkefeld ceramic filters see a reduction in flow when filtering water with high levels of sediment and turbidity, they can be removed and the surface gently cleaned to restore the flow rate.

What are Absolute and Nominal Micron Ratings?

Absolute Micron Rating:

What is meant by an absolute rating is the maximum size of particles that can pass through a filter. For instance, a 1-micron absolute filter is designed to block anything larger than 1 micron from passing through – it is usually expressed at a certain efficiency so for example:

1 Micron Absolute Filtration Defined as >99.99%

In contrast, a nominal filter provides a less precise assurance regarding the particle size that can be retained, quite often it is expressed as a size range. Again, this is usually expressed at a certain efficiency so for example

0.5-0.8 Micron Nominal Filtration Defined as >99.9%