LOG reduction; what is it?

At PB, we often get the question ‘your filter captures 99.9999% of the bacteria, but the millionth bacteria still gets through? An interesting question which we will answer in this article! We will also tell you more about what LOG reduction is and how you can use our filter technology to ensure that you can always provide your guests or customers with clean and safe water.

What does LOG reduction mean?

LOG reduction is a term we use to indicate the degree of capture and/or elimination of all forms of micro-organisms (microbes), including pathogens. The word ‘LOG’ here is an abbreviation of the word ‘logarithm’, which indicates how many times a number must be multiplied by itself to arrive at another number. By comparing the quality of the filtered water (permeate side) with the feedwater, we determine the log reduction factor. A reduction value of 5 LOG then means a reduction factor of 10 to the power of five (105). A reduction factor of 105 equals a reduction percentage of 99.999% of all micro-organisms in the water.

The overview below shows the reduction rate, the reduction factor and the reduction percentage for each LOG:

LOG reductionReduction ratioReduction percentage

From 3 LOG onwards, we speak of disinfection; from this point onwards, the microbe level is no longer harmful to humans. PB’s filters capture 99.9999% of bacteria, which is equal to LOG 6 reduction, or sterilization. We define sterilization as ‘the elimination of all forms of microbiological life. Complete elimination is difficult to demonstrate, so we define sterilization in terms of a number.

What are pathogens?

Pathogens are pathogenic microorganisms. Examples of these pathogens are E-coli and Legionella. If not (properly) filtered, microbiological pathogens that are active in water can make people very ill. All the more reason to filter these pathogens out of your water.

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LOG reduction calculation

We express the reduction values of the microbes on a logarithmic scale. With a logarithmic scale distribution, we place steps of 10 at equal distances on a scale. In this way, not the numerical value of the quantity is given, but a logarithm of the quantity to a reference value. This allows both very small and very large numbers to be placed.

LOG reduction in practice

The application of membrane filtration is extremely suitable for removing bacteria, microplastics and particles such as E.coli, Pseudomonas and Legionella bacteria from your water, but how does this work step by step in practice?

Step 1

The retention of a filter is investigated by cultivating a bacterium, for example, Legionella, on it.

Step 2

The quantity of colony-forming units (bacteria) per liter or milliliter of liquid is then measured in LOG units.

Step 3

The fluid, containing the bacteria, is pressed through the filter, after which colony-forming units remain in the filter.

Step 4

The fluid that has been through the filter is called permeate water. The permeate water is also measured in LOG units, after which the difference in LOG units (reduction percentage) between the feed water and the permeate water is determined.

Step 5

Finally, the reduction percentage indicates the degree of reduction of the colony-forming units in the water, and you have calculated the LOG reduction of the filter.

Calculate your LOG reduction

Do you want to calculate the LOG reduction factor yourself? Please use the formula below:

LOG reductie berekenen formule

Calculation example
Because we never speak of 0 Cfu/L but of <100 Cfu/L for example, the calculation is as follows:

Measured on supply: 6,500,000. Cfu/L = 6 log
Measured on permeate: <10 Cfu/L = 1 log
Reduction: 6 log – 1 log = 5 log

Or calculate according to the above formula:

Reduction = (1-(10/6500000)) * 100% = 99,9998461 = >99,999% = >5 log

Ultrafiltration and LOG reduction

With ultrafiltration, PB strives for clean and safe drinking water. We use ultrafiltration to filter all small undissolved particles, such as bacteria and viruses, from the water. The inside of the membrane works like a very fine sieve, with a pore size of 0.015 to 0.03 µm. With a pressure of approximately 2 bar, the water is forced through these pores. By taking it a step further, as with reverse osmosis, dissolved substances are also filtered out of the water.

Improving parameters with filters from PB

Ultrafiltration is suitable for improving various parameters, depending on the nutritional quality of the water. Below is the total list of suitable applications:

  • Viruses: 4 LOG reduction;
  • Bacteria: 6 LOG reduction (In Dutch drinking water, the membrane filter is frequently used to remove Legionella, Pseudomonas and E-coli bacteria);
  • Removal of oxidised iron;
  • TSS (turbidity) < 0.1 NTU (TSS and turbidity are different quantities. TSS are countable particles in PPM);
  • Lower SDI value (Sludge Density Index) < 2
log reductie effectiviteitstabel

High LOG reduction with filters from PB

Ensure clean and safe drinking water for your customers, guests, and colleagues with a Legionella filter from PB with the highest achievable LOG reduction factor of 6 LOG. Our filters meet the strict quality requirements and are therefore KIWA BRL K14010-1 certified.

At PB, we have an extensive standard range but we also like to be challenged with specific issues. Offering the best solution for your challenge is our strength!

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