Filtering Pool Water with Consecutive Dilution

It is important to know and recognize that all the pool water is not filtered out on a single turnover. When filtering begins, the first portion of the filtered water will be all dirty water. Shortly the dirty water in the pool will be diluted by the returning filtered water. For this reason the filtration and purification of a pool is accomplished by what is known as consecutive dilution.

For example in a pool containing a given amount of soil, about 67% will be removed on the trip through the filter, 86% on the second and so on as follows:

In actual operation, however, the pool will be in use while the water is being filtered and dirt will therefore be continuously added and filtered out. So that after a period of time, the dirt content of the pool will reach equilibrium and remain relatively constant, subject to fluctuations in the bathing load.

If the amount of dirt added every day is just equal to the amount present in the pool when filtration started, the following table shows what can be expected if filter efficiency is 100% and distribution of water is perfect:

It can be seen therefore, what effect turnover rate has on the clarity and sparkle of the pool water and why filters should be operated 24 hours per day.