Pollution causes water molecules to gather together in larger clusters than it would naturally, as the water "wraps up" (dissolves) the pollutant.
Even if the pollutant is filtered the water molecule cluster still remains in an unnaturally large cluster due to its lasting electromagnetic frequency influence on the water, this frequency keeps the water molecules in the same unnatural structure, as they were when the pollutant was present, despite its absence.
This is the pollution no one considers.
Water pollution comes in many forms, chemicals, farm run-off, Thermal, frictional and electromagnetic. Even methods or devices that we typically attribute with the removal of pollution from water are themselves contributors to water pollution on the molecular/frequency level.
Pollution saturates water with unnatural amounts of substances and electromagnetic influences, that all leave their influence in the form of frequency on water, reducing its capacity to dissolve and transport nutrients, food and oxygen and also to clean the cells within our bodies.
In the larger environment waterways no longer function as they should, because pollutants cause unbalancing of ecosystem nutrient and energy cycling.
As water becomes over polluted it can no longer clean or regenerate itself, it is simply too full of the frequency influence of pollution, causing larger than natural water molecule clusters.
If water cannot dissolve and transport oxygen effectively it can become anaerobic. If this occurs in any environment, it also may become anaerobic.