- 1) reservoir zone : Where water will be poured into the filter.
- 2) resting water zone : This water maintains wet sand, and lets oxygen through to the biological layer.
- 3) biological zone : Develops on the 5-10 first cm of the sand surface. Sand eliminates pathogens, particules and other contaminants. As in slow sand filters, a layer of micro-organisms (also known as schmutzedecke) develops in the 1-2 cm of the sand surface.
- 4) non-biological zone : Do not contains micro-organisms (or very few) because of oxygen and nutrients deprivation.
- 5) gravel zone : Maintains the sand and protects the output pipe from clogging.
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