Sand is the oldest filtration media still in general use, and it survives because a properly graded bed removes suspended solids well, cleans itself by backwashing and lasts for years with no consumable. The word grading is doing a lot of work in that sentence, because a bed of uniform sand performs far worse than a layered one.
How a graded bed works
The bed is built in layers, coarse gravel at the bottom supporting the media and preventing it washing into the underdrain, then progressively finer material rising to the finest sand at the top. Water flows downwards, so large particles are caught in the upper layers and finer ones penetrate further before being trapped. That gives depth filtration across the whole bed rather than blinding at the surface, which is what allows a long run between backwashes.
Why backwashing must reclassify it
Backwash reverses the flow fast enough to fluidize the bed, releasing trapped material to drain. Crucially, the rinse that follows lets the media settle back into the correct order by density, with the finest at the top, which is what reclassification means. A bed that is backwashed at too low a flow does not fluidize and simply compacts, and one that is not rinsed properly settles disordered and then channels, passing water with little contact while pressure drop looks normal.
What sand catches and misses
Suspended solids down to a moderate size, so sand, grit, rust particles and coarser silt are removed well. Fine silt and clay largely pass, and anything dissolved passes entirely: hardness, dissolved iron, manganese, tannins and sulfur smell are untouched. Bacteria pass. Where finer filtration is needed, zeolite filters finer than sand at lower weight per tank, and multi media beds layer several densities to catch a wider particle size range in one vessel.
Where it belongs and what it needs
First in a treatment train, taking the mechanical load off everything after it, with a centrifugal separator ahead of it where the well makes coarse sand. It needs a flow rate high enough to fluidize the bed for backwash, which on a low yield well can exceed what the borehole delivers, and a drain that will take that flow. Expect a cartridge stage after it to polish what the bed passes. Our well filtration guide covers sizing to a well's yield.
Questions people ask about sand for water filtration
Why is a sand bed layered rather than uniform?
Grading coarse at the bottom to fine at the top gives depth filtration across the whole bed instead of blinding at the surface.
What does reclassification mean?
The rinse after backwash lets the media settle back in the correct density order, finest at the top, so it does not channel.
What does sand fail to remove?
Fine silt and clay largely pass, and everything dissolved passes entirely: hardness, iron, manganese, tannins and sulfur smell.
Is there a better media than sand?
Zeolite filters finer at lower weight, and multi media beds layer densities to catch a wider particle range in one vessel.