A sand filter on a well is a backwashing tank of graded media that traps grit, silt and rust as the water passes down through the bed, then reverses the flow periodically to wash the accumulated material out to drain. It is the standard answer to a well that produces sand, and it works well, but it is worth asking why the sand is there before installing one.
Ask where the sand is coming from
A well that has always made a little fine silt is one thing. A well that has recently started producing coarse sand is telling you something: a failed or wrongly sized screen, a pump set too close to the bottom of the borehole and drawing from the sediment layer, a collapsing formation, or a pump running at a rate the aquifer cannot sustain. Filtering the output treats the symptom while the pump wears out on abrasive water and the borehole degrades. Get the well looked at first, then filter what remains.
Where it sits and what it protects
First in the treatment train, immediately after the pressure tank and ahead of everything else. Softener resin channels and abrades on grit, membranes foul, UV sleeves cloud and lose transmissivity, and cartridge filters downstream blind within weeks if sand reaches them. A backwashing sand bed absorbs that load continuously without a cartridge to buy, which is precisely why it is used on wells rather than a cartridge only arrangement. On very sandy wells a centrifugal separator goes ahead of even the sand filter.
Sizing, backwash and the drain
The bed must be sized to the household's peak flow, since water pushed through too fast simply carries particles into the bed and out the other side. Backwash needs both a flow rate the well can actually deliver and somewhere for the discharge to go, and on a low yield well the backwash requirement can exceed what the borehole produces, which is a real constraint rather than a detail. Alternative media such as zeolite filter finer than sand at lower weight. Our well water filtration guide covers matching a tank to a well's yield.
What it will not do
A sand filter is mechanical. Dissolved iron passes through clear and stains fixtures and laundry after it oxidizes in the air, hardness passes through and scales, sulfur smell passes through, and bacteria pass through entirely. Each of those needs its own stage: an oxidizing filter for iron, ion exchange for hardness, and UV or chlorination for pathogens. Getting sand out is the first step in a train, not a treatment on its own, and a well producing sand should be tested for the rest at the same time.
Questions people ask about sand filter for water well
Why has my well started making sand?
Usually a failed or wrongly sized screen, a pump set too low in the borehole, a collapsing formation, or over pumping. Investigate before filtering.
Where in the system does it go?
First, right after the pressure tank, so grit never reaches the softener, membrane, UV sleeve or cartridge filters downstream.
Does it remove iron or hardness?
No. It is mechanical only. Dissolved iron, hardness, sulfur smell and bacteria all pass through and need their own treatment stages.
What limits its use on a low yield well?
Backwash. The bed needs a flow rate to clean itself, and on a poor borehole that requirement can exceed what the well actually delivers.