A whole house sand filter is a tank of graded media with a control valve on top, plumbed into the main line so all the water passes through it. Sediment is trapped in the bed, and instead of being thrown away with a cartridge it is periodically flushed to drain by reversing the flow through the tank.
Media bed versus cartridge
The difference is what happens to the dirt. A cartridge holds it and you replace the cartridge, which is cheap to install and expensive to run on dirty water. A media bed holds it and then backwashes it out, which costs more upfront and almost nothing per year in consumables. On a well that produces steady sediment, the media tank wins comfortably, both on running cost and on flow, because a deep bed passes more water at a given pressure loss than a cartridge of the same footprint. It also does not need anyone to notice a clog, since the control valve backwashes on its own. The trade is that a media tank is bulky, needs a drain, needs power, and uses water every time it flushes.
What the bed is actually made of
Filter sand is one option, but most residential tanks sold as sand filters use a graded blend or a specialty media chosen for the water. Multimedia beds layer coarse over fine so the water meets progressively finer material as it descends, which spreads the dirt through the depth of the bed instead of blinding the top layer. Some beds use a granular media that traps very fine particles for their weight and backwashes cleanly. The choice is set by particle size and by whether the water carries anything besides grit, since a bed chosen for sediment will not touch iron, hardness or odor. Vendors sell these as backwashing filter tanks in a range of tank sizes with matching valves.
What backwashing needs from your well
This is where installs go wrong. Backwashing means running water backward through the bed fast enough to lift and tumble the media so the trapped dirt lets go and leaves with the flush water. That requires a real flow rate and real pressure, sustained for the length of the cycle. A well and pump that cannot deliver it will produce a filter that never truly cleans itself, and a bed that gradually cements. It also needs a drain that can accept the full backwash flow, ideally by gravity, and the discharge cannot simply be dumped where it will run back toward the wellhead. Check the pump's capability against the tank's backwash requirement before you buy the tank, not after.
When something simpler is the better buy
A media tank is not always the right answer. If the water is producing coarse sand rather than fine silt, a spin down separator at the head of the line costs a fraction as much and flushes in seconds. If sediment is occasional rather than constant, a big-body cartridge housing will handle it without a drain, power or backwash water. If the real problem is iron, manganese, sulfur or hardness, a plain sediment bed will not touch any of them and you need a different media or a different machine entirely. Test first, look at what the water is actually producing, and size the sediment stage to protect whatever treatment sits behind it.
Questions people ask about whole house sand filter
Does a whole house sand filter remove iron?
Not on its own. A plain sediment bed catches particles, so it only removes iron that has already oxidized into solids. Dissolved iron needs different media.
How much water does backwashing use?
Enough to matter on a weak well, since the cycle must lift and tumble the whole bed. Check your pump can sustain the tank's required backwash flow before buying.
Do I need to replace the media?
Not often, but beds do wear, channel or foul over years. A tank that no longer clears sediment after a proper backwash usually needs the media replaced.
Is a sand filter better than a cartridge filter?
On water with constant sediment, yes, because it backwashes instead of consuming cartridges. On occasional sediment, a cartridge housing is simpler and cheaper.