Walk into a treated basement and you will see two, three or four upright vessels that look broadly identical. They are not. Water treatment tanks fall into four distinct jobs: media tanks that hold filtration or softening media, brine tanks that make regenerant, retention tanks that buy contact time for a chemical, and pressure tanks that belong to the well pump rather than the treatment at all.
Media tanks, the working vessels
A media tank is a fiberglass-wrapped pressure vessel with a control valve on top and a bed of media inside: softener resin, carbon, calcite, greensand, or an iron-removal medium. The valve does all the thinking, running service, backwash, rinse and refill cycles. From outside they are indistinguishable, so the label on the valve and the paperwork are the only way to know what a given tank contains. Media is replaced periodically; the tank and valve outlast several beds of it.
The brine tank and why it is different
The short, wide tank with a lid and a float assembly is a brine tank, and it belongs to a softener. It holds salt and water, makes a saturated brine, and hands that brine to the resin tank on each regeneration to strip off the hardness minerals and recharge the resin. It is not a filter and no household water flows through it on the way to the taps. Bridged salt and a crusted mush at the bottom are the two common faults, and both stop regeneration while everything still looks normal.
Retention and contact tanks
Where a chemical is injected, chlorine for disinfection or soda ash for pH, a retention tank holds the water long enough for the reaction to complete before it reaches the house. It is a plain vessel with no media and no control valve, sized to the household's peak flow rather than to any capacity rating. Skip it or undersize it and the chemical arrives at the tap still working, which shows up as chlorine odor with iron still dissolved and passing through the filter behind it.
Pressure tanks are not treatment
On a well system the tank with a schrader valve on top is the pressure tank, and it is part of the pump system: it stores a cushion of pressurized water so the pump is not starting every time somebody runs a faucet. It filters nothing. A waterlogged pressure tank causes short cycling that people often blame on the treatment equipment. Our water treatment service guide covers who to call for which of these vessels, since the pressure tank belongs to a pump contractor and the media tanks do not.
Questions people ask about water treatment tanks
How do I tell what is inside a media tank?
You generally cannot from the outside. The control valve label, the installer's paperwork or a look through the top port after depressurizing are the ways to identify the media.
Does water flow through the brine tank on its way to my taps?
No. The brine tank only supplies concentrated salt solution to the resin tank during regeneration, and household water never passes through it in service.
Why does my system need a retention tank?
Because injected chlorine or soda ash needs time in contact with the water to act. Without that holding volume the chemical is still reacting when it reaches your fixtures.
Can I remove the pressure tank if I install treatment?
No. The pressure tank serves the well pump, stopping it from short cycling, and has nothing to do with treatment. Removing it wears the pump out quickly.