A water filter and salt based water softener system are usually sold together and are often mistaken for one appliance. They are not. The filter takes things out of the water by trapping or adsorbing them; the softener trades hardness minerals for sodium on a resin bed and regenerates itself with brine. Each one fails at the other one job, and the order you install them in matters.
Two different jobs, two different failure modes
The softener exists to remove calcium and magnesium, which is what scales the water heater, the kettle and the shower glass, and what makes soap refuse to lather. The filter exists to deal with everything you can taste, smell or see: chlorine, sediment, organic compounds, sometimes iron or sulfur. A softener will not improve chlorine taste. A carbon filter will not stop scale. Households that fit only one and are disappointed have almost always fitted the one that does not address their actual complaint.
Which comes first in the line
In a combined installation the filter normally goes upstream of the softener, so the softener resin sees water that has already had chlorine and sediment taken out. That protects the resin, which chlorine slowly degrades and sediment fouls. The exception is a well with dissolved iron, where the treatment order depends on which form the iron is in. Penn State Extension notes that softeners can remove dissolved iron when pH is above 6.7, hardness sits between 3 and 20 grains per gallon and dissolved iron is under 5 mg/L, but that oxidized iron fouls the resin, which is why an iron filter belongs ahead of a softener whenever the water has already seen air.
Combined cabinets versus separate tanks
Some manufacturers sell one cabinet containing both media beds, and some sell two tanks plumbed in series. The single cabinet is smaller and needs one connection, which is a real advantage in a tight utility room. Separate tanks let you size each stage independently, replace one without the other, and swap the filter media when the water changes. Where the filter is a backwashing tank rather than a cartridge, it needs its own drain connection and its own backwash flow, and both of those come out of the same well or service line the softener uses.
Sizing and the service space you will need later
The softener is sized on measured hardness, iron content and household demand, so a hardness test is the first step, not an afterthought. The filter is sized on flow, because a cartridge or tank that cannot pass the peak flow of the house drops pressure at exactly the moment two showers are running. Whatever you install, leave a bypass and standing room in front of both tanks. Resin gets replaced, brine tanks get scrubbed out, and media gets changed, and every one of those jobs is far worse in a cabinet built tight to the wall.
Questions people ask about water filter and salt based water softener system
Do I need both a filter and a salt based softener?
Only if you have both problems. Scale, poor lather and spotting point to hardness and need a softener; taste, odor, chlorine or sediment complaints need a filter. A water test tells you which you actually have.
Which goes first, the filter or the softener?
The filter usually goes first, so chlorine and sediment are removed before they reach the softener resin. Well water with oxidized iron is the case where an iron filter must lead.
Will a softener alone fix chlorine taste?
No. Ion exchange resin swaps hardness minerals for sodium and has no effect on chlorine taste or odor, which needs carbon or another filtration stage to address.
Is a single combined cabinet as good as two separate tanks?
It saves space and connections, but separate tanks let you size, service and replace each stage on its own, which is usually worth the extra footprint in a house with changing water.