Whether filtered water is hard or soft depends entirely on the type of filtration technology used. Common household filters and specialized whole-house systems operate using fundamentally different physical and chemical principles. To determine a filter’s effect, one must understand what makes water hard and the specific mechanism a system uses to treat it.
What Makes Water Hard or Soft
Water hardness is a measure of the concentration of dissolved multivalent cations, which are positively charged metal ions. The two primary minerals responsible for water hardness are calcium and magnesium. These ions enter the water supply as it travels through mineral-rich geological deposits like limestone and gypsum. Soft water, by contrast, contains a very low concentration of these specific ions.
The total concentration of these minerals is often measured and expressed as an equivalent amount of calcium carbonate. Common units for measuring hardness include parts per million (ppm) and grains per gallon (gpg). Water is classified as hard when the concentration of these dissolved minerals exceeds a certain threshold. The presence of these ions causes scale buildup in pipes and appliances and inhibits the lathering action of soap.
How Standard Household Filters Affect Water Chemistry
Most common household filtration devices, such as pitchers, refrigerator filters, and faucet-mounted units, primarily rely on activated carbon (AC) and sediment filtration. The purpose of these systems is to remove particles, improve taste, and eliminate odors. Activated carbon works through adsorption, where contaminants like chlorine and organic chemicals stick to the porous surface of the carbon granules.
The dissolved calcium and magnesium ions that cause water hardness are too small and chemically distinct to be removed effectively by adsorption. Since these minerals are dissolved ions, they pass right through the carbon and sediment filters unchanged. Therefore, water that runs through a standard activated carbon filter remains essentially as hard as the source water. Some pitcher filters may include a small amount of ion-exchange resin for a modest reduction in hardness, but the main AC component does not target these minerals.
Specialized Systems That Alter Water Hardness
To achieve truly soft water, a system must employ a specialized process designed to chemically or physically remove the dissolved hardness ions. Two main technologies accomplish this: ion exchange and reverse osmosis. These systems are distinctly different from the standard carbon filters found in household devices.
Ion Exchange Softeners
Ion exchange water softeners are the most common technology engineered to remove hardness minerals from the entire household water supply. This system contains a resin bed charged with sodium or potassium ions. As hard water flows through the resin, the beads attract the multivalent calcium and magnesium ions, swapping them out for the monovalent sodium or potassium ions. Because the hardness minerals are chemically replaced by softer ions, the resulting water is classified as soft.
Reverse Osmosis (RO)
Reverse osmosis (RO) systems also produce chemically soft water through a physical separation process. RO forces water under pressure through a semi-permeable membrane with extremely fine pores, sometimes as small as 0.0001 microns. The membrane is designed to block almost all dissolved solids, including the larger calcium and magnesium ions, while allowing water molecules to pass through. This process can remove 95 to 99% of dissolved solids, effectively stripping the water of its hardness minerals. While RO is highly effective at producing soft water, it is typically installed to treat drinking water at a single tap, rather than the entire home supply.