Water treatment terms like “filtered water” and “soft water” are often confused, but they refer to fundamentally different aspects of water quality. Filtration primarily removes contaminants affecting safety and aesthetics. Softening focuses exclusively on altering the water’s mineral composition. Filtered water is not necessarily soft water.
What Defines Filtered Water?
Filtered water has passed through a physical or chemical process designed to remove impurities, contaminants, and suspended particles. This process aims to improve the water’s taste, odor, and safety. Common household filtration often uses activated carbon.
Activated carbon works by adsorption, where contaminants like chlorine, pesticides, and volatile organic compounds (VOCs) chemically stick to the carbon’s porous surface. Sediment filters physically trap larger particles like rust and dirt. Filtration effectiveness is measured by its ability to reduce these unwanted substances.
Understanding Hardness and Soft Water
Water hardness measures the concentration of dissolved minerals, specifically the divalent cations calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)). Water collects these minerals as it travels through geological deposits like limestone. The mineral content is typically measured in parts per million (ppm) or milligrams per liter (\(\text{mg/L}\)) of calcium carbonate (\(\text{CaCO}_3\)).
Water is classified as soft when the hardness level is below 60 \(\text{mg/L}\); levels above 120 \(\text{mg/L}\) are considered hard. These mineral ions cause practical issues, such as scale or limescale buildup on heating elements and appliances. Hard water also reacts with soap, preventing effective lathering and leaving behind soap scum.
Comparing Filtration Methods to Water Softening
The fundamental difference lies in what each process removes. Standard filtration systems, such as carbon filters, trap physical particles and adsorb chemical contaminants but are not designed to remove dissolved mineral ions. The calcium and magnesium ions responsible for hardness pass right through these common filters, meaning the water remains hard. Standard filtered water is therefore not soft water.
True water softening requires a process that specifically targets these dissolved ions. The standard for whole-house softening is ion exchange, where water passes through a resin bed. The resin beads swap the hard mineral ions (\(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\)) with non-hardness ions, typically sodium (\(\text{Na}^{+}\)). This chemical exchange effectively removes the hardness.
An exception among filtration methods is Reverse Osmosis (RO), which uses a semi-permeable membrane to filter out nearly all dissolved solids, including hardness minerals. Since RO systems remove calcium and magnesium, the resulting water is both highly filtered and soft. However, the term “filtered water” in common usage usually refers to less intensive, non-softening carbon or sediment filtration. Many households combine a whole-house water softener with a separate point-of-use filter or RO system for drinking water.