Chloride is a negatively charged ion (anion) that is highly soluble and naturally present in water sources. It is distinct from chlorine, which is a highly reactive greenish-yellow gas used primarily as a disinfectant in municipal water systems. When chlorine reacts with substances like sodium, it forms stable compounds such as sodium chloride (table salt), which then release the chloride ion in water. Chloride enters water supplies from various sources, including saltwater intrusion in coastal aquifers, natural mineral deposits, agricultural runoff from fertilizers, and de-icing road salts. Removing this specific ion requires specialized treatment methods that target dissolved inorganic salts, distinguishing it from the simple removal of chlorine, which is often accomplished with a basic carbon filter.
Understanding Chloride and Acceptable Limits
The need to remove chloride typically stems from its undesirable sensory and technical impacts, rather than direct health concerns at common levels. Water with elevated chloride often tastes noticeably salty; this effect becomes apparent to most people when concentrations exceed 200 to 300 milligrams per liter (mg/L). The U.S. Environmental Protection Agency (EPA) set a non-enforceable guideline, known as a Secondary Maximum Contaminant Level (SMCL), of 250 mg/L for chloride.
Chloride concentrations above this SMCL can also lead to significant technical problems, particularly in household plumbing and appliances. The presence of chloride ions promotes the corrosion of metal components, including copper pipes, stainless steel fixtures, and water heaters. This corrosive effect can shorten the lifespan of expensive equipment and lead to damage over time. Furthermore, high chloride levels can negatively affect plant life, making water unsuitable for agricultural irrigation or lawn care.
Technologies for Chloride Reduction
Because chloride is a dissolved inorganic salt, its removal requires technologies that can separate water molecules from the ionic compounds. The most effective methods involve either a physical barrier or a specialized chemical exchange process. These solutions are generally applied at the point-of-use, such as a dedicated drinking water tap, or through a whole-house system designed for highly saline water.
Reverse Osmosis (RO)
Reverse Osmosis (RO) is the most common and effective method for residential chloride reduction, often removing 90% or more of the ions. The system works by forcing water under pressure through a semi-permeable membrane that has extremely small pores. Since the chloride ion is hydrated, its resulting size is too large to pass through the membrane, effectively rejecting the salt while allowing pure water molecules to pass. RO systems are typically installed as under-the-sink units that treat water specifically for drinking and cooking.
Distillation
Distillation offers a high-purity alternative by mimicking the natural hydrologic cycle. In this process, water is boiled to create steam, leaving virtually all non-volatile dissolved solids, including chloride salts, behind in the boiling chamber. The pure steam is then collected and condensed back into liquid water. While this method achieves a very high removal rate, it is a slow process that requires significant energy to boil the water.
Specialized Ion Exchange
Specialized Ion Exchange systems can also target chloride, though they are less common for residential whole-house applications. These systems use an anion exchange resin, which is specifically designed to attract and swap the negatively charged chloride ions with another benign anion, such as hydroxide or bicarbonate. For high concentrations of chloride, these resins require frequent and costly regeneration cycles, making them less practical or economical for daily use compared to the continuous nature of an RO system.
Methods That Do Not Remove Chloride
Many common water treatment solutions effectively address other contaminants but are completely ineffective against dissolved chloride ions. Standard activated carbon filters, which are popular for improving water taste and odor, do not remove chloride. Carbon works primarily through adsorption, where organic compounds stick to the filter’s surface, allowing inorganic salts like chloride to pass straight through.
Boiling water does not remove chloride; in fact, it concentrates the problem. When water is boiled, pure water evaporates as steam, but the non-volatile chloride salts remain in the reduced volume of liquid. This action increases the concentration of chloride, making the salty taste and corrosive potential worse. Boiling is effective only for killing biological contaminants or removing highly volatile chemicals.
Traditional residential water softeners are also not designed to remove chloride. Water softeners use a cation exchange process to swap positively charged hardness minerals, specifically calcium and magnesium, for sodium or potassium ions. Since chloride is a negatively charged anion, the softener’s resin bed does not capture it, meaning the softener has no effect on the chloride concentration.