Water is often called the universal solvent because of its remarkable ability to dissolve substances it encounters, leading to the presence of invisible impurities. Even water that appears crystal clear can contain a variety of materials at the molecular level. While many dissolved substances are harmless minerals, others can affect taste, cause scaling, or indicate unwanted contaminants. Removing these integrated impurities requires specialized processes that target dissolved solids, which is different from filtering out larger suspended particles.
Defining Total Dissolved Solids (TDS)
Total Dissolved Solids (TDS) quantifies the combined content of all organic and inorganic substances dissolved in water. These solids primarily consist of inorganic salts, metals, minerals, and electrically charged ions such as calcium, magnesium, sodium, and chloride. Unlike suspended solids, which are removed by mechanical filtration, dissolved solids are small enough to pass through filters with pores as fine as two micrometers.
TDS concentration is commonly measured in parts per million (ppm) or milligrams per liter (mg/L). Water acquires these materials from several sources as it moves through the environment. Natural sources include the weathering of rocks and soil, which introduces minerals like limestone and gypsum.
Human activity also contributes to TDS levels through agricultural runoff, urban runoff, and industrial discharge. Plumbing infrastructure can also introduce metals through corrosion. A high TDS reading serves as a metric for overall water quality and the presence of ionic contaminants.
Removal Through Phase Change (Distillation)
Distillation mimics the natural hydrologic cycle by using a phase change to separate water from dissolved impurities. The process begins by heating the source water until it converts into steam. Non-volatile dissolved solids, including heavy metals and inorganic minerals, cannot vaporize at this temperature and are left behind in the boiling chamber.
The steam, which is nearly pure water vapor, rises and enters a condenser. Inside the condenser, the vapor is cooled, reverting into purified liquid water. This liquid is collected, resulting in water with extremely low TDS levels, often exceeding 99.9% removal efficiency.
While effective against inorganic solids, distillation struggles with certain Volatile Organic Compounds (VOCs). If a contaminant has a boiling point similar to water, it may vaporize and re-contaminate the product. Therefore, some systems incorporate a specialized vent or a carbon post-filter to manage these volatile chemicals.
Removal Using Pressure and Selective Membranes (Reverse Osmosis)
Reverse Osmosis (RO) is a highly effective method for removing dissolved solids by applying pressure to overcome natural osmosis. Normally, water moves across a semipermeable membrane from low solute concentration to high solute concentration. In an RO system, a pump applies mechanical pressure to the contaminated water, reversing this flow.
This pressure forces water molecules through a synthetic, thin-film composite membrane. The membrane’s extremely small pores allow only water molecules to pass while physically blocking the larger, charged ions of the dissolved solids. This process achieves a rejection rate between 95% and 99% of dissolved salts and ionic impurities.
Pre-filtration is necessary to protect the RO membrane from damage and fouling. Sediment filters remove particles that could clog the membrane, and carbon filters remove chlorine, which degrades the material. A concentrated stream of rejected solids is continuously flushed away to the drain as a reject stream, preventing impurity accumulation and maintaining efficiency.
The purified water, known as the permeate, is collected in a storage tank. RO is a common point-of-use solution for residential drinking water. The necessary operating pressure is directly related to the concentration of dissolved solids in the feed water, requiring more pressure for higher salinity.
Removal Through Ion Exchange and Deionization
Ion exchange (IX) and deionization (DI) systems use a chemical process to remove ionic dissolved solids, unlike physical separation methods. This process involves synthetic resin beads chemically charged to attract and swap undesirable ions in the water. This method is highly specific and does not rely on particle size.
Deionization systems use two types of resins: a cation resin and an anion resin. As water flows through the cation resin, positively charged ions like calcium and sodium are captured and exchanged for hydrogen ions (H+). The water then passes through the anion resin, where negatively charged ions such as chloride and nitrate are exchanged for hydroxyl ions (OH-).
The hydrogen and hydroxyl ions released by the resins combine immediately to form a pure water molecule (H2O), removing the dissolved solids without introducing new ionic contaminants. While water softeners only replace hardness minerals with sodium ions, true deionization targets nearly all ionic TDS. DI is primarily used in laboratory, medical, and industrial settings requiring extremely high-purity water.