Tap water is not treated with reverse osmosis (RO) by public utilities. Municipal treatment methods focus on large-scale production and disinfection to ensure public health safety. Reverse osmosis, by contrast, is a technology designed for creating highly purified water, often used in industrial applications or as a point-of-use system in homes and businesses. This distinction means tap water undergoes a significantly different purification process than water filtered through an RO system.
Standard Public Water Treatment
The process of making water potable in a public utility setting is a multi-stage operation. In the United States, this process is governed by the Safe Drinking Water Act (SDWA). This federal law mandates that public water systems meet stringent standards to limit the levels of over 90 contaminants, primarily focusing on pathogens and harmful chemicals. The goal is to ensure the water is safe for consumption and distribution throughout the community.
The initial stage involves coagulation and flocculation, where chemicals like aluminum sulfate are added to neutralize the electrical charges of suspended particles. This action causes tiny particles of dirt and organic matter to stick together, forming larger clumps called floc. Following this, the water moves into sedimentation basins, where the heavy floc particles are allowed to settle to the bottom under the influence of gravity.
After the largest solids have been removed, the water then flows through a filtration system, typically consisting of layers of sand and gravel. This physical barrier removes any remaining suspended matter, including smaller particles and microorganisms. The final and most important step is disinfection, where a chemical agent like chlorine or chloramine is added to destroy any remaining disease-causing bacteria, viruses, and parasites before the water is sent into the public distribution network.
How Reverse Osmosis Filtration Works
Reverse osmosis is a purification method that relies on a physical process to separate water molecules from almost all other substances. It works by applying pressure to overcome the natural osmotic pressure that would otherwise cause water to flow from a less concentrated solution to a more concentrated one. This “reverse” action forces the water through a semi-permeable membrane.
This specialized membrane contains pores so small that they allow water molecules to pass through but physically block larger molecules and ions. This effectively rejects dissolved salts, heavy metals, and trace contaminants. The applied pressure drives the water across the membrane, leaving the concentrated impurities behind in a separate stream, which is sent to drain.
The technology is exceptionally effective, capable of removing 95% to 99% of total dissolved solids (TDS), making it a gold standard for high-purity water applications. Because the process is slow and requires significant energy input to maintain the necessary pressure, RO is not practical for the immense volumes handled by municipal water treatment plants. This method can eliminate a wide range of contaminants, including nitrates, fluoride, and even certain pharmaceuticals.
Comparing the Final Water Profiles
The end product of municipal treatment and reverse osmosis filtration results in two distinct water profiles, particularly concerning their Total Dissolved Solids (TDS) and mineral content. Tap water, having undergone a process focused on pathogen removal and clarification, retains a moderate to high level of TDS. This includes naturally occurring minerals like calcium and magnesium, which are important for taste and contribute to the water’s overall hardness.
Reverse osmosis water, conversely, has an extremely low TDS count, often near zero, because the RO membrane aggressively removes virtually all dissolved inorganic solids and ions. This high level of purity means the water is essentially demineralized, which can affect its taste, often described as flat or neutral. While tap water is primarily treated to meet health-based standards for microbial safety, RO targets a much broader spectrum of contaminants, including dissolved ions, which are the main components of scale and hardness.
The differing goals of the two processes create the contrast in water quality. Municipal treatment ensures bulk safety and potability, while RO is designed for superior chemical purity. Consumers who prefer the taste of water with dissolved solids and retained minerals may favor tap water, whereas those seeking the lowest possible concentration of any dissolved substance will prefer RO water.