Reverse osmosis (RO) is a highly effective water purification technology that uses applied pressure to force water through a semipermeable membrane. This process separates water molecules from dissolved salts and large particulate matter, which are rejected by the membrane’s fine pores. Despite its reputation for producing extremely clean water, the effectiveness of RO is not universal across all types of contaminants. The system has specific limitations concerning the size, chemical properties, and state of certain substances that allow them to bypass the membrane.
Volatile Organic Compounds and Dissolved Gases
RO membranes reject dissolved solids based on size and electrical charge. However, this mechanism is largely ineffective against many volatile organic compounds (VOCs) and dissolved gases. These contaminants, which include substances like chlorine, chloramines, radon, and industrial solvents such as benzene, present a challenge due to their chemical nature.
Many VOCs are non-polar and possess a low molecular weight, allowing them to pass directly through the membrane rather than being physically blocked. Some organic molecules can even dissolve into the membrane material itself, a process called permeation, before diffusing out on the purified side. Dissolved gases, such as hydrogen sulfide or carbon dioxide, are also not effectively blocked.
Carbon dioxide, for instance, can pass through the membrane and then dissolve back into the purified water, where it forms a weak carbonic acid. Because of this structural weakness against gaseous and volatile compounds, a robust multi-stage system is mandatory. A separate pre-filtration stage, typically using activated carbon filters, is integrated to adsorb these specific contaminants before they reach the membrane.
Non-Ionized Low Molecular Weight Contaminants
RO is efficient at removing charged particles (ions) because the membrane material naturally repels them through charge exclusion. This repulsion works well for charged species like sodium, calcium, and fluoride ions, which are heavily rejected. However, many compounds lack an electrical charge, meaning the membrane must rely solely on physical size exclusion for removal.
Contaminants that are non-ionic (neutral charge) and have a low molecular weight can easily bypass the semipermeable barrier. This category includes certain trace pharmaceuticals, pesticides, and agricultural herbicides. Specific molecules, such as some types of alcohol or small organic industrial chemicals, are not effectively retained if their size is near or below the nominal pore size of the membrane.
The rejection rate for these substances varies significantly depending on the specific compound and the condition of the RO membrane. While a high-quality membrane may remove most contaminants above 200 Daltons, smaller, neutral molecules can still slip through. The lack of an electrical charge eliminates the primary defense mechanism of the RO process, leaving only the physical barrier to prevent their passage.
The Removal of Essential Minerals and pH Impact
A limitation arises from the system’s efficiency in removing dissolved solids. Reverse osmosis does not distinguish between harmful contaminants and beneficial dissolved solids, such as essential minerals. The process removes nearly all naturally occurring minerals like calcium, magnesium, and potassium, which are important for both human health and the taste of the water.
This comprehensive removal results in water with extremely low total dissolved solids (TDS), often referred to as demineralized water. The lack of these dissolved minerals causes a subsequent change in the water’s chemical characteristics. Specifically, the removal of bicarbonate and carbonate compounds eliminates the water’s natural buffering capacity.
Without these buffering minerals, the purified water becomes susceptible to a decrease in pH, making it slightly acidic. The pH of RO water typically falls between 5 and 7, compared to the neutral or slightly alkaline pH of natural water sources. To counteract this effect, which can make the water taste flat and increase corrosiveness toward plumbing, post-treatment solutions are often employed to reintroduce a balanced amount of beneficial minerals.