Reverse osmosis (RO) is a water purification technology that uses a semi-permeable membrane to remove dissolved solids and contaminants from water. By forcing water under pressure through this fine filter, the system separates water molecules from impurities, resulting in highly purified water. While RO is effective at removing heavy metals, chemicals, and unwanted salts, it fundamentally alters the water’s chemistry. Consumers should consider the operational drawbacks and changes to water quality before installing an RO system.
Depletion of Essential Minerals
The primary health-related disadvantage of reverse osmosis water is its lack of beneficial dissolved minerals. The RO membrane is highly efficient, removing up to 99% of total dissolved solids (TDS), which includes naturally occurring elements like Calcium and Magnesium. These minerals are important for human health, and their near-complete removal can have long-term nutritional implications if a person’s diet does not provide sufficient compensation.
A significant portion of the population already has inadequate intake of these minerals, and drinking demineralized water may exacerbate potential deficiencies. Research suggests that minerals in water are often more readily absorbed by the body than those found in food because they are present as free ions. Studies have linked water low in Magnesium and Calcium to increased risks of cardiovascular disease.
The lack of dissolved solids also impacts the water’s palatability, often described as having a “flat” or “bland” taste. Minerals like calcium and magnesium contribute to the characteristic flavor of drinking water. Using demineralized water for cooking can cause a higher loss of essential elements from foods, with losses potentially reaching 60% for calcium and magnesium. Some RO systems address this by incorporating a post-filter that adds trace amounts of minerals back into the water, a process known as remineralization.
Elevated Water Acidity
The removal of dissolved minerals by reverse osmosis systems results in water with a lower capacity to buffer against changes in pH. Natural water contains buffering minerals, such as bicarbonates, which help maintain a neutral pH. When RO strips these compounds away, the resulting water often becomes slightly acidic, typically falling within a pH range of 5 to 7.
This slight acidity is not generally considered a direct health concern for consumption, as many common beverages like coffee and orange juice are far more acidic. However, the absence of dissolved solids makes the water more chemically aggressive or “hungry.” This high purity increases the water’s tendency to absorb carbon dioxide from the air, which forms carbonic acid and further lowers the pH.
The main consequence of this pure, slightly acidic water is its potential corrosiveness to plumbing and fixtures. The water will attempt to re-establish a mineral balance by leaching trace amounts of metals from any piping it contacts, especially copper or older metal pipes. This corrosive action can lead to increased maintenance costs and cause heavy metals like lead and copper from the plumbing to leach into the purified drinking water. To mitigate this, some industrial and home systems employ post-treatment to raise the pH and hardness of the water.
Operational Inefficiency and Water Wastage
Reverse osmosis is an inherently inefficient process regarding its water input-to-output ratio, leading to significant water wastage. To prevent the semi-permeable membrane from becoming clogged with rejected contaminants, a continuous stream of water, known as “brine” or “concentrate,” is required to flush the impurities down the drain. This process is known as cross-flow filtration.
For typical residential under-sink RO systems, the standard waste-to-purified water ratio often ranges between 3:1 and 4:1. This means that for every one gallon of purified water produced, three to four gallons of water are sent to the wastewater drain. While modern, high-efficiency systems are available that can reduce this ratio to 1:1 or less, the majority of traditional units still operate with this high level of water consumption.
The process also has a relatively slow production rate compared to other filtration methods. Since the system relies on water pressure to force molecules through the membrane, the purified water, called permeate, is collected slowly over time. This necessitates the use of a pressurized storage tank to ensure that purified water is available on demand, adding to the system’s size and complexity. The waste ratio can also be influenced by factors like low water pressure or high total dissolved solids in the source water.
System Maintenance and Contamination Potential
Maintaining a reverse osmosis system requires regular attention and can involve substantial recurring costs. The system relies on multiple filter stages, including sediment and carbon pre-filters, which must be replaced on a schedule to protect the RO membrane. Neglecting the replacement of the pre-filters can lead to chlorine, sediment, or other debris reaching the membrane, which can damage or “foul” it, necessitating an expensive replacement.
The RO membrane itself typically needs replacement every two to five years, depending on the quality of the source water and the system’s usage. Beyond the filters, there is a potential for bacterial contamination, particularly within the storage tank or post-filter stages. Because the RO process removes disinfectants like chlorine, the purified water is susceptible to the growth of microorganisms and the formation of biofilm if the system is not properly maintained and sanitized.
If the water sits in the storage tank for extended periods, the risk of consuming contaminated water increases. Regular maintenance, including cleaning and sanitizing the storage tank, is necessary to ensure the system continues to produce safe drinking water. Failure to adhere to the recommended maintenance schedule can result in diminished water quality, reduced flow rate, and a shorter overall lifespan for the unit.