Reverse osmosis (RO) is a water purification method recognized for producing extremely clean water. This process forces water through a semi-permeable membrane, stripping out a vast array of dissolved solids and contaminants like heavy metals, pesticides, and microbial cysts. While highly effective, this pursuit of purity comes with significant, often unacknowledged, drawbacks. Users must be aware of the consequences of consuming demineralized water, the system’s operational characteristics, and the need for consistent maintenance.
The Removal of Essential Minerals
The RO membrane is an indiscriminate filter, removing nearly all dissolved solids, including beneficial nutrients. This process typically strips 92% to 99% of naturally occurring minerals like calcium, magnesium, and potassium from the water. These elements are important for taste and are biologically active, contributing to electrolyte balance and nutrient absorption.
The long-term consumption of demineralized water is a primary health concern because it fails to contribute to the body’s necessary mineral intake. Studies have suggested that low magnesium and calcium intake from water can lead to acute health complaints within months, including cardiovascular disorders, weakness, and muscular cramps. Furthermore, the lack of buffering minerals causes the purified water to become slightly acidic, often lowering the pH to between 5.5 and 6.5. This low-mineral, slightly acidic water can increase the leaching of toxic metals from the plumbing materials it contacts.
Operational Inefficiency and Water Waste
Residential RO systems are inherently inefficient because the purification process requires a continuous flush to prevent the semi-permeable membrane from fouling. To push water through the membrane and carry rejected contaminants down the drain, a substantial amount of feed water must be used. For many standard under-sink models, the waste-to-pure water ratio can be as high as 4:1 or 5:1, meaning four or five gallons are sent to the drain for every one gallon produced.
This high “reject rate” results in a significant increase in household water usage and corresponding utility costs, especially where water conservation is a priority. The system’s reliance on a pressurized storage tank also affects its usability. The purification process is slow, often yielding water at a rate of only a few gallons per hour. Users experience this as a slower flow rate from the dedicated faucet, which depends on the water level remaining in the storage tank.
Potential for Contaminant Reintroduction
While the RO process produces highly pure water, the system itself can create conditions that foster microbial growth if not rigorously maintained. The purified water stored in the holding tank is essentially sterile, lacking the residual chlorine or chloramine that municipal water suppliers use to inhibit bacterial growth. This absence of a disinfectant residual makes the purified water vulnerable to recontamination.
If the pre-filters or the membrane are not replaced according to the manufacturer’s schedule, the system’s components can become a breeding ground for bacteria, forming a biofilm. This biofilm can then contaminate the stored, purified water in the tank, potentially introducing pathogens. Timely filter and membrane replacement is necessary for safe operation.
Mitigation Strategies for RO Users
Fortunately, there are straightforward strategies to address the primary drawbacks of using a reverse osmosis system. To counteract mineral depletion, users can install a remineralization or alkaline post-filter. These filters automatically reintroduce beneficial minerals like calcium and magnesium back into the water, restoring a balanced pH and improving the taste. Alternatively, liquid mineral drops can be added manually to individual glasses or pitchers of RO water.
To tackle the issue of water waste, consumers can upgrade to high-efficiency RO units, which often feature a more favorable waste-to-pure water ratio, sometimes as low as 1:1. Installing a permeate pump is another effective measure, as this small device increases the pressure on the membrane, significantly reducing wastewater. Maintaining the system by adhering to the manufacturer’s schedule for filter and membrane replacements is required to prevent microbial contamination and ensure the system operates at peak efficiency.