Reverse Osmosis Systems: Membranes, Contaminants, and Waste Water

Reverse osmosis is the most thorough point-of-use treatment most homes will install. It pushes water through a semipermeable membrane so fine that dissolved salts, metals, and many other contaminants are left behind, which is exactly why it also removes beneficial minerals and sends a stream of concentrate to the drain.

How the membrane works

RO reverses the natural direction of osmosis with pressure. Culligan describes reverse osmosis as a water purification technology that uses high pressure to force water through a semipermeable membrane. Fresh Water Systems explains the same step, noting that high pressure is applied to the feed water against the semipermeable membrane, which forces water molecules through while the membrane is designed to allow only water to pass. Culligan adds that the purified water passing through the membrane is called permeate, while the remaining water containing the rejected contaminants is called concentrate or brine.

What reverse osmosis removes

The membrane rejects a wide range of dissolved contaminants. Crystal Quest reports that RO typically reduces total dissolved solids by 90 to 99 percent, and lists reductions of 95 to 99 percent for lead, 94 to 99 percent for arsenic V, 85 to 95 percent for fluoride, and 85 to 95 percent for nitrates. Those contaminants carry federal limits: the EPA sets a maximum contaminant level of 0.010 mg/L for arsenic and 10 mg/L for nitrate, and regulates lead through a treatment technique with an action level of 0.010 mg/L. RO does have blind spots. Crystal Quest notes that RO does not effectively remove dissolved gases such as carbon dioxide and radon, which can slip past the membrane.

The stages of a point-of-use system

A residential RO unit is a stack of filters, not a single membrane. Fresh Water Systems describes a five-stage flow in which water first passes through prefilters that remove sediment and chlorine to protect the membrane, then through the RO membrane that separates water from dissolved solids, with rejected contaminants flushed to the drain, followed by storage in a tank and a final carbon post-filter before the tap. Culligan describes the same prefiltration logic, noting sediment filters remove dirt, rust, and sand while carbon filters remove chlorine and chloramine ahead of the membrane. Fresh Water Systems notes RO is most commonly installed at the point of use, such as under a kitchen or bathroom sink or connected to a refrigerator.

Waste water is built in

Rejecting contaminants means sending water to the drain. Fresh Water Systems states that the contaminants that do not pass through the semipermeable membrane are flushed to the drain, and that most RO systems waste about four gallons of water for every gallon produced, though a permeate pump can reduce this by 75 to 80 percent. Culligan gives a similar picture, describing traditional systems at a ratio of about 4 to 1 or higher, meaning four or more gallons wasted per gallon purified, while EPA WaterSense-certified models achieve 2.3 to 1 or lower. If water conservation matters in your region, the waste ratio is a real number to compare between models.

Remineralization and mineral loss

Because RO strips dissolved solids indiscriminately, it also removes minerals many people want. USGS notes that hardness is dissolved calcium and magnesium, the very minerals RO takes out. Crystal Quest states that RO removes calcium and magnesium along with harmful contaminants, and that a remineralization post-filter adds back beneficial minerals like calcium and magnesium. Culligan describes modern systems that add a small, balanced amount of healthy minerals and electrolytes to the purified water, which improves taste and raises pH. A remineralization stage is optional, but for households drinking RO water daily it is the common way to restore taste and mineral content.