Reverse osmosis is a membrane treatment process. Pressure moves feed water across a semipermeable membrane, producing a treated stream and a concentrate stream.
The membrane is central, but the surrounding stages and operating conditions determine whether the complete system is a good fit.
Pressure drives separation
In RO, applied pressure helps water molecules pass through the membrane while many dissolved ions and other substances are retained to different degrees. The treated stream is called product water or permeate. The remaining stream carries concentrated material away as reject water or concentrate.
The US EPA describes RO as pressure-driven treatment through a semipermeable membrane.
Why pretreatment matters
Sediment and carbon stages can protect the membrane from particulate loading and oxidants, depending on the system and source water. Pretreatment does not make every RO system identical; it supports the membrane under the conditions the product was designed for.
Product water and reject water
RO does not send all incoming water to the drinking-water outlet. Some becomes product water and some carries concentrated material away. Recovery ratio describes the relationship between these streams.
Efficiency depends on membrane design, pressure, temperature, feed-water chemistry, storage and flow control. Compare tested system figures rather than assuming every membrane performs the same way.
What RO may reduce
RO can reduce many dissolved ions and selected substances, but the exact claim must be product-specific. EPA WaterSense requires additional contaminant claims to be tested and certified.
Do not treat a lower TDS reading as a complete water-safety score. TDS does not identify which substances are present.
Maintenance is part of performance
Prefilters, the membrane, post-treatment stages, storage and sanitisation all require system-specific care. Replacement timing depends on the model, incoming water and use.
Next: read about RO recovery ratio, compare methods in the Filtration Guide, or review WAA drinking-water products.