Reverse osmosis removes impurities primarily by forcing water through a semi-permeable membrane under pressure, which filters out contaminants based on their size.
Reverse osmosis (RO) is a highly effective water purification process that uses a multi-stage approach to significantly reduce dissolved solids and other contaminants from water. The core of its removal capability lies in its specialized membrane and preparatory filtration steps.
The Multi-Stage Filtration Process
The removal process in a reverse osmosis system involves several stages, ensuring that the water is progressively purified.
Pre-filtration
Before reaching the main reverse osmosis membrane, water will pass through pre-filters. These filters are crucial for removing larger particles that could damage the delicate RO membrane or reduce its efficiency. As stated in the reference, these pre-filters are designed to remove sediment, large particles and chlorine.
Common pre-filters include:
- Sediment Filters: Capture visible particles like rust, dirt, sand, and silt.
- Carbon Filters: Typically remove chlorine, chloramines, volatile organic compounds (VOCs), and improve taste and odor. Removing chlorine is vital as it can degrade the RO membrane over time.
The Reverse Osmosis Membrane
After pre-filtration, the water moves on to a semi permeable RO membrane. This is the heart of the system, responsible for the majority of contaminant removal. The membrane has tiny pores that are large enough for water molecules (H₂O) to pass through, but too small for most dissolved salts, minerals, heavy metals, bacteria, and other impurities.
This membrane can remove most impurities down to .001 microns. By applying pressure greater than the natural osmotic pressure, water is forced from a more concentrated solution (untreated water) to a less concentrated solution (purified water) across the membrane, leaving the contaminants behind.
Post-filtration (Optional but Common)
Many RO systems include a post-filter, often another carbon filter, to polish the water and ensure the best possible taste and odor before it is dispensed.
What Reverse Osmosis Removes
The effectiveness of reverse osmosis lies in its ability to reject a wide range of contaminants based on their size and charge.
- Dissolved Solids: Salts (like sodium, chloride), minerals (calcium, magnesium), metals (lead, mercury, arsenic).
- Microorganisms: Bacteria, viruses, cysts (though typically pre-filtration and potentially post-filtration are also barriers for larger microbes).
- Other Impurities: Chlorine (removed by pre-filter), sediment, pesticides, herbicides, nitrates, sulfates, and many other chemicals.
By combining pre-filtration with the fine filtration of the semi-permeable membrane, reverse osmosis systems provide highly purified water by effectively removing a vast majority of contaminants.
