Reverse Osmosis (RO)
High-pressure semi-permeable membrane process for removing dissolved salts, minerals, and organic contaminants — producing high-purity water for drinking, process, and Zero Liquid Discharge applications
Overview
What is Reverse Osmosis (RO)?
Reverse Osmosis (RO) is a membrane-based water purification process that removes dissolved salts, minerals, heavy metals, organics, and almost all dissolved contaminants from water by forcing it under high pressure through a semi-permeable membrane. The membrane pores are so small (approximately 0.0001 microns — 1 ångström) that they reject virtually all dissolved ionic species while allowing water molecules to pass through. This makes RO the most effective technology available for reducing Total Dissolved Solids (TDS), achieving water quality far beyond what physical filtration or biological treatment can accomplish.
RO operates on the principle of overcoming osmotic pressure — the natural tendency of water to move from a low-concentration solution to a high-concentration solution through a semi-permeable membrane. By applying hydraulic pressure (typically 10–80 bar, depending on feed salinity) in excess of the osmotic pressure, water is forced through the membrane in the reverse direction, leaving behind the dissolved contaminants in a concentrated reject (brine) stream. Spiral-wound thin-film composite membranes are the predominant module design for industrial and municipal RO systems.
RO systems require carefully pre-treated feed water to protect the membranes from fouling, scaling, and damage. Pre-treatment typically includes particle filtration (pressure sand filter), activated carbon filtration (for dechlorination), and ultrafiltration (UF) to remove all suspended solids and biologicals before the high-pressure RO feed pump. Antiscalant dosing prevents precipitation of sparingly soluble salts (calcium carbonate, barium sulphate) on the membrane surface. With proper pre-treatment and operation, RO membranes can achieve a service life of 3–7 years before replacement.
In industrial applications, RO is increasingly deployed as a core component of Zero Liquid Discharge (ZLD) systems, where the RO permeate is recovered for reuse while the concentrate is further processed (through evaporators and crystallisers) to eliminate liquid discharge entirely — a regulatory requirement in many Indian and global jurisdictions for industries such as pharmaceuticals, textiles, and chemical manufacturing.
Process
How Reverse Osmosis Works
Pre-treatment Train
Feed water passes through the pre-treatment train: a pressure sand filter or multimedia filter removes suspended solids; activated carbon filtration removes residual chlorine (which would damage RO membranes); and ultrafiltration provides an absolute barrier to particles and biologicals. Antiscalant chemical is dosed to prevent mineral scale precipitation on membrane surfaces.
High-Pressure Pumping
A high-pressure pump raises the pre-treated water to the operating pressure required to overcome osmotic pressure and force water through the membrane. Required pressures range from 6–12 bar for low-TDS brackish water to 50–80 bar for seawater desalination.
Membrane Separation
Pre-treated, pressurised water flows through spiral-wound RO membrane modules. Water molecules pass through the dense semi-permeable membrane layer; dissolved ions, salts, organics, and micropollutants are rejected and concentrated in the brine (reject) stream that flows along the membrane surface.
Permeate Collection
Water that permeates through the membrane — the product water — is collected from the permeate spacer and exits the module at low pressure. Permeate TDS is typically 95–99% lower than feed TDS, depending on membrane selectivity and operating pressure. Permeate proceeds to storage or further treatment.
Reject (Brine) Management
The concentrated reject stream — containing all rejected salts and contaminants — exits the membrane array at a volume typically 15–25% of the feed flow. In standard systems, the reject is discharged to an approved effluent stream. In ZLD systems, reject undergoes further concentration in multiple-pass RO, mechanical vapour recompression (MVR) evaporation, or spray drying.
Energy Recovery
In high-pressure seawater RO systems, energy recovery devices (pressure exchangers or Pelton turbines) transfer pressure energy from the high-pressure reject stream back to the incoming feed, reducing overall energy consumption by up to 60%. For brackish water systems, energy consumption is inherently lower and energy recovery is less critical.
Benefits
Key Advantages
- Removes 95–99% of dissolved salts, TDS, hardness, heavy metals, and ionic contaminants
- Effective against virtually all dissolved contaminants including organics, micropollutants, and pharmaceuticals
- Produces consistently high-purity permeate regardless of feed water variability
- Essential technology for Zero Liquid Discharge (ZLD) systems — enables complete wastewater elimination
- Enables seawater desalination for coastal water-stressed regions
- Reduces fresh water consumption through treated wastewater reuse in industrial processes
- Modular skid-mounted systems scalable from small industrial units to large municipal plants
- Automated operation with CIP (Clean-in-Place) for minimal operator intervention
- Long membrane service life (3–7 years) with proper pre-treatment and chemical dosing
- Proven technology with decades of global installations across all industrial and municipal sectors
Applications
Industries & Use Cases
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