Desalination Technologies: Turning Seawater into Drinking Water
As freshwater runs out, the ocean is our limitless reservoir. Compare Thermal Desalination (MSF/MED) vs. Membrane Desalination (SWRO) and the energy costs involved.
97% of the earth's water is salt water. Desalination is the technology that unlocks this resource. Once incredibly expensive, it is now standard in the Middle East, Australia, and parts of the USA.
1. Thermal Desalination (Old School)
Boiling the ocean.
- MSF (Multi-Stage Flash): Seawater is heated and passed through chambers of decreasing pressure. It "flashes" into steam instantly.
- Pros: Robust; uses waste heat from power plants.
- Cons: Massive energy consumption. Mostly maintained in oil-rich Gulf nations.
The Process of SWRO (Modern Way)
Step 1: Intake & Pretreatment
- Intake: Drawing water from the sea. Subsurface intakes (beach wells) are better because the sand naturally filters the water.
- Pretreatment (The Key): Seawater is full of algae and silt. We must use Ultrafiltration (UF) or Multimedia Media Filters to clear it. If you send dirty water to an SWRO membrane, it will foul in hours.
Step 2: High Pressure Pumping & RO
- The Barrier: SWRO membranes are tight. To overcome the natural osmotic pressure of salt (~25 bar), we need to push at 60-70 bar.
- The Separation: Fresh water (Permeate) is squeezed through. Salt water (Brine) stays behind.
- Recovery Rate: usually 40-50%. (100 Liters of Seawater = 45 Liters Fresh + 55 Liters Brine).
Step 3: Energy Recovery (The Game Changer)
Old plants used huge energy. Modern plants use an ERD (Energy Recovery Device).
- How it works: The waste Brine leaves the membrane at high pressure (59 bar). Instead of dumping this pressure, we use it to spin a ceramic rotor that helps push the incoming seawater.
- Impact: Cuts energy use by up to 60%.
Step 4: Post-Treatment
The water coming out is acidic and distilled. We run it through Limestone (Calcite) Filters to add Calcium and raise the pH.
SWRO vs BWRO: What's the Difference?
| Feature | Seawater RO (SWRO) | Brackish Water RO (BWRO) |
|---|---|---|
| Source Salinity | 35,000 ppm (Ocean) | 2,000 - 10,000 ppm (Borewells) |
| Operating Pressure | High (60 - 80 bar) | Medium (10 - 25 bar) |
| Material Cost | Super Duplex Steel (Anti-rust) | SS316 or SS304 |
| Energy Cost | High (3 kWh/m3) | Low (0.5 - 1.5 kWh/m3) |
| Recovery | 40 - 50% | 70 - 85% |
The Environmental Challenge
1. Brine Disposal
For every liter of drinking water, we create a liter of "super-salt" water (70,000 ppm).
- Problem: Brine is denser than seawater. If dumped near the shore, it sinks and smothers the seabed ecosystem.
- Solution: Long outfall pipes with Diffusers that spray the brine high into the water column to mix it rapidly.
2. Intake Entrainment
Open ocean intakes can suck in fish larvae and plankton. Solution: Use fine wedge-wire screens and low intake velocities (< 0.15 m/s).
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