Desalination & Brackish Water Treatment
Technologies for converting saltwater and brackish groundwater into potable water.
Why Desalination?
As freshwater resources decline, desalination provides a climate-independent water source. The global desalination market produces over 100 million m³/day of freshwater from seawater and brackish sources.
Brackish vs Seawater
| Parameter | Freshwater | Brackish | Seawater |
|---|---|---|---|
| TDS | <1,000 mg/L | 1,000-10,000 mg/L | 35,000-45,000 mg/L |
| Source | Aquifer, surface | Coastal aquifer, inland | Ocean, Gulf |
| RO pressure | N/A | 10-25 bar | 55-80 bar |
| Energy | N/A | 1-3 kWh/m³ | 3-6 kWh/m³ |
| Cost | $0.5-2/m³ | $1-3/m³ | $2-6/m³ |
Reverse Osmosis (RO) — Primary Technology
RO membranes reject 95-99% of dissolved salts under pressure:
- Membrane types: Thin-film composite (TFC) — most common; cellulose acetate (legacy)
- Configuration: Spiral-wound elements in pressure vessels — 6-8 elements per vessel
- Array design: Multiple stages — first stage concentrate feeds second stage for higher recovery
- Recovery rates: Brackish: 75-90% | Seawater: 40-50%
- Energy recovery devices (ERDs): Capture pressure energy from concentrate stream — reduce energy 30-60%
RO System Components
- Pre-treatment: multimedia filter → cartridge filter → antiscalant dosing — protects membranes
- High-pressure pump: Supplies feed pressure — 15-25 bar (brackish), 55-80 bar (seawater)
- Membrane array: Pressure vessels containing spiral-wound elements
- Energy recovery: Pelton wheel, pressure exchanger, or turbocharger
- Post-treatment: Remineralization (calcite contactor) → pH adjustment → disinfection
- CIP system: Clean-in-place — chemical cleaning of membranes every 1-6 months
Energy Recovery Devices
ERDs are critical for seawater RO economics — they recover up to 98% of the energy in the concentrate stream:
- Pressure exchanger (PX): Isobaric device — high-pressure concentrate pressurizes incoming feed (95-98% efficiency)
- Turbocharger: Turbine driven by concentrate powers feed pump impeller (85-90% efficiency)
- Pelton wheel: Pelton turbine driven by concentrate jet — generates electrical power (80-85%)
- Impact on cost: ERDs reduce energy cost from $2.50/m³ to $0.50/m³ — make seawater RO viable
Brackish Water Desalination
Lower pressure requirements make brackish RO significantly cheaper than seawater RO:
- Feed pressure: 10-25 bar (vs 60-80 bar for seawater)
- Energy: 1-3 kWh/m³ (vs 4-6 kWh/m³ for seawater)
- Membrane life: 5-7 years (vs 3-5 years for seawater membranes)
- Common sources: Coastal aquifers, mining pit dewatering, agricultural drainage
- Brine disposal: More concentrated than feed — disposal options depend on local regulations
Brine Disposal Options
- Ocean outfall: Surface or diffuser discharge — must meet mixing zone regulations
- Deep well injection: Into deep confined formation — isolated from freshwater
- Evaporation ponds: Solar evaporation — land-intensive, suitable for arid regions
- Crispification: Zero-liquid discharge (ZLD) — crystallizer produces solid salt
- Brine mining: Extract valuable minerals (Li, Mg, K, NaCl) — turns waste into revenue
Small-Scale Desalination
- Containerized RO: Pre-assembled in shipping containers — rapid deployment (2-4 weeks)
- Community systems: 100-10,000 m³/day — powered by grid, diesel, or solar
- Emergency units: 5-50 m³/day — mobile, rapid deployment for disaster response
- Solar-powered RO: PV + battery or direct-drive — off-grid coastal communities
- Brackish well systems: Treat local brackish aquifer — lower energy than seawater
Global Desalination Leaders
Saudi Arabia (5.7 M m³/day), UAE (3.0 M), Israel (1.5 M), Spain (1.2 M), Australia (1.0 M). Israel's Sorek plant produces 624,000 m³/day at $0.58/m³ — the world's lowest seawater desalination cost.
Water Quality Considerations
- Remineralization: RO permeate is corrosive — add calcium and alkalinity before distribution
- Fluoridation: RO removes natural fluoride — may need post-treatment addition
- Blending: Mix RO permeate with untreated brackish water to achieve target TDS
- Mineral recovery: Opportunity to extract valuable minerals from brine — lithium, magnesium, potassium