Membrane desalination: seawater, brackish water and the plants between
Reverse osmosis now carries most new desalination capacity worldwide. Seawater and brackish duties share the membrane — and almost nothing else.
Calling a plant “RO” says little more than calling a vehicle “diesel”. A seawater plant at 45 g/L and a brackish groundwater plant at 3 g/L share membrane chemistry and high-pressure pumps; their design drivers, failure modes and economics barely overlap. Reviews go wrong when experience from one is transplanted onto the other.
Two different problems
| Parameter | Seawater RO | Brackish RO |
|---|---|---|
| Feed TDS | 35,000–47,000 mg/L | 1,000–10,000 mg/L |
| Recovery | 40–50 % | 70–90 % |
| Feed pressure | 55–70 bar | 8–20 bar |
| Energy (with ERD) | 3.0–4.5 kWh/m³ | 0.5–1.2 kWh/m³ |
| Limiting risk | Biofouling, bloom events | Scaling — CaCO₃, CaSO₄, silica |
| Brine problem | Volume — marine outfall design | Destination — often no sea to send it to |
Seawater design is an energy and biology problem: isobaric energy recovery devices took the specific energy from eight kilowatt-hours to nearer three, and pretreatment — dissolved air flotation ahead of ultrafiltration on bloom-prone coasts — decides whether the nameplate is ever met. Brackish design is a chemistry and geology problem: recovery is pushed until the least soluble salt says stop, antiscalant programmes are engineering not consumables, and the well field deserves as much design attention as the membranes, because feed salinity that drifts upward strands the plant design.
Brine is the second product
Every desalination plant makes two streams and must sell one and own the other. Coastal SWRO brine goes back to sea through diffusers designed for dilution — a marine engineering task with real cost. Inland BWRO has no such relief: evaporation ponds, deep-well injection or a concentrator train are the choices, and at high recovery the brine, small as it is, can dominate whole-life cost. Zero liquid discharge is the limiting case and has its own economics — and its own article.
Where the technology is moving
- Larger elements and internally staged designs shaving specific energy toward 2.5 kWh/m³ at seawater strength.
- Closed-circuit and semi-batch RO lifting brackish recovery past 90 per cent without chemical excess.
- Hybrid plants pairing membranes with existing thermal assets for bloom resilience and peak flexibility.
- Solar-coupled BWRO for inland communities, where the energy profile of RO finally matches an intermittent source.
- Voutchkov, N., Desalination Engineering: Planning and Design. McGraw-Hill.
- International Desalination Association, IDA Water Security Handbook — capacity and technology data.
- Author's desalination selection and review records, Gulf region, 2002–2026.
Kassem, M. S. (2026). ‘Membrane desalination: seawater, brackish water and the plants between’. dr-kassem.com/writing/swro-bwro-desalination. Accessed 18 Sep 2026.