Table 2 Performance and estimated technology readiness level of brine concentration or crystallization processes

Process	Function		Specific energy consumption^a,b	Technology readiness level^c
Process	Concentration	Crystallization	Specific energy consumption^a,b	Technology readiness level^c
High-pressure RO (up to 120 bar)	×	–	5–10 kWh_e m⁻³ (ref. ¹¹)	7–9
Ultrahigh-pressure RO (up to 300 bar)	×	–	5–10 kWh_e m⁻³ (ref. ¹¹)	3–4
Counter-flow RO	×	–	3–70 kWh_e m⁻³ (refs. ^1,138)	6–7
Low-salt-rejection RO	×	–	3–15 kWh_e m⁻³ (refs. ^16,29,143)	6
Mechanical vapour compression	×	×	20–40 kWh_e m⁻³ (conc.) 50–60 kWh_e m⁻³ (cryst.)^7,110	9
Membrane distillation	×	×	>100 kWh_t m⁻³ (refs. ^112,113)	6
Solvent extraction	–	×	40–200 kWh_t m⁻³ (refs. ^119,120)	4–5
Freeze crystallization	–	×	20–1,100 kWh_e m⁻³ (refs. ^182,183)	4–6
Evaporation pond	–	×	NA^d	9
Enhanced evaporation	–	×	NA^d	4–5
Wind-aided intensified vaporization	–	×	NA^d	9
Electrodialysis	×		20–30 kWh_e m⁻³ (ref. ¹)	9
Electrodialytic crystallization^e	–	×	20–30 kWh_e m⁻³ (ref. ¹¹⁵)	3–4

NA, not available; RO, reverse osmosis. ^aThe specific energy consumption (SEC) values should be interpreted with caution because some are based on pilot-scale studies, whereas the others are based on theoretical modelling, depending on the technology readiness level, and because the values depend on factors such as salt type, starting concentration, water recovery and the optimization of the process. ^bThe unit kWh_e represents electrical energy and the unit kWh_t represents thermal energy. Processes that rely on thermal energy input could also consume substantial amounts of electrical energy for flow circulation and maintenance of vacuum, which is not reported here. ^cThe technology readiness levels are based on our best estimate using scientific literature and publicly available information, and using the technology readiness definitions given by the US Department of Energy¹⁸⁴. ^dHeat from solar radiation or the environment is the primary energy input for evaporation ponds and enhanced evaporation. The energy cost for pumping in wind-aided intensified vaporization is not considered. ^eElectrodialytic crystallization can currently only crystallize certain salts effectively (such as Na₂SO₄, KNO₃ and K₂SO₄)¹¹⁵. The reported SEC is for Na₂SO₄ only. Electrodialytic crystallization can crystallize other salts (such as NaCl) but likely with a much higher SEC.

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