Advances in design are incremental and slow-moving from idea to testing, to industrial scale-up. Some of them are ready for immediate use in the grass-root projects of tomorrow, if they meet adequate engineering expertise in problem solving.
16" membranes
I think the main divide separating the desalination past from the future is the size of the RO membranes.
Today the 8" membranes and adaptors contribute to over 25% of the plant unavailability. Moving to the 16" membranes will decrease this number to below 8%. Extra spare of 17% is a good reason to globally re-consider all the practices of the plant design for reliability.
Much higher reliability and safety with 16" membranes may be obtained by moving from pressure vessels array (each vessel holding up to 8 membranes) to a single shell arrangement encapsulating all the SWRO membranes. It is similar to Veolia Barrel design.
Two-stage SWRO
The majority of SWRO desalination plants implement a single-stage reverse osmosis. As shown in numerous reports, moving to two-stage reverse osmosis makes desalination more energy-efficient. Combined with higher recoveries (above 52%), it decreases the seawater consumption and the plant footprint. Cons are more sophisticated start-up procedure and control.
SWRO trains number
Most of SWRO mega-plants are driven by high pressure pumps equipped with motors of around 1200kW. This size defines the number of SWRO trains. Available on the market sizes of 1400-1600 kW and 2200-2600 kW are more efficient, reliable, and require less maintenance. Combined with 16" membranes these sizes lead to mega-trains leveraging economies-of-scale.
Ultrafiltration
Ultrafiltration is gradually ousting multi-media filtration due to predicted filtration quality, lesser environmental impact and smaller footprint. Another advantage is that ultrafiltration does away with micron filtration before SWRO membranes. Cons of UF are more sophisticated operation and maintenance and susceptibility to off-spec seawater quality.
Brine use for backwash
Backwashing of multi-media filters or ultrafiltration membranes with brine is not an option anymore due to substantial volume of feedback from plants where backwashing with brine has been used for years.
How is two-stage SWRO desalination implemented?
As shown in P&ID seawater feed flow is split into 2 streams. First is pressurized in the high pressure pump and fed to the first pass of SWRO membrane array. The second stream enters the ERI energy recovery device (PX300 or similar) where its pressure is increased through the energy recuperation from the brine reject of the second pass of SWRO membrane array. This pressurized seawater stream enters the first pass of SWRO membrane array.
The brine flow after the SWRO array first pass is pumped by the booster pump to the turbocharger where the brine pressure is increased to above 72 Barg due to the energy regeneration from the brine of the SWRO second pass. After the turbocharger this brine flow enters the ERI energy recovery at the pressure of slightly above the feed pressure of the first pass.
The advantage of this scheme is that it may be used for retrofitting of existing desalination plants. For new plants it provides a reasonable compromise between energy efficiency and the operation flexibility.