Reverse osmosis

===Energy recovery===
[[File:ReverseOsmosis with PressureExchanger.svg|thumb|upright=1.3|Schematics of a RO desalination system using a [[pressure exchanger]].<br>''1'': Sea water inflow,<br>''2'': Fresh water flow (40%),<br>''3'': Concentrate flow (60%),<br>''4'': Sea water flow (60%),<br>''5'': Concentrate (drain),<br>''A: Pump flow (40%),<br>''B'': Circulation pump,<br>''C'': Osmosis unit with membrane,<br>''D'': Pressure exchanger]]
[[File:Reverse Osmosis with Pressure Recovery Pump.jpg|thumb|upright=1.3|Schematic of a RO desalination system using an energy recovery pump.<br>''1'': Sea water inflow (100%, 1 bar),<br>''2'': Sea water flow (100%, 50 bar),<br>''3'': Concentrate flow (60%, 48 bar),<br>''4'': Fresh water flow (40%, 1 bar),<br>''5'': Concentrate to drain (60%,1 bar),<br>''A: [[Pressure exchanger|Pressure recovery pump]],<br>''B'': Osmosis unit with membrane]]

Energy recovery can reduce energy consumption by 50% or more. Much of the input energy can be recovered from the concentrate flow, and the increasing efficiency of energy recovery devices greatly reduces energy requirements. Devices used, in order of invention, are:
* [[Water turbine|Turbine]] or [[Pelton wheel]]: a water turbine driven by the concentrate flow, connected to the pump drive shaft provides part of the input power. Positive displacement axial piston motors have been used in place of turbines on smaller systems.
* Turbocharger: a water turbine driven by concentrate flow, directly connected to a [[centrifugal pump]] that boosts the output pressure, reducing the pressure needed from the pump and thereby its energy input,<ref name="Yu Jenne p=132">{{cite journal | last1=Yu | first1=Yi-Hsiang | last2=Jenne | first2=Dale | title=Numerical Modeling and Dynamic Analysis of a Wave-Powered Reverse-Osmosis System | journal=Journal of Marine Science and Engineering | publisher=MDPI AG | volume=6 | issue=4 | date=8 November 2018 | issn=2077-1312 | doi=10.3390/jmse6040132 | page=132| doi-access=free }}</ref> similar in construction principle to car engine [[turbocharger]]s.
* [[Pressure exchanger]]: using the pressurized concentrate flow, via direct contact or a piston, to pressurize part of the membrane feed flow to near concentrate flow pressure.<ref name="Stover 2007 pp. 168–175">{{cite journal | last=Stover | first=Richard L. | title=Seawater reverse osmosis with isobaric energy recovery devices | journal=Desalination | publisher=Elsevier BV | volume=203 | issue=1–3 | year=2007 | issn=0011-9164 | doi=10.1016/j.desal.2006.03.528 | pages=168–175}}</ref> A boost pump then raises this pressure by typically 3&nbsp;bar&nbsp;/&nbsp;50&nbsp;psi to the membrane feed pressure. This reduces flow needed from the high-pressure pump by an amount equal to the concentrate flow, typically 60%, and thereby its energy input. These are widely used on larger low-energy systems. They are capable of 3 kWh/m<sup>3</sup> or less energy consumption.
* [[Energy recovery|Energy-recovery]] pump: a reciprocating [[piston pump]]. The pressurized concentrate flow is applied to one side of each piston to help drive the membrane feed flow from the opposite side. These are the simplest energy recovery devices to apply, combining the high pressure pump and energy recovery in a single self-regulating unit. These are widely used on smaller low-energy systems. They are capable of 3&nbsp;kWh/m<sup>3</sup> or less energy consumption.
* Batch operation: RO systems run with a fixed volume of fluid (thermodynamically a [[closed system]]) do not suffer from wasted energy in the brine stream, as the energy to pressurize a virtually incompressible fluid (water) is negligible. Such systems have the potential to reach second-law efficiencies of 60%.<ref name=WarsingerBatch /><ref name="Cordoba Das Leon Garcia 2021 p=114959">{{cite journal | last1=Cordoba | first1=Sandra | last2=Das | first2=Abhimanyu | last3=Leon | first3=Jorge | last4=Garcia | first4=Jose M | last5=Warsinger | first5=David M | title=Double-acting batch reverse osmosis configuration for best-in-class efficiency and low downtime | journal=Desalination | publisher=Elsevier BV | volume=506 | year=2021 | issn=0011-9164 | doi=10.1016/j.desal.2021.114959 | page=114959| s2cid=233553757 }}</ref><ref name="Wei Tucker Wu Trueworthy 2020 p=114177">{{cite journal | last1=Wei | first1=Quantum J. | last2=Tucker | first2=Carson I. | last3=Wu | first3=Priscilla J. | last4=Trueworthy | first4=Ali M. | last5=Tow | first5=Emily W. | last6=Lienhard | first6=John H. | title=Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation | journal=Desalination | publisher=Elsevier BV | volume=479 | year=2020 | issn=0011-9164 | doi=10.1016/j.desal.2019.114177 | page=114177| hdl=1721.1/124221 | s2cid=213654912 | hdl-access=free }}</ref>