Reverse osmosis vs evaporation for desalination ('Membrane vs...' series, 6)
Channel: Judd Water and Wastewater Consultants
Date: 22/04/2024

This video is the sixth in our "Membrane versus.." series, in which membrane processes are compared with the conventional alternatives. This one compares reverse osmosis with evaporation for seawater desalination.

There are two ways of separating salt from highly saline solutions such as seawater. The water can either be evaporated, or permeated through a reverse osmosis (RO) membrane.

The seawater entering the membrane produces a concentrated brine waste stream and a permeate potable water product stream. This permeate stream requires post-treatment to stabilise the pH by adding alkalinity.

Evaporation uses heat to turn the water into steam, which is condensed to produce pure water. Like the RO permeate, this condensate product needs to be stabilised to be usable as potable water.

Now, in terms of energy consumption, you can think of RO permeation as being high, and evaporation as being really high. So, in practice, RO is always used for desalination unless a low-cost thermal energy source is available.

For evaporation the thermal energy needed for the process can be provided as low-grade, waste steam from a power station. So, where seawater evaporation is implemented for large-scale drinking water supply, it is most often associated with electrical power generation - along with a low-cost primary source of thermal energy.

For both processes, energy consumption is reduced by recovering energy from the brine stream. In the case of evaporation, energy is recovered in the form of heat from the brine through a series of evaporative stages. The potential heat energy recovery is then proportional to the difference in temperature between the initial brine temperature and the final temperature at which it is discharged.

In the RO process, the latent mechanical energy is recovered from the brine stream using a turbine. This converts the flow of high-pressure brine from mechanical into electrical energy.

So, RO is the favoured technology for seawater desalination based on energy consumption. Whilst RO energy efficiencies have improved over the years, the lower limit of energy consumption is imposed by the seawater salinity. The pressure required to pass water through the membrane relates to the water osmotic pressure, which is proportional to the salt concentration. This makes it practically challenging to get the specific energy consumption down below around 3.3 kWh per m3 permeate product for a normal seawater feed. Added to the clarification pretreatment - normally by membrane filtration - gives an overall specific energy consumption (SEC) of over 3.5 kWh/m3.  On the other hand, municipal wastewater is much less saline - less than 0.1%, which equates to an SEC of around 1.1 kWh/m3 or less. Biological pretreatment - either by conventional activated sludge followed by membrane filtration or by MBR technology, demands relatively little energy - yielding a total SEC of below 1.8 kWh/m3.

So, freshwater produced from sewage is more energetically efficient than from seawater. But of course, there are other factors to consider besides just energy when it comes to drinking water supply.

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Other videos in our 'Membrane versus..' series:

1. Metrics for assessing water and wastewater treatment technology – Introduction:
https://youtu.be/CG15V9cp4WQ

2. MBRs vs CAS - Membrane bioreactor versus conventional activated sludge technology for wastewater treatment:
https://youtu.be/Y28A_5t-6_0

3. Membranes vs. conventional clarification #1 - Principles of membrane filtration versus media filtration and sedimentation for water treatment:
https://youtu.be/4gd0yDPeaog

4. Membrane filtration vs conventional clarification - footprint:
https://youtu.be/dIHcyrfYs6E

5. Membranes vs UV and chemical disinfection for water treatment:
https://youtu.be/D0UboxFYRPs

7. MABRs vs MBBRs - membrane-aerated bioreactor versus moving bed bioreactor technologies for wastewater treatment:
https://youtu.be/GOCUHsoNNCo