| Summary: | Reverse osmosis (RO) desalination can help to ensure secure water resources, but the process remains costly. From 2007-2017, global desalination capacity nearly doubled, from 47 to 92 million m3 /day, with RO accounting for two thirds of installed capacity. Despite this growth, the total volume of treated water accounts for less than half a percent of global freshwater consumption. To be part of a sustainable water supply, RO must be made cheaper. RO energy consumption can never fall below the thermodynamic least work of separation, which is 1 kWh/m3 for 50% recovery of seawater. Practically speaking, RO energy consumption will not reach the thermodynamic limit but may be further reduced through improvements in system design.
Batch RO is the most energy-efficient RO process. It saves energy because the feed pressure varies over time with the osmotic pressure. In this thesis we further develop the batch RO technology to identify its benefits and limitations. We demonstrated the first batch RO system using a flexible bladder and validated theoretical models of energy consumption and water production. Next, we investigated practical losses associated with batch operation. This work shows that current batch RO designs are not attractive due to the combined inefficiencies of salt retention and water loss. Incomplete flushing of brine from cycle-to-cycle leads to an elevated feed salinity relative to the feed intake, boosting energy consumption by about ∼10%. De-pressurization during the reset phases of the batch RO cycle leads to water loss via osmosis. This water loss is significant (∼10%) under seawater conditions. We introduce an improved batch RO design which rapidly flushes the system to reduce downtime and water loss. Unfortunately, there does not appear to be a practical way to avoid the salt retention penalty. Batch RO has more economic value in increasing plant productivity, rather than reducing energy consumption. We conclude that batch RO is a promising technology and identify future directions for research and commercialization.
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