Temporally Multi-staged Batch Counterflow Reverse Osmosis for High Recovery Desalination

Osmotically assisted reverse osmosis (OARO) or counterflow reverse osmosis (CFRO) are recent RO configurations that uses saline streams on both sides of the membrane in counterflow. This reduces the osmotic pressure difference that needs to be overcome for permeation and allows water recovery from high salinity feeds at regular RO pressure. Batch RO is a new, transient RO configuration that closely follows the osmotic pressure profile of the feed and is marked by high energy efficiency. In this work we extend a transient version of CFRO, Batch CFRO for high recovery (~74%) desalination of seawater using a temporally multi-staged version of the process for the first time. In doing so, we introduce the first configuration to achieve Batch CFRO using entirely available components, including a pressure exchanger rather than high pressure tanks. Using a reduced order model, the terminal salinity of the brine leaving the system is calculated to be 183 g/kg. The key feature of this new configuration is that it is multi-staged in time rather than space. As such it can use the same hollow fiber membrane module for the different stages and hence reduce the component (pumps and pressure exchangers) count of the process. The brine produced in each stage is stored in inexpensive atmospheric pressure tanks. This is in contrast with other multi-stage processes where the number of flow devices usually scale with the number of stages needed for higher recovery and usually leads to high cost. Notably, the choice of membrane type can make a significant difference, as common narrow hollow fibers can experience large pressure drops that become significant. This leads to the conclusion that module design must be key to achieve the top-energy numbers of other batch CFRO configurations by the team, such as spiral wound membranes, turbulence-inducing spacers, or using feed on the shell side of the fibers.