Whereas the conventional RO process is a continuous, steady-state (i.e. static) process, there are a family of dynamic RO processes which have entered the market. These time-varying processes require actuated valves to switch between different states. This additional complexity provides anti-scaling benefits, such as in flow-reversal RO. In batch-type processes, there are energy savings to be had as well.
A fully batch RO process features a moving barrier (bladder or piston) to prevent the mixing that occurs in a semi-batch process. It also uses a circulation pump throughout the desalination step to ensure sufficient crossflow velocity across the membranes, to prevent fouling and scaling. Failure of the circulation pump or bladder is not necessarily catastrophic since the system can continue to operate in an impaired state, as shown above.
|Hydraulic work: pressurizing pump
In pulse flow RO, water is treated in batches as feedwater is pressurized in a dead-end arrangement. Brine is flushed out of the membranes between each cycle. This process does not require a circulation pump (an expensive piece of equipment) but there is no crossflow velocity across the membrane during the desalination step.
Semi-batch RO, also known as closed-circuit RO, was introduced in the early 2000s and was commercialized by Desalitech. In this process, water is treated in batches and system pressure rises over the course of each batch. It is referred to as “semi” batch because feedwater continuously enters the system during the production (or pressurization) phase. The mixing between feed and brine is a source of entropy generation, so semi-batch is not as efficient as a full batch process.