(538b) On the Operational Flexibility of Process Systems for Membrane-Based Water Treatment and Desalination

Membrane-based processes are continuing to be applied for water treatment and desalination of a growing portfolio of underutilized, geographically distributed water sources. With diversified source waters, the conventional paradigm of centralized water treatment via reverse osmosis (RO) membranes using large-scale process systems is increasingly being challenged. The alternative is the use of smaller distributed but more robust RO water treatment/desalination systems. The flexibility of a single RO process system to operate over a wide range of feed water conditions is therefore becoming increasingly important. High operational flexibility that maintains high energy efficiency and compact system footprint, however, remains challenging due to the inherent limitations of conventional membrane system designs. Membrane process systems are composed of integrated process components that are highly interdependent. Optimal operation of each system component is often limited to a narrow flow range, which in turn restricts the operational range of the integrated process system. Pumps (and associated motors) and energy recovery devices, for example, typically have narrow flow ranges at which energy efficiencies are optimal (i.e., best efficiency points). Furthermore, membrane modules are inherently limited with respect to the flow ranges for optimal operation close to the thermodynamic limit, while maintaining conditions that protect physical integrity, maximize membrane area utilization, minimize concentration polarization, and avoid excessive membrane fouling and/or mineral scaling. Such module-level operational limits can significantly restrict water recovery and permeate productivity ranges of the integrated system, which is highly dependent on the specific membrane array design. Existing methods for improving operational flexibility rely on RO concentrate recycling (to RO feed); this provides an additional degree of freedom for steady-state process regulation and also for enabling unsteady-state cyclic operation. At present, however, the impact of operational flexibility on systems energy and membrane footprint has not beem fully explored and remains unclear in the published literature. Accordingly, the present work presents a fundamental analysis framework for quantifying operational flexibility and its realizable improvements. Using the framework, the interdependence between system operational flexibility with energy consumption were explored. The resulting analysis indicated that concentrate recycling, whether applied in a steady-state or cyclic mode of operation, can significantly enhance system operational flexibility. However, increased operational flexibility leads to either higher energy consumption or larger system footprint when compared to a less-flexible system with no concentrate recycling. This finding is consistent with the higher expected axial average osmotic pressure in the membrane array of systems with concentrate recycling. Analysis also shows that an energy recovery device can impose a restriction on operational flexibility; such restriction can be overcome through process decomposition. It is concluded that high operational flexibility is well suited for small- to medium-scale applications or distributed deployments, especially where effective handling of feed water variability to maintain system optimality is critical. In such systems, it is imperative to balance system operational flexibility with the expected increased energy consumption and/or membrane footprint. Less-flexible membrane systems with high efficiencies with respect to energy consumption and membrane area utilization may better serve large-scale, centralized applications with low feed water variability or where feed water equalization tank/basin can be cost effective.