(580c) Overcoming Operating Pressure Barrier in High Recovery Membrane Desalination Via Hybrid RO-NF Processes

In the application of reverse osmosis (RO) membrane technologies for inland brackish water desalination, achieving high product water recovery (up to 90 % or higher) is critical. Even a small enhancement in water recovery has significant impacts on concentrate volume reduction, which in turn directly affect the capacity requirements and thus overall costs of concentrate management and disposal solutions. RO water recovery in inland water desalination is often limited due to two major challenges: a) membrane mineral scaling, and b) mechanical limit of operating pressure. The challenge of membrane mineral scaling can be overcome via antiscalant treatment, as well as the integration of intermediate concentrate demineralization technologies to remove mineral scale precursors from primary RO concentrate. Given that osmotic pressure rises rapidly with water recovery, mechanical limit of operating pressure in membrane systems imposes an upper limit to product water recovery, in addition to the high capital cost of high pressure pumps. New methods are needed for addressing such pressure-imposed water recovery limit in extending the operating range (and thus applicability) of pressure-driven membrane technologies. The present study addresses the issue of concentrate volume reduction in inland brackish water desalination via high recovery hybrid RO-NF process integration. The hybrid RO-NF processes can operate at higher water recovery levels relative to conventional RO processes, while keeping the required operating pressures within the limits imposed by process components (e.g., pumps, membrane elements, and pressure vessels). For example, process analysis indicates that a two-stage hybrid RO-NF process for desalting inland brackish water (e.g., subsurface agricultural drainage water, mine drainage water, produced water, etc.) at up to 90% water recovery can have up to 54% lower operating pressure (e.g., 700 psig vs. 1,500 psig for 15,000 mg/L feed TDS) relative to a conventional two-stage RO process. However, in order to maintain high product water quality, the reduction in the required applied pressure necessitates recycling of NF permeate to RO feed, which increases the required volumetric feed capacity of pumping (and thus energy consumption). Notwithstanding, the above increase in energy requirement remains modest (i.e., 17% higher than a two-stage RO process). Based on process modeling, analysis, and proof-of-concept experimental data, a set of fundamental principles for process design and optimization of hybrid RO-NF processes will be presented with the aim of demonstrating the promising applicability of pressure-driven membrane technologies for desalination of highly saline waters.