Hybrid RO-PRO for Energy-Efficient Desalination

Abstract

Specific energy consumptions (SECs) in seawater RO desalination employing staged RO and RO-PRO with energy recovery devices are investigated using systematic mathematical methods. The SECs in both configurations are minimized by solving a constrained nonlinear optimization model which optimally selects operating conditions and allocates membrane area between different membrane units. It is shown that both staged RO and RO-PRO are noticeably advantageous over single-stage RO only if a dimensionless parameter total = A total Lp_0/Q0 is sufficiently large. The RO-PRO outperforms staged RO when internal concentration polarization is not severe, water recovery is low (e.g., 30%) and/or membrane area is abundant (e.g., total _ 1.6). The staged RO is likely to excel at a high water recovery (e.g., 60%) even though the high-salinity brine enhances the driving force for osmotic energy recovery in the RO-PRO. Both configurations have comparable SECs based on water recoveries of 40% and 50% and a total of 0.8, a representative value in industrial seawater RO plants. An multistage RO-PRO (e.g., RO-RO-PRO-PRO) may allow a profile of the applied pressure more closely track that of the concentrate osmotic pressure than RO-PRO, therefore improving energy efficiency. Challenges and opportunities in further development of hybrid RO-PRO processes are discussed.