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Distributed RO/NF Water Desalination and Purification Systems:  Energy and Pressure Optimal Operation, Operational Flexibility and Fouling Mitigation

Distributed RO/NF Water Desalination and Purification Systems: Energy and Pressure Optimal Operation, Operational Flexibility and Fouling Mitigation

Yoram Cohen
University of California, Los Angeles, USA and Ben Gurion University, Israel

Inland water desalting and purification based on reverse osmosis (RO) and nanofiltration (NF) membrane technologies generally require operation at high recovery to increase water productivity and reduce the challenge of brine (i.e., concentrate) management. However, one must also strive for energy optimal operation, in addition to overcoming operating pressure limitations and effective mitigation of mineral scaling/fouling. Effective RO feed pretreatment can reduce fouling and alleviating the recovery constraint due to scaling is also possible, to some degree, via antiscalant dosing, pH adjustment, and concentrate demineralization from a primary desalting, followed by secondary desalting. Even with successful scale mitigation, the rise in osmotic pressure with water recovery elevates the required applied pressure. However, there are practical recovery limits imposed by RO operating pressure constraints (e.g., imposed by current RO membranes and element construction), the high cost of high- pressure pumps and lack of pumps suitable for operation above the limits of most current generation of RO feed pumps. In addressing the above issues, two potential RO and RO/NF process configurations were explored for distributed deployment. A hybrid RO-NF configuration (with stream recycling with and without interstage pumping) was first assessed suggesting the potential for significant feed pressure reduction (by ~30-60%) relative to conventional RO over a recovery range of 50-95%. Minimum pressure requirement for the RO-NF configuration was governed by the NF membrane intrinsic salt rejection with respect to which an optimal pressure reduction exists. Experimental studies with a spiral-wound RO-NF pilot system (~13 m 3 /day capacity) confirmed the above behavior. The second system was a flexible RO (FLERO) configuration integrated with a pressure intensifier and with partial recycle and in semi-batch operation. The FLERO system was evaluated, for desalting over a wide range of water salinity (brackish to seawater) demonstrating flexibility for operation of a single system over a wide recovery range with a single system using a relatively low pressure feed pump. Laboratory and field tests, consistent with process modeling, revealed that the specific energy consumption (SEC) can be significantly reduced relative to conventional RO. Moreover, with the above RO/NF and FLERO systems and their respective operational strategies overcoming mineral scaling is feasible via automated periodic permeate flushing, antiscalant deed dosing and/or feed flow reversal when desalting source water sufficiently undersaturated with respect to the mineral scalants of concern.