Submerged Reverse Osmosis Desalination

Abstract

Over the past three decades, advances is reverse osmosis technology have led to dramatic increases in the number and capacities of reverse osmosis desalination plants. One key component in all efficient traditional reverse osmosis plants is an energy recovery turbine that recovers energy from the waste stream of the reverse osmosis process. The works documented herein describe and analyzes a system in which no recovery turbine is needed for efficient operation. The proposed system requires submerging a reverse osmosis element to a depth sufficient to provide the pressure required to overcome the osmotic pressure of the salt water. The fresh water that passes the reverse osmosis membrane is then pumped to the surface with a high pressure pump. First and second law analysis is performed on the proposed system and compared to traditional systems. The energy analyses include parametric studies to determine energy optimized recovery rates. The effect of adding stages to traditional systems is analyzed and compared to the proposed system. An experimental apparatus is designed, built, and used to simulate submerged reverse osmosis desalination. The experimental work shows good quality fresh water (TDS< 300 ppm) from brackish water (TDS=10 ppt) at low recovery rates (3%-20%), and low net driving pressure (100 psi). The experimental work for artificial seawater resulted in iii permeate with concentrations of 1000 ppm to 2000 ppm for net driving pressures up to 200 psi. Variations for the design are also presented. The primary design includes submerged pretreatment and submerged reverse osmosis. The depth required to achieve good permeate quality is found to be 1100 ft to 1500 ft for typical seawater. A design alternative is presented to accommodate surface pretreatment and submerged desalination. A land based design alternative is also described in which the reverse osmosis membrane is submerged in a seawater or brackish water well.