DESALINATION FLUX MEMBRANES PERMEABILITY RESEARCH REVERSE OSMOSIS TECHNOLOGY DEVELOPMENT WATER FORUM

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Abstract

Editor’s note: The Water Forum is a new column we are launching on ultrapurewater.com. This column will explore relevant issues in high-purity and industrial water treatment, as well as other timely topics impacting the industry. Please contact Mike Henley at mhenley@globalwaterintel.com if you would be interested in making a contribution.)

The growth of the reverse osmosis (RO) industry has been robust and rapid over the past 30 years. It has moved from an industry in the 1980s with specialty uses for small applications requiring up to a few hundred spiral-wound elements to today’s mega plants with as many as 55,000 elements, such as the Melbourne seawater RO (SWRO) plant in Australia. This current state would have been hard to envision for the plant designers back in the 1980s or 1990s, but the step-by-step incremental improvement in the technology has made RO increasing the best solution for producing low-cost, high-purity water.

Technological improvements have come across the entire range of the RO industry. Key developments include advanced pretreatment, using ultrafiltration as one example; pressure exchangers to recover energy in the brine stream; advanced cleaning and antiscalant chemicals; complex algorithms to control plant operation automatically; and side-ported pressure vessels to minimize piping and piping losses. These are just a few changes, but the most dramatic improvements are due to the RO membranes themselves. 

The membrane producers have developed membranes with higher rejection, going from previous values of 99% rejection for seawater, to as high as 99.85%. This has reduced salt passage 95%. In addition, the water permeability has been significantly improved, going from 5,000 gallons per day (gpd) to 10,000 gpd at standard seawater test conditions. This reduces the needed net driving pressure in half for SWRO. 

On top of the membrane improvements, the design of the spiral-wound elements and associated materials has greatly advanced. In the early days, seawater elements were made with 315 square feet (ft²), but now they come with as much as 440 ft² of area, a 40% increase. Add to these material improvements, the implementation of automated product manufacturing, and you have a highly controlled, optimized RO spiral-wound element that makes significantly higher quality water at much lower energy than achieved back in the 1990s. 

With all this advancement, it begs the question: “What is next for RO?”

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