Since the manufacturing of the first reverse osmosis (RO) elements in the 1960s, steady advances in RO technology have significantly reduced the cost of desalinating water. One of the biggest leaps forward came in the late 1970s with the introduction of the composite polyamide chemistry that nearly doubled the permeability of the membrane compared to that of the widely used cellulose acetate membrane, while also giving better salt rejection.
Other advances came in the 1990s with the introduction of the energy-saving polyamide (ESPA) membrane and low fouling composite (LFC) membrane.
During the past five years, improvements in RO performance have come from less dramatic, more incremental advances in both the design of the RO element and the formulation of membrane chemistry. Though the recent advances do not equal the impressive breakthroughs of the past, the incremental improvements of today continue to generate significant benefits for the end user, especially when factored over the life of the RO plant.
Improving the RO element
Recent improvements in the design and construction of the RO element have come by focusing attention on the details of glue line placement, feed spacer configuration and the selection of the permeate carrier. By optimizing these components, membrane area is increased and pressure losses are reduced, leading to an improvement in element productivity without modifying the membrane itself.
The move by most RO element manufacturers to automate element production has also led to an increase in element performance by precise control of fabrication variables. Automated production contributes, among other things, to an increase in the active membrane area contained in a single element. This can lead to higher productivity.
By comparing the performance of the recently released ESPA2+ with that of the well-known ESPA2, these improvements in both element design and manufacturing are clearly demonstrated. The ESPA2+, which includes many of the improvements discussed above, is manufactured with ESPA2 membrane chemistry, but produces 33% more permeate flow per element.
RO membrane improvement
Coupled with the improvements in element efficiency and increased element area, advancements in the existing polyamide membrane chemistry have yielded state-of-the-art performance. These improvements result in higher permeability and therefore lower pressures. As an example of these improvements in both element design and membrane chemistry, Table 1 illustrates the evolution of the Hydranautics seawater (SWC) element over the past four years. The increase in element permeate flow in the SWC3+ came from the same design and manufacturing advances as those applied to the ESPA2+. The increase in permeate flow as seen in the SWC5 came from further optimization of membrane chemistry.
Though the improvements are incremental and built upon existing technology, their cumulative benefit can lead to significant savings for the end user, depending on how the improvements are utilized.
For example, with more membrane area packed into a single element, fewer elements are required to achieve the average design flux for the system. Fewer elements lead to fewer pressure vessels and less piping, all of which translates into capital savings.
Likewise, if an existing RO system is replaced with newer high area, high productivity membranes, the result is a lower average flux, which leads to lower feed pressure and less fouling—both of which translate into operational cost savings.
Upgrading existing plants
One existing plant that took advantage of the advances in membranes is a 1.4 mgd municipal RO system in central Florida. The plant treated high salinity groundwater to augment the local municipal water supply.
After eight years of successful operation using CPA2 elements, there was a need to increase plant output while decreasing energy requirements. To do so, the old CPA2 elements were replaced with high permeability ESPA4 elements.
Table 2 compares the performance of the plant before and after membrane replacement. After replacement, plant output increased by 5.2% while feed pressure decreased by 30%.
In terms of savings, the cost of producing a gallon of water from the plant during the next eight years will be about 41% less.
Targeting specific ions
As the limitation of membrane permeability and element design improvements is approached, membrane manufacturers turn to improvements in membrane rejection and membrane modifications that target the rejection of specific ions.
The ESNA2, a new high permeability nanofiltration membrane, selectively rejects divalent salts while readily passing monovalent salts. When treating a typical seawater feed, for example, the ESNA2 rejects 99.7% of the sulfate ions while removing only 18% of the chloride ions. The membrane also has a molecular weight cut-off of 160 Daltons, making it useful for the recovery of certain organic components from salt solutions.
A membrane developed to specifically reject the boron ion is the ESPA B. Boron is present in seawater at an average of 5 mg/L but readily passes through an RO membrane. Built on Hydranautics proven ESPA technology, the ESPA B operates at lower pressures like other low energy brackish elements, but with the highest boron rejection of comparable membranes in the industry. With an elevated pH of 10, the ESPA B can achieve a boron rejection of 96%, resulting in fewer elements or less chemical addition in the second pass of a seawater desalination system.
With today’s RO membrane industry in a mature phase, advances in element construction, membrane permeability and specific ion rejection will continue to incrementally optimize RO system performance.
Other advances will come by creatively developing selective rejection membrane technology, such as the low-fouling ESNA1-LF nanofiltration softening membrane being used at multiple sites in Florida. But no matter the quality or quantity of technological improvements, the end user will inevitably realize those advancements in the form of capital savings, operational savings or both.
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