Mar 21st 2023
New ultra-high pressure reverse osmosis (UHP RO) membrane elements are entering the market, rated for 50% higher pressures than previously available. These 1,740 psi (120 bar) membranes raise the practical osmotic pressure and brine concentration limit to 130,000 mg/L total dissolved solids (TDS). This is a 1.6x increase relative to conventional “seawater” RO membranes rated for 1,200 psi (80 bar), which tap out at roughly 80,000 mg/L TDS. However, there are important design and technical considerations, which we review in this article.
Saltworks is one vendor that offers these new membrane systems. Our product, XtremeRO, delivers the latest UHP RO membrane technology in a reliable, low cost, modularized package. It incorporates all of the important design considerations below, in addition to proprietary enabling features, and expert knowledge.
Reverse osmosis is the most widely commoditized, cost effective, and lowest energy desalination technology; however, it has limits. These limits can include sensitivity to organics, oxidants, scaling ions, and declining productivity with increasing salinity (TDS). As the brine concentration increases, flux through the membrane and permeate quality decrease. This is shown below in Figure 1. The data in Figure 1 was gathered on a pilot plant developed by Saltworks, based on 4” membrane elements, shown in Figure 2. Two leading manufacturers’ membrane products were tested. Similar relationships in flux were observed but one company produced high-quality permeate in a membrane element with more reliable and consistent performance. Saltworks builds this knowledge into our XtremeRO reverse osmosis systems for our customers.
The increase in permeate salinity with brine concentration is expected. The RO membrane surface rejects a percentage of salt, for example 99.8%. This means roughly 0.2% of salt permeates through the membrane with the water. Naturally, at higher brine concentration more salt gets through the membrane. Also, operation at lower flux results in lower rejection. The slightly higher salinity permeate can be blended with other lower salinity permeates, using as flush source for upstream ROs, or further refined in a low-pressure polishing RO process. The primary objective and cost driver when applying UHP RO should be brine concentration and volume reduction.
Figure 1 also shows the flux declining at higher salinity. Flux values in liters per m2 per hour (LMH) represent proprietary knowledge available only to Saltworks’ customers and partners. Nevertheless, the lower flux trend in Figure 1 is clear and translates into the need for either higher pressure, more membrane area, or both. With the advent of new 1,740 psi UHP RO membranes, designers can apply higher pressures to improve flux at higher TDS, and ultimately concentrate a sodium chloride solution to 130,000 mg/L TDS. If sulfates are present as an appreciable mass fraction, the brine concentration limit increases up to 160,000 mg/L TDS. At these high TDS ranges, flux decreased by three times relative to RO systems operating at 30-60K mg/L TDS. This means more membrane elements, pressure vessels, and pipework in addition to the higher pressure rated equipment.
Ultimately, capital cost of UHP RO systems is notably higher than the cost of brackish or seawater RO systems (SWRO). Although more expensive than SWRO, UHP RO still has a much lower cost than thermal evaporators. For this reason, UHP RO can be applied upstream of evaporators, to reduce capacity and energy of more expensive evaporation, or reduce volumes sent to disposal or evaporation ponds. It is important to keep in mind that evaporators can achieve higher brine concentrations and produce lower volumes than UHP RO. For larger flow rates, a combination of technologies, each operating at an optimum capacity, may be the most economical. Typical brine concentration limits and volume reduction ratios are summarized in Table 1.
Table 1: Typical brine concentration limits and volume reduction ratios
| Technology | TDS Brine Limit (mg/L) | Brine Volume Reduction Relative to SWRO |
|---|---|---|
| SWRO (1,200 psi) | 80,000 | 1.0x |
| UHP RO (1,800 psi) | 130,000 | 1.6x |
| Conventional Evaporator (thermal or MVR) | 220,000 | 2.8x |
| SaltMaker Evaporator (thermal) | 450,000 or solids | 5.6x |
Reverse osmosis systems have been capable of 1,740 psi operation for a number of years as disk tube RO (DTRO) technology. DTRO employs the same membrane material as SWRO, with the membranes arranged as a series of vertically stacked disks in specialty pressure vessels. The primary benefit of UHP RO is that the membranes are spiral wound—the most widely applied RO technology included in every seawater RO desalination plant worldwide. DTRO is a specialty niche manufactured product, while spiral wound RO membranes are widely produced, available and interchangeable from multiple manufacturers.
Figures 2 to 5 compare spiral wound and DTRO membrane construction.
Spiral wound membranes have captured over 99% of the global RO membrane market share. Their production has been perfected over the years, resulting in lower cost, high quality, and broad availability. In addition, spiral wound membranes make use of the most common 4” or 8” pressure vessels used in RO, versus a specialty product. Put simply, the end user deciding between either spiral wound or disk tube RO should consider if they prefer commonplace or specialty products. Spiral wound membranes are employed in over 95% of all global desalination plants. DTRO offers some claimed shear benefits and the ability to operate on more turbid waters; however, a well built spiral wound system with high membrane cross flow can achieve the same. Saltworks builds our XtremeRO systems with spiral wound membranes to assure the customer a widely available product that is not dependant on a single manufacturer. We also build in chemical, process, and mechanical considerations that are required to maintain membrane longevity.
When reaching for ultra-high recovery, scaling ions and organics are concentrated to very high levels. Combined with the ultra-high pressure, they can impinge on the membrane surface at a high force. One must be extremely cautious to manage any membrane scaling or fouling risk. Organics may require a combination of pre-treatment, or a Saltworks continuous-removal approach that is optionally built into XtremeRO. As for removing scaling ions that can block the membrane surface, a variety of both mechanical and chemical methods are available. First, one must know the scaling ion concentration and its solubility. The solubility of scaling ions can be reviewed on our Periodic Table of Scaling Elements. If scaling ions can exceed four times their solubility in the RO brine, even with anti-scalants, they should be reduced to a safe level.
One method of removing scaling ions is to apply a chemical softening technology, such as BrineRefine, to remove or reduce ions such as silica and calcium. When chemical softening is applied before RO, it is important to:
BrineRefine technology manages all of the above. It was specifically developed to be paired with ultra-high recovery RO systems, as described in this article. In addition to chemical matters, hydraulics and energy are also important considerations. Hydraulically, designers should aim for high cross flows, which provide high brine velocity to wash away any boundary layers of stagnant water next to the membrane surface where scale and fouling can occur. Steps can be taken to reduce energy.
Although energy recovery devices (ERDs) rated for 1,740 psi do not exist yet, an ERD can be avoided and additional benefits realized by including a high-pressure circulation pump as shown below. The high-pressure recirculation pump recycles and upgrades the pressure of a portion of the RO brine. This prevents the breaking of all of the brine’s high-pressure energy before recirculation, lowers feed pump energy, and also beneficially improves cross flow velocity across the membrane surface. Saltworks’ XtremeRO incorporates all of these considerations. If the reader would like to learn more about state-of-the-art, technological advancements in RO design, they can read this accompanying article.
Important engineering considerations also include:
Saltworks considers all of the above and more when designing an XtremeRO system for our clients. We welcome the opportunity to work with partners, including integrating with an upstream lower-pressure RO that may release a scaling brine. The figure below shows an example system where an existing RO-1 is in place, but additional recovery of brine volume is desired. This is accomplished by the integration of BrineRefine and a second RO-2 optionally employing UHP RO technology. This system has built-in intelligence to continuously adjust operation to prevent scale formation and ensure peak performance, while also communicating between upstream and downstream treatment assets to deliver value to your entire treatment train. As a result, minimal liquid discharge (MLD) and peak RO brine concentrations of 130,000 mg/L TDS are achieved.
Contact Saltworks to review options for your project. All we need is your chemistry and capacity, or a live water sample for testing at our lab.
Saltworks Technologies is a leader in the development and delivery of solutions for industrial wastewater treatment and lithium refining. By working with customers to understand their unique challenges and focusing on continuous innovation, Saltworks’ solutions provide best-in-class performance and reliability. From its headquarters in Richmond, BC, Canada, Saltworks’ team designs, builds, and operates full-scale plants, and offers comprehensive onsite and offsite testing services with its fleet of mobile pilots.
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