Guest Column | October 16, 2014

Understanding Salt Solubility Reaps Benefits In RO System Performance

Harold Fravel

By Harold Fravel, executive director, American Membrane Technology Association

Co-author: Karen Lindsey, VP, Avista Technologies, Inc.

No matter where it originates, the feed water to downstream reverse osmosis (RO) and nanofiltration (NF) membranes contains some level of dissolved salts.  This is the primary reason why membrane processes have become the treatment of choice in supplying end users with a low total dissolved solids (TDS) permeate and why RO system operators benefit from a fundamental understanding of how several key factors influence salt solubility. 

TDS is a measure of the dissolved organic and inorganic substances present in water and values vary depending on the feed water source. Measured components for TDS include salts, minerals, and metals and, as the name implies, excludes suspended solids.

When salts dissolve in water, they dissociate into two ionic species called cations (positive ions) and anions (negative ions). The water molecules that have a slight dipole surround the salt ions and effectively buffer their ionic charges, reducing their natural affinity to bond and form larger particles. However, this dissociation reaction is reversible.  When salt concentrations exceed the waters buffering capacity, the ions are free to bond and form a solid that eventually comes out of solution.  In RO, this phenomenon is referred to as scaling and scale fouling in membrane applications can have a detrimental effect on overall RO system performance and efficiency.

Calcium carbonate (CaCO3) scale

Factors That Influence Salt Solubility

Salt solubility is dependent on a variety of factors.  For instance, every compound has its own unique solubility property, dissolving in water at different degrees depending on the actual composition of the salt.  Sodium chloride (NaCl) dissolves at a rate of 26.5 g/L in 25 degrees C water, compared to barium sulfate (BaSO4), which dissolves at a rate of 0.002 g/L.  When a compound has reached the upper solubility limit, it is said to be 100 percent saturated or supersaturated. 

Ionic strength is a function of the concentration of all the ions present in a solution and is another key factor.  Salt solubility increases with ionic strength which is why salt precipitation in RO seawater applications is not usually a primary concern. While specific seawater salt concentrations vary around the globe, the average salinity is 3.3 percent, giving seawater a very high ionic strength. In addition, seawater RO systems are typically designed to operate at lower relative recoveries, ≤ 50 percent, further decreasing the likelihood of precipitation.

Water temperature also influences salt saturation properties. While calcium carbonate (CaCO3) becomes less soluble as the water temperature increases, the saturation of most salts increases as temperatures rise.  Sodium chloride (NaCl) will dissolve at a rate of 26.5 g/L at 25 degrees C, however, this rate increases to 28.5 g/L in 100 degrees C water. Salt precipitation due to temperature variations is a particular risk in RO when the source is derived from aquifers and the feed water cools as it flows from the point of origin through the distribution line. 

Hydrochloric acid (HCl) reacts to CaCO3 scale.

Forecasting Scale Formation

Predicting solubility limits in RO applications presents a unique challenge for system designers because the salinity of RO feed water increases as it passes through the membrane system.  In the reverse osmosis process, feed waters are separated into two streams, a high-quality permeate and a higher salinity concentrate.  As feed water flows along the membrane series, salts are rejected into the concentrate stream.  While the volume of permeate water produced increases, there is a corresponding decrease in the volume of concentrate water and the total volume of salt remains unchanged, though most is now directed to the decreasing volume of concentrate water.  As salts become supersaturated, they begin to fall out of solution, forming scale in crystalline and other structural forms.  These can act as a catalyst to accelerate additional scale formation and exponentially increase the rate of precipitation.  As scale forms, the compounds settle on membrane surfaces and within feed channel spacers, disrupting flow and velocity and reducing permeate quality.  As the flow through the fouled membrane becomes restricted, operators respond by increasing feed pressures to force water through the membranes and maintain desired permeate flows.  The effects of scale fouling are confirmed by high TDS values in the permeate and/or high delta pressures across the system. 

Calcium sulfate and silica seen via Chromatic Elemental Imaging (CEI) technology

Preventing scale formation

The industry has responded to the adverse effects and economics of scale fouling by embracing the use of antiscalants and dispersants formulated specifically for membrane applications.  Many of these formulators provide customized software programs that will mathematically predict the percent saturation of individual compounds based on a variety of site specific parameters including feedwater constituents, temperature, pH and desired system recovery.  The software then offers an optimal antiscalant and dose rate, typically between 2 and 5 ppm.  The chemical is injected into the RO feedstream to effectively delay the precipitation of sparingly soluble salts and extend system run times between cleanings.  In many cases, the use of antiscalant chemistries can allow operators to increase system recoveries and improve overall plant efficiency.  Reduced fouling rates also help operators maintain consistent feed pressures and enjoy the corresponding energy savings.

Operators who understand salt solubility and saturation and recognize the advantages of effective scale control chemistries can achieve improved RO membrane performance, longer system run times, and reduced cleaning frequencies. 

Harold Fravel accepted the position of executive director for the American Membrane Technology Association (AMTA) after working for Dow Chemical /FilmTec Corporation for 36 years.  He has a PhD in Organic Chemistry from the University of North Carolina and a BS in Chemistry from Florida State University. He resides in Jupiter, FL.

Karen Lindsey is an executive member of the American Membrane Technology Association (AMTA) board of directors.  She is the VP and co-founder of Avista Technologies and has 30 years’ experience in the water treatment industry, working with companies that cast cellulose acetate membrane, produced polyamide elements, and formulated specialty chemicals.