Over the past few decades the Reverse Osmosis (RO) industry has observed an increase in applications in challenging waters. Water sources that were never considered suitable for RO are now commonplace, including municipal waste streams, industrial waste, and even mining waste. Furthermore, regulatory restrictions, environmental obligations, solid waste expenses, and water supply shortages have forced the industry to push system recoveries to higher and higher limits. Many systems are now running at recoveries in the 85% to 90% range to meet these demands. The combination of higher recoveries and stressed waters increases the probability of forming “scale” within the RO membranes. Scales are comprised of sparingly soluble salts that can form in a reverse osmosis system as the water is concentrated. Additionally, the forms of scale and the complications resulting from many scales forming in the same system make the selection and the proper use of the correct chemistry paramount.
Antiscalants provide threshold inhibition and dispersant properties as some of the key mechanisms to prevent scale formation in the RO system. Scaling calculations are the backbone for predicting the types of scales that may form in the RO system and are used to determine the chemistry required to prevent it from forming. For high recovery or nonstandard feed streams calculating scale can be a difficult task because basic scaling indices can give erroneous results and/or incorrect chemical dosages. To predict scaling properly, many factors need to be examined. Thermodynamic and kinetic models need to be considered since they control how much and how fast scale will form and precipitate. With today’s high recovery systems, it is not only important to consider the integrity of the membranes but also the fluidity of discharge lines. The results of extensive laboratory studies will be discussed to demonstrate novel approaches and techniques that confirm not all chemistries for preventing scale are the same. Laboratory studies will include looking at various chemistries commonly used for RO antiscalants. Field data will also be reviewed, demonstrating the importance of proper scaling calculations and antiscalant dosing for optimal system performance, and to keep downstream discharge lines free of scale as well.
The Reverse Osmosis (RO) industry has observed changes in feed water sources and increases in RO system recovery. Highly stressed waters containing extremely high saturations of sparingly soluble salts are becoming the norm. Antiscalants are one cost effective method for controlling these types of compounds. Sparingly soluble salts (scales) vary in behavior including reaction rate, precipitation mechanisms and interaction with antiscalants; and certain antiscalant components can inhibit one type of scale but may fall short for another. Antiscalants do not change the saturation of the sparingly soluble salts. Instead, they delay the precipitation of super saturated salts. Therefore, choosing the wrong antiscalant can have detrimental economic consequences ranging from overfeeding an antiscalant which can foul a system, to scaling a system to the point a clean is required.