As higher recoveries are pushed on RO systems, scale becomes the primary bottleneck to recovery improvements. As discussed in this RO blog, there have been many innovations within the RO field that seek to manage scale through operational ‘tweaks’ in order to achieve higher recoveries. The authors build such RO systems; they understand that operational tweaks and anti-scalants do not remove the underlying risk, but rather delay the symptoms. Operating at or beyond saturation limits means that any misstep could result in membrane fouling, decreased capacity, higher energy costs, plant downtime, and more repairs. RO systems may also not be operated at their peak performance, reducing membrane recovery and wasting money on expensive brine treatment downstream of the process.
Before spending money to remove scale causing ions, consider which of the three categories you may fit in:
1. Basic RO: can achieve the desired recovery with basic RO and a sound anti-scalant program (lowest cost).
2. Advanced RO: applies operational changes such as: cycling concentrations while draining some brine, high cross-flow, automated flushes and CIPs; which may push an RO to its recovery goal. Alternatively, a scaling risk may limit the brine concentration and prevent the maximum achievable recovery, which can add system costs and complexity.
Knowledge tip: maximum RO recovery is defined by osmotic pressure limits (the pressure at which water stops permeating through the membranes). New generation RO membranes can operate up to 1800 psi, or concentrate non-scaling fluids to 130,000 mg/L (NaCl) and 150,000 mg/L (Na2SO4).
3. Chemical Softening + RO: If done correctly, chemical softening can now maximize an RO system’s recovery to reach the osmotic pressure limits. Scale causing compounds are removed by chemical softening as reviewed below, which also reduces the operational risk of membrane damage. As a note of caution, only the correct amount of scaling ions should be removed; excessive dosing will result in higher chemical costs and harmful sludge generation. Chemical softening typically adds between $2 and $5/m3 to capital and operating costs, so it is only a worthwhile investment if the added costs of chemical softening and a secondary RO are lower than the alternative brine disposal or thermal brine concentration costs (typically a minimum of $20/m3).
Design tip: assess the RO brine, and only remove the right amount of scale: we aim for 85% scaling potential of the downstream RO brine (with 100% representing the onset of precipitation).
Correctly applying chemical softening can remove scale and will eliminate/mitigate the fundamental risk of precipitation on a membrane surface, allowing downstream systems to operate at high recoveries and greater reliability. Scale removal is most commonly carried out via chemical softening.
It is important to acknowledge that as water is concentrated, organics will also concentrate which can foul an RO. In more severe cases, solvents concentrate to where they become insoluble and damage the RO membrane. This article focuses on scale removal to achieve ultra-high recoveries, but concentrated organics also need to be checked. Consideration of an organic removal ‘kidney loop’ to treat out organics as they concentrate may be warranted. Nevertheless, do not fret too greatly about the organics as assessments can be completed. Contact the authors for more details.
The most important and first step is to understand your water chemistry: the scale causing compounds it contains and concentration factor at which they will precipitate. Scale comprises low solubility salts that precipitate as water is concentrated. Precipitated salt, if uncontrolled, will plate-out on membranes. This impedes performance and decreases membrane life. Depending on water chemistry, different scale will pose different challenges. Figure 1 below is a “Periodic Table of Scaling Compounds” that shows the solubility of some of the most common scale compounds in industrial wastewater. Readers can use it to check the solubilities against their brine water chemistry.
Calculation Tip: concentrated brine causes scaling, not the feed. So, concentrate your water chemistry individual ions by the same factor of volume reduction. For example, if operating at 75% recovery, that is 4X volume reduction, meaning all the ions in your water will be concentrated by 4X. Multiply your raw chemistry data by 4 and compare to the solubility limits in the Periodic Table of Scaling Compounds above. If two scaling ion pairs are both higher than the concentration in the table above, you could have scale form. Typically, anti-scalants can delay the formation of scaling by 2X above the theoretical concentrations.
The most common scale encountered is typically: silica and calcium-based salts, and metals (such as aluminum and iron). The table below discloses chemical methods to remove specific scaling ions.
Table 1: Chemical Methods to Remove Scaling Ions
| Scalant | Chemical Removal Solution | Concentration (Post-Softening) |
|---|---|---|
| SiO2 | pH 11 + magnesium if not sufficiently present | < 5 mg/L |
| Mg | pH 11 (not a critical scalant but will increase base consumption) | 10–300 mg/L (heavily dependent on Mg levels) |
| Al | pH 11 | Case-specific |
| Ca | pH 9–11 + soda ash if required | 20–80 mg/L |
| Ba | pH 9–11 + soda ash if required | < 1 mg/L (depends on initial levels) |
| Sr | pH 9–11 + soda ash if required | < 5 mg/L (depends on initial levels) |
| Mn and Fe | pH 9–11/oxidation/greensand | |
| F | pH 9–11 | < 2 mg/L |
| CO32- | pH 5 |
Conventional chemical softening systems have over 100 years of history. They are used throughout water treatment plants ranging from municipal to industrial wastewater. The process consists of:
1. Addition of lime to increase pH to 11 to precipitate out silica and heavy metals (aluminum and iron). Some calcium may also precipitate.
2. Addition of soda ash to precipitate out hardness ions (calcium, barium, strontium etc.) if the high pH step does not reach the calcium goal.
Steps 1 & 2 occur in large reaction vessels, often with a suspended agitator. Be cautious of mechanical forces on the agitator and ensure the metallurgy of the tanks and impellers will not corrode in your brine.
3. Clarification + coagulants/flocculants to separate the precipitated solids and water (beware this step if RO is downstream).
Step 3 occurs in a clarifier, which may be vertical laminar plate type for smaller flows, or large circular type shown below in Figure 5. A higher solids brine settles to the bottom which is then directed to a filter press. The clarified fluid leaving the “top” clarifier is often directed to micro or ultrafiltration before an RO system. However, dissolve residual coagulants and flocculant byproducts will not be filtered and could foul the downstream RO.
4. Solids management to handle the precipitated solids for disposal etc.
Step 4 occurs in centrifuges and/or filter presses, with the solids sent to landfill. Metallurgy and operability is extremely important, as is making sure the solids pass a paint filter test for landfill disposal (no free water droplets). If normally occurring radioactive material (NORMs) are present in the raw water, they may concentrate in the filter press solids and should be checked for radioactivity.
Conventional chemical softening, though effective, has some challenges when applied to modern RO systems
Tips:
Recent innovations have been developed to focus on addressing the chemical softening challenges discussed above. BrineRefine, an example of advanced chemical softening, offers a safer, more compact, and smarter system for removing scaling ions at the optimal cost.
Single package: BrineRefine receives inlet feedwater, and outputs high quality filtrate that meets RO’s SDI requirement, and filter cakes.
Pilot test units are available for BrineRefine, including both up and downstream RO systems as shown in Figure 7.
A case study is included below from a mine water treatment project. A primary RO could be used and was recommended by vendor A, however the client still sought further volume reduction. The primary RO could concentrate the brine to 50-55,000 mg/L total dissolved solids (TDS) with anti-scalants, but without chemical softening. At the ~55K TDS, calcium sulfate or gypsum scale could initiate. If according to Figure 4 above, the primary RO can be followed by BrineRefine and a secondary RO, further reducing brine volume in a membrane system by 2X (50% recovery = 50% less brine).
The secondary chemical softening and RO systems will be higher cost than the primary RO, but will be four times lower cost than a thermal evaporation system. The chemistry resulting from BrineRefine and the secondary RO brine is also shown below. Although calcium can be reduced to ~20 mg/L in BrineRefine, this target was not necessary. A lesser calcium reduction to ~135 mg/L was sufficient to ensure the downstream RO does not suffer gypsum fouling when concentrated to within 85% of the gypsum scale potential (i.e. calcium concentrating beyond 400 mg/L). The result is less chemical use and sludge generation, while still maximizing recovery and protecting the downstream RO.
Every industrial water treatment project, chemistry, and goals may differ. It is important to understand the economics of brine management and the cost of each option. Do not spend more money concentrating brine than the savings that would be achieved from reduced disposal. For example, if it costs $5/m3 to achieve ultra-high recovery with an advanced chemical softening and RO system, that should only be done if the brine disposal costs are greater than $5/m3. Thermal systems are at least 4X more costly than $5/m3, so advanced membrane systems make a lot of sense to pre-concentrate before an evaporator.
In addition to understanding water chemistry, think about variability. A good way to do this is to take water samples at periods of both high and low flow and send them into a lab for a detailed chemistry analysis such as the one above.
Finally, although there are many vendors and options on how to package both chemical softening and reverse osmosis, look for a company that does it frequently. Engineers can read online design guides and advise on different mechanical design options, but lessons learned from those with past implementation experience can be invaluable and save your project from repeating the mistakes of the past. Vendors with modular designs developed for repeat dispatch may have a more mature and tested product line. These vendors are available to help you understand your total cost of ownership (capital plus operating cost) and assess if membrane brine concentration is worth it.
Please feel free to contact Saltworks for a detailed review of your project, risk and opportunities, as well as options to limit your brine management costs.
Reverse osmosis-based brine concentrators are reaching new performance levels not seen before. Saltworks is pleased to report that a substantial pilot plant is currently demonstrating 99% freshwater recovery on cooling tower blowdown (CTB).
Saltworks announces ScaleSense: our real-time, ion-specific sensor that works on saline waters i.e. brines. By measuring scaling species live, it enables users to optimize processes in real time as conditions change—without the need for human intervention, lab work, or guesswork.
BrineRefine is a continuous stirred tank reactor (CSTR) that executes diverse chemical reactions efficiently and with precision.
Scale is a crust that forms on membrane and evaporator surfaces, blocking performance. Scaling occurs when ion pairs precipitate out of solution. Know your scaling potential with this free periodic table of scaling compounds and the conditions that cause them to precipitate.