Modern Chemical Softening to Maximize RO Recovery: Scale Removal in Smart, Compact, Modular Systems

Nov 5th 2018

Key Takeaways

  • With the advent of ultra-high-pressure reverse osmosis and the growing importance of brine management, modernized chemical softening systems can enable economic ultra-high recovery reverse osmosis, minimizing RO brine volume, disposal costs, and reliability risks.
  • Chemical softening systems (conventional and advanced) are described, including chemical dosing and tips on how to remove scaling risk.
  • Conventional chemical softening settling times are reviewed as well as the risks posed by coagulant/flocculants that can foul downstream ROs.
  • BrineRefine is introduced: an advanced chemical softening system that addresses many of the challenges faced by conventional chemical softening; there are no coagulants used or foulants introduced (reduce RO fouling risk); it is compact (no clarifiers), modular (minimal site installation); intelligently automated (precise dosing and recovery tuning); and includes a simplified solids management system to remove sludge management concerns.

Why Care About Scale?

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. Saltworks builds such RO systems; we 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 ions that cause scale, consider which of the three categories you may fit into:


1. Basic RO: Desired recovery can be achieved 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 clean-in-place cycles (CIPs), each of 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 1,800 psi, or concentrate non-scaling fluids to 130,000 mg/L (NaCl) and 150,000 mg/L (Na2SO4). 


3Chemical Softening + RO: If done correctly, chemical softening can now maximize an RO system’s recovery to reach 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 that can foul an RO will also concentrate. In more severe cases, solvents concentrate to where they become insoluble and damage the RO membrane. 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 Saltworks for more details.

Scale Management vs. Scale Removal

The first and most important step is to understand your water chemistry—the scale causing compounds the water contains and the 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 shows the solubility of some of the most common scaling compounds in industrial wastewater. Readers can use the figure to check the solubilities against their brine water chemistry.

Periodic table of scaling compounds
Figure 1. Periodic Table of Scaling Compounds (Click image to download PDF)

Calculation Tip: Concentrated brine causes scaling, not the feed. So, concentrate the individual ions in your water chemistry by the same factor of volume reduction. For example, if operating at 75% recovery, that is 4x volume reduction, meaning that 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 shown above, you could have scale forming. 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). Table 1 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 101 and its Challenges

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 the following:


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 a vertical laminar plate type for smaller flows, or a large circular type (see Figure 5 below). 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 (i.e. no free water droplets). If normally occurring radioactive materials (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.


  • Fouling coagulant chemicals are added: Due to the poor settling nature of chemically softened solids, large clarifiers are required. The addition of flocculants/coagulants is often used to help decrease the size of clarifiers needed. See Figure 2 for settling times of chemical softening solids, with and without the aid of flocculants/coagulants. These specialty chemicals improve the settling times of the solids by approximately 3x, reducing clarifier size by roughly 3x. However, iron and aluminum-based coagulants, as well as added polymers, pose a fouling risk to downstream ROs by forming a gel on the membrane surface and requiring almost daily chemical clean-in-place (CIP) cycles. This common occurrence adds downtime, operator frustration, and increased chemical cost. One should not assume that 20-year-old chemical softening textbooks and design guidelines can be applied to RO systems, as these older systems were not designed for utilization with RO.
Table of photos showing differences in settling of solids in test tubes
Figure 2. Conventional Chemical Softening Settling Time with and without Coagulant/Flocculant addition, by Saltworks
  • Large clarifiers: installed to enable settling and downstream solids management. In higher hydraulic capacities, these need to be built at site, incurring higher installation costs and space requirements.
  • Imprecise chemical dosing: the manual steady state nature of operating a conventional chemical softening plant means that there is higher risk of over-dosing chemicals (waste of chemicals) or under-dosing (scaling risk for RO as the scaling ions will not be adequately removed). If inlet water chemistry changes, so should the chemical dosing set points.
  • Solids and sludge management: low-flux filter presses are commonly used, resulting again in a large equipment footprint and more frequent operator attention.


  • Determine the most economic method/chemical for removal. Example: If there are high levels of sulfates and magnesium in the wastewater, the use of lime would be a lower cost option. However, if there are low levels of sulfates and high levels of calcium, sodium hydroxide may be more cost effective if followed by soda ash softening. This is because sodium hydroxide does not add calcium, whereas lime will, resulting in increased usage of more expensive soda ash.
  • Understand your solids disposal costs. Solid waste from the chemical softening process will also need to be disposed. The more chemicals that are added, the more solid wastes are required for disposal.

Advanced Chemical Softening: Introducing BrineRefine

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.

  • No coagulants or flocculants: No chemicals are added that could foul a downstream RO. Rather than use gravity and time to settle, BrineRefine includes a robust, high-flux mechanical separation step, producing a filtered non-scaling brine that is suitable for direct feed into an RO system.
  • Compact: BrineRefine eliminates the need for large reaction vessels and clarifiers. See Figure 3 below for BrineRefine separation of solids from liquid without the use of clarifiers or coagulants/flocculants.
Table comparing chemical softening settling in test tubes
Figure 3. BrineRefine Solids-Liquid Separation without Clarifier and without use of Flocculants/Coagulants
  • Modular: BrineRefine consists of pre-built ISO container-sized blocks and does not need clarifiers. Higher capacity plants simply add more blocks. The modular and factory tested skids minimize site installation, ensure quality, and provide economies of scale across a standardized factory-built fleet, rather than higher cost project-by-project custom designs.
  • Intelligent: The controls are integrated with downstream and upstream unit operations to communicate changes in process conditions and allow the entire system to adjust to allow for maximum reliability. BrineRefine senses and reacts to changes in the feed chemistry, optimizing operation costs and downstream RO recovery while providing the operator with remote control and a simple human machine interface (HMI). Figure 4 is an example of how BrineRefine integrates controls with an RO system.
Process flow diagram showing how BrineRefine enhances reverse osmosis freshwater recovery
Figure 4. Integration of BrineRefine Controls with Upstream and Downstream RO

Single package: BrineRefine receives inlet feedwater, and outputs high quality filtrate that meets RO’s SDI requirement, and filter cakes.

Photo showing the use of a clarifier in a treatment facility as used in conventional chemical softening
Figure 5. Conventional chemical softening, with large settling clarifiers.
Figure 6. BrineRefine ISO-based design, without large reaction vessels or clarifiers.

Pilot test units are available for BrineRefine, including both up and downstream RO systems, as shown in Figure 7. 

Photo of a Saltworks engineer inspecting a BrineRefine module used for chemical softening
Figure 7. BrineRefine Pilot Unit

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 ~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, brine volume in a membrane system can be further reduced 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 at 4x lower cost than a thermal evaporation system. The chemistry resulting from BrineRefine and the secondary RO brine is shown in the table below. Although calcium can be reduced to ~20 mg/L in BrineRefine, this target was not necessary. A lesser calcium reduction to ~136 mg/L was sufficient to ensure that 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 use of fewer chemicals and reduced sludge generation, while still maximizing recovery and protecting the downstream RO.

Every industrial water treatment project, chemistry, and goal 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 ultra-high recovery can be achieved with an advanced chemical softening and RO system for $5/m3, 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 to a lab for a detailed chemical analysis, such as the assay shown above. 


Finally, although many vendors have options on how to package both chemical softening and reverse osmosis, look for a company that does this 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 who have modular designs that were 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 cost plus operating cost) and assess if membrane brine concentration is worth the investment.


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.

About Saltworks

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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