Sulfate Discharge: Measurement and Cost-Optimized Removal

Sulfates Selective Removal

Sulfate Discharge:

Measurement and Cost Optimized Removal

June 2nd 2020

Key Takeaways

  • Sulfate levels in many wastewater flows need to carefully monitored and moderated. In high concentrations, they are a threat to the natural environment and livestock, and they are therefore subject to regulations.
  • There are many sulfate treatments available, with different advantages and disadvantages. Factors for consideration include capital and operational costs, solid Vs. liquid brine reject for disposal, if the need is seasonal or year-round, and suitability for adverse operating conditions.
  • A ‘surgical’ chemical precipitation approach can be a very good choice for some wastewaters, such as in mining. It removes sulfates and produces a solid by-product, without a liquid brine waste.
  • Selective membrane separations such as nanofiltration do an excellent job at rejecting sulfates while passing other TDS to the permeate for discharge, reducing brine management costs in comparison with reverse osmosis.
  • Advanced sensing and controls can allow even more cost-effective performance and adaptability, specifically by only treating a portion of the flow, minimizing costs, and enabling the plant to adjust as inlet sulfate concentrations change.

Sulfates Under Scrutiny

Many human and natural processes increase the levels of sulfates in waters. Sulfates are used, produced or of concern in many processes e.g. mining, oil & gas, and the manufacturing of fertilizers, dyes, paper, soaps, cosmetics, pesticides, and more. In fossil fuel power plants, sulfates are collected in flue-gas desulfurization (FGD) systems. In mining, metals are often extracted from minerals containing sulfur—and processing means oxidation to sulfates.


Although not generally very dangerous to humans, moderate levels (250–500 mg/L) of sulfates in drinking water are associated with an undesirable taste, and higher levels can cause illnesses such as diarrhea (>1000 mg/L). In recent years, elevated sulfate levels have been closely associated with negative environmental outcomes. Sulfates can kill aquatic plants while feeding algal blooms, causing severe disruptions to eco-systems. Sulfates can be very dangerous to ruminants like moose and cattle because their digestive systems can convert sulfates to toxic hydrogen sulfide. Sulfates can also form precipitants on stream beds, covering spaces that aquatic organisms need for habitat and breeding.


For these reasons, sulfates are increasingly being subjected to regulatory guidelines and public scrutiny. Sulfate discharge limits are imposed on many wastewater flows to ensure that environmental and health impacts of sulfates are minimized. These limits are often dependent on hardness; in British Columbia for example, maximum sulfate limits typically range from 128–429 mg/L. Safety, regulation-compliance, and cost-effectiveness must all be considered in finding appropriate solutions for sulfates in industrial, mining, and other wastewater processes.

FGD coal power plants
Sulfates are produced from many industrial processes

Treatment Options for Sulfates

Fortunately, a variety of treatment options exist to bring total sulfate levels into regulatory compliance. Choosing the right option—or combination of options—is complex, but experts can help you assess and navigate your options. The seasonality, incoming sulfate concentrations, discharge targets, and residuals management options are just some of the factors to be considered. By residuals management, we specifically mean where will the sulfates end up? As a solid to be landfilled, or as a liquid brine that could be added to an existing tailings or evaporation pond? Below, we discuss some of these options:


Sulfate-reducing bacteria bioreactors (SRBRs) use a biological process to convert sulfates to sulfides, which then react with metal species. The precipitation of metals and metal sulfides is useful for recovery while having the added benefit of lowering sulfate levels—helping to meet discharge requirements. However, many SRBRs may have strict operating requirements such as a pH range of 6–8, an anaerobic environment, moderate temperatures, and a supply of organic carbon. Installation and operating costs can be high, and they depend heavily on geography and local water chemistry. Performance may be less predictable than physical and chemical treatment systems.


Both Nanofiltration (NF) and reverse osmosis (RO) methods can reject sulfates. NF rejects multivalent ions such as sulfates, while allowing monovalent ions such as sodium chloride to pass through. NF can operate at extremely high brine concentrations on sulfates, with a unique ability to concentrate chlorides in the permeate in some cases. NF modelling requires expertise and Saltworks is here to help. However, keep in mind that NF/RO will produce a sulfate-rich liquid brine reject. In some cases, this brine can be used to manufacture sodium sulfate, which is used in other industrial processes such as pulp and paper. In other cases, the brine may be ponded, or must be processed in a zero liquid discharge (ZLD) evaporative-crystallizer plant to make a solid waste by-product.

Reverse Osmosis System
Reverse osmosis or nanofiltration can concentrate sulfates

Ion exchange technologies can remove sulfates using an anionic resin. Ion exchange does not require pre-treatment, its energy consumption and other costs tend to be low, and its residues tend to be harmless, requiring little disposal effort e.g. gypsum. However, ion exchange resins do require routine regeneration and are vulnerable to fouling by solids and organics. Fouling is of particular risk when the feed water is from lakes or rivers due to the large amounts of dissolved organics. Furthermore, without supporting equipment such as ultrafiltration, resins can accumulate organics, leading to their support of bacterial growth.


Electrocoagulation can be used for the removal of sulfate ions, producing a solid waste. However, electrocoagulation really struggles to remove high proportions of the sulfate content quickly. Furthermore, electrocoagulation requires precise tuning in response to its operating conditions and can have high electricity and consumable costs.  

Barium Sulfate Reaction
Barium precipitation of sulfate

Chemical precipitation can be an excellent option for the selective removal of one—or a few—specific ions. It produces a low-volume, solid ‘filter cake’ residue that can be landfilled. Almost one hundred percent of the water processed can be discharged, resulting in no brine liquid waste. In physical-chemical processes, specific ions are precipitated out by the addition of a suitable reagent. In the case of sulfates, one can add barium chloride to precipitate barium sulfate. Barium chloride is not cheap, but if the need is seasonal, this method can save on capital costs and prevent the need for brine management. This method will also increase chlorides in the discharge, on a molar equivalent basis to sulfates in the inlet. Saltworks’ engineers can help you model that—and compare it with other options such as NF.


Our BrineRefine system is a prime example of an advanced, intelligently automated chemical softening system. Input your water & chemicals, it will output treated water and solid filter cake.

BrineRefine modern chemical softening
BrineRefine, our chemical precipitation system

Don’t Over Do It! Blend and Save

Treating for sulfates does not mean treating all water and removing all sulfates. Your plant can be optimized to treat a side stream and then blend to meet (but not overtreat!) your target. This can save both capital and operating costs, regardless of the sulfate treatment method selected.


If your flow rate and sulfate concentrations change with time, we have sensor and control solutions to help with that. Our ScaleSense real time ion selective sensor can help a plant adjust its treatment and blending rates. We can help with the process integration and automated control of your treatment assets, in partnership with a ScaleSense real time sulfate ion sensor.

Sulfate Treatment Sidestream Blend
A high-level PFD of a side stream-blend approach to sulfate treatment

Saltworks 'Keyhole Surgery' Approach Example

A “keyhole surgery” approach can be used for sulfates, treating just the discharge limit excess using a small, cost-effective plant. To show how we can do this, consider the example below.


A mine is producing a sulfate-laden wastewater flow with 300 mg/L of sulfate, but the discharge limit is 250 mg/L. To reach this target, a large plant would need to remove one sixth of the sulfate from the entire flow. However, it is unnecessary to use a large plant to treat the entire flow. Instead, we can direct a side stream into a much smaller sulfate removal plant. There we can treat one sixth of the flow to ~0 mg/L.

Sulfates Selective Removal
A Saltworks 'keyhole surgery' approach to sulfate treatment

With our advanced sensing and control systems (supported by ScaleSense), we can:

  • treat the side stream precisely,
  • optimize the treatment dose,
  • and measure for the correct re-blending ratio to ensure continuous compliance.

Treatment costs are reduced significantly because a much smaller plant achieves the same target. With smart design and automation, the lower outlay is combined with low operations costs i.e. consumables and disposal of residue.

How Can We Help?

We do this kind of work every day, so you don’t need to become an expert. Contact Saltworks so we can help you assess, map, and cost your options. If you have your detailed water chemistry, plant capacity, and treatment goals then we can get started immediately. If you have more general enquiries, we can help you with those too.

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SulfateSelect Sulfate Extraction System

Request More Information Learn More SulfateSelect removes sulfates from wastewater, in some cases without brine by-products, thereby removing the expense of brine management. We offer a suite of sulfate processes,

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real-time sensors analyzers scaling ions

Real-Time Sensing and Process Control: Introducing ScaleSense

Real-time measurement provides prompt feedback for controls. A real-time sensor can help to optimize your process, reduce risk, and minimize operational and maintenance demand. Saltworks has developed a new real-time sensor—that is simple and robust—to operate in the high-TDS range.

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Comparison of FGD wastewater, one container is brown, murky and the second is clear water.

Flue Gas Desulfurization (FGD) Wastewater Pilot Test Results

Saltworks completed an off-site Flex EDR Selective pilot test to treat flue gas desulfurization (FGD) wastewater from a coal fired power plant in China. The objective was to reduce chlorides such that the FGD wastewater could be internally recycled and final treatment costs reduced notably.

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Real-Time Sensing and Process Control: Introducing ScaleSense

real-time sensors analyzers scaling ions

Real-Time Sensing and Process Control:

Introducing ScaleSense for Ca2+, Ba2+, SO42-, and SiO2

May 4th 2020

Key Takeaways

  • Real-time measurement provides prompt feedback for controls. A real-time sensor can help to optimize your process, reduce risk, and minimize operational and maintenance demand.
  • Many real-time measurement methods struggle in challenging environments, with waters that have high total dissolved solids (TDS), or waters that are otherwise highly impaired.
  • Real-time measurement and controls on sulfates, calcium, barium, or silica can be useful in a host of applications explored in this blog—from mine water treatment, to boosting reverse osmosis (RO) water production while generating less brine.
  • Saltworks has developed a new real-time sensor—that is simple and robust—to operate in the high-TDS range.

Real-Time Measurement & Controls

If “that which is measured, improves”, then that which is measured in real-time can also be improved in real-time. Obtaining and analyzing data faster—and adjusting a process accordingly—yields substantial efficiency gains. Furthermore, data acquired continuously does not just present one ‘snapshot-in-time’ but can show trends and variations. This type of continuous data facilitates forecasting, allowing anticipatory actions to be taken. Modern operational technologies such as real-time sensors allow process control with unprecedented agility.


While some wastewater treatment or discharge processes use real-time controls, there remain many flows that are difficult or even impossible to measure continuously. Developing a sensor for high-TDS flows that is pH-resistant, easy to-clean, and exhibits high-throughput has proven challenging. In this blog we discuss real-time measurement, briefly review some existing sensor technologies, introduce a new ion selective sensor product (ScaleSense), and then explore some case studies in which a new sensor can be of great benefit.

real-time sensors analyzers scaling ions
Figure 1. Our new ScaleSense real-time sensor.
real-time sensor analyzer scaling ions
Figure 2. ScaleSense can be used to measure Ca2+, Ba2+, SO42-, and SiO2.

Sensors in Impaired Water

There are many sensing and detection methods—with different advantages and disadvantages—used for measuring ions and other chemical species in industrial wastewaters. Among them are chemical titration and colorimetry (spectrophotometry operating within the visible range). Titration is usually a slow process, requiring a lot of skilled operator time. Colorimetry can be performed manually by an operator, or automatically using an appropriate system. However, signals in colorimetry become saturated for high-TDS (>50,000 mg/L) wastewaters, leading to poor performance for highly impaired water. Although this can be mitigated by dilution with deionised or distilled water, this has disadvantages, such as decreased accuracy. The table below shows a comparison of titration, colorimetry, and our new ScaleSense product.

Parameter Titration Manual Colorimetric Auto-Colorimetric ScaleSense
Real-Time Digital Feedback Offline Offline Real-Time Real-Time
Resolution/Uncertainty (±%) ~0.1–1 0.1–1 2 ~2–5%*
Temperature (°C) -5–100 0–40 5–50 -5–80
Testing Volume (mL) ~1–100 ~2–50 Continuous Continuous
Testing Rate (mL/min) Static Static 100–500 Up to 300
Analysis Cycle Time (mins) ~5 ~2–100 ~7 ~5
High TDS Operation Not Accurate

ScaleSense: A New Real-Time Sensor

ScaleSense is a novel, specific real-time sensor for saline waters. It can measure calcium (Ca2+), barium (Ba2+), and sulfate (SO42-) ions and silica (SiO2). ScaleSense was developed to operate on the most challenging waters, especially those in which other real-time sensors do not function effectively. It functions accurately and precisely, even at extremely high-TDS. It is simple, robust, and easy-to-clean. ScaleSense is also corrosion-resistant and operates well over broad temperature (5–80°C) and pH (0–14) ranges. ScaleSense response is sharply defined and accurate, providing fast and continuous feedback for a control that can work well, even with a fluctuating wastewater chemistry.

sulfate-sensor-response 2
Figure 3. ScaleSense response to sulfate injection.

Application Case 1: RO Reliability and Recovery Maximisation

Reverse osmosis (RO) is an essential and widespread water treatment technology. Recently, ultra-high-pressure RO has been developed to achieve ultra-high recoveries. When high recoveries are sought in RO systems, they are usually bottlenecked by scale. For the safe operation of RO systems, the brine concentration should not exceed that which produces scaling with compounds such as CaSO4—they damage the membrane.


As inlet chemistry changes, an ion-specific sensor in communication with the RO system can adjust brine volume and recovery on the fly. If, for example, a chemical softening system was employed, an ion specific sensor can optimize chemical consumption, while protecting the membranes. ScaleSense can behave as a “virtual anti-scalant”, providing dynamic recovery control, to get the best out of RO and softening processes while protecting membranes.

scaling sensor analyzer recovery control reverse osmosis
Figure 4. ScaleSense maximising recovery and protecting an RO membrane system from scaling.

Application Case 2: Mining and Industrial Wastewaters

Some industrial or mining facilities may need to reduce sulfates in discharges. Continuous monitoring enables better decisions with data. If treatment is required, hydraulic capacity can be minimized and cost saved by treating and blending, in an automated fashion to always meet the discharge limit.  For example, in a sulfate process which uses nanofiltration, a sensor could maximise recovery; or alternatively in a barium precipitation system the chemical costs can be minimized by knowing sulfate concentrations and dosing accordingly.

target compliance cost-effective treatment sensor analyzer
Figure 5. ScaleSense optimizing sulfate discharge in industrial and mining processes.

Application Case 3: Cooling Tower Blowdown Reduction

The vast majority of freshwater used in all of industry passes through cooling towers, which are widely used in processes such as thermoelectric power generation (coal, oil, natural gas, nuclear etc.). One of the greatest opportunities to reduce freshwater consumption is by optimizing cooling tower function.


In cooling tower operation, hot water is sprayed in, some of which evaporates. The cooler water is collected at the bottom. Over time, this can become high in total dissolved solids (TDS). To mitigate this effect, high-TDS “blow-down” water is removed, while “make-up” or low-TDS water is injected. Some cooling towers are scale-limited and require the use of anti-scalant chemical consumables, often in excess quantities to ensure that scale does not form.


The image below shows how a real-time sensor can help to improve cooling tower function. An accurate measurement of scaling ions means that the real scaling risk—rather than assumed risk—can be known. Blowdown cycles can be optimized, enabling freshwater savings and less blowdown. Superfluous freshwater consumption and anti-scalant treatment can therefore be safely reduced on scale-limited towers.

cooling tower blowdown scaling sensor analyzer anti scalant
Figure 6. ScaleSense reducing cooling tower blowdown, reducing water consumption.

Application Case 4: Protecting Disposal Wells

Disposal wells are widely used to manage many fluids, by placing them deep underground. A problem which often arises in the disposal well industry is the inadvertent plugging of active wells, which can be caused by the mixing of incompatible wastewaters that produce a solid. Disposal wells may receive wastewaters from different sources. For example, one wastewater might be rich in sulfate ions, while another is rich in barium ions. If allowed to mix or “co-mingle”, the dissolved solids may react to form barium sulfate, a water-insoluble solid which precipitates and risks plugging the well.

Similarly to the above Figure 5, a real-time sensor can protect a disposal well. In the example of a disposal well, a wastewater that is high in one of barium or sulfate can be diverted into a separate stream for management including pre-treatment. The disposal well is therefore protected from inadvertent plugging caused by the co-mingling of different wastewaters.

Application Case 5: Protecting Offshore Oil & Gas Reservoir Assets

During the extraction of oil from offshore reservoirs, the injection of seawater is used to maintain output by preserving pressure. Sometimes the seawater is desalinated, but more often it is treated with nanofiltration. Seawater contains sulfates, which can promote the growth of sulfate-reducing bacteria. This leads to the formation of sulfides such as hydrogen sulfide. The presence of such sulfides causes the oil to become ‘sour’, i.e. the total sulfur level is >0.5%. This sulfur content needs to be removed before the oil can be refined into petrol—which is costly, but necessary to meet regulations. Injecting sulfates can also form scale or plug the reservoir in certain cases. Therefore, it is important to know if sulfates are inadvertently being injected, and quality assure the injection water system.


ScaleSense can monitor sulfates, allowing their controlled treatment for reduction before injection, protecting valuable reservoir assets and minimising the need for any subsequent treatment. It’s small footprint makes it a good fit for offshore operation.

scalesense protecting offshore oil and gas assets from sulfates
Figure 7. Protecting offshore oil & gas assets with ScaleSense.

How Can We Help?

Our process and sensing experts are ready to assess your process. Talk to us about our Process Engineering & Control Services today to find out more about our ScaleSense solutions. Contact us below.

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Anaerobic Digester Wastewater Management During Biogas Production

Biogas Digester

Anaerobic Digester Wastewater Management During Biogas Production

Targeted and Comprehensive Solutions

April 28th 2020

Key Takeaways

  • Digester wastewaters are by-products of biogas production in anaerobic digesters. They require treatment prior to disposal. To meet regulation compliance, treatment options range from minor interventions such as selective contaminant removal, to major interventions such as minimum and zero liquid discharge (MLD/ZLD).
  • Ammonia in digester wastewaters can be selectively removed by stripping, biological treatment, and chemical methods. Selection of options requires careful consideration of local conditions, energy sources, economic usability of the end-products, and most importantly the specific wastewater chemistry, such as ammonia concentration and co-contaminants (e.g. bicarbonate, phosphate, organics, and metal ions).
  • An experienced wastewater treatment team can help you to review and economically assess the treatment options for digester wastewater, from selective contaminant removal to closed-loop ZLD, and everything in-between.

Anaerobic Digesters & Wastewater

Biogas is primarily composed of methane (CH4) and carbon dioxide (CO2). It is produced in an anaerobic digester (AD) using a biological process, degrading organic matter in the absence of oxidant (e.g. oxygen). Figure 1 below is a simplified PFD of biogas production with conventional digester wastewater management.


While some of the organic matter will be broken down into biogas, digestate slurry remains. It is a mixture of solids (leftover organic matter, biomass) and wastewater, in which inorganic salts are dissolved. Solids are separated and sent for disposal/used as fertilizer.


However, without further treatment the digester wastewater cannot usually be re-used directly or discharged into a public sewer. Typical issues include total suspended solids (TSS), total dissolved solids (TDS), ammonia, biological oxygen demand (BOD), and chemical oxygen demand (COD). These characteristics of the digester wastewater are impacted by the composition of the organic matter feed, for example: municipal organic waste, livestock waste, poultry waste, manure etc.


biogas digestate treatment pfd
Figure 1. A high-level PFD of biogas production.

Digestate Wastewater Management

As in other wastewater treatment processes, such as for landfill leachate, digester wastewater treatment involves the application of various technologies and can range from highly targeted selective contaminant removal, up to a comprehensive ZLD process. Figure 2 shows an overview of a comprehensive treatment solution for digester wastewater, including options for zero/minimal liquid discharge (ZLD/MLD).


The most common digester wastewater constituents-of-concern are fine suspended solids, ammonia, and salt ions. However, depending on wastewater characteristics, and project requirements, not all treatment steps may be needed. For example, a low-strength wastewater, with only ammonia concentrations beyond discharge limit compliance, will only require ammonia removal technology. To decide on an appropriate and cost-effective treatment process, it is critical to know the wastewater chemistry in detail and understand the treatment requirements. Saltworks can review water chemistry and specific project requirements so that a digester wastewater treatment process is delivered that balances simplicity, cost-effectiveness, and reliability.

biogas digestate treatment flow
Figure 2. Overview of a comprehensive treatment solution for digester wastewater with MLD/ZLD options (not all steps are necessarily required).

Removing Suspended Solids

Suspended solids in digester wastewater pose scaling/plugging problems in any downstream equipment, especially for ammonia removal and total dissolved solids (TDS) treatments. Additionally, suspended solids make up a significant portion of BOD and COD. They can be removed from the digestate wastewater through traditional physical/chemical separation processes, namely coagulation-flocculation using ferric chloride and polymers, followed by clarification/sedimentation and media filtration of the clarifier overflow.


Alternatively, a more advanced membrane technology, XtremeUF, can be used to remove suspended solids in the digester wastewater without the use of coagulation-flocculation. XtremeUF, shown in figure 3, is a ceramic-based ultrafiltration module that can concentrate suspended particulate into a dense slurry. It is highly automated, compact, and modular, offering an alternative to traditional physical/chemical separation processes for projects requiring low footprint, low maintenance, and process simplicity. A more detailed explanation on ceramic ultrafiltration can be found here.

XtremeUF Ceramic Ultrafiltration System
Figure 3. XtremeUF, an advanced ceramic membrane ultrafiltration system that can treat for organics.

Ammonia Removal

The selection of an appropriate ammonia removal method depends on the concentration, the presence of other contaminants (e.g. bicarbonate, phosphate, organics, and metal ions), local conditions, energy sources, and the economic usability of the end-product (i.e. can it be re-used as a fertilizer). An overview of these ammonia removal options is provided in Table 2, below.

wastewater aeration biogas
Figure 4. Wastewater in a biological aeration tank.
Technology Process Description Considerations & Applicability
Air Stripping - Bicarbonate is removed as CO2
- Base is added to increase pH
- Wastewater and air flow countercurrent in a stripping tower, ammonia is stripped from the wastewater into the air stream
- The air stream then enters a scrubbing tower and flows countercurrent to a sulfuric acid stream
- More economical for high ammonia concentrations (>2,000 mg/L N)
- Ammonia can be recovered as ammonium sulfate fertilizer
- Small footprint, widely practiced
- Acid/base consumption can be high
- Potential for calcium carbonate and/or calcium sulfate scaling in the stripping column
Steam Stripping - High temperature steam flows countercurrent to the wastewater stream, ammonia and bicarbonate are stripped out of wastewater as ammonia gas and CO2
- Ammonia gas, carbon dioxide, and water vapor are condensed out as ammonium bicarbonate solution and/or ammonia solution
- More economical for high ammonia concentrations (>20,000 mg/L N)
- Can recover ammonia as ammonium bicarbonate solution or ammonia solution without chemicals
- Wastewater TDS also reduced, easing the load/cost of any downstream TDS removal process
- Steam stripping & condensation can be further compacted with a mechanical vapor recompression system
- Market share of ammonium bicarbonate or ammonia water fertilizer is smaller than ammonium sulfate fertilizer
- High energy consumption for steam and ammonia vapor condensation
Biological Nitrification-Denitrification - Nitrification: Ammonia is biologically oxidized into nitrate by bacteria in aerobic conditions
- Denitrification: Nitrate is biologically reduced into nitrogen gas under anoxic conditions
- Typically used for lower ammonia concentration wastewaters (<500 mg/L N)
- Ammonia is converted to nitrogen gas, no additional ammonia-based salt to manage
- Organics, phosphorous, and other co-contaminants are also removed
- Requires large footprint
- Large volume of sludge generation
Anammox - Ammonia is directly converted into nitrogen gas under anaerobic conditions - Best suited for ammonia concentration 500–2,000 mg/L N
- Ammonia is converted to nitrogen gas, no additional ammonia-based salt to manage
- More energy efficient and less sludge production than biological nitrification-denitrification process
- Slow startup and extremely temperature sensitive
Breakpoint Chlorination - Ammonia is chemically converted into nitrogen gas with the use of bleach (hypochlorite) - Best suited for low ammonia & organics
- No temperature sensitivity
- Simple installation, minimal start up time, and fast ammonia removal rate
- Bleach consumption is expensive; organics will consume bleach, increasing chemical costs
Struvite Precipitation - Ammonium is chemically precipitated out as struvite (magnesium ammonium phosphate) - Best suited for high concentrations of phosphate
- Struvite fertilizer market share is smaller than other ammonia-based fertilizers

Total Dissolved Solids (TDS) Removal Through Reverse Osmosis and Evaporation

The most common TDS removal technologies are reverse osmosis (RO) and evaporators. Due to wide availability, low cost, and energy efficiency, RO is typically recommended upstream of evaporators. You can read more about how reverse osmosis can decrease treatment costs here.


Digester wastewater enters a seawater reverse osmosis (SWRO) system for TDS and further organics removal. The SWRO operates at pressures up to 1,200 psi, producing a brine stream with around 70,000 mg/L TDS and a high quality permeate water stream. In a typical biogas plant, the SWRO operates at 80% permeate recovery. The permeate can be re-used within the biogas plant or be safely discharged. The SWRO-brine can be disposed of in the compositing process of digestate solids waste, or in a disposal well, depending on which option the economics favor.

XtremeRO Brine Concentrator MLD
Figure 5. XtremeRO, UHP RO system with spiral wound RO membranes.

If MLD/ZLD is required, the SWRO brine can be further treated in an ultra-high pressure reverse osmosis system, such as our XtremeRO system, shown in figure 5. XtremeRO uses spiral wound RO membranes that allow operation up to 1,800 psi and can further concentrate the SWRO-brine, up to a TDS of about 140,000 mg/L in some conditions. Compared to existing thermal evaporation technology, XtremeRO is about 3× more energy efficient for decreasing brine volume, providing a significant decrease in the energy cost of a downstream evaporation system. Saltworks’ intelligent automation for SWRO and XtremeRO make them especially well-suited for the treatment of digester wastewater. Flux monitoring and automated self-cleaning ensure that the membranes remain free of organic fouling.


To achieve ZLD, XtremeRO brine is then treated by a thermal evaporation system. Many options exist for evaporator systems, such as Saltworks’ SaltMaker AirBreather and MultiEffect products, shown in figure 6. The distillate from the evaporator crystallizer will be high quality and can be discharged or re-used within the biogas plant. All contaminants will be reduced to minimal liquid volume or solids for final disposal or land application.

Figure 6. SaltMaker MultiEffect, a thermal evaporator that achieves ZLD.

Contact Us

If you have wastewater needs downstream of an anaerobic digester, contact us. Our process engineering experts will provide options analysis, taking your treatment goals and economics into account.

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Removing Highly Toxic Phenolic Compounds: Wastewater Treatment Options

phenol structure

Removing Highly Toxic Phenolic Compounds:

Wastewater Treatment Options

April 24th 2020

Key Takeaways

  • Phenol and its derivatives (phenols, or phenolic compounds) are moderately water-soluble pollutants, common to the wastewaters of various industries, including oil & gas, paint manufacturing, phenolic resin production, paper and pulp factories, and pharmaceutical industries. Phenolic compounds are used in low concentrations in disinfectants, and are also present in many alcoholic beverages, pharmaceuticals, and cosmetics.
  • Excessive phenolic compounds are harmful to human health and the environment. Chlorophenols, by-products of chlorinating phenol-containing water, are carcinogens.
  • Existing treatment options include biodegradation, distillation/evaporation, adsorption and extraction, membrane separation, and chemical oxidation. A treatment system needs to be chosen and engineered carefully, with consideration of specific wastewater chemistry, operating conditions, and economics.
  • Recent advances in membrane separation technology can help optimize the treatment of phenolic compounds-contaminated wastewaters, making zero liquid discharge (ZLD) or minimum liquid discharge (MLD) options more cost-effective.
phenol structure
Phenol (C6H5OH), the simplest member of the phenols family.

Uses, Toxicity, & Wastewater

Phenolic compounds are profoundly toxic to humans, animals, and aquatic life, and can also form carcinogenic chlorophenols in the presence of chlorine. They are considered priority water pollutants by the EPA and the NPRI in the USA and Canada, respectively. There are strict discharge limits for phenols in many jurisdictions, typically <0.5 mg/L.


The phenolic compounds consist of an aromatic hydrocarbon group bonded to a hydroxyl group (–OH). Phenols are often moderately water-soluble and smaller phenol molecules can be volatile. Phenols occur naturally in small, relatively harmless concentrations. They are also synthesized on an industrial scale for use in disinfectants, medicinal products, and as ingredients for many polymers, resins, and rubbers for the plywood, tire, construction, automotive, and appliance industries. Small quantities are present in alcoholic beverages, pharmaceuticals, and cosmetics. They can be found in the wastewaters of these industries and others, such as oil & gas.

discharge limits
Phenols face strict discharge limits.

Treatment Options for Phenols in Wastewaters

There are many technologies available for the treatment of phenols-laden wastewaters—with highly varied advantages and disadvantages. For example, some are destructive, while others allow the recovery of phenols for potential reuse. The economics of the different processes vary significantly depending on water chemistry, scale, treatment requirements, and other operating conditions. Options need to be carefully analysed for a specific application before treatment decisions are made. Many of the available phenol treatment processes are discussed below and summarized in this table. 

Treatment Description Recovery Pros Cons
Bio- Degradation Uses microorganisms to biodegrade phenols Destructive
  • Cost-effective
  • Safe & easy
  • Simple, harmless products
  • Not suitable for high phenols concentration/ TDS/acidity etc.
  • May require other chemical treatment/aeration etc.
  • Large space requirements
  • Membrane Separation Uses high pressure & fine membranes to reject phenols, other compounds, & ions Possible
  • Cost-effective & reliable
  • Small footprint, scalable
  • Produces low volume of concentrated brine
  • Risks such as fouling & scaling need to be managed
  • Cannot meet very low discharge requirements, but can be combined with other steps
  • Distillation/
    Uses heat & difference in volatility to separate phenols from water Possible
  • Good for high concentrations
  • Good potential for recovery & reuse
  • Costs relatively high
  • Energy intensity
  • Adsorption & Extraction Phenols adsorb to solids such as activated carbon (AC)

    Solvent extraction uses relative solubilities to separate liquids
    Adsorption: typically unfeasible

    Extraction: possible
  • AC/solvents cost-effective for low concentrations.
  • Recovery from solvents possible
  • Polish to extremely low concentrations
  • Not economic for high concentrations due to high consumable costs
  • Requires disposal/recycling of spent AC/solvents
  • Other organics increase cost
  • Chemical Oxidation Uses one of a variety of oxidising methods, e.g. the Fenton reaction Destructive
  • Different strategies available
  • Scalable
  • Costs can be high for some chemistries
  • High consumable or energy costs
  • Products may require further treatment/precipitation/solids management
  • Biological and Enzymatic Degradation

    Biological treatment is a highly established, successful method for the removal of phenols from water. It uses either aerobic or anaerobic microorganisms to biodegrade phenols. It is relatively inexpensive, safe, easy-to-operate, and environmentally friendly. It generally produces simple, harmless products. Membrane bioreactors (MBRs) are a variant of conventional bio-degradation. They use an activated sludge process combined with a membrane filtration step to produce very high-quality water with a small footprint.


    Another phenol treatment uses enzymes to catalyze phenol polymerization for precipitation from water. Depending on the circumstances (e.g. availability and cost of appropriate enzymes) it can be even more cost-effective than biological treatments.


    However, both biological and enzymatic treatments are not suitable for all water chemistries, for example, high concentrations of phenols, high salinity, and high acidity. They may also require interventions such as further chemical additives and aeration. This means that they are not suitable or cost-effective for all applications.

    wastewater aeration biogas
    Biological treatment aeration tanks


    Membrane methods use reverse osmosis membranes to reject phenols, as well as other compounds and ions. They are typically cost-effective and reliable, with low power consumption, a small footprint, and scalability. Although they have good rejection performance for many phenols, their performance can be pH dependent. Furthermore, a polishing step may help to further lower the phenol concentration to reach discharge limits, especially for smaller phenol-family molecules. They are particularly suited to treating high concentrations that are unsuitable for biological treatment.


    Some risks must be considered such as membrane fouling and scaling. However, comprehensive membrane treatment processes with appropriate pre-treatment can manage the risk effectively, making reverse osmosis (RO) and nanofiltration (NF) very cost-effective and convenient for the treatment of phenols. 

    XtremeRO Brine Concentrator MLD
    XtremeRO reverse osmosis system


    There is a variety of evaporation/distillation options possible for the separation of phenols from wastewater. They use the relative volatility of some phenols to purify water. Distillation methods generally have high energy costs and are typically viable for high phenol concentrations only. Due to the spread of volatility across different members of the phenol-family, such methods may not be suitable for all phenols.


    Pervaporation (PV) applies a vapor pressure difference to pull phenol vapor through a membrane, exploiting the difference between the affinities of phenol and water to the membrane. With relatively low energy consumption and simple operation, it is effective for separating low volatility organics, including some phenols.

    SaltMaker MultiEffect evaporator

    Adsorption and Extraction

    Adsorption is commonly used to remove phenols using solid consumables such as activated carbon (AC).  The economics depend on the cost of the adsorbent and the cost of its disposal or recycling. Once spent, the management of phenol-saturated solids is critical and includes options for reuse or disposal. Although usable for the treatment of any concentration of phenols, adsorption is most cost-effective for removing low concentrations, making it an excellent final step polish. The presence of other organics in the water will increase the cost since these may also occupy adsorption sites in the AC. 


    Solvent extraction is a similar, liquid-liquid process. It is standardized and non-destructive  for phenols compounds, useable over a wide range of concentrations. However, it is only cost-effective in some circumstances, typically on a small scale.

    activated carbon for phenol treatment
    Activated carbon

    Oxidation and Fenton’s Reaction

    Oxidation is a destructive method for the treatment of phenols and other organics. It can use a variety of conventional oxidising agents (ozone, chlorine, permanganate etc.), catalysts, and conditions, including irradiation. The choice of oxidation strategy depends heavily on the economics, wastewater chemistry, and other conditions. Costs are typically low to moderate, and oxidation can be used in varied operating conditions. Reaction products may require further treatment or be precipitated (meaning that consideration may need to be given to appropriate solids management).


    The Fenton reaction (and variants) use H2O2 and Fe2+ to produce hydroxyl radicals, which oxidize organics such as phenols. As a phenol treatment method, it is usually cost-effective, and suitable for high concentrations. However, it is pH dependent and may not suit all water chemistries. As with many other chemical treatments, sludge is produced that requires management.

    Contact Saltworks for Phenols Solutions

    Saltworks’ process experts support our clients by providing options analysis—we help you to decide on the right treatment for your phenols-laden wastewater. Providing you with the right solution is what matters most, so our recommendations may include solutions from third-party vendors rather than our own products (or combinations of both). We have extensive experience developing specifications for third party vendors for our clients. Given our position in the water industry, we know who builds the best sub-processes and process elements.


    Saltworks offers two ‘in-house’ solutions for phenols that are adaptable for new or existing treatment chains:

    • Option 1 uses reverse osmosis/nanofiltration, combined with our ultra high pressure XtremeRO system to remove most of the phenolic compounds. We optionally add polishing to remove the rest cost-effectively. Option 1 is better suited for low TDS wastewater.
    • Option 2 uses our SaltMaker evaporator crystallizers to reduce the volume of high TDS wastewater to minimal or zero liquid discharge. Post-treatment systems can polish the phenol-containing condensate. Option 2 is better suited for high TDS wastewater.

    Once a treatment technology is chosen, we can bring our testing, automation, and piloting expertise to the project. Get in touch below to see how Saltworks can help you to meet your phenols challenges.

    Saltworks Engineers Working at an Ammonia Splitter Pilot Plant 1
    Saltworks engineers operating a pilot plant

    Phenols Related Resources

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    Ultra High Pressure Reverse Osmosis for Landfill Leachate

    Reverse osmosis (RO) is the best available technology to treat landfill leachate for surface discharge. Possible trace volatile organic compounds (VOCs) and ammonia emerging in the RO permeate can be removed with a polishing step to meet the highest discharge standards.

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    Economic Ceramic Ultrafiltration for Highly Impaired Water

    ceramic ultrafiltration of petrochemical oil and gas wastewaters

    Economic Ceramic Ultrafiltration for Highly Impaired Water

    January 16th 2020

    Key Takeaways:

    • Ultrafiltration (UF) is widely used in water treatment to filter out particles within a size range of about 0.01 to 1 µm e.g. suspended solids, bacteria, viruses, oils and grease.
    • The vast majority of UF systems employ polymeric membranes, especially in municipal water treatment. However, polymeric UF membranes are restricted in their field of application. Polymeric membranes are less reliable in harsh chemical environments, with slurries that can plug the membrane elements, and when performing oil & grease removal.
    • Ceramic ultrafiltration membranes open the UF application range to the above severe conditions and provide a polymeric ultrafiltration alternative for the treatment of highly turbid water. However, they have historically been viewed as expensive.
    • Recently ceramic membranes have become more widely available at lower costs. In addition, companies such as Saltworks have developed robust process and control systems to keep the ceramic membranes clean. This opens opportunities to employ ceramic UF to filter highly impaired waters where polymeric UF cannot cope.
    • This article introduces a new type of ceramic membrane system and explains how it opens the application range. An example of treatment of oil and grease in produced water is provided.

    XtremeUF: Ultrafiltration for highly impaired waters

    What Do I Need to Know About Ultrafiltration?

    Ultrafiltration (UF) uses a pressurized membrane to achieve TSS (total suspended solids) removal in wastewater within a size exclusion range of about 0.01 to 1 µm. This is a critical regime for the screening of particulate, bacteria, viruses, oils and grease. UF is therefore an essential step for many processes, including municipal wastewater treatment and pre-treatment for reverse osmosis.

    Polymeric and Ceramic Membranes

    The majority of UF membrane installations are polymeric which imposes some important technical limits. Operation at high temperature, exposure to high/low pH, high TSS concentration, or oils and grease may restrict the longevity and performance of polymeric membranes or make them unsuitable altogether. A typical polymeric membrane module consists of thin hollow fibres (imagine a handful of hollowed-out spaghetti) with flow channels 0.2–3 mm in diameter. Failure of polymeric ultrafiltration membranes occurs when these fibres are damaged or blocked, especially likely in severe applications.


    Ceramic UF membranes are a solution in the treatment of a wide range of highly impaired wastewaters such as highly turbid water, oil and grease in produced water and slurries. Usage of ceramic membranes has historically been restricted to especially challenging conditions, largely on the grounds of cost. Recent advances in ceramic membrane manufacturing and availability mean that they are now a commodity item, widely available at a lower cost than in the past. For challenging waters, ceramic membranes can now be a much more viable economic choice.


    A typical ceramic membrane is composed of a sintered material such as alumina or zirconia (imagine a porous rock that can take a real beating). Unlike polymeric materials, ceramics are stable at high temperatures, will not swell and deform in the presence of oil and grease, and can repeatedly withstand harsh chemical cleans. In addition, commodity ceramic membranes are extruded with flow channel openings 2–8 mm in diameter, making them less susceptible to blockage in high TSS applications. A ceramic membrane is highly robust and—with the correct automated processes—can recover from performance degradation to maintain operations.

    Image Credit | 4ewip [CC BY-SA]

    Image Credit | Wikiwayman [CC BY-SA]

    Polymeric (left) and ceramic (right) membranes

    Keeping Ceramic Membranes Clean

    To keep ceramic membranes operating at their best, it is important to employ a combination of automated-cleaning methods. The first defense is to achieve high crossflow, minimizing the stagnant zone or boundary layer near the membrane surface. Higher crossflow, or velocity at the membrane surface, will wash particulate away. There are two crossflow velocities: operational; and a forward flush—whereby velocity is dramatically increased for a short duration by 2-3× to wash away debris. This can require very high flow rates and energy consumption.


    Saltworks has developed a novel hydraulic system that minimizes energy consumption and back pressure while maximizing velocity at the membrane surface. Our forward flushes are done in a smart way, to increase the crossflow to wash away any particulate while minimizing the operational impact on the system.


    Backwashing is another cleaning method which incorporates hydraulic pressure to push particulate off the membrane surface. Chemical cleaning is comparatively expensive and used less frequently. Nevertheless, we build-in and automate all of these systems.


    Saltworks’ roboticized XtremeUF performs cleaning in an intelligent and responsive manner, based on continuous online monitoring of membrane performance. The system selects from multiple levels of cleaning cycles to enable the membrane to maintain flux without irreversibly fouling. We clean only when and how it is necessary. This maintains performance and uptime while minimizing energy consumption.

    xtremeUF ceramic ultrafiltration skid
    Our XtremeUF System


    UF performance is quantified by a measure of filtrate flux: a normalized unit that represents the filtrate flow rate per unit area of membrane. The two most common flux units are LMH (L/m2/h) and GFD (gallons/ft2/day). A typical polymeric membrane will operate in the range of 40–80 LMH (24–47 GFD). A ceramic membrane will operate in the range of 50–1000 LMH (29–588 GFD). Pinpointing where an application will fall on this spectrum is a function of particle size, oil/organics type, concentrations and operating pressure. Lower flux applications will require more membrane area and more energy to meet the same nominal filtrate flow rate. Using experimental data, Saltworks has developed a performance map of expected flux ranges for different industries.

    xtremeuf performance flux ranges ceramic
    XtremeUF flux ranges for a selection of waters from different industries

    The Origins of XtremeUF

    Saltworks focuses on treating the toughest waters. We identified a gap for XtremeUF on the basis of three observations: significant demand for treating waters too challenging for polymeric UF; reduction in ceramic membrane prices; and an opportunity to apply our know-how to implement intelligent automation in cleaning.

    To produce an XtremeUF system, we package commodity ceramic UF membranes into well-engineered and intelligently automated systems that can take slurry concentrations to new levels.

    We needed XtremeUF to meet the following criteria:

    1. Removal of suspended solids and organics from the most challenging slurries and impacted wastewaters
    2. Concentration and thickening of slurries
    3. Performance maintenance through our proprietary process of intelligently automated cleaning
    4. High tolerance to a wide range of waters and conditions, i.e. turbidity, oils, grease, chemicals, pH and temperature
    5. Corrosion-resistance appropriate for high total dissolved solids (TDS) water

    XtremeUF Ceramic Ultrafiltration Feed and Filtrate EOR Produced Water
    XtremeUF feed and filtrate: EOR produced water (left) and filtrate (right)

    Choosing the Right Materials for You

    The choice of construction materials of pipework and pumps is critically important for the performance and longevity of UF systems. For high chloride waters, corrosion-resistant materials are required. Saltworks selects and incorporates the correct materials for your application. This may include Stainless Steel 316L, Super Duplex Stainless Steel, 6 Moly Alloy, Titanium or even non-metallics such as CPVC. For short-lived or cost-sensitive projects, this may include an economic trade-off to accept some corrosion, especially with known replacements.

    XtremeUF Ceramic Ultrafiltration System
    Our XtremeUF system

    What Differentiates XtremeUF from Other UF Processes?

    1. Robust and suitable for a very wide range of applications and operating conditions, Saltworks’ XtremeUF removes suspended solids and organics from the most challenging slurries and impaired wastewaters. XtremeUF can concentrate slurries up to 10% TSS (100,000 mg/L) with a high tolerance to turbidity, oils, grease, chemicals, pH and temperature.
    2. Using our intelligently automated self-cleaning controls, XtremeUF cleans itself as it operates, maintaining flux with minimum power consumption and operator intervention. We can treat wastewaters that no one else will touch.
    3. Commoditised, widely available ceramic membranes to enable a wide selection of vendors into the future.
    4. Versatile and adaptable: XtremeUF can be constructed with a variety of corrosion-resistant materials for use with high salinity, or other corrosive water. We offer filtration specifications of 0.01, 0.05, 0.1, 0.5, and 1.2 µm. It is available in two capacities: 100 and 600 m3/day.

    enhanced oil recovery pilot oil and gas
    XtremeUF pilot at an enhanced oil recovery site

    Example Application: Preventing Reservoir Plugging in Enhanced Oil Recovery

    Enhanced Oil Recovery (EOR) maximizes reservoir production by injecting water to push out more oil. While reinjecting the water may be desirable, the water becomes saline and picks up particulate during the process. The water may also have been treated with polymers to increase its viscosity, to push out even more oil.


    This is a prime example of where traditional polymeric UF membranes don’t fit, while ceramic membranes do. XtremeUF can produce clean filtrate—removing oils, grease, polymers and particulate—from produced water. This allows the injection/reinjection of high-quality water into a reservoir undergoing enhanced oil recovery, reducing the potential for reservoir plugging. It can also facilitate the ocean discharge of produced water, by removing all of the above prior to discharge.


    Here you can read about a pilot test where an XtremeUF system was dispatched to a live oil field in an EOR scenario, and its reliable operations were successfully demonstrated.

    How Can we Help?

    Can the XtremeUF help you to meet your targets? Is ceramic or polymeric filtration right for you? Our Saltworks experts are ready to test your water to establish feasibility and indicate site performance. Contact us to learn more.


    You can also see our spec sheet to see where polymeric membranes stop and ceramic UF membranes start.

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    How to Manage Brine Disposal & Treatment

    The many options for managing brine, a term for saline wastewater from industrial processes, fall under two categories: brine treatment and brine disposal. Brine treatment involves desalinating the brine for reuse and producing a concentrated brine (lower liquid waste volume), or residual solids (zero liquid discharge).

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    Achieving Minimal Liquid Discharge (MLD) with Advanced Reverse Osmosis Membrane Systems

    Membrane System - Reverse Osmosis

    Achieving Minimal Liquid Discharge (MLD) with Advanced Membrane Systems at Maximized Volume Reduction: 5X, 20X, 40X, and 70X!

    December 23rd 2019


    Saltworks Technologies Inc. (Saltworks) completed a pilot on cooling tower blowdown demonstrating minimal liquid discharge (MLD) of up to 70x volume reduction using advanced membrane systems. The MLD paper presents brine management results and economics for water plant designers. Readers will learn how to concentrate brines to 130,000 mg/L total dissolved solids (TDS) with reverse osmosis membrane technology while avoiding scaling and fouling. The work is intended to inform widening use of membrane-based brine concentration systems in order to offset more expensive evaporation or disposal methods.


    Pilot test results are presented for cooling tower blowdown brine at 1,800 mg/L TDS. The pilot test consisted of ways to achieve multiple volume reduction factors (recovery):  5X (80%), 10X (90%), 20X (95%), 40X (97.5%), and 70X (99%). Each jump in volume reduction adds plant complexity and cost. Each step will be explained and mapped so readers can learn about the technology and investment required to take the next step in recovery improvements.


    A MLD treatment plant was trialed and will be reviewed. The plant consisted of a robotized chemical softening system – BrineRefine designed for use with variable water chemistry. This system includes a real time calcium sensor and precipitation management system. The plant also consists of ultra high-pressure reverse osmosis – XtremeRO spiral wound membranes rated for 1,800 psi (120 bar) to achieve ultra high recoveries.


    For the complete paper including pilot test results, sign-up below to download the free paper.

    Download the Minimal Liquid Discharge (MLD) Paper

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    Fluoride Removal from Industrial Wastewater Using Advanced Chemical Precipitation and Filtration

    fluoride etched silicon wafer microelectronics

    Fluoride Removal from Industrial Wastewater Using Advanced Chemical Precipitation and Filtration

    December 3rd 2019

    Key Takeaways: ​

    • Microelectronics production generates wastewater with elevated levels of fluoride often requiring treatment before discharge.
    • Fluoride removal is practiced in drinking water via adsorption, ion-exchange and/or reverse osmosis. These technologies should also be considered for industrial wastewater treatment but are generally better fit for low concentrations of fluoride.
    • Calcium fluoride precipitation and coagulation is another option to remove fluoride from industrial wastewater to meet discharge requirements. This is suited for the much higher fluoride concentrations typically associated with microelectronics industry.
    • New modular, intelligent, and automated chemical precipitation with ceramic membrane filtration system eliminates the challenges of chemical overdosing and large footprint requirements.


    Fluoride Treatment Options

    In the microelectronics industry, fluoride wastewaters are generated during hydrofluoric acid etching of semiconductors. Hydroflouric acid is also employed in solar cells and metal plating industries. Typical concentrations of fluoride in these wastewaters can range from 100 mg/L to more than 10,000 mg/L. In general, fluoride discharge limits are less than 20 mg/L, if the wastewater can be discharged into a public sewer system, and less than 5 mg/L, if the wastewater is discharged into an aquatic environment. In some jurisdictions, the fluoride discharge limit can be less than 2 mg/L. It is noted that some municipalities add fluoride to drinking water (0.5 – 1.5 mg/L) due to its beneficial prevention of dental cavities. However, excessive fluoride is harmful to human health causing skeletal fluorosis (bone disease).

    Existing removal options for fluoride wastewaters include: 1) calcium fluoride (CaF2) precipitation and coagulation, 2) adsorption, 3) ion-exchange, and 4) membrane-based processes such as reverse osmosis and electrodialysis. An industrial wastewater with a high fluoride concentration is often treated through the CaF2 precipitation and coagulation method. The adsorption, ion-exchange and reverse osmosis are more often used in drinking water applications and for final fluoride polishing.

    An overview of the pros and cons for these fluoride removal options are summarized in Table 1.

    Table 1. Comparison of Fluoride Removal Technologies
    Technology Pros Cons
    CaF2 Precipitation & Coagulation • A commonly practiced method through using lime (Ca(OH)2) and/or calcium chloride (CaCl2) to precipitate calcium fluoride (CaF2) down to its solubility limit. This is followed by aluminum based coagulation to further reduce fluoride to meet low discharge limits.

    • Removes other contaminants such as acid, silica and heavy metals (cadmium, copper, chromium, lead, mercury, and zinc).
    • Two separated reaction processes with long hydraulic retention times.

    • CaF2 and Al(OH)3 precipitates result in fine particulates that take long times to settle, requiring large sedimentation clarification tank footprints.

    • Overdosing of lime and coagulation agents, resulting in high chemical costs and excess wet sludge
    Adsorption & Ion-exchange • High availability of adsorbent options, such as activated alumina, modified activated carbon, hydroxyapatite, zeolites, char, and fluoride specific ion exchange resin.

    • Reduces fluoride to 1 mg/L.

    • Best fit for removal of low concentrations of fluoride.
    • Not cost effective for industrial wastewater with high fluoride concentration due to high consumption of the adsorbent and/or high cost to regenerate ion exchange resins.

    • Operation pH limited between 5 and 8. Other anions (e.g., chloride, nitrate, sulfate) present in the wastewater reduces fluoride removal efficiencies.

    • An ion exchange regeneration wastewater brine requires management.
    Reverse Osmosis & Electrodialysis • Reduces fluoride to 1 mg/L.

    • Remove other contaminants including total dissolved solids.

    • Minimal chemical consumption.

    • More compact footprint and greater automation over other options.

    • A good polishing method if required.
    • Membrane is not compatible to hydrofluoric acid or fluorosilicic acid in the wastewater.

    • Possible membrane fouling by other inorganics and organics in the wastewater.

    • Pretreatment often required.

    • A concentrated brine reject is produced requiring management.

    During CaF2 precipitation, lime is used to neutralize any waste acids in the fluoride wastewaters. With fluoride concentrations less than 1,000 mg/L, lime is often selected as the sole calcium source to precipitate fluoride. Lime has a low solubility at 0.18% by weight, so it is used together with calcium chloride, which has a high solubility, for wastewater with high fluoride concentrations. However, excessive lime addition often results in undesired consumption of aluminum coagulants in the downstream coagulation step and increases sludge quantities.


    Calcium fluoride precipitation can reduce the fluoride down to about 8 mg/L to 20 mg/L, depending on the total dissolved solids concentration in the wastewater. A second coagulation step, using aluminum-based coagulation agents, is required to further reduce fluoride to less than 5 mg/L.


    The calcium fluoride precipitation and coagulation method, however, has two disadvantages: 1) CaF2 from the precipitation step and Al(OH)3 from the coagulation step are both very fine particles, so their sedimentation and clarification times are very long, requiring large footprint clarifiers; and 2) overdosing of lime and coagulation reagents results in a higher chemical cost and a large volume of wet sludge for final dewatering treatment.


    Recent technology and process improvements in chemical precipitation and filtration can address both disadvantages associated calcium fluoride precipitation and coagulation.

    fluoride etched silicon wafer microelectronics

    Fluoride Treatment Using Advanced Chemical Precipitation and Filtration

    Saltworks developed a cost optimized solution for fluoride removal using advanced chemical precipitation BrineRefine and ceramic ultrafiltration XtremeUF. An example process flow diagram is presented in the figure below. The incorporation of BrineRefine and XtremeUF in calcium fluoride precipitation and coagulation eliminates the need for large clarification/sedimentation tanks and multimedia filtration and/or cartridge filtration, reducing footprint, maintenance and operating costs.

    Fluoride wastewater treatment PFD

    BrineRefine is an improvement to existing chemical precipitation for fluoride removal. BrineRefine uses intelligent controls for automated precise dosing, avoiding lime and calcium chloride under/over-dosing to precipitate CaF2. XtremeUF is ultra-robust ceramic ultrafiltration that self cleans while it operates. XtremeUF filters the CaF2 and Al(OH)3 fine particles from the coagulation process. The clean water filtrate of XtremeUF could be discharged or reused directly in hydrofluoric acid etching of semiconductors. BrineRefine and XtremeUF are fully integrated into a single smart and automated plant.


    Please contact Saltworks for a detailed review of your fluoride wastewater treatment project.

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    Ultra High Pressure Reverse Osmosis for Landfill Leachate

    Trash being dumped into a landfill

    Ultra-High-Pressure Reverse Osmosis for Landfill Leachate

    November 1st 2019

    Key Takeaways: ​

    • Reverse osmosis (RO) is the best available technology to treat landfill leachate for surface discharge. Possible trace volatile organic compounds (VOCs) and ammonia emerging in the RO permeate can be removed with a polishing step to meet the highest discharge standards.
    • Membrane fouling and RO leachate brine volume management need to be considered up front, and will influence treatment costs.
    • A robust and fouling resistant ceramic ultrafiltration module can pretreat the foulants in landfill leachate to make existing spiral wound RO more reliable.
    • RO leachate brine volume can be minimized through a membrane brine concentrator: ultra-high-pressure reverse osmosis in conjunction with advanced scaling-removal chemical softening. It can concentrate leachate total dissolved solids (TDS) up to 140,000 mg/L – around 10X volume reduction for the raw leachate depending on its water chemistry.

    landfill leachate evaporation pond

    Current State of Reverse Osmosis for Landfill Leachate

    Landfill leachate is a foul-smelling wastewater generated when water trickles down through buried solid wastes. Landfill leachate contains a wide range of pollutants, such as ammonia, heavy metals, organic matter, personal care and pharmaceutical chemicals, pesticides, per- and polyfluorinated substances (PFAS), total dissolved solids (TDS), and many more. In many jurisdictions, landfill leachate is prohibited from discharge into a public sewer without treatment. However, landfill leachate can be treated to meet regulation guidelines for safe discharge into an aquatic environment.

    Existing landfill leachate treatment generally integrates the following options:


    1. physical-chemical (phys/chem) treatment using adsorption, air-stripping, flotation, coagulation/flocculation, chemical precipitation, membrane separation (ultrafiltration, nanofiltration, and reverse osmosis), and advanced oxidation;
    2. biological treatment method using anaerobic, aerobic and anammox treatments; and
    3. evaporation using mechanical vapor recompression and submerged combustion evaporation.

    When landfill leachate must be treated to meet surface water discharge quality, reverse osmosis (RO) is the best available technology. RO offers an absolute separation barrier for all pollutants, more compact footprint and greater automation over other options. A final polishing step after RO can remove any trace volatile organic compounds (VOCs) and ammonia that may slip through the RO membrane into the permeate.


    RO however also has two disadvantages for landfill leachate: membrane fouling and high disposal cost for a large volume of RO leachate brine. RO with unique modules such as disc-tube RO (DTRO) has claimed to address the membrane fouling issue to a certain degree. DTRO equips larger feed channels than existing spiral wound RO. During operation, landfill leachate is recirculated in DTRO module under high turbulence and crossflow to clean membrane surface. However, active membrane area of DTRO is much smaller than that of a spiral wound RO.  DTRO requires a big footprint and high energy consumption with excessive brine recirculation. Operators also allege that DTRO require frequent cleaning maintenance and internal rebuilds. DTRO has not addressed the RO brine issue. The RO brine is usually about 20 – 40% by volume of the original leachate depending on the leachate chemistry. RO leachate brine is oftentimes disposed by evaporation, incineration, advanced oxidation, and/or solidification/stabilization, which are energy intensive and with high cost.


    Recent technology and process breakthroughs in desalination can help address both RO disadvantages while treating landfill leachate. There are several emerging commercial versions of ceramic membrane modules (for example, XtremeUF) that enable foulant removal pretreatment and the use of commoditized spiral wound RO for wastewaters with heavy foulants (for example, produced water from oil/gas production). Ceramic membranes are long-lasting, fouling-tolerant, and easy to clean. After many competitive suppliers entered the ceramic membrane market, membrane costs lowered tremendously over the years. As for RO brine volume reduction, a membrane concentrator, which uses a process of ultra-high-pressure RO (for example, XtremeRO) in conjunction with scaling-removal chemical softening (for example, BrineRefine), can  now further concentrate RO brine up to  a TDS of 140,000 mg/L, around double the upper TDS limit of existing spiral wound RO.

    Ultra-High-Pressure RO for Landfill Leachate

    Saltworks developed a more cost-effective solution for landfill leachate. The solution integrates three recent breakthrough water treatment technologies with four existing options: phys/chem treatment, XtremeUF, membrane bioreactor (MBR), Seawater RO (SWRO), BrineRefine, XtremeRO, and evaporation. Each step addresses a specific pollutant in landfill leachate to deliver an optimized treatment train. The process flow diagram present in Figure 1 shows all steps to provide a comprehensive view to the reader. Some steps are optional, depending on specific project requirements. 


    Saltworks engineers can review specific projects to deliver the balance of a simple but cost-effective treatment train.

    Landfill Leachate treatment

    The phys/chem treatment, if required, precipitates heavy metals that are toxic or inhibitory to microorganisms in the downstream MBR. Some fouling organics (for example, natural organic matters (NOM)) and scaling inorganics (for example, calcium carbonate) are also partially precipitated out. 


    XtremeUF is an ultra-robust ceramic based ultrafiltration module. It removes the solid slurries from the phys/chem precipitation and most organic foulants (NOM, oil and grease) that will otherwise foul downstream MBR and RO. XtremeUF is built with a well-engineered and intelligently automated package that cleans itself during operation. XtremeUF provides a pretreated “clean” landfill leachate for downstream treatment.


    Membrane bioreactor (MBR) biologically degrades ammonia and biochemical oxygen demand (BOD) based pollutants. The MBR is divided into two reaction zones in series: a smaller anoxic zone for nitrogen removal, and a larger aerobic zone where the pretreated leachate is oxygenated for BOD removal. If ammonia removal is not required, the MBR is optional. As the reader can see, every case is unique and may only need a simple treatment train.


    Seawater RO (SWRO) operates at less than 1200 psi to first remove dissolved solids, and non-volatile and non-degradable organics in the leachate after MBR. SWRO produces a brine about 20-40% by volume of the raw leachate as well as a treated water. The treated water may require an additional polishing step for trace ammonia and/or VOCs. This added polishing step is nominal in cost; Saltworks has successfully applied it in several projects to meet the highest discharge standards, including offering onsite pilots that result in successful full-scale projects.


    BrineRefine removes potential scaling components in the SWRO brine for the downstream ultra-high-pressure RO. BrineRefine is an improvement to existing chemical softening: it helps avoid using coagulants that foul downstream RO systems, and incorporates a simple solid management system that reduces the sludge volume, and improves sludge settling and dewaterability.


    XtremeRO is an ultra high pressure RO that operates at up to 1800 psi and is designed with widely available ultra-high-pressure RO spiral wound membrane modules. XtremeRO concentrates the BrineRefine treated brine to approximately 140,000 mg/L TDS. Compared to any existing thermal evaporation method, XtremeRO is about 3 times more energy efficient. The volume of landfill leachate brine after the Xtreme Reverse Osmosis is about 10% of the raw leachate volume, enabling a significant downsizing of final brine treatment which could include recirculation back to the landfill, solidification with fly ash or cement, or a final leachate evaporation by an evaporator/crystallizer


    Saltworks has successfully installed  SaltMaker evaporator-crystallizers at several US landfill sites to evaporate RO leachate brine for minimum liquid discharge (MLD) or zero liquid discharge (ZLD).


    Please feel free to contact Saltworks for a detailed review of your landfill leachate project, including risks and opportunities, as well as options to manage the resulting brine residuals.

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    Coal Ash Pond Water Treatment: Technology Options

    coal ash pond

    Coal Ash Pond Water Treatment:

    Technology Options

    October 29th 2019

    Key Takeaways: ​

    • Coal ash pond water may require treatment as leached pollutants from ash pond water pose human health and ecological risks.
    • Each ash pond water has unique water chemistry and requires an integration of multiple treatment solutions. Treatment methods for the most common pollutants of concern in coal ash pond waters are discussed.
    • Consider membrane concentrators, minimal liquid discharge (MLD), and zero liquid discharge (ZLD) technologies to reduce brine volumes from treating leachable pollutants.

    coal ash pond wastestreams EPA
    Process Flow Diagram by USEPA

    Treatment Solutions for Coal Ash Pond Waters

    The combustion of coal in thermal power plants generates large volumes of coal combustion residuals (CCR), such as fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) materials. These CCR are collectively referred to as coal ash. For decades, coal ash has been managed using river or lake water to sluice it into a large surface water impoundment (also called “coal ash pond”) for its final settlement. Coal ash contain substances such as arsenic, boron, selenium, and heavy metals (cadmium, copper, chromium, lead, mercury, among others). As a result, coal ash pond water, if released into the environment through embankment breaches and seepage of the water through the pond bottom as leachate, may contaminate soil, rivers, lakes, and groundwater. Several U.S. power plants are currently required by the regulators to clean up their coal ash ponds and to treat coal ash pond waters as soon as possible.

    Prior to designing a coal ash pond water treatment plant, it is important to know its water chemistry. This will identify the constituents of concern for treatment and the potential scaling risks. A single treatment method unlikely removes every pollutant to meet all requirements. 

    Rather a plant should be designed to meet the treatment objectives and be cost optimized for the project. This will involve an integration of multiple treatment methods. We hereby provide a “tool box” of treatment methods for the most common pollutants of concern in coal ash pond waters.

    Arsenic: Arsenic may exist in coal ash pond waters as inorganic arsenite As(III), arsenate As(V) or organic methylated arsenic compounds. The species of arsenic present in the water will determine the treatment method. There are three options for removing inorganic arsenite and arsenate:

    • physical-chemical (phys/chem) precipitation using lime-softening and/or aluminum or iron coagulation;
    • physical adsorption using ion exchange (IX) resins, activated alumina, green sand or zero-valent iron-based adsorbents; and
    • reverse osmosis (RO).

    The phys/chem precipitation method is often preferred as it can reduce arsenate down to 0.01 mg/L. When arsenite is the predominant species in the water, an oxidation step is required to convert arsenite into arsenate prior to the phys/chem precipitation process.

    Reverse osmosis has the benefit of being able to remove both inorganic and organic arsenic. Phys/chem precipitation or physical adsorption is not effective for organic methylated arsenic compounds. Although work is being done on advanced oxidation processes to target only organic arsenic removal (for example, Fenton oxidation), these are not yet commercial and may be expensive.

    Boron: Two options for boron-removal are commonly used:

    • ion exchange (IX) using boron-selective resin that can reduce boron concentrations to less than 0.1 mg/L, the spent resins require regeneration with acid and base; and
    • reverse osmosis (RO) under a basic condition (pH > 10), a polish step for the treated water may be required to reduce boron to < 0.5 mg/L (a compliance level regulated by some jurisdictions), the polish step could be IX or a secondary RO; at an acidic condition (pH < 7), boron exists as boric acid, which is too small in radius to be rejected by an RO, boric acid will pass through RO membranes into the treated water.

    Both IX and RO options will generate a waste brine. Option for managing these brines are summarized in this article.

    Selenium: Selenium may exist in coal ash pond waters as selenite Se(IV) or selenate Se(VI). Selenite is more reactive and easier to remove than selenate. Phys/chem precipitation using ferrous or ferric reagents is effective in removing selenite but not for selenate. Existing selenate removal processes are based on converting selenate into elemental selenium through biological reduction. Although electrochemical reduction processes have been studied to remove selenate, they are not yet commercialized and generate a large volume of solid waste requiring disposal of. Reverse osmosis will remove both selenite and selenate, management of the reverse osmosis brine needs to be considered. In same cases the brine can be returned to the coal ash pond, however this will increase the pond salt contents over time. Brine management techniques are covered in this article.

    Heavy metals: Heavy metals (cadmium, copper, chromium, lead, mercury, and zinc) are generally removed by phys/chem precipitation as metal oxides or metal hydroxides. The phys/chem process may not meet stringent discharge requirements, depending on the specific metal and its precipitate solubility. To further reduce heavy metal concentrations after the phys/chem process, a polishing step with reverse osmosis or scavenging agents that reacts and binds with metals may be required. 

    Most metal scavenging agents are toxic so care must be taken to ensure that they do not end up in the treated water.

    Practical Guidance and Considerations

    Each coal ash pond water treatment project will have its own unique water chemistry and required treatment goals. Additional constituents of concern not described above may require removal, such as organics, total suspended solids and total dissolved solids. An experienced wastewater treatment team can engineer and price a solution including an initial desktop study to treat coal ash pond water to meet project objectives and identify risks.

    An exemplary end-to-end treatment train is presented below for the removal of pollutants of organics, inorganics and suspended solids. Not all process steps are needed, depending on the specific project requirements, but are included to demonstrate integration of multiple technologies to meet stringent treatment objectives and end of life for residuals.

    coal ash pond pfd process

    (1) Treat for arsenic, heavy metals and TSS


    • Solids Management


    (2) Treat for selenate, nitrate, and organics

    • Anoxic and oxic biological systems

    • Solids Management


    (3) Treat for residual elements, boron, and TDS

    • Prefiltration 

    XtremeRO Reverse Osmosis


    (4) Polish of treated water to meet stringent limits

    • Reverse Osmosis or IX


    (5) Remove scaling compounds: 

    • BrineRefine 

    • Solids Management


    (6) Reduce brine volumes with membrane system

    XtremeRO Reverse Osmosis


    (7) Minimal liquid discharge (MLD) or 

    Zero liquid discharge (ZLD)
    • SaltMaker MultiEffect

    • SaltMaker AirBreather


    (8) Dispose of residuals

    • Solids to landfill

    • Low volume brine to injection well or offsite disposal company

    Saltworks has the experience and product solutions to treat coal ash pond waters. Our modular advanced desalination technologies (BrineRefineXtremeRO/NF) can achieve ultra high brine volume reduction and evaporator crystallizers (SaltMaker AirBreather, SaltMaker MultiEffect) for minimal liquid discharge (MLD) / zero liquid discharge (ZLD) can help economically treat coal ash pond waters for discharge and manage residuals for end of life disposal. 


    Please feel free to contact Saltworks for a detailed review of your coal ash pond project, risk and opportunities, as well as options to manage the resulting brine residuals.

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    Enhanced Oil Recovery (EOR) Produced Water Desalination

    enhanced oil recovery pilot oil and gas

    Enhanced Oil Recovery (EOR) Produced Water Desalination

    October 7th 2019

    Saltworks completed two successful live oil field pilots, desalinating enhanced oil recovery (EOR) produced water. The objectives were to lower the clients’ polymer consumption costs and improve the quality of injection water, in order to improve hydrocarbon recovery and lower operating cost. These objectives were met, with over 97% up-time. A video walk through of the pilot can be viewed here.

    For further background, some EOR practitioners add polymer to increase the viscosity (“thickness”) of injection water in order to recover more oil. Water returns to the surface as produced water, often more saline. Operators add expensive polymer back to this water to meet their injection viscosity goal. However, polymer consumption and costs increase dramatically with salinity. Polymer costs alone can be over $4-5/m3 injected. By desalinating the produced water prior to polymer addition, the following benefits can be realized:

    1. Reduced polymer consumption to meet the injection viscosity goal.
    2. Re-activate and recycle residual polymer in the produced water.
    3. Deliver an injection water with lower hardness, multivalent ions, and suspended solids which can improve well performance to recover more hydrocarbon.
    4. Increased water recycling, resulting in less produced water disposal and freshwater withdrawal.

    The technology and products employed equally apply to other types of produced water desalination, where operators are seeking to treat produced water for discharge and/or to reduce disposal costs. Sign-up to download a presentation report, and/or contact Saltworks with your produced water challenge!

    Download the free Enhanced Oil Recovery (EOR) Pilot Results

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