Sulfate Removal & Discharge:
Measurement and cost-optimized removal
June 2nd 2020, Updated May 18th 2021
- Sulfate levels in many wastewater flows need to be 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, whether 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 total dissolved solids (TDS) to the permeate for discharge, reducing brine management costs in comparison with reverse osmosis.
- Advanced sensing and controls allow even more cost-effective performance and adaptability, specifically by only treating a portion of the flow, and enabling the plant to adjust as inlet sulfate concentrations change.
- For solutions optimized for specific sulfate flows, also see our SulfateSelect family of solutions.
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 results in 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 (>1,000 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 ecosystems. Sulfates can be very dangerous to ruminants like moose and cattle because their digestive systems convert sulfates to toxic hydrogen sulfide. Sulfates 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, Canada, 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.
Treatment Options for Sulfate Removal
Fortunately, a variety of treatment options exist to bring total sulfate levels into regulatory compliance. Choosing the right solution or combination can be complex, but experts will help you assess and navigate your options. 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 consideration of where the sulfates will 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 reject sulfates. RO rejects all dissolved solids to a concentrated brine, generating a freshwater permeate. 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 the unique ability to concentrate chlorides in the permeate in some cases. RO and NF modelling requires expertise and sulfate-rich liquid brine reject requires management. In some cases, brine can be used to manufacture sodium sulfate, which is used in other industrial processes such as pulp and paper. In particular, our ChilledCrys hybrid membrane crystallizer can produce sodium sulfate solids without the costs of evaporation. In other cases, the brine may be ponded, or processed in other zero liquid discharge (ZLD) systems—including from our SaltMaker platform—to produce a solid by-product.
Ion exchange technologies 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, for example 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 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.
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 may 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 will help you model this and compare it with other options such as NF.
Our BrineRefine system is a prime example of an advanced, intelligently automated chemical precipitation system. Input your water & chemicals and it will output treated water and solid filter cake from a compact-footprint and modular 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, our sensor and control solutions are a solution. Our ScaleSense real time ion selective sensor helps 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.
Saltworks 'Keyhole Surgery' Approach
A ’keyhole surgery‘ approach can be used for sulfates, treating just the discharge limit excess using a small, cost-effective plant. Consider the following example: a mine produces 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 system that treats one sixth of the flow to ~0 mg/L.
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 capital outlay is combined with low operations costs i.e. consumables and disposal of residue.
To learn more about smart sulfate solutions that combine sensors, membranes treatment, and chemical precipitation see our SulfateSelect solutions.
How Can We Help?
We do this kind of work every day, so you don’t need to become an expert. Contact Saltworks to get help assessing, mapping, and costing 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.
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.
Saltworks completed an off-site FlexEDR 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.