Measurement and Cost Optimized Removal
- 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.
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.
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.
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.
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.
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.
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.