As water flows naturally underground, it picks up some of the minerals it dissolves from the surrounding soil and rocks. These minerals typically include sulfates, which are present in almost every type of water found in nature, including groundwater, oceanwater, and rainwater, and even crude oil and brines. They are also present in many industrial wastes as a result of the facility’s process—mainly in mining and mineral processing applications where sulfates are unearthed via minerals like calcium sulfate (gypsum), magnesium sulfate (Epsom salt), and sodium sulfate (Glauber’s salt). In high concentrations, sulfates can be harmful to human health and the environment and corrosive to pipes and equipment, so they are something many industrial facilities look to remove whether the intent is for release into the local POTW or environment or reuse in the facility’s process.
You might be wondering, what are the best technologies for removing sulfate from your industrial process and wastewater? Here, we look at the different methods used and whether or not your facility should consider the sulfate-removal method so you can make the best decisions possible for your process.
Removing sulfates with adsorption
Adsorption harnesses the molecular forces of attraction to capture contaminants from a liquid stream. The process consists of passing a liquid stream through some type of porous, adsorbent media, and when the soluble contaminants are more attracted to the adsorbent media than they are to the water in the stream, the contaminants bind to the surface of the media while the liquid effluent flows through.
Adsorption is effective for removing sulfates at relatively low concentration levels and can be achieved with technologies like granular activated carbon filtration. Although the technology is generally cost-effective, the media will likely need to be changed out often, which can add up in both cost and time.
Removing sulfates with hydrated lime
Many facilities prefer to remove sulfates, especially with larger concentrations, by adding hydrated lime (Ca(OH)2), which will precipitate out the sulfate as gypsum. This can be useful for facilities that also have large amounts of heavy metals in their waste because the sulfate can be precipitated out at a pH below what the metals would need to precipitate out. Many facilities will see an 80% reduction in sulfate with hydrated lime before they even need to start worrying about the metals, which will minimize the toxicity of initial secondary wastes and sludge created in the precipitation process.
This is often the first step used in the well-known Cost-Effective Sulfate Removal (or CESR) process, which can be integrated into many wastewater treatment trains.
After the initial step is completed (reducing the sulfates with hydrated lime–induced precipitation), the CESR process typically proceeds with a pH adjustment aimed at metals removal and a further sulfate-removal step with a pH increase and a second lime addition and treatment of any secondary wastes. Many facilities like treating sulfates with lime because lime is relatively low-cost and readily available.
Reducing sulfate to lower levels can be achieved in a polishing step with barium salts, which can be added to precipitate barium sulfate (which has very low solubility in water). Barium addition is used in a polishing step as barium salts are relatively higher in cost.
Removing sulfates with distillation
With distillation, the sulfate-saturated liquids are heated to boiling, then the resulting water vapor is cooled in a condenser, and the purified water is captured in a sterile container. Where some separation processes remove the contaminant from the liquid stream, distillation removes the liquid from the contaminants, which are left behind in the still after the water has evaporated away.
Distillation is extremely effective at removing sulfates, but keep in mind that for most industrial applications, distillation requires significant energy expenses for heating, circulation, and cooling, especially for the large volumes of water needed to support production on an industrial scale. Over the last few decades, innovations such as vapor compression and multiple effect distillation setups have led to greater energy efficiency, but they are still costly to run compared to other purification technologies.
Removing sulfates with electrocoagulation
As a simple definition, electrocoagulation is a process of purifying water using electricity. When submerged metal plates and electrical currents charge the surrounding contaminated water, the particles cling together and fall from the solution as an easily removable floc. Sometimes used by itself, the technology can also be incorporated into a pre-or post-treatment phase (before reverse osmosis, for example) to lessen the contaminant removal needs of your membranes or to polish water that needs to be at a higher purity. This can be beneficial to facilities looking to lessen the need for chemicals.
Removing sulfates with ion-exchange
Ion exchange (IX) systems are used across a variety of industries for water softening, purification, and separation purposes. While the chemistry of individual ion-exchange reactions varies from one application to the next, IX is a treatment process whereby dissolved ions are replaced by other, more desirable, ions of a similar electrical charge. This reaction typically takes place in an IX column or vessel where a process or waste stream is passed through a specialized resin that facilitates the exchange of ions.
When it comes to the ideal ion exchange system for removing sulfates, you’ll typically want to see a resin system filled with a combination of cationic (to remove calcium and magnesium) and anionic resins (to then remove sulfates). IX process works best for polishing and with a background stream that is low in other competing salts.
Removing sulfates with membrane filtration
Membrane filtration (like reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF)), is one of the most effective technologies for removing sulfates from industrial processes and wastewaters. The technology can also be a cost-effective way to treat sweaters and brine during the oil extraction process when the sulfates need to be removed efficiently to prevent system scaling and the creation of hydrogen sulfide, a harmful gas that can be created during the oil-extraction process when bacteria break down sulfates.
Often used in combination with other technologies to lessen the membrane fouling outcomes and reduce the need for backwashing or membrane replacement, your facility might use a range of filters to gradually narrow down the particle size allowed through—for example, some facilities, depending on the sulfate concentration, might find it helpful to pretreat their water and brine streams with UF before passing through an NF or RO unit.
To know more, please check SAMCO.