By David Kratochvil
New regulations can change the way we approach wastewater treatment and disposal. Sulfate discharge is under increasing scrutiny, and legislation surrounding the removal and processing of sulfates in wastewater is taking hold in jurisdictions around the world.
As concern grows, industry is forced to reconsider its treatment options. Regulatory agencies have put the spotlight on industries like mining, power generation, and chemical manufacturing to adopt rigorous water treatment practices that improve water quality, encourage reuse, and reduce consumption.
Sulfate can be generated in a wide range of industrial activities. For example, in the mining industry, sulfate is a common by-product of the mining process, as most metals are won from ore bodies containing sulfur-based minerals that oxidize to sulfate during the metal extraction process or due to natural oxidation in waste rock and tailings. In the fossil-fuel-fired power generation sector, sulfate can be produced in flue-gas desulfurization (FGD) circuits within emissions scrubbers and in cooling water.
Once considered relatively harmless to humans, studies have shown that sulfates in wastewater discharged to the environment can play a role in reducing crop yields and fertility rates in livestock. For industry, sulfate in process waters can cause scaling in equipment, resulting in reduced performance and premature equipment failure. This has resulted in regulations that limit the discharge of sulfates into the environment, to as low as 100 ppm in some jurisdictions.
There are several technologies that have been used successfully to reduce sulfate concentrations in water. Membrane technology, while effective, is expensive and can create a residual waste that requires special disposal. New ion exchange technology, on the other hand, is already proving to be a highly effective and environmentally sustainable option to solving the sulfate dilemma.
Sulfates and membrane technology
Industry has typically turned to membrane or reverse osmosis systems to tackle the sulfate dilemma. Membrane systems operate by pushing water through a series of very fine filters that capture contaminants and concentrate them into a supersaturated liquid. Once the system collects the residual liquid, the liquid is either discharged to a storage pond, or heated to evaporate the water content, which produces a crystalline brine by-product that often requires special handling and disposal.
During this process it is not uncommon to see up to 50 percent or more of the water used end up as part of the waste product stream. This is not a sustainable practice in a world where water conservation and reuse are becoming the norm, and it is increasingly difficult for industries to acquire licenses for access to fresh water resources.
Although effective, reverse osmosis systems can be expensive for industrial applications. The crystallization and evaporation process consumes high amounts of energy, and produces a waste product that can result in expensive disposal costs. Consumables cost can also be high, as industrial wastewater tends to comprise a complex cocktail of contaminants that can require frequent replacement of membranes.
The ion exchange alternative
Ion exchange technology operates on an entirely different principle from membrane systems in removing calcium and magnesium sulfates from wastewater. Rather than using membranes to remove contaminants, ion exchange uses chemistry. In very simple terms, it is a two-stage process that uses cationic and anionic resins to remove calcium or magnesium and sulphate respectively, and then regenerate the resins. The technology can be used in a number of ways in water treatment. For example, cation-exchange resins can be used to soften water.
The ideal ion exchange solution for sulfate-reduction is one that combines both anionic and cationic resins. In that case, the process would work as follows:
• Feed water is passed through a series of contactors containing cation exchange resin to remove calcium and magnesium
• The water is then passed through a second set of contactors containing anion exchange resins to remove sulfate
• The clean water is then recycled or discharged safely to the environment
A new ion exchange process called Sulf-IX™ uses low cost, off-the-shelf resins (e.g., sulphuric acid and lime) to remove calcium and sulfate ions from water in various concentrations. It requires no pre-treatment, and leaves no residual waste for special disposal. Besides clean water, the only other by-product of this process is gypsum, a saleable resource used in the production of fertilizer and building materials.
It also consumes up to 90% less energy than a reverse osmosis system and can achieve 95% water recovery rates, thereby reducing the impact on local water supplies. This process can reduce operating and capital costs by at least half for certain resource and industrial applications.
The mining industry is among the leaders in piloting this technology. A large mining operation in the southwestern U.S. for example has responded to pending sulfate regulation by piloting an ion exchange system for removing contaminants from its wastewater streams. In Chile -- where the government has established strict policies for sulfate discharge -- a refinery is in the process of applying ion exchange technology at their water treatment facilities to ensure they stay in line with current legislation.
The newly elected government in the U.S. has made its commitment to environmental stewardship very clear. This sentiment is also being expressed by governments around the world. As a result of this renewed interest, we can expect to see a surge in interest and demand for technologies that safely and effectively address issues such as sulfate discharge.
The challenge will be finding solutions that not only remove contaminants, but also support environmental and fiscal sustainability -- without depleting precious water and energy resources in the process. Ion exchange is one technology that holds a great deal of promise for the industrial sector in managing their wastewater streams.
About the Author: David Kratochvil is the President and Chief Operating Officer of BioteQ Environmental Technologies. He holds a doctorate in chemical engineering, and is a specialist in wastewater treatment and chemical processing, with fifteen years experience in plant design and operations at locations around the world.
April 2009
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