We have been fielding many questions about our Selen-IX™ process for selenium removal, as it is the only commercially available non-biological technology in use by mining and power generation at present to remove the selenate form of selenium to less than 0.5 ppb, without generating liquid brine and fixing the selenium into a stable solid by-product suitable for re-use. We have given numerous presentations on Selen-IX™ and general awareness about the technology is increasing among both industries and regulators. For those who have not yet been able to ask us directly, we have pulled together some of the most often asked questions and our responses.
Q: Does the Selen-IX™ process remove selenite? If not, why doesn’t BQE Water develop a process that also targets selenite rather than just selenate?
A: Selen-IX™ selectively removes selenate from wastewater. We developed Selen-IX™ to target selenate because selenite is removed easily and efficiently using ferric iron co-precipitation. Selenate is the most common selenium species encountered in mine waters and the most difficult form to remove.
Q: How does Selen-IX™ compare with biological systems in terms of costs?
A: This is best answered on a case-by-case basis, as costs vary greatly depending on numerous site-specific factors including the influent water chemistry and target effluent quality. When assessing any water challenge, we take a holistic approach that looks at integrating treatment into the broader water and waste management plan through the full life of the project. In doing so, we factor in the long-term risks and liabilities associated with treatment to create solutions that are adaptable to changes in mine plans or regulatory environments. With biological systems transforming some of the selenium into a more toxic form and generating unstable residues and excess nutrients, the costs and risks associated with managing these outcomes must also be considered when comparing costs.
Q: When is using NF or RO membranes a better option than using IX to pre-concentrate selenium into a stream for ERC treatment?
A: Selen-IX™ is generally more suitable for waters with Total Dissolved Solids (TDS) below 4,000 mg/L. Under these conditions the ion exchange (IX) process concentrates the selenium in solution by at least one or two orders of magnitude more than membranes. For these low TDS waters, the IX regenerant solution has a much higher electrical conductivity than RO/NF brine, resulting in more efficient and cost-effective electroreduction (ERC) treatment to remove selenium. Membranes followed by ERC is more applicable to wastewater with TDS greater than 4,000 mg/L and/or when simultaneous removal of selenium and sulphate is required.
When TDS exceeds 4,000 mg/L, the electrical conductivity of the RO/NF brine approaches that of the IX regenerant and selenium pre-concentration by IX is less efficient. In instances when simultaneous removal of selenium and sulphate is needed, the RO/NF membrane fulfills two functions – first it pre-concentrates the selenium upstream of the ERC, and second it creates supersaturation of sulphate in the brine, enabling sulphate removal by precipitation of gypsum. This combination of membrane plus ERC has been successfully implemented on an industrial scale to treat tailings seepage and underground mine water at a site in the US.
Q: Is Selen-IX™ capable of removing other contaminants of concern besides selenate?
A: Through laboratory test work with different water samples, we have identified that Selen-IX™ could remove arsenic, antimony, bismuth, chromium, and molybdenum. The removal mechanism is similar to selenate – the IX resin captures the targeted contaminant which is released and concentrated in the regenerant for removal by ERC. The results conclusively demonstrate the removal of these contaminants together with selenium, preventing their concentration in the recycled regenerant.
Q: What happens to nitrate in the Selen-IX™ process?
Selen-IX™ has some capacity to remove nitrate but the extent of removal is limited typically to less than 30 mg/L. If more than 30 mg/L nitrate removal is required, Selen-IX™ is combined with other treatment steps to achieve this goal. While the ERC step of Selen-IX™ converts nitrate in the regenerant solution into ammonia, very little of this ammonia reports to the treated water as the regenerant loop is largely isolated from the clean water production in the IX step. In most cases, the increase in effluent ammonia level attributed to Selen-IX™ is less than 1 mg/L. When zero increase in ammonia is needed, ammonia removal from the ERC solution can be incorporated into the overall design.
We have applied Selen-IX™ successfully in both mining and power generation to remove selenium to comply with limits specific to each project. As wastewater challenges evolve, we will continue to innovate to combine one or both treatment steps of the technology with other treatment systems to broaden the applicability to address a greater array of selenium removal needs. Do you have questions about Selen-IX™ not answered here? Ask us!
H.C. Liang, PhD, PChem and Kresimir Ljubetic, PhD, PEng
Ferric co-precipitation – Using ferric iron to remove contaminants and clarify the water.
Nanofiltration (NF) – Semipermeable membrane filtration process that is highly selective of contaminants based on size and charge for removal.
Reverse Osmosis (RO) – Water treatment process that forces water through a semipermeable membrane to remove contaminants.
Electroreduction – Use of electric current to cause a chemical reaction.
Categorías: selenio, sulfato, tratamiento de aguas, normativa