Formatted Title
Predictable and Sustainable ZVI Treatment Approach for Selenium in the Mining Industry
Background/Objectives
More stringent regulations are making treatment of selenium in wastewater a bigger challenge in industries such as power, refining, mining and construction. With increasing focus on water quality, the federal, state and local regulatory agencies are moving towards tighter selenium discharge limits to as low as 3 µg/L (ppb). Although the US EPA has designated biological treatment as the Best Available Technology (BAT) for selenium treatment, a more efficient, simple, reliable and cost-effective treatment technology is still lacking in the market.
In water and wastewater, selenium generally exists as soluble oxyanions, selenite (SeO32−) and selenate (SeO42−). Organo-selenium compounds are also present in some wastewater, such as coal mining and refining waste streams. Among these, selenate is the more mobile and prevalent species, which is also more difficult to remove. This abstract provides an insight into an innovative selenium treatment media, which has high selenium removal efficacy and capacity regardless of the presence of interfering co-contaminants in the water, such as sulfate, nitrate, phosphate, molybdenum, mercury, and most other heavy metals. The new media developed for selenium removal is an engineered and surface-modified iron-based media, which has high surface area, reactivity and contaminant capacity. Reduced iron is the functional unit in this media. The media functions by reducing selenate/selenite to elemental selenium, which is then strongly adsorbed on the media surface.
Approach/Activities
This technology was evaluated with both bench and pilot studies. One pilot study was conducted to evaluate the treatment of selenium in CCR effluent. General process arrangement for the flow-through pilot unit involves six (6) up-flow columns connected in series, filled with the media up to 70% of the column height. The influent pH was maintained between 4.5 and 5.0 by dosing acid before the first, third and the fifth column. The pilot unit was fed with the CCR effluent, which contained 374 ± 38.2 µg/L of selenium (all selenate) along with co-contaminants such as molybdenum, arsenic, boron, sulfate, and nitrate. The treatment target for selenium was 12 µg/L. The total operational EBCT was 60 min (10 min in each column). The pilot unit was operated continuously for 30 days with selenium consistently removed below 12 µg/L. Additionally, the pilot also consistently removed molybdenum, a co-contaminant of concern, from 3,000 µg/L to almost non-detect levels.
A second pilot study was conducted on effluent wastewater at a Kaolin Mining location in Georgia. The pilot was ongoing at the time of this abstract submittal, but had been succesfully operating with no selenium breakthrough for 21 days. This abstract will be updated based on the results of that pilot.
Results/Lessons Learned
Contaminant breakthrough was not observed during the study period. However, based on the media isotherm studies, the media is estimated to last for 3 to 9 months for such wastewaters. In summary, the pilot study demonstrated successful use of this new selenium removal technology for the treatment of CCR wastewater at relatively short EBCTs, demonstrating its potential to be established as a disruptive selenium treatment technology.
On the mining wastewater, the technology successfully demonstrated selenium removal using the ZVI solution. The ZVI material was reconditioned successfully, extending the life of the media and reducing the wastefootprint and cost associated with selenium removal.