Treatment Of Selenium-Containing Coal Mining Wastewater With Fluidized Bed Reactor Technology

In the coal mining industry today, selenium, in its many forms, is gaining attention. New regulations covering selenium discharge are either in place or being developed. It has been said that handling selenium wastes will increasingly become a limiting factor on the ability of the coal mining industry to grow. While a number of physical/chemical approaches show promise in selenium treatment, when considering optimal cost and treatment efficacy issues, biological treatment – particularly fixed-film biological treatment - is one of the most promising approaches to managing selenium-containing wastewaters. Envirogen Technologies offers a patented fixed-film fluidized bed reactor (FBR) biological treatment technology that has been proven as a „best in class? approach for handling medium to high flow rates of selenium-containing wastewater. Envirogen FBR technology has several distinct features that make it ideally suited for the coal mining industry.

The noteworthy features of Envirogen FBR technology include its ability to consistently reduce selenium levels to less than 5 µg/L, shorter required residence times for treatment and a smaller overall footprint. Its flexibility in the choice of electron donor chemicals can translate into capital and operating cost savings with reduced solids generation. It also responds well to changes in feed flow and composition, consistently achieving discharge limit conditions. In addition, these systems can be modular, with all-weather protection where desired. These features, combined with decades of experience by Envirogen?s team, allow the design and installation of FBR systems with capital costs that are a fraction of the cost of competitive fixed-film biological systems.

Waste rock is generally the primary source of selenium in coal mining operations. When exposed to water, various selenium species will leach or migrate from the rock and enter the environment. Selenium occurs in various valence states from -2 to +6. The speciation of selenium plays a critical role in the effectiveness of any approach for removal, especially to low levels. In aqueous environments, selenium is most often found as the oxygenated anions of selenite, Se(IV) and selenate, Se(VI). Selenium in mine runoff most often occurs in the soluble form .Selenium is commonly found in mining wastewaters in concentrations ranging from 3 to >12,000 µg/L. The US National Primary Drinking Water Standard MCL is 50 µg/L for selenium. The National Fresh Water Quality Standard is 5 µg/L for selenium. The U.S. Fish and Wildlife Service has recommended that the National Fresh Water Quality Standard be lowered to 2 µg/L to protect fish, waterfowl, and endangered aquatic species. Several states have followed with enforcement actions at these same low levels. In Canada, permits may require stakeholders to monitor levels in water or biota, or to comply with Canadian Water Quality Guidelines of 1 µg/L in surface waters. Province and local regulations may vary from these requirements.