EOSi can help mining companies develop compliance strategies in anticipation of regulatory changes expected to greatly reduce the amount of selenium that can be discharged to the environment. MicroC® supplemental carbon sources enable biological treatment for selenium removal, as well as for the biodegradation of nitrates and sulfates commonly found in mining wastewater.


Selenium, nitrogen, and sulfur compounds are naturally-occurring constituents of soil and rock. Disturbance from mining operations can oxidize some of these compounds, making them more likely to leach into water. In addition to any wastewater generated by mineral extraction processes, mining operations often have to collect and treat stormwater runoff from waste rock piles and tailings impoundments, groundwater from mine dewatering, and groundwater from mining-impacted aquifers.

USEPA is currently updating the national recommended water quality criterion for selenium. It is likely that mining operations will need to meet wastewater discharge limits for selenium in the low parts per billion range within the next few years. Mining operations that are already removing selenium from wastewater will likely have to upgrade their treatment systems to achieve such low discharge limits, and other mining operations will need to add a new process to their existing wastewater treatment system.

Biological treatment is a cost-effective and established technology for removing selenium from wastewater as well as nitrates and sulfates. Under anoxic conditions, microbes will first convert nitrates to harmless nitrogen gas, carbon dioxide gas, and water. When the nitrate is depleted, the microbes will begin using less preferred electron acceptors such as selenate and selenite, the most common forms of selenium found in water. The microbes reduce selenate and selenite to elemental selenium, which either precipitates or is trapped in the biomass. Bioreactors designed for nitrate and selenium removal will also support sulfate-reducing bacteria. These biological treatment systems require a supplemental carbon source that serves as an electron donor for these microbiological processes.

How EOSi can help

MicroC® supplemental carbon sources provide a consistent, readily-available source of food and energy to support the microbial populations needed for biological treatment of selenium, nitrate and sulfate. MicroC® is also used for BOD augmentation to help establish an acclimated biomass during startup of new or upgraded biological treatment systems. EOSi can conduct nutrient deficiency analyses and, if necessary, design a custom blend that includes essential microbial nutrients not present in the mining wastewater.

EOSi offers optimization services that maximize the treatment effectiveness of existing biological systems that remove selenium, nitrate and sulfate from mining wastewater. If mine operators need to install new or upgraded biological treatment systems, EOSi can perform bench-scale treatability studies, modeling and simulations, and equipment selection and procurement for the supplemental carbon program. EOSi also delivers complete solutions through our Objective Based Programs such as ongoing technical and application support, remote process monitoring, troubleshooting and optimization programs to our MicroC® customers.