Selenium compounds can be removed from water by physical, chemical, or biological processes. Some of these methods, however, can be cost-prohibitive and are not always successful at meeting discharge limits. BRT’s successful method of removing selenium from water is “biological transformation”. This involves changing the soluable oxide forms of selenium (selenate and selenite) to the elemental form, which is non-soluble in water, and as a result causes selenium to precipitate out of solution to meet discharge limits. Microbiological methods of selenium detoxification have been documented to be both successful and cost-effective.
In anaerobic environments (those devoid of oxygen), bacteria have evolved respiratory mechanisms which utilize compounds other than oxygen to generate energy. Certain bacteria have been found that can breathe using selenate and selenite as a terminal electron acceptor instead of oxygen.
As part of their metabolism, and in the presence of our proprietary nutrients and substrate, the bacteria will then reduce selenate and selenite to elemental selenium. Therefore, as a result of the activity of selenate-respiring bacteria, selenate and selenite are converted from water-soluble compounds into non-soluble elemental selenium which precipitates out of the water.It has been well documented that the growth and metabolic activity of the intrinsic selenate-reducing bacteria can be promoted and maintained by BRT’s proprietary nutrients and substrate. Our process requires no electricity, produces no sludge and allows discharge limits to be obtained, even in winter months.
AMD results from the oxidation of pyrite (FeS2) or other sulfide minerals by oxygen. Resultant groundwater has high concentrations of sulfate (SO42-), ferrous iron (Fe2+), and other metals. The ferrous iron in the contaminated groundwater is subsequently oxidized to ferric iron (Fe3+) upon contact with oxygenated waters. The precipitation of ferric oxyhydroxides produces acidic conditions. This acidic water, high in metal content, exits the mine pool, contaminating the receiving ground and tributaries. The following chemical reactions provide a detailed view of the generation of AMD.
The Use of Sulfate Reducing Bacteria to Abate AMD at the Source
Sulfate-reducing bacteria (SRB) can be used to treat AMD. SRB use organic carbon compounds [(CH2O)n] as carbon and energy sources and sulfate (SO42-) as an electron acceptor for respiration. SRB produce hydrogen sulfide (H2S) and alkalinity (HCO3-) as byproducts of their respiration. The release of hydrogen sulfide into waters containing high concentrations of metals (Me2+) can lead to metals precipitation as metal sulfides. Overall, the activity of SRB leads to decreased concentrations of iron, sulfate, and heavy metals as well as an increase in pH and alkalinity.
An active area of scientific research involves utilizing SRB to treat AMD. SRB can play an important role in the geochemical cycles occurring within a mining environment. Multiple studies have documented the presence of SRB within the mining environment. Although SRB generally prefer anoxic, neutral conditions, SRB have been documented to tolerate oxygen as well as low pH conditions. SRB have been isolated from anoxic mine tailings with a neutral pH, as well as from oxic, more acidic tailings.It has been well documented that SRB activity can be promoted and maintained by the addition of organic carbon sources. The activity of SRB has been shown to have a dramatic impact on AMD water quality.
Acidic waters consume alkalinity and can compromise the ability of SRB to promote the reduction of sulfate (thereby eliminating the desired generation of alkalinity and the precipitation of metal sulfides). Therefore, BRT takes the approach of promoting sulfate reduction within the naturally occuring eco-system prior to exposure of the waters to oxygen and the resulting acidification. To build upon existing data about the documented benefits of enhanced SRB activity, while eliminating potential problems with permeable reactive barriers, BRT treats mining waste on a site specific bases. By promoting the activity of SRB through the addition of an available carbon source as well as a source of proprietary nutrient blend to the mine pools prior to discharge, BRT’s approach in effect creates an in situ reactor to reduce concentrations of sulfate and dissolved metals, while increasing alkalinity in the pool waters.
Bioreactors can be a powerful tool for stimulating the activity of selenate-respiring bacteria as a means of meeting discharge limits for selenium. They ensure contact of impacted water while providing a controlled ecosystem where the bacteria can thrive. However, like all living systems, this ecosystem can be impacted by external or internal factors and become less efficient. Through BioRemedial Technologies, Inc.’s (BRT) unique, site-specific approach, these ecosystems are examined and enhanced to ensure optimal performance. The following is a brief description of each diagnostic step BRT performs. Detailed information and procedures are available on each procedure upon request.
BRT can monitor the health of the bacterial population within a bioreactor system through bacterial enumeration and / or DNA identification. Enumeration ensures that the bacterial community is present in robust numbers (both beneficial and non-beneficial bacteria). DNA techniques allow for identification of specific bacteria that may be required for the desired activity in the bioreactor system. Both techniques provide evidence of change in bacterial community structure within a bioreactor that can correlate with changes in bioreactor performance.
Selenate-Respiring Bacteria Seeding
Using site water, BRT can select and cultivate selenate-respiring bacteria native to the specific site. These bacteria can be used as seed culture to enhance selenate-respiratory activity within a bioreactor system. This seeding can be particularly advantageous during time periods when activity in the bioreactor may decline, such as cold weather months.
The carbon substrate is a particularly important variable in the optimization of selenate-reducing bacterial activity, as selenate-reducing bacteria require an external carbon source to support their respiratory activity. A laboratory “Treatability Study” is designed to be a kinetics investigation that evaluates the impact of various carbon substrates and / or nutrient sources on bacterial selenate-reduction activity. The “Treatability Study” allows for the determination of the substrate yielding optimal bacterial activity for the lowest cost. Carbon supplementation information is particularly useful in cold water conditions and / or high flow rate situations when a bioreactor may need amendments to remain in compliance.
Oxygen Addition to Combat BOD/COD Issues
A potential negative side effect of a bioreactor may be the Biological Oxygen Demand / Chemical Oxygen Demand (BOD/COD) at startup. The BOD/COD removal efficiency of aerobic biological treatment processes depends on a number of factors including (but not limited to): influent BOD loading; nutrient to biological mass (FM) ratio; temperature; nutrient levels; and dissolved oxygen (DO) concentrations. BRT can monitor these factors and recommend the changes needed for optimal treatment. Typically, oxygen is the limiting factor. BRT can provide oxygen systems for rent, to reduce capital expense at start up. BRT units incorporate state of the art oxygen generators in lieu of air sparging. This optimizes overall system efficiency and eliminates the need for constructing a large holding pond to handle the BOD/COD at start up.
This process uses the carbon supplement and selenate-respiring bacteria seeding techniques discussed above. The difference being that these amendments are injected up-gradient to the bioreactor. Up-gradient treatment is useful when the size of the bioreactor is limited by available space and when the raw selenium concentrations are too high for the bioreactor to reduce to compliance levels.