Patent Application
20120241376
This invention relates to a method for removal of selenium from water. More particularly, this invention relates to the use of a combination biological reduction process followed by an adsorption process to remove selenium from water.
Selenium is a commonly occurring element that is widely dispersed in rock and is frequently encountered in coal and coal mining influenced water. Selenium can be found in one of four oxidation states (Se+6, Se+4, SeO, and Se−2) and often is found as the water soluble ions selenate (SeO4−2) and selenite (SeO3−2). Elemental selenium (Se0) and the most reduced form, selenide (Se−2), are much less soluble in water. Selenium can be found in both inorganic and organic compounds.
Elevated levels of selenium in surface water bodies are potentially harmful to aquatic life. As a result, the federal Environmental Protection Agency as well as various state environmental regulatory bodies have promulgated standards to limit selenium content in lakes and streams. Under these standards, streams must have a maximum selenium content of 5 parts per billion (ppb).
One common technique to treat selenium-containing water is through chemical reduction. In a typical chemical reduction process, a reagent is contacted with an aqueous source to react with a selenium compound. However, such techniques are generally effective only with particular selenium compounds and often experience operating difficulties and/or are not economically viable.
A second technique to remove selenium from aqueous sources is adsorption. A demonstrated adsorptive technique, Ferrihydrite Co-Precipitation, has shown effectiveness in selenium removal, but not to concentrations as low as 5 ppb. Additionally, high operational costs, sludge disposal, and sludge stability concerns discourage use of this process. However, most adsorbent materials, including activated carbon, have been found ineffective in removing selenium, typically in the form of selenate and selenite, to the required levels.
Table 1 presented below presents the results of a series of tests performed on samples of aqueous samples containing selenium, mostly selenite and selenate. The samples were fed into a 55 gallon drum filled with granulated activated carbon. Measurements of the selenium content were taken of the feed going into the drum and of the discharge liquid exiting the drum. These tests showed inconsistent removal of the selenium which generally was insufficient to meet the 5 ppb discharge standards.
A third technique to remove selenium from aqueous sources is biological reduction. It has been found that selenate and selenite can be reduced to elemental selenium and other reduced and organic forms in biological processes using certain microorganisms. Such a biological reduction process can be used to convert the soluble forms of selenium to insoluble elemental selenium which can then be precipitated out of solution. However, although this procedure removes a substantial amount of selenium from waste streams, unless the system is actively controlled, a significant amount of selenium remains in soluble form.
Selenium can be removed from an aqueous source for discharge into a lake, pond, river or stream by a combination of biological reduction of selenium followed by adsorption. The aqueous source first passes through a bioreactor which reduces the selenium compounds in solution to elemental selenium, selenide and organic forms of selenium. An adsorber, such as activated carbon, is then used to remove the organic selenium and selenide from solution.
In the bioreactor, selenium is reduced to the elemental, organic and selenide forms through the action of anaerobic bacteria. In this biologic reduction process, selenium compounds such as selenate and selenite are reduced to elemental selenium, organic forms of selenium, and selenide. The selenide and any organic selenium that was present in the aqueous source largely remain in solution.
It has been unexpectedly found that residual soluble selenium from an anaerobic biological treatment system can be removed using an activated carbon adsorbent. Although such an adsorbent is not effective on the more oxidized forms of selenium, it has been found that the highly-reduced selenide and organic selenium compounds are readily adsorbed by activated carbon and effectively removed from an aqueous source.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawing:
While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.
Upon exiting the bioreactors, the treated water 18 may be subjected to an ultraviolet light source 20 which is adapted to inactivate bacteria in the bioreactor discharge. The bioreactor discharge, with or without ultraviolet light treatment, is then introduced into an adsorption unit 22 preferably containing activated carbon such 12×40 Mesh, reagglomerated coal base virgin activated carbon. Additional adsorbing agents may also be used. Adsorption unit 22 will absorb the selenide and organic selenium compounds contained in the bioreactor discharge. Provided sufficient hydraulic residence time, such as 30 hours, to establish sufficient anaerobic conditions and biological activity to allow selenium reduction, and sufficient contact with suitable adsorbents, the treated discharge 24 will contain less than 5 ppb selenium.
Optionally, if desired, prior to feeding the aqueous source 12 into the first bioreactor 14, a pump injects a carbon source into bioreactors 14 and 16 to initiate and sustain microbiological metabolic processes. A dilute solution of molasses has been found to work well as a suitable carbon source. The addition of this carbon source has been found to enhance the ability of the anaerobic bacteria to biologically reduce the selenium.
Although the identity of the bacterium performing the selenium reduction has not been determined, it is currently believed that the bacterial species that perform selenium reduction overlap the bacterial species that perform nitrate reduction (also known as denitrifiers). Some research has been conducted to identify selenium reducing species based on 16s rDNA identification. These methods compare DNA found in samples to the DNA of known bacteria to identify bacterial species found in the sample. It is important to note that 16s rDNA methods only identify which bacteria are present in sample and do not necessarily identify which species are actively reducing selenium. The 16s rDNA results have shown that species capable of selenium reduction are very diverse, and can be found in three major phyla: Proteobacteria, Actinobacteria, and Fimicutes. There is also some evidence that a group of bacteria and archaea called haloalkaliphiles, which aren't frequently found in surface fresh water systems, may perform selenium reduction.
Some highlighted genera in the literature that are thought to reduce selenium, based on the 16s rDNA methods include: Bacillus spp., Pseudomonas spp., Enterobacter spp., Aeromonas spp., Brevundimonas spp., Staphylococcus spp., Arthrobacter spp., Paracoccus denitrificans, Planococcus spp., Serratia fonticola, Escherichia spp., Citrobacter braakii, Klebsiella spp., and Stenotrophomonas maltophilia.
The system of
The data from the experiments is presented in Table 2 below.
As shown in Table 2, the bioreactors T-1 and T-2 did not sufficiently reduce the selenium content of the water. The typical sample exiting the bioreactors had a selenium content that was roughly on average twice the permissible amount of 5 ppb. However, when the biologically reduced samples were subjected to activated carbon adsorption, the discharge not only met the selenium content limits but, for the most part, was too low to be detected.
While not being bound to any particular theory of operation, the biological reduction of the selenium compounds to selenide and organic selenium compounds enhanced the ability of the activated carbon adsorber to remove the selenium compounds from the water. This can be seen by comparing the results in Table 1 with those in Table 2. As shown in Table 1, activated carbon is not adequate to remove selenate and selenite from the water source. However, as shown in Table 2, if the selenate and selenite is biologically reduced to selenide, then the activated carbon can remove the selenium from the water. Thus the combination of the biological reduction followed by adsorption with activated carbon accomplishes what neither biological reduction nor adsorption could do individually.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.
All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. Use of the term “about” should be construed as providing support for embodiments directed to the exact listed amount. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
