Biological Selenium Removal via Redox Cycling and Surface Complexation
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Solution Overview
Problem
Current selenium removal processes are inefficient and costly, often producing toxic elemental selenium and organic selenium species, and struggle to achieve sub-ppb selenium levels in wastewater, which are required by stringent water quality standards.
Innovation Solution
A biological selenium removal process that reduces selenium +6 species to selenium +4 species using a biomass reactor under anaerobic or anoxic conditions, followed by a precipitation reactor with a coagulant to adsorb selenium +4 species onto solids, and a second aerobic reactor to oxidize residual selenium species back to the less toxic selenium +6 form, minimizing the production of elemental and organic selenium.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional biological selenium removal processes are used, then selenium can be removed from wastewater, but toxic elemental selenium and organic selenium species are produced
Solution Approach 1:
The patent changes the oxidation state parameter of selenium during the removal process. By reducing selenate (Se+6) to selenite (Se+4) through biological reduction, and then adsorbing selenite onto solids, the process avoids producing elemental selenium (Se0) and organic selenium compounds. The oxidation state is further adjusted in the second reactor to convert any remaining selenite back to selenate, ensuring only less toxic inorganic selenium species are present in the effluent.
Solution Approach 2:
The patent introduces solids with surface complexation sites (such as metal oxides or hydroxides) as an intermediary substance. These solids serve as a mediator to adsorb selenite from the wastewater, converting dissolved toxic selenite into solid-phase adsorbed selenite that can be easily separated. This intermediary approach prevents the formation of elemental selenium and organic selenium species while effectively removing selenium from the water.
2Quantity of substance
If stringent water quality standards requiring sub-ppb selenium levels are implemented, then water quality improves, but treatment costs increase significantly
Solution Approach 1:
The patent divides the selenium removal process into two distinct biological reactors with different functional purposes. The first reactor is dedicated to biological reduction of selenate to selenite, while the second reactor performs oxidation of remaining selenite back to selenate. This segmentation allows each reactor to be optimized for its specific function, achieving sub-ppb selenium removal efficiency through the combined effect of both stages without requiring overly complex or expensive single-stage treatment systems.
Solution Approach 2:
The patent utilizes naturally occurring biological processes and readily available solids with surface complexation sites to perform selenium removal. The biological reactors employ indigenous microorganisms to carry out reduction and oxidation reactions, while the solids (such as metal oxides) provide surface complexation sites for selenite adsorption. This self-service approach leverages natural phenomena rather than requiring expensive specialized equipment or reagents, making sub-ppb selenium removal economically viable.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively reduces selenium concentrations to sub-ppb levels, minimizing the formation of toxic selenium species and achieving compliance with stringent water quality standards while reducing operational costs.
Implementation Method 1
Selenium +6 species are biologically reduced by the biomass to selenium +4 species (selenites)
Implementation Method 2
A coagulant, such as a ferric or aluminum salt, is mixed with the water. Solids having adsorption sites precipitate from the water.
Implementation Method 3
Solids having adsorption sites precipitate from the water
Implementation Method 4
Selenium +4 species are adsorbed onto the adsorption sites of the solids
Implementation Method 5
Solids having surface complexation binding sites precipitate from the water
Implementation Method 6
the water is subjected to reoxygenation resulting in oxidizing the organo-selenium and any residual selenium +4 species back to selenium +6 species
Data Source
AI summary
A process for removing selenium from water is described. Through a biological reduction process, selenium +6 species are reduced to selenium +4 species. A coagulant is mixed with the water and as a result, solids having complexation binding sites are formed. The reduced selenium +4 species is adsorbed onto the complexation binding sites of the solids. Thereafter, the solids having adsorbed selenium +4 species is separated from the water and ultimately separate from the process. The resulting effluent is subjected to a second biological treatment under aerobic conditions which converts residual selenium and organo-selenium to selenate.