WATER MANAGEMENT & TREATMENT
compete with selenites and selenates ; it should be removed prior to selenium reduction to optimise the biological reaction . Elemental selenium particles are difficult to separate due to their size and lack of charge . It is also easily oxidisable , which make sludge disposal complicated as to prevent selenium release over time ; reaction kinetics are slow , which translates into large reactors and hence high costs .”
It continues : “ In addition , due to the low selenium concentration targeted at the effluent , the reductive conditions in the biological system are as such that the transformation of selenates to elemental selenium can lead to the formation of selenide , notably due to longest retention time and in the attempt to transform very low concentrations to elemental selenium . Selenide is a known precursor to organo-selenium species .”
Veolia like BQE Water points out that the issue of selenium toxicity is complex . “ In recent years a number of large-scale bird mortality incidents have been associated with selenium . Examples include birds feeding and nesting at the Kesterson Reservoir in the San Joaquin Valley , CA ( 1983-1985 ), eared grebes at the Salton Sea , CA ( 1992 ) and , again , eared grebes at the Great Salt Lake , UT ( 2011 ). While selenium has been associated with the die-offs , the mechanism involved is not yet well understood . It is , however , likely that these die-offs are complex events caused by bioaccumulation of selenium through the food chain up to a point where the selenium concentration became high enough that it caused acute toxicity in the predators . It is generally understood that the selenium tendency to bioaccumulate depends on its form , with organo-selenium much more likely to bioaccumulate than any other selenium sources .”
While decreasing total selenium concentration is important , it is also important to try to lower the potential for bioaccumulation of the residual selenium after treatment . To address this issue , Veolia Water Technologies has developed Tracer™ Se , an “ innovative selenium removal process which not only aims at lowering total selenium concentration but to limit and address part of the production of organo-selenium species prior to discharge .”
The Tracer Se process combines biological reduction of selenates to selenites , removal of selenites and biomass under reducing conditions , and biological reoxygenation of the water . The Tracer Se process was initially designed to limit the biological reduction , aiming the production of selenite instead of elemental selenium . It has the advantage of considerable footprint reduction , due to higher kinetics .
The mine effluent is first directed to a biological reduction reactor containing biomass and operated under denitrifying ( anaerobic ) conditions , typically using a Moving Bed Biofilm Reactor ( MBBR ). In this biological reduction reactor , selenates are biologically reduced by the biomass to selenites or absorbed on said biomass . The oxidation-reduction potential ( ORP ) in the biological reduction is controlled to minimize further reduction of selenite species to elemental selenium and selenide .
The water containing the selenium and sloughed biomass is then directed to a precipitation reactor . While maintaining reducing conditions , a coagulant such as a ferric or aluminium salt is mixed with the water . By controlling the pH and using sludge recirculation to age the sludge , solids having adsorption sites are formed . Some selenium species such as selenites are adsorbed onto these sites . The solids with adsorbed selenium , in addition to the biomass containing adsorbed selenium , are separated from the water , typically using a ballasted flocculation settler such as Veolia ’ s
Actiflo ® High-Rate Clarifier .
The water is further treated in a second biological reactor under aerobic conditions where the water is subjected to reoxygenation . In this stage , reduced selenium species present in the water are oxidized , thus limiting the potential of releasing organo-selenium to the environment , in addition to providing dissolved oxygen back to the water so it will not be toxic to aquatic life .
Veolia states : “ Extensive laboratory testing , which has been conducted on a typical metallurgic coal mine effluent , has shown that Tracer Se mostly captured the selenate from the influent water within the biological treatment biomass , operating at high kinetic . The adsorbed selenium , as well as some selenite from the biological reduction , were efficiently captured in the metal precipitation and ballasted flocculation step . Finally , the reoxidation biological reactor was shown to be highly efficient in oxidising reduced selenium species such as harmful organo-selenium , and the overall treatment chain can achieve a global selenium removal down to 5 μg / L with low organo-selenium concentrations .”
It concludes : “ Selenium removal is a challenge as there is no ultimate water treatment applicable for every situation . The importance of selenium removal is increasing , as its toxicity is getting more documented but with little certainty as for how selenium is so toxic . Veolia ’ s Tracer Se aims to prevent selenium toxicity by acting both on providing a low selenium concentration at the effluent as well as lowering as much as possible the organo-selenium concentration , which is one of the main hypotheses to explain most of the die-off events observed in the past few years .” IM
International Mining | MARCH 2024