Mathematical Modeling of Biological Selenium Removal from Flue Gas Desulfurization (FGD) Wastewater Treatment

Due to increased enforcement of selenium regulations and understanding of selenium health and environmental effects, treatment of selenium contaminated waters has become a very important issue. The biological selenium reduction process is capable of transforming selenate into selenite and subsequently to nontoxic insoluble elemental selenium. Treatment of Flue Gas Desulfurization (FGD) wastewater is challenging due to high levels of metals such as selenium and total dissolved solids. Biological processes under anoxic/anaerobic conditions have been successfully applied to FGD wastewaters for removal of nitrate/nitrite and selenium. Unlike many other biological processes such as nitrification, denitrification and phosphorus removal, a mechanistic mathematical model as a design and analysis tool is presently lacking in the literature for biological selenium removal. In this work, a mechanistic mathematical model is proposed, added to the BioWin process simulator, and calibrated based on the operational data from a full-scale FGD Wastewater Treatment Plant (WWTP). This paper presents full-scale WWTP data on selenium concentration and speciation, and proposes a model that addresses the reductive competition between denitrifiers (nitrate and nitrite reduction) and Selenium Reducing Bacteria (SeRB). Four new state variables and five biological processes are proposed in this model to mechanistically simulate the two stage reductive pathways of selenate to elemental selenium.