Modeling Saturated Rock Fills with SeSANS

Selenium oxyanions (i.e., selenate and selenite) can be released into the environment from surface mining. Selenium is an essential micronutrient, but high selenium in water has adverse health effects for aquatic animals and humans. Mine-influenced water is often co-contaminated with high concentrations of nitrate, selenium oxyanions, and sulfate. The Saturated Rock Fill (SRF) is a treatment technology that utilizes waste rocks from surface mining to create a biological treatment system that can be effective at removing nitrate and selenium-oxyanions from the mine-influenced water. The Selenium, Sulfur, and Nitrogen species (SeSANS) model can be used to estimate the respiration, synthesis, and endogenous decay of biomass in an SRF. The goal of this thesis is to simulate SRF biofilms using a biofilm version of SeSANS. Three nitrate loads (100, 250, and 450 kg NO3-N/day) with a low flow rate (1000 m3/d) or a high flow rate (5000 m3/d) -- a total of six scenarios -- were simulated for 5000 days of operation. The influent water contained 0.18 g Se/m3 of selenate, 0.02 Se/m3 selenite, and 800 S/m3 of sulfate; the input nitrate concentration was 100, 250, and 450 g N/m3 for the low flow rate and 20, 50, and 90 g N/m3 for the high flow rate. Methanol was injected as the electron donor. These criteria were used to define a successful simulation: effluent nitrate < 3 mg N/L and total dissolved Se < 0.029 mg Se/L, minimal sulfate reduction, and an average biofilm-biomass density of 96 kg TS/m3. To achieve those criteria, the following model parameters were adjusted: rate for methanol addition, biofilm thickness, SRF volumes, and biofilm-detachment rates. The most important parameter for achieving all the goals was the methanol addition ratio: 3.56 g COD/g NO3-N. Another important outcome was that the high-flow-rate scenarios required a larger total SRF volume to achieve target nitrate and Se-oxyanion reductions. The results of the simulations can be used to estimate biofilm characteristics and optimize the SRF configuration and treatment operation.