Treatment of Selenium-Laden Agricultural Drainage Water Using a Full-Scale Bioreactor

Soils in the San Joaquin Valley of California have been a source of selenium (Se) release through dissolution into agricultural drainage, requiring treatment before discharge to the environment. A full-scale primarily anaerobic, granular activated-carbon (GAC) bioreactor with an empty bed contact time ranging from 3.4 to 5.0 h was fed $757\mathrm{L}/\min$ of second-pass agricultural drainage water (ADW) at the San Luis Demonstration Treatment Plant (SLDTP) in Firebaugh, California, for 1,062 days. This study summarized startup of an actual water treatment plant with a process scheme that had never been designed or operated at this scale. ADW nitrate (${\mathrm{NO}}_3^{-}$) and dissolved oxygen (${\mathrm{O}}_{2(\mathrm{aq})}$) concentrations largely dictated the externally dosed carbon (EDC), or glycerin dose, which served as the electron donor to achieve selenate (${\mathrm{SeO}}_4^{2-}$) reduction to elemental Se (${\mathrm{Se}}^0$). Total Se (${\mathrm{Se}}_{\mathrm{T}}$) influent concentrations between 111 and $332\mu \mathrm{g}/\mathrm{L}$ were treated to average effluent ${\mathrm{Se}}_{\mathrm{T}}$ concentrations of $7.4\mu \mathrm{g}/\mathrm{L}$ across the bioreactor. Bioreactor effluent ${\mathrm{Se}}_{\mathrm{T}}$ concentrations were further reduced across a downstream ultrafiltration membrane system (UFMS) to an average permeate ${\mathrm{Se}}_{\mathrm{T}}$ concentration of $2.9\mu \mathrm{g}/\mathrm{L}$. It was determined that most of the ${\mathrm{Se}}_{\mathrm{T}}$ removed from the ADW was retained on the bioreactor GAC and was not amenable to removal through backwashing, rendering it hazardous waste by both California and federal standards. Removal of other trace metals such as chromium and uranium were investigated.