Background/Objectives. A full-scale Groundwater Extraction and Treatment System (GETS) was designed and built to provide ex situ biological treatment of groundwater containing perchlorate, hexavalent chromium, and selenium at a former ordnance manufacturing facility.
The treatment system incorporated continuously-stirred bioreactor vessels coupled with polishing by ultraviolet disinfection, activated carbon, and sand filtration. The operational complexity of the GETS and low effluent permit limits for perchlorate and selenium resulted in technical challenges and an average uptime of 49% during initial operation. Geosyntec was retained to evaluate the system and modify its operation to meet performance goals. Geosyntec identified multiple factors that contributed to unstable operations, including variability in: influent flow rates, internal bioreactor chemistry, and hydraulics. Principles of chemistry, thermodynamics and microbiology were applied to stabilize bioreactor chemistry, resulting in reliable GETS operation and performance. Empirical relationships were derived to support real-time monitoring of key parameters that ultimately stabilized and optimized GETS operations. Approach/Activities. Physical configuration of the bioreactors and operable limits for hydraulic residence time (HRT), mean cell residence time (MCRT), food-to-microorganism ratio, stoichiometry, and redox state were established based on the Geosyntec operational philosophy. The bioreactors were converted from continuous stirred-tank reactors to a two-stage fixed-film reactor system, and the electron donor dosing system was upgraded and automated. Flow rates were stabilized by retrofitting extraction well pumps withvariable frequency drives (VFDs), and bioreactor hydraulics were modified to maintain consistent HRT and MCRT. The retrofitted electron donor dosing system uses real-time instrumentation to automatically control the electron donor feed rate based on stoichiometric electron donor demand.Selenium removal was accomplished via bio-chemical reactions within the bioreactors and down-stream filtration.In addition, a direct empirical relationship was established between the redox state (measured via oxidation-reduction potential (ORP) within the bioreactors) and perchlorate removal, resulting in the use of real-time ORP sensors as reliable indicators of perchlorate treatment.Feedback from the ORP sensors is tied to alarms that discontinue effluent discharge when redox conditions drift outside of acceptable limits. Results/Lessons Learned.Factoring principles of chemistry and biology into GETS operations has improved uptime from an average of 49% to 99% and has resulted in reliable attainment of permit discharge limits. Use of real-time instrumentation has allowed for targeted poising of bioreactor redox and prevention of system upsets. This presentation will focus on the use of scientific principles coupled with empirical observations and real-time instrumentation to stabilize and optimize bioreactor systems. Results from re-start testing and ongoing optimization will be presented, including empirical data used to establish key relationships and operational procedures that have resulted in reliable operation, and reduced operations and maintenance costs.