Engineered systems are often needed to supply an electron donor, such as hydrogen (H 2 ), to the subsurface to stimulate the biological dehalogenation of perchloroethene (PCE) to ethene. A column study was performed to evaluate the ability of gas permeable hollow-fiber membranes to supply H 2 directly to PCE-contaminated groundwater to facilitate bioremediation. Two glass columns were packed with soil obtained from a trichloroethene-contaminated site at Cape Canaveral, Florida, and were fed a minimal medium spiked with PCE (7μM) for 391 days. The columns were operated in parallel, with one column receiving H 2 via polyethylene hollow-fiber membranes (lumen H 2 pressure of approximately 1atm) and a control column receiving no H 2 . PCE was initially dechlorinated at a similar rate and to a similar extent in both columns, likely due to the presence of soil organic matter that was able to support dechlorination. After 265 days of operation, dechlorination performance declined in the control column and the benefits of membrane-supplied H 2 became evident. Although the membrane-supplied H 2 effectively stimulated PCE dechlorination at the end of the experiment (days 359-391), the system was inefficient in that only 5% of the supplied H 2 was used for dechlorination. Most of the remainder was used to support methanogenesis (94%). Despite the dominance of methanogens, nearly complete dechlorination of PCE to ethene was observed in the H 2 -fed column. In addition to the inefficient use of H 2 , operational problems included excessive foulant accumulation on the outside of the membrane fibers and water condensation inside the fibers. Use of alternative membrane materials and changes to the operating approach (e.g. pulsing or supplying H 2 at low partial pressures) may help to overcome these problems so that this technology can provide effective and stable remediation of aquifers contaminated with chlorinated ethenes.
