Anaerobic bioventing for treatment of vadose zone soils contaminated with highly chlorinated organic compounds

Microbial destruction of highly chlorinated organic compounds must be initiated by anaerobic dechlorination followed by aerobic oxidation. In situ remediation of vadose zone soils requires, among other factors, the establishment of highly reductive anaerobic conditions in the unsaturated subsurface. Such conditions may be established by delivering appropriate gas mixtures to the subsurface, i.e., “anaerobic bioventing.” This study is conducted as a feasibility study for anaerobic bioventing, using tetrachloroethene (PCE) as a test contaminant and H₂ as the electron donor. Using a soil column inoculated with anaerobic dechlorinating bacteria, to simulate the vadose zone, experimental results show that by passing a gas composed of 1% H₂ and >0.1 CO₂ in N₂, methanogenic conditions are established and that PCE (fed as a vapor in the gas stream) is rapidly converted with terminal products vinyl chloride (VC) and trans-dichloroethene (trans-DCE).; Sequential anaerobic aerobic bioventing principles have been applied to completely dechlorinate tetrachloroetheylene vapors in the unsaturated zone. The aerobic step results in rapid oxidation of the VC and trans-DCE to carbon dioxide. Hydrogen was delivered in the gas phase as a reducing agent for the anaerobic step at levels of 1%, and oxygen at a percentage of 4.2% was used as an electron acceptor in the aerobic step. PCE and VC half lives in the anaerobic and aerobic steps respectively, were less than 10 min.; A numerical and experimental study of transport phenomena underlying anaerobic bioventing is presented. In particular the establishment of an anaerobic zone of influence by injecting a multicomponent gas mixture in the vadose zone is investigated. Oxygen exclusion experiments are performed in a pilot scale flow cell, using different venting flows, and an open and partially covered outflow boundary. Injection gas velocities varied from (0.25–1.0) × 10−3 cm/s and are correlated with the radius of influence. Numerical simulations are used to confirm those correlations, and to relate the injected radius of influence to the zone of cleanup during anaerobic bioventing. In general, reasonable agreement is found between observed and predicted oxygen concentrations. Use of impervious caps can significantly reduce the volume of forcing gas used.