The objectives of this demonstration were to identify a suitable site for passive bioventing, install a single-vent well system, measure relevant design parameters, and compare estimated costs for a full-scale passive bioventing system to those for a full-scale conventional bioventing system.
Passive bioventing occurs when a pressure differential exists between the atmosphere and subsurface environment. When atmospheric pressure is greater than subsurface pressure, air is introduced into the subsurface (usually on a diurnal basis in the morning or at high barometric pressure). Conventional bioventing, on the other hand, forces air into the subsurface via blowers. Passive bioventing is applicable for remediation of aerobically biodegradable compounds. It requires adequate soil-gas permeability (e.g., sandy to gravel soils), soil oxygen concentrations less than 5 percent, and adequate microbial degradation. Vent wells include a one-way passive valve to prevent air movement to the surface.
A successful passive bioventing system has a peak airflow rate of at least 1 cubic foot per minute (cfm) per well, or a total daily airflow rate of at least 1,200 cubic feet per day (cfd) per well, based on 1 pore volume of air exchange per day with a radius of oxygen influence (ROI) of 10 feet (ft), a contaminated thickness of 15 ft, and an air-filled porosity of 0.25 percent (i.e., air-filled pore volume of 1,200 cubic feet). Fifteen Department of Defense (DoD) sites were selected with conditions deemed favorable for passive bioventing, and of these, Castle Airport, CA; Tinker Air Force Base, OK; and Finland Air Force Station, MN, met the minimum condition of peak airflow greater than 1 cfm. However, only Castle Airport contained petroleum hydrocarbons as well as a laterally continuous clay/silt layer between 20 and 25 ft below ground surface (bgs) to prevent equilibration of barometric pressure between the atmosphere and deeper soils. One vent well was installed at 25 to 65 ft bgs, and eight vent monitoring wells were installed in transects from the vent well. Peak airflow ranged from 5.1 to 15 cfm (primarily before noon), with an average daily rate of 3,409 cfd. The oxygen levels increased from less than 1 to approximately 12 percent at 16 ft from the vent well and increased to 5.5 percent after 49 days 42 ft from the vent well.
Based on a 3-year period, an 85 ft ROI requiring six vent wells was used to estimate full-scale passive bioventing. The full-scale system cost estimate was $366,000 with a unit cost of $2.49 per cubic yard. The full-scale conventional bioventing system was estimated to have a 110 ft ROI, half the vent wells, and a $397,207 cost with a unit cost of $2.71 per cubic yard. The cost differential reflected trenching costs, electrical utilities, and higher operations and maintenance (O&M) costs. The conversion of injection wells from a conventional to passive bioventing system may be cost effective when the rate of contaminant mass recovery and in situ respiration decrease to a predetermined point.
Benefits of passive bioventing as compared to conventional bioventing include elimination of electrical equipment, blowers, vacuum manifold systems, associated O&M costs, use of ambient air without pretreatment, and the aboveground off-gas treatment requirement. Disadvantages include the potential for more vent wells, reduction in airflow in soils with high moisture levels and low permeability, and a longer time requirement to reach cleanup goals. (Project Completed - 2004)