from Tech Trends March 1995

Cold Climate Soil Vapor Extraction/Bioventing

A field demonstration of soil vapor extraction (SVE) and bioventing to remove hydrocarbons from above the ground water table at Gulf Canada Resources Limited's Strachan Gas Plant represents the first documented evidence in Canada of the practicality of using these technologies in colder climates. The results show that the addition of bioventing to conventional SVE enhances remediation and reduces costs by as much as 50% over conventional SVE. Bioventing uses lower air flow rates than conventional SVE; as a result, blower and operating costs are reduced compared to SVE, with little or no off-gas treatment required. The SVE/bioventing demonstrations are part of a series of field demonstrations of remedial technologies applicable to the unique problems encountered in the natural gas processing industry. The project is a joint project of the Canadian Association of Petroleum Producers (CAPP), the Government of Canada and the U.S. Department of Energy.

The site, located in Alberta, Canada, is contaminated by a complex mixture of natural gas condensate liquids, polyaromatic hydrocarbons (PAHs), dissolved volatile organic compounds (including the BTEX compounds of benzene, toluene, ethylbenzene and xylene) and gas-processing chemicals such as amines and amine degradation products. There is free-phase natural gas condensate in addition to a dissolved hydrocarbon plume in glaciofluvial sands and gravels and fractured bedrock. In the hydrocarbon plume, BTEX concentrations average about 10 to 20 milligrams per liter. The SVE/bioventing demonstrations focus on removing hydrocarbons from above the ground water table. The first phase of work was not optimized for removal by SVE and bioventing. After the first month of conventional SVE operation, the system was shut down for one month to allow subsurface conditions to equilibrate and to allow an extended respiration test to be performed. This type of test involved determining whether aerobic biodegradation was occurring in the subsurface by measuring changes in the oxygen (O2) and carbon dioxide (CO2 ) concentrations in the soil gas over time. Biodegradation can be indirectly monitored by observing changes in O2 and CO2 over time. Oxygen levels in the soil were seen to decrease over this period, from approximately 21% (atmospheric concentration) to less than 3%, while CO2 concentrations increased from less than 1% to approximately 8-12%. Based on these calculations, it was estimated that about 450 kilograms (kg) of hydrocarbons were biodegraded.

The next step in the project was to examine the effects of bioventing (Phase I bioventing). Data collected over the next four month period spanning the relatively warm late summer to mid-winter 1994 indicated that about 3,500 kg of contaminant were degraded through bioventing. Air extraction rates below 50 liters per second were insufficient to maintain rapid biodegradation. A series of additional respiration tests were then run over the winter months, during which time the subsurface temperatures dropped from 9 degrees Celsius (C) in October to around 4 degrees C by March. The data showed that biological degradation continued to occur at significant rates. The oxygen concentration dropped substantially from atmospheric levels (21%). The corresponding rise in CO2 concentrations indicated that the changes were due to bioactivity.

Continuing cold climate testing from December 1994 to the present shows that the biodegradation rates remain very high, averaging approximately 200 kg of hydrocarbon biodegraded per day during the winter. During this period surface temperatures dropped as low as -35 degrees C, although subsurface temperatures ranged from a high of a 10 degrees in the fall to a low of 5 degrees C in the spring.

In economic terms, SVE with bioventing removed over 17,000 kg of contaminant for a total capital cost for the SVE test cell system (covering about 1 hectare) of $150,000 and semiannual operation and maintenance costs of approximately $15,000. This represents a mass removal unit cost of about $10/kg. Compared to many other techniques, SVE with bioventing is clearly an economical method of removing volatile hydrocarbons from subsoils. The goal of the Strachan bioventing demonstration in the future (Phase II bioventing) is to determine how clean bioventing can get the soil at Strachan, and how long it will take. In December 1993 and October 1994 the subsurface soils were sampled and analyzed for hydrocarbon concentration and composition, bacterial concentrations, nutrients and soil moisture. The subsoils will be re-sampled in the same locations after the O2 depletions rates indicate that most or all of the biodegradable hydrocarbons have been removed.

For a more detailed description of background, range and extent of the field demonstrations at the Strachan Gas Plant (including the use of horizontal wells), see the July 1994 issue of "Subsurface Remedial Technologies Newsletter," published by the Canadian Association of Petroleum Producers Public Affairs Group in Calgary, as well as the upcoming April 1995 issue of the same Newsletter. The information presented in this article borrowed heavily from the CAPP newsletters. To obtain a copy of CAPP's newsletters on this project, call Michele White at CAPP at 403-267-1154. Also, the results will be presented at Canada's 5th Annual Symposium on Groundwater and Soil Remediation in Toronto, Canada in October (see "International Symposia Alert" section, p. 4 of this issue of TECH TRENDS for Symposium details).

For more technical information on the project, call Alex Lye, GASReP Manager at 905-336-6438.