RESEARCH

HISTORY
In Trail, British Columbia, Canada, a historical vegetation-capped landfill, near an operating Teck Cominco lead zinc smelter produces a leachate that contains toxic metals (Zinc, Cd and As).

To evaluate an alternative treatment technology for this leachate Nature Works Remediation Corporation was contracted by Teck Cominco in 1997 to design and build a demonstration scale engineered wetland system. The initial system was completed during the summer months and the system was watered with clean water from a nearby stream to ensure adequate plant root development. A number of test species were planted during this year to assess their viability as terrestrial plants in a sub-surface flow wetlands (hydroponics) environment. Including in the initial planting scheme were Brassica juncea, Helianthus annuus, Tripsicum dactyloides and Typha latifolia. During the winter months companion research projects were undertaken.

At the University of Guelph greenhouse the recently constructed system was modeled and three effluent concentrations were tested using the chosen plant species. At lab facilities at Teck Cominco anaerobic digestion was investigated. The research there examined the ability of bacteria to produce insoluble metal sulphides using waste biosolids from a local pulp mill as the carbon source.

When the plant research showed that they would survive and sequester metals if the effluent metal concentration was reduced by 90% and the research at Teck Cominco showed that an anaerobic digester could reduce metal concentrations in the effluent by that amount it was decided to add an anaerobic digester using vertical sub-surface flow wetland design principles. This component was added in 1998 using funds provided by the National Research Council IRAP program.

After a full year of operations the system experienced some problems with too high metal concentrations entering the plant cells and it was decided to add a second anaerobic digester that combined aspects of an anoxic limestone drain. The effluent contained spike loads of up to 800 ppm zinc and this metal requires a pH of between 7.2 and 7.6 to maximize sulphide production. A multi-stage process is also important in removing zinc concentrations that are this high. Construction of this bioreactor ensured that the pH requirements were met. Funding for this bioreactor was received as part of an ambitious winterizing project that was assisted by grants from Environment Canada.

The single-train engineered wetland now consists of six cells: two anaerobic bioreactors (AB1 and AB2) followed by three horizontal sub-surface flow (HSSF) cells, and a pond wetland cell. AB1 is 30m x 17m and contains 750 m3 of substrate. AB2 is 25m x 18 m and contains 600 m3 of substrate. HSSF1 is 50 m2 and contains a mixture of plants that are being assessed for metal uptake potential as well as Tripsicum dactyloides. HSSF2, also 50 m2 in area is planted with Calamagrostis canadensis. HSSF3 (300 m2) is planted with cattails (Typha latifolia). Invasive volunteers are controlled in HSSF3 by springtime flooding of the normally dry surface. Water from the final holding pond irrigates a tree farm planted with hybrid poplars and other fast growing species used for reclamation work. The original bioreactors were designed with a 1 m layer of water covering the substrate to provide winter protection from freezing temperatures.

During the summer of 2000 and 2001, an extensive program of plant sampling was completed to assess various plants metal sequestration abilities. This important research was funded by Environment Canada as part of their phytoremediation investigation program.

Operations began in 1997. Following the successful operations during an 8-week trial period in 1998, the system was operated for a full summer (18 weeks) in 1999. It operated from June 8, 2000 to Jan. 20, 2001 and from May 8, 2001 continuously to April 10 2002. Following reconstruction of the initial anaerobic bioreactor completed in June 2002, the system has operated effectively and continuously to this date.

ANAEROBIC BIOREACTOR CELL DESIGN

A design flow rate of 20,000 L per day (5,200 US Gal/d) was used to estimate anaerobic bioreactor cell areas. The sizing calculation was based on the removal of 0.3 mol/m3/d of metal using standard design methods (Dvorak et al, 1991, Hedin et al, 1989, Gusek and Wildeman, 1997). Pulp mill biosolids (65%) were used as the carbon source in the substrate along with 35% sand and 5% manure.