RESULTS

FLOW RATES
The mean flow rates of the landfill leachate being treated were substantially the same during the two reporting periods but total volume treated was higher during 2001 - 2002 due to a longer operational period (Table 1). Daily flow rates during 2002 were lower due to efforts made to more accurately determine the system’s operational limits, to reduced flow rates at start-up, and to reduced flows when pH changes resulted in lower Zn removal efficiencies. Evaporation pan measurements were used to determine the volume of water evaporated.

Table 1. Comparison of flow rates during similar operating periods for 2 years (2000, 2001) showing total volume treated, daily treatment rate during summer and amount evaporated over summer period, total volume treated during winter and total for period. Volume in litres.

MEASURED OPERATING PARAMETERS
Table 2 shows mean pH and dissolved oxygen levels (D.O.) in each of the treatment cells during the two-year reporting period. D.O. levels and pH measurements were monitored (three times a week at eight points during the summer and weekly at five points in the winter) in the system and levels used as an indication of system operability. Normal system operation would show an increase in pH as leachate moved from cell to cell and a decline in D.O.

Table 2. Showing mean pH and dissolved oxygen levels in each of the treatment cells during the two-year reporting period. Data shown are the results of three times weekly measurements during summer months followed by weekly measurements during winter. For 2001 data are given until August 8, 2001

For efficient Zn removal the pH in the cells had to be between 7.2 and 7.6. This pH level is achieved by the outflow of HSSF3 and as a result that cell demonstrated strong zinc removal ).

The high D.O. levels in HSSF3 were, in part a reflection of the presence of standing water on the surface of the cell through part of the year, together with the oxygen pumping action of the plants.

METAL REMOVAL
Combined metal concentrations and percentage removals during operating periods from 07/05/00 to 20/01/01 (2000) and from 14/05/01 to 20/01/02 (2001) are shown (Table 3). The percentage of each metal removed was calculated as the amount removed in each cell based on values at input and outflow from that cell. Concentrations are from the output of each cell and are reported as ppm (n=56, year 2000; and n=72, year 2001). Total mean metal concentration of the influent to the entire system for the three contaminants was 429.7 ppm, and for the effluent out of the pond wetland cell was 16.4 ppm for a net % removal of 96.2%. The bulk of the metal removal takes place in the first two anaerobic bioreactor cells; 93% of the As, 98% of the Cd and 84% of the Zn was removed.

The HSSF cells following the bioreactors also assist in metal removal via a number of processes including: physical filtration (any suspended solids), sequestration, root adsorption, and anaerobic bacterial processes. Testing of aboveground plant biomass growing in the HSSF cells has been completed and results were reported previously (Mattes et al, 2002). None of the plants tested had tissue metal concentrations that approach the minimum amount for hyperaccumulation >1% total metals by dry weight (Baker and Brooks, 1989, Baker et al., 1991).

Table 3. Mean metal uptake in selected plants (all cells combined) for all harvests in summer of 2001. Metal levels for Pb are included for information purposes only as it is not a primary contaminant in the collected leachate. All values are in ppm.