Iron and Al contributed to the majority of metal (98.0-99.0%) and acidity (median 79.2% (66.8- 81.3%)) loading to each system. Iron and Al loading correlated generally well (R2 = 0.984) with Al loading exceeding Fe loading an average of 1.40 times throughout this study. Influent and effluent Fe and Al concentrations and removal efficiency for each BGCR are displayed in Figure 4.16. A comparison of Fe and Al percent removal efficiencies from each BGCR are shown in Figure 4.17 where data points below the dashed lines indicate a greater percentage of Fe removal, and data points above the dashed lines represent a greater percentage of Al removal.
Removal efficiency was generally stable for Fe and Al (with exception of S1); however, treatment effectiveness was better for Al except at the highest loading rate tested for S4 (1.36 mol total metals/m3/day and 2.73 mol sulphate/m3/day) and for P3. A decrease in Fe removal efficiency was more notable, especially during the final sampling event, for most BGCRs (at metal loading>0.8 mol/m3/day) and throughout the operation of S1. Excluding S1, Al removal was always >99% following the first flush (third sampling event; week 5.2 onwards) and prior to the highest loading rates tested. Iron removal during the same period was less effective and more variable and typically
ranged between 94% and 99%, which further demonstrates the more effective removal efficiency of Al compared with Fe.
Figure 4.16. Influent and effluent Fe and Al concentrations during the mesocosm-scale treatability tests. Removal efficiencies were computed from concentration reductions.
Figure 4.17. Relationship of Fe and Al percent removal efficiencies from each BGCR. Data points below the dashed lines indicate a greater percentage of Fe removal, whereas data points above the dashed lines represent a greater percentage of Al removal.
A summary of influent and effluent metal and sulphate concentrations from each reactor during stable operating conditions (weeks 5.2-16.0; metal loading rates 0.23-0.83 mol/m3/day; and acidity loading rates 21-80 g CaCO3/m2/day) are summarised in Table D.1 in Appendix D. Data excludes the first three (weeks 1.1-3.2) sampling events due to effects from the first flush and the final (week 16.7) sampling events due to system overloading. Effluent data from P3 and S1 includes samples collected from week 5.2 until BGCR operation ceased (during week 11.3 for P3 and week 12.4 for S1).
Influent and effluent dissolved Fe and Al concentrations during stable operating conditions are shown in Figure 4.18. The x-axis illustrates influent AMD and effluent from each BGCR. The y-axis shows Fe (dark grey bars) and Al (light grey bars) concentration ranges (on a logarithmic scale). Horizontal black lines represent median Fe and Al concentrations. Dissolved metal influent (AMD) and effluent concentrations and calculated removal efficiencies from BGCRs P1, S2 and S3 (20-30% mussel shells as sole alkalinity amendments to substrate) during metal loading rates of 0.23 to 0.83 mol/m3 substrate/day and acidity loading rates of 25 to 80 g as CaCO3/m2/day are shown in Table 4.8. These represent the results of BGCRs containing mussel shells solely as an alkalinity amendment and represent recommended operational ranges for metal removal. Metal removal was most effective for Al, Cu, Ni, Zn, Cd and Pb, but a substantial amount of Fe (96.5-99.8 %) was also removed. There was typically a net export of As, likely as a result of leaching from substrate materials. Effluent As
concentrations were greatest from BGCRs containing mussel shells (P1, S2 and S3) during the first three (weeks 1.1-3.2) sampling events, averaging 0.026 mg/L (range of 0.013-0.042 mg/L), while the lowest As concentrations were from S1 averaging 0.010 mg/L (range of 0.006-0.018 mg/L). For sampling events thereafter, effluent As concentrations were 0.002-0.003 mg/L.
Figure 4.18. Influent (AMD) and effluent (P1, P2, P3, S1, S2, S3 and S4) dissolved Fe and Al concentrations from mesocosm-scale BGCR experiments during metal loading rates from 0.23 to 0.83 mol/m3 substrate/day and acidity loading rates from 25-80 g (as CaCO3)/m2/day. Data represents samples collected between weeks 5.2-16.0 during stable operating conditions. Effluent data from P3 and S1 includes samples collected from week 5.2 until BGCR operation ceased (during week 11.3 for P3 (0.41 mol metals/m3/day and 45 g CaCO3/m2/day) and Week 12.4 for S1 (0.56 mol metals/m3/day and 46 g CaCO3/m2/day)).
Table 4.8: Dissolved metal influent (AMD) and summarised effluent concentrations and removal efficiencies from BGCRs containing 20-30 vol.% mussel shells (P1, S2 and S3) during metal loading rates from 0.23-0.83 mol/m3 substrate/day and acidity loading rates from 25-80 g CaCO3/m2/day. Median concentrations were computed assuming sample concentrations detected below laboratory PQLs were equal to one-half the PQL values.
AMD Conc. (mg/L) Effluent Conc. (mg/L) Removal Efficiency (%)
Median Median Min Max Range
Fe 70.7 1.04 0.05 3.46 96.5-99.8 Al 51.8 0.031 0.0170 0.277 99.5-99.9 Cu 0.199 0.00025 <0.0005 <0.001 >99.7->99.9 Ni 0.210 0.001 <0.0005 0.0020 99.3->99.7 Zn 1.23 0.002 <0.001 0.005 99.7->99.9 Cd 0.00169 0.000025 <0.00005 <0.00005 >98.3-98.9 Pb 0.0150 0.00005 <0.0001 0.0001 99.5->99.7
A summary of influent and stoichiometric equivalent effluent sulphate concentrations and removal efficiencies from each BGCR are shown in Figure 4.19. Sulphate removal was not as effective as Fe, Al and most other metals. Sulphate removal was less effective as loading rates increased. This was likely due to decreasing organic matter decomposition, which resulted in lower DOC concentrations (Figure D.11 in Appendix D). This trend differed from those observed for metals where steady removal efficiencies were achieved until distinct loading rates were reached. Aluminium removal was greater than sulphate removal throughout the experiments; however, sulphate removal was better than Fe removal on several occasions during the first three sampling events (weeks 1.1-3.2) when sulphate removal averaged 0.30 mol/m3/day (excluding P3).
Figure 4.19. Influent and effluent sulphate concentrations and removal efficiencies from each BGCR. The y-axis scale is different for BGCR P3 due to the extensive export of sulphur from the NSD within its substrate mixture.
4.2.2.3 Removal Effectiveness of Fe, Al and Sulphate Based on Hydraulic