Acid mine drainage (AMD) at Stockton Coal Mine, located near Westport, New Zealand, is generated
from the oxidation of pyrite within sedimentary overburden exposed during surface mining. The
pyrite oxidation releases significant acidity, Fe, and sulphate together with trace metals to the
receiving environment. Aluminium is also elevated in drainage waters due to acid leaching from
overburden materials. Thirteen AMD seeps emanating from waste rock dumps, and associated
sediment ponds were monitored at Stockton Coal Mine to characterise water chemistry, delineate their
spatial and temporal variability, and quantify metal loads. Dissolved metal concentrations ranged
from 0.05-1430 mg/L Fe, 0.200-627 mg/L Al, 0.0024-0.594 mg/L Cu, 0.0052-4.21 mg/L Ni, 0.019-
18.8 mg/L Zn, <0.00005-0.0232 mg/L Cd, 0.0007-0.0028 mg/L Pb, <0.001-0.154 mg/L As and 0.103-
29.3 mg/L Mn and the pH ranged from 2.04-4.31. Currently this AMD is treated further downstream
by a number of water treatment plants employing a combination of ultra fine limestone and calcium
hydroxide; however, in the interest of assessing more cost-effective technologies, passive treatment
systems were investigated for their treatment and hydraulic efficacy and as potential cost-effective
options.
Biogeochemical reactors (BGCRs) were selected as the most appropriate passive treatment system for
ameliorating AMD at Stockton Coal Mine. Results of mesocosm-scale treatability tests showed that
BGCRs incorporating mussel shells, Pinus radiata bark, wood fragments (post peel), and compost
increased pH to ≥6.7 and sequestered ≥98.2% of the metal load from the Manchester Seep located
within the Mangatini Stream catchment. The following design criteria were recommended for BGCRs
incorporating 20-30 vol. % mussel shells as an alkalinity amendment: 1) 0.3 mol sulphate /m3
substrate/day for sulphate removal (mean of 94.1% removal (range of 87.6-98.0%)); 2) 0.4 mol
metals/m3/day for metal (mean of 99.0% removal (range of 98.5-99.9%)) and partial sulphate (mean of
46.0% removal (range of 39.6-57.8%)) removal; and 3) 0.8 mol metals/m3/day for metal (mean of
98.4% removal (range of 98.2-98.6%) and minimal sulphate (mean of 16.6% removal (range of 11.9-
19.2%)) removal. At the maximum recommended loading rate of 0.8 mol total metals/m3/day an
average of 20.0 kg/day (7.30 tonnes/year) of metals and 85.2 kg acidity as CaCO3/day could be
removed from the Manchester Seep AMD by employing BGCRs. The design hydraulic residence
time (HRT) would be 3.64 days. On an acidity areal loading basis, a design criterion of 65 g/m2/day
was recommended.
Tracer studies conducted on the BGCRs indicated ideal flow characteristics for cylindrical drumshaped
reactors and non-ideal flow conditions for trapezoidal-shaped reactors indicative of shortcircuiting,
channelised flow paths and internal recirculation. Consequently, this resulted in
compromised treatment performance in the trapezoidal-shaped reactors. The relaxed tanks in series
(TIS) model could be successfully applied to model the treatment performance of drum-shaped
reactors; however, the model was unsuccessful for trapezoidal-shaped reactors. Because most pilot and full-scaled vertical flow wetlands (VFWs) have consisted of trapezoidal-prism basins excavated
into the ground, the rate-removal methods previously recommended (e.g. mol metals/m3/day) should
be applied to BGCR design, evaluation and operation rather than results of hydraulic and reactor
modelling.
Overall, a staged passive treatment approach is recommended. The first stage should consist of a
sedimentation basin to remove sediment, the second stage a BGCR to remove acidity and metals and
the third an aerobic wetland to provide oxygenation and tertiary treatment of metals (primarily Fe)
from BGCR effluent. Preliminary analysis indicates that BGCRs are potentially a more cost-effective
means of treating AMD at Stockton Coal Mine compared with the current active lime-dosing plant by
over $125/tonne of acidity ($197/tonne for BGCRs versus $324/tonne for lime dosing (60%
efficient)); however, their successful implementation would need to recognise current treatment goals,
required areal footprint and inherent maintenance requirements.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/5378 |
| Date | January 2011 |
| Creators | McCauley, Craig |
| Publisher | University of Canterbury. Civil and Natural Resources Engineering |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Craig McCauley, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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