some recent advances in passive treatment of mined drainage arthur w. rose pennsylvania state...
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Arthur W. RosePennsylvania State University
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Topics
Advances in last 10 years since ADTI Handbook
Acidity Oxic limestone drains Upflow limestone beds, automated
flushing Sizing and flushing of vertical flow ponds Sulfate-reducing bioreactors Steel slag and chitin as reactants Economic recovery of Fe sludge
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ACIDITY
A key parameter for evaluating and sizing passive systems
Combines effects of pH, Fe, Mn, Al, alkalinity
Acidity by Standard Methods 1998 is a net acidity
Acidity can be calculated from pH,Fe,Mn,Al and alkalinity if suspended solids are low
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Oxic Limestone Drains
Useful for large flows of low-metal AMD (few mg/L)
May be helped by periodic flushing
Bed of limestone fragments
Soil or coverAerated AMD
Cover retains CO2 and improves effluent alkalinity
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Iron Removal
Net alkaline – aerate, degas CO2
Net acid – use low-pH Fe oxidation by aerating and providing large surface area for microbes, then react with limestone
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FLUSHING LIMESTONE BED
Limestone Bed
Flushing Control
Capable of treating high-Al-Fe AMD
Siphon or Smart-Drain-System
Improved by occasional “washing” of limestoneCan be designed in upflow mode
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Flushing Limestone Bed
Flushing control can be -Manual - <5% of Al is flushed -An automatic siphon started when pond is full -An Agridrain Smart Drain System using solar panel to operate a clock and open valve
Flushing can remove 50% of accumulated precipitateSee Hedin Environmental website
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• Smart Drainage System
Flushing SiphonTimed Flusher
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Vertical Flow Ponds (VFP’s, SAPS)
watercompost
limestone
Compost reduces ferric Fe and accomplishes sulfate-reductionLimestone neutralizes acidityOutflow goes to pond in which Fe oxidizes and precipitates
Inflow
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Sizing of VFP’s
Early VFP’s were designed with 12-24 hr. retention in limestone (ALD’s)
Some VFP’s release acid effluent despite long retention times
Field data shows a maximum acidity loading rate of 35 g/m2/day to generate alkaline effluent
Modeling shows this limit is explained by slow limestone reaction at pH greater than about 6, and CO2 pressure
Design VFP’s with loading of 25-35 g/m2/d
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Problems of VFP’s
1. Al >10-20 mg/L precipitates and coats limestone fragments
Manual flushing (monthly) Automatic flushing (siphon or Smartdrain) Add preceding flushed limestone pond2. High influent Fe precipitates on top of
compost and plugs flow Capture in oxidation pond(s) before VFP,
using low-pH iron oxidation Add preceding flushed limestone pond
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Problems of VFP’s (cont.)
3. Effluent is acidic despite long retention time
VFP is too small (use 35 g/m2/d) Actual flow exceeds design flow (accurate
flows and acidities needed for design)4. Effluent is acidic, short retention time
from testing Short circuiting thru compost (along
outside of cleanout pipes, thin compost, riprap thru compost, broken pipes).
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Reliability of Large Passive Systems
Some consider passive treatment of more than minor flow and metals to be unreliable – Field evidence.
Most “failures” were not properly designed or constructed.
Some large systems do work. Need concerted effort to evaluate
“failures” and successes – Underway? Use results to improve design.
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SULFATE-REDUCING BIOREACTOR
Like VFP but only a single layer of compost mixed with fine limestone
Sulfate reduction is a major provider of alkalinity
Al precipitates as fine dispersed material that does not plug or coat limestone
Heavy metals removed as sulfide Usually must be larger than VFP
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SLAG BEDS
Some steel slag has high CaO, generates very alkaline water (1000 mg/L)
Run good water into slag bed and mix high alkalinity effluent with AMD
Use beds and dams of slag to neutralize AMD
Still experimental
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CHITIN BEDS
Chitin (skeleton of crabs and other crustaceans) has finely interlayered CaCO3, chitin (organic matter) and N source. Lab and pilot tests show excellent neutralization and metal removal.
Still experimental
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METAL RECOVERY
Hedin Environmental is selling selected Fe sludge, after processing, as pigment
Process is commercially profitable Stream Restoration has recovered
Mn oxides and sold small amounts for pigment, ceramics, etc.
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Written Review
Available on ADTI website- wvwri.nrcce.wvu.edu/programs/adti/index.cfm
Paper in 2010 ASMR Proceedings
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CONCLUSIONS
o Many large passive systems have not performed to expectations, but reasons are becoming understood
o Better design and construction, plus new technologies, allow passive treatment of a wider range of AMD