SOME RECENT ADVANCES IN PASSIVE TREATMENT OF MINED

SOME RECENT ADVANCES IN PASSIVE TREATMENT OF MINED DRAINAGE Arthur W. Rose Pennsylvania State University

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

ACIDITY A key parameter for evaluating and sizing passive systems ¡ Combines effects of p. H, Fe, Mn, Al, alkalinity ¡ Acidity by Standard Methods 1998 is a net acidity ¡ Acidity can be calculated from p. H, Fe, Mn, Al and alkalinity if suspended solids are low ¡

Oxic Limestone Drains Useful for large flows of low-metal AMD (few mg/L) May be helped by periodic flushing Aerated AMD Soil or cover Bed of limestone fragments Cover retains CO 2 and improves effluent alkalinity

Iron Removal ¡ ¡ Net alkaline – aerate, degas CO 2 Net acid – use low-p. H Fe oxidation by aerating and providing large surface area for microbes, then react with limestone

FLUSHING LIMESTONE BED Capable of treating high-Al-Fe AMD Limestone Bed Flushing Control Siphon or Smart-Drain-System Improved by occasional “washing” of limestone Can be designed in upflow mode

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 precipitate See Hedin Environmental website

Flushing Siphon Timed Flusher • Smart Drainage System

Vertical Flow Ponds (VFP’s, SAPS) Inflow water compost limestone Compost reduces ferric Fe and accomplishes sulfate-reduction Limestone neutralizes acidity Outflow goes to pond in which Fe oxidizes and precipitates

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/m 2/day to generate alkaline effluent Modeling shows this limit is explained by slow limestone reaction at p. H greater than about 6, and CO 2 pressure Design VFP’s with loading of 25 -35 g/m 2/d

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 pond 2. High influent Fe precipitates on top of compost and plugs flow § Capture in oxidation pond(s) before VFP, using low-p. H iron oxidation § Add preceding flushed limestone pond

Problems of VFP’s (cont. ) 3. Effluent is acidic despite long retention time § VFP is too small (use 35 g/m 2/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).

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.

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 ¡

SLAG BEDS ¡ § § § Some steel slag has high Ca. O, 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

CHITIN BEDS Chitin (skeleton of crabs and other crustaceans) has finely interlayered Ca. CO 3, chitin (organic matter) and N source. Lab and pilot tests show excellent neutralization and metal removal. ¡ Still experimental ¡

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. ¡

Written Review Available on ADTI websitewvwri. nrcce. wvu. edu/programs/adti /index. cfm ¡ Paper in 2010 ASMR Proceedings ¡

CONCLUSIONS o o Many large passive systems have not performed to expectations, but reasons are becoming understood Better design and construction, plus new technologies, allow passive treatment of a wider range of AMD