Arthur W. RosePennsylvania 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 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

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

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

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

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

• Smart Drainage System

Flushing SiphonTimed Flusher

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

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

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

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).

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 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

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

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 website- wvwri.nrcce.wvu.edu/programs/adti/index.cfm

Paper in 2010 ASMR Proceedings

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