The complicated role of CO 2 in mine water treatment

Transcription

1 The complicated role of CO 2 in mine water treatment Benjamin C Hedin 1,2 Robert S Hedin 1 1 Hedin Environmental 2 University of Pittsburgh

2 CO 2 in PA coal mine drainage Site Log CO 2 partial pressure Atmosphere Eastern U.S. groundwater (Appelo & Postma) Mine Waters Marchand Crabtree Pine Run Brinkerton Howe Bridge Morrison Wingfield Phillips Cravotta, 2008 (90 samples w/ alk > 0) Median to -2.45

3 CO 2 over time Crabtree Discharge log pco

4 Presentation topics CO 2 and lime treatment CO 2 and passive Fe and Mn oxidation CO 2 and limestone dissolution CO2 calculated Assume alkalinity is HCO 3 - H 2 CO 3 <-> H + + HCO 3 - k 1 ~ (temp dependent) CO 2(g) + H 2 O <-> H 2 CO 3 K co2 ~ (temp dependent)

5 H 2 CO 3 + CaO CaCO 3 + H 2 O

6 Lime Treatment Inefficiency due to CO 2 Nine lime treatment sludges from Pennsylvania 10 92% CaCO 3 equivalency, average 56% Clyde lime treatment plant (Pennsylvania) 25% of lime addition calcite formation $117,000/yr in extra reagent cost But Hollywood lime treatment plant (Pennsylvania) needed to discharge more alkalinity, so the plant s CO 2 aeration was scaled back Schleenbain lime treatment plant (Germany) produces a calcite-containing iron sludge that is valued for its neutralization potential in the mine pit

7 CO 2 and passive treatment of net alkaline mine waters Fe 2+ and Mn 2+ oxidation rates are both directly affected by ph HCO 3 - CO 2(g) + OH - Degassing of CO 2 from alkaline water increases ph

8 Fe (mg/l) Fe ph Pond A in Pond B in Pond C in Pond D in Pond E in Pond F in Wet in Wetland out ph

9 pco2 ph Pond A in Pond B in Pond C in Pond D in Pond E in Pond F in Wet in Wetland out Log(pCO2) ph

10 CO 2 and alkalinity generation with limestone CO 2(g) + H 2 O <-> H 2 CO 3 H 2 CO 3 + CaCO 3 2HCO Ca 2+ H + + CaCO 3 HCO Ca 2+ How important is the CO 2 reaction?

11 ALKasts

12 Woodlands treatment system at the Pittsburgh Botanic Garden

13 Collected from pipe out of abandoned underground mine

14 Do Alkasts mimic limestone beds? Influence of particle size? Alkalinity, mg/l CaCO Woodlands Effluent Small Stone Alkasts Large Stone Alkasts Incubation Time, hours

15 Alkalinity, mg/l CaCO Does CO 2 matter? Fresh AMD Stale AMD Woodlands Effluent Incubation Time, hours Condition Alkalinity (mg/l CaCO 3 ) St. Dev. log pco 2 Woodlands Effluent Fresh AMD (>4 hr incubation) Stale AMD (>4 hr incubation)

16 Fall Brook north/south Treatment System North underground collection system South aboveground collection system

17 North: Boring buried pipe South: ~250 ft channelized flow before in stream collection

18 Fall Brook Fall Brook North = Fresh Fall Brook South = Stale ALKasts on North/South DLB influent/effluent ALKasts on South AMD at source (fresh) ALKasts on South AMD at variable flows Influent flow ph Acid Fe Al Mn SO4 gal/min CaCO mg/l Woodlands Pipe FB North Pipe FB South Stream 2, FB South Stream

19 How do ALKast measurements on the influent water compare to alkalinity generated by the system? FBN FBS FBS System influent water type Fresh Stale Stale System theoretical retention, hr System effluent, alk Retention time = hours. Alk = alkalinity as mg/l CaCO 3

20 How do ALKast measurements on the influent water compare to alkalinity generated by the system? FBN FBS FBS System influent water type Fresh Stale Stale System theoretical retention, hr System effluent, alk Alkast, system influent, alk Retention time = hours. Alk = alkalinity as mg/l CaCO 3

21 How do ALKast measurements on the influent water compare to alkalinity generated by the system? FBN FBS FBS System influent water type Fresh Stale Stale System theoretical retention, hr System effluent, alk Alkast, system influent, alk Alkast, system effluent, alk Retention time = hours. Alk = alkalinity as mg/l CaCO 3

22 How do ALKast measurements on the influent water compare to alkalinity generated by the system? FBN FBS FBS System influent water type Fresh Stale Stale System theoretical retention, hr System effluent, alk Alkast, system influent, alk Alkast, system effluent, alk Alkast, fresh influent, alk Retention time = hours. Alk = alkalinity as mg/l CaCO 3

23 CO 2 and Limestone Conclusions What happens when the fresh influent is allowed to aerate and become stale before ALKast testing? Wood FBN FBS FBS System influent water type Fresh Fresh Stale Stale System theoretical retention, hr System effluent, alkalinity mg/l Alkast, system influent, alkalinity mg/l Alkast, system effluent, alkalinity mg/l Alkast, fresh influent, alkalinity mg/l Alkast, stale influent, alkalinity mg/l Loose ~ 93 mg/l alkalinity if allow water to aerate

24 What happens if water is collected from the source and kept fresh for Alkast testing? Wood FBN FBS FBS System influent water type Fresh Fresh Stale Stale System theoretical retention, hr System effluent, alkalinity mg/l Alkast, system influent, alkalinity mg/l Alkast, system effluent, alkalinity mg/l Alkast, fresh influent, alkalinity mg/l Alkast, stale influent, alkalinity mg/l Gain ~ 100 mg/l alkalinity if collected at source Lower CO 2 with high flow rate

25 How to handle CO 2 in treatment systems? Lime system Degassing CO 2 substantially decreases chemical costs Preserving CO 2 increases effluent alkalinity and makes sludge more alkaline Fe 2+ or Mn 2+ passive oxidation system Degassing CO 2 increases ph and oxidation rates Limestone system Preserving CO 2 substantially increases alkalinity generation Maximize alkalinity generation vs maximize lifespan of bed

26 Recommendations Measure CO 2 or simply assume fresh AMD has high CO 2 content Consider the effect of CO 2 on the treatment processes and handle it appropriately

27 Use ALKasts to experiment with alkalinity generation Alkalinity (mg/l as CaCO 3 ) 1, Alkalinity Carbonated Spring Water ph ph