1

Alkaline industrial waters and wetlands:

prospects for effective treatment

Will Mayes1 & Jon Aumônier 2

1 Centre for Environmental and Marine Sciences, University of Hull, YO11 3AZ

W.Mayes@hull.ac.uk

2 Mineral Industry Research Organisation

• Successful programme of treating abandoned mine waters

• Circum-neutral pH, high Fe concentrations

Treatment wetlands and post-industrial pollution

Taff Merthyr coal mine treatment scheme, S. Wales

>50 full scale coal mine water treatment schemes

in UK, many incorporating wetlands;

>2000 tonnes Fe year prevented from stream

discharge

Inflow

Outflow

Gravel

substrate

Aerobic wetlands: shallow water planted with

reeds / rushes; enhances Fe settlement; residual

Fe uptake

2

• Weathering of lime-rich residues produced in major global

industries: e.g. steel slags, fly ash, lime spoil, C&D waste,

Solvay Process waste

• CaO hydrolyzes and liberates OH- in solution

Ca(OH)2 → Ca2+ + 2OH-

• NaOH-derived alkalinity at some sites: Bayer Process sources

Alkaline leachates - sources

• High rates of calcite (CaCO3) precipitation as waters take in atmospheric CO2

Ca2+ + CO32- → CaCO3

• Potential elevation of trace elements – e.g. As, Cr, Se, V

• V potentially a key issue given pentavalent form prevails

Extremely alkaline waters

Drainage from a

former lime works at

Harpur Hill near

Buxton, UK

The Dene Burn

downstream of the

former Consett

Steelworks, UK

3

• Acid dosing, recirculation of waters over spoil, aeration

• Requires sustained capital input

• Not usually suitable beyond economic life of workings

Conventional Management

F&N Hawley Award for Environmental Engineering, September 2007

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pH

Dene BurnHowden Burn

Workings closureWater pH in two

streams draining the

former Consett

Steelworks, Co. Durham,

UK

Statutory surface water

quality limit

Mayes et al. (2008). Water

Air Soil Pollut. 195: 39-40.

Natural wetland at Thrislington Limeworks, Co. Durham

The basis for treatment wetlands

• ‘Volunteer’ wetlands

• pH 12 lime spoil leachate

• pH 12 steel slag leachate

• Calcite precipitation from mass balance and direct measurement using immersed limestone blocks

Environ. Sci. & Technol. (2006)

40: 1237-1243

0 100 200 metres

Leachate

source zone

Pond

Wetland flow

zone

1

2

4

3

5

6

Culvert

Base of slag heaps

Base of slag

heaps

Extent of

wetland area

Sample location

Stream

Slag heap base

Hownsgill

Valley, Consett

Wetland extent

Flow

4

Volunteer wetlands

• Precipitation rates between 0.8 and 9.9 g/day/m2 of wetland area from limestone blocks

• Rates from mass balance varied between 1.7 and 13.6 g/d/m2 of wetland area

• Highest rates coincide with areas where vegetation/algae begin to establish

• Infers importance of biological activity in enhancing removal

SOURCE LAGOON WETLAND

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2 3 4 5 6Site

pH

pH

Median pH values during study period. Y

error bars show range, n = 30 for each.

• Sufficient removal for economically viable wetland sizes (~50m2

and 14000m2 for 12 monitored discharges in UK)Mayes et al. (2006). Environ.

Sci. Technol. 40: 1237-1243

Pilot constructed wetland - Redcar

Mayes et al. (2009). Water

Sci. Technol. 59: 2253-2263.

5

• Brackish coastal setting

• Two leachate sources: input increases alkalinity (>1500mg/L as CaCO3), pH (>12) and Ca2+ (>600mg/L)

• CM2: Pilot CW sized by CaCO3 removal rates from volunteer system

• CM3: Monitored through natural wetland

• Much improved flow rate measurements

Pilot constructed wetland - Redcar

pH E.C. Alk Ca Mg Na Cl SO4 Fe Al Sr

CM1 8.2 2203 250 40 9 316 375 138 0.9 0.3 0.3

CM2 11.9 2203 664 80 0.4 284 326 59 0.3 0.4 0.4

CM3 12.4 7879 1930 601 0.1 258 116 52 0.5 0.5 2.2

Typical water quality composition of upstream reference site (CM1), and leachate source areas (CM2, CM3). All

values mg/L except pH, electrical conductivity (‘E.C’: μS/cm2), total alkalinity (‘Alk’: mg/L as CaCO3)

• PVC liner, 0.2m indigenous boulder clay

• Transplantation of Phragmites australis (Common Reed)

• Low flow rate – suitable for pilot testing

• Flow monitoring through 90o V-notch, tracer tests

Pilot constructed wetland

MARCH 2008 SEPTEMBER 2008

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• No ‘honeymoon period’ - modest decline in pH

• Improved performance over time

Early performance

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pH

pH IN Cell 1pH OUT Cell 1

• Alkalinity removal efficiency

Performance and metrics

% Total Alkalinity removal: CELL 1

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Trea

tme

nt E

ffic

ienc

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)

% Total Alkalinity removal: CELL 1

Alkalinity removal to

reach background

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• Potential mechanisms:

– increased precipitation

nuclei: emergent plants

and micro-organisms /

debris

– increased microbial

activity

– Hydraulic efficiency

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0 200 400 600 800 1000

Ca2+ IN (mg/L)

CaC

O3 r

emo

val (

g/m

2/d

ay)

Seasonality

Consett winter Coatham winter

Coatham growing

season

Consett growing

season

Constructed

wetland

Natural wetland

Mayes et al. (2009). Water

Sci. Technol. 59: 2253-2263.

• Alkaliphilic bacteria shown to be present in many hyperalkaline settings

• Cultures used in treatment of alkaline pulp and textile waste

• Acid generation processes:

– Carbohydrate degradation: e.g. glucose metabolised to gluconic acid

• Scope for developing cultures to use in wetlands?

Microbially-mediated bufferingBurke et al. (2012). Geomicrobiol. J.

in press.

Kumar et al. (2011) World Academy

of Science, 76: 503-506

8

• Lab tests using water and substrate from Coatham

Controlling mechanisms

8.0

8.5

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pH

CTRL CTRL_SSED1 SED1_S

Above pH~12: OH-

predominates

pH 10.3-12: CO32-

predominates

pH 6.4-10.3: HCO3- predominates

~7 days

~14 days

• Leachate buffering

~50% more rapid

when in contact

with wetland

substrate than

atmospheric CO2

transfer

• Good evidence of

importance of

microbial

processes

• Most trace elements below detection limits in water (e.g. Cd, Co, Pb, Se). Some Ni lost across wetland.

• Calcite-dominated secondary precipitates

• Supersaturation with a range of phases (e.g. CaCO3, SrCO3, Ni(OH)2

and a range of Fe and Al oxyhydroxides)

Trace elements

CM2 CM3 SC1Ca 373700 390800 264350Mg 817 767 7781Fe 16701 7326 8144Al 2457 2976 2451Ba 70 66 130Cr 10 15 12Cu 3 3 7Mn 2545 2370 7199Ni 6.4 4.4 7Sr 6216 3542 2785V 17 15 44Zn 41 51 70 Composition of digested secondary precipitates. Selected

elements in mg/kg.

9

• Scunthorpe, Yarborough Landfill, constructed in Spring 2012; area ~1600m2

• Batch flow system – currently pumped from borehole

• Better residence time estimates for constraining alkalinity / Ca removal rates

• Aughinish, Ireland: trial cell for red mud leachate: 2012

Larger trial systems

• Increasing evidence of efficacy of wetlands for alkaline waters

• Some removal of trace metals in wetland through uptake in

secondary calcite precipitates

• More empirical data: robust sizing estimates for full scale

• Improved mechanistic understanding – process rates /

modelling; biotic / abiotic processes

Conclusions