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Copyright [insert date set by system] by [CH2M HILL Entity] • Company Confidential
Passive Treatment Systems for the Removal of Selenium: Selenium Removal Mechanisms with Biochemical Reactor Substrate
R.C. Thomas, J.J. Tudini, J.S. Bays, M.A. Girts, K.B. Jenkins, L.C. Roop, and T. Cook
June 3 2013
Overview of Presentation
Outline
Barrel-Study Overview Aqueous Results Volumetric/Aerial Loading Rates Substrate Se Removal Volatilization Substrate Se Speciation Modeling Calibration/Removal Rate
Development Secondary Parameters Conclusions
NAMC Selenium Report 2010
http://www.namc.org/docs/00062756.PDF
Case History (2011-2012)Two Concurrent BCR Pilot Studies for Coal Mine Drainage
Outlet A: 389 days
Pumped Inflow
Outlet B: 180 days
Gravity Inflow
Influent
Effluent ManifoldEffluent Sample Ports
Influent – Gravity Feed
Effluent Sample Ports
TDS 1 g/L Se in 6–17 µg/LNO3-N 0.47 mg/LTP 0.071 mg/L
Source:R. Thomas (2011)
Outlet 022: Variable Flow and Limited Flow Control Resulting in Variable Se Reduction
0.1
1.0
10.0
5/27/2011 6/26/2011 7/26/2011 8/25/2011 9/24/2011 10/24/2011 11/23/2011
Sele
nium
Con
cent
ratio
n (p
pb)
Outlet 022 Pilot: 180 Days
Barrel A Barrel B Barrel C Barrel D Monthly Avg Limit Max Daily Limit Influent
Outlet 033: Consistent Se Reduction Through Winter
8.2 µg/L
Outlet 033 Pilot: 389 days
0.8-0.9 µg/L
Source:CH2MHILL (2012)
8.2°C mean4°C min<0°C air
Focus on Winter Water Minimum Temperature 4°C
Avg Water Temp 8.2°CAvg Min Air Temp 4.2°C
Average Se Mass Removed Per Day – 022
4,106
542
3,564
Outlet 022 - Barrel A, Avg. Se Mass (µg/day)
Se Influent
SeEffluent
Se Removed
87% removed
3,827
338
3,489
Outlet 022 - Barrel BAvg. Se Mass (µg/day)
Se Influent
SeEffluent
Se Removed
92% removed
3,523
678
2,844
Outlet 022 - Barrel CAvg. Se Mass (µg/L)
Se Influent
SeEffluent
Se Removed
83% removed
3,027
288
2,739
Outlet 022 - Barrel DAvg. Se Mass (ug/L)
Se Influent
SeEffluent
Se Removed
90% removed
Average Se Mass Removed Per Day – 033
1,726
139
1,587
Outlet 033 - Barrel A, Avg. Se Mass (µg/day)
Se Influent
SeEffluent
Se Removed
91% removed
2,348
179
2,169
Outlet 033 - Barrel BAvg. Se Mass (µg/day)
Se Influent
SeEffluent
Se Removed
92% removed
1,761
198
1,563
Outlet 033 - Barrel CAvg. Se Mass (µg/L)
Se Influent
SeEffluent
Se Removed
89% removed
1,752
151
1,601
Outlet 033 - Barrel DAvg. Se Mass (ug/L)
Se Influent
SeEffluent
Se Removed
90% removed
Se Mass Removed vs Volumetric Loading
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035
Se R
emov
ed (g
/m3 -
d)
Se Loading (g/m3-d)
022 HRT12 022 HRT18 022 HRT24 033 HRT12 033 HRT18 033 HRT24
Winter Barrel A
Winter Barrel B
Reaction Fronts in BCR Substrates Over Time
Time
Se Mass Removed vs Surface Aerial Loading
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.000 0.005 0.010 0.015 0.020 0.025
Se R
emov
ed (g
/m2 -
d)
Se Loading (g/m2-d)
022 HRT12 022 HRT18 022 HRT24 033 HRT12 033 HRT18 033 HRT24
Winter Barrel A
Winter Barrel B
Barrel Selenium Profile
012345678910
Se Concentration (mg/kg)
Outlet 022 Substrate Total Se Concentration (180 days)
022A
022B
022C
022D
Top
Middle
Bottom
upflo
w
Sampled 11/22/11(180 days)
Flow Initiated 5/26/11
Barrel Selenium Profile
024681012
Se Concentration (mg/kg)
Outlet 033 Substrate Total Se Concentration (180 & 389 days)
033C
033D
033A
033B
Series5
Top
Middle
Bottom
dow
nflow
Surface
Sampled 6/18/12(389 days)
Sampled 11/22/11(180 days)
Flow Initiated 5/26/11
Selenium Mass Balance vs Sediment Core: Gap Suggests Volatilization Loss
B-A B-B B-C B-D A-A A-B0
100
200
300
400
500
600
700
CoreBalance
Core
Se
(mg
)
022 - A 022 - B 022 - C 022 - D 033 - C 033 - D
Vertical Distribution and Speciation of Selenium: Reduction, Sorption, Volatilization
Source:CH2MHILL (2012)
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
mg/
kg To
tal S
e
mg/
kg S
e
Substrate Selenium Speciation after 180 days
Salt Weakly adsorbed S2Se/Elemental Se Selenides Total Se
123% 88%105%131%87% 88%
Percent Recovery of Sequential Extractions versus the total digests
87% 105% 108% 88% 94%
In In Out In OutIn
Vertical Distribution and Speciation of Selenium: Reduction, Sorption, Volatilization
Source:CH2MHILL (2012)
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
mg/
kg To
tal S
e
mg/
kg S
e
Substrate Selenium Speciation after 389 days
Salt Weakly adsorbed S2Se/Elemental Se Selenides Residual Total Se
84% 189%95%121%94%Percent Recovery of Sequential Extractions versus the total digests
167% 99% 67% 80%
In Out In Out
Model Rate Constant Calibration
Empirical data from barrel studies used to calibrate treatment rates in the development of a sizing model for a full-scale treatment system
General terms of mass conservation for a plug flow system are used while maintaining the first order model as the basis
Empirical data was modeled using first order area-based treatment wetland model of Kadlec & Knight (1996) & Kadlec & Wallace (2009)
Model estimates the potential BCR effluent concentration for a given flow rate and concentration for a given BCR area, as adjusted for temperature
J = k(Ci – Ce) where:
J = zero-order contaminant removal rate [g/m2/yr])
k = first-order, area-based rate constant (m/yr)
Ci = influent concentration (g/m3)
Ce = effluent concentration (g/m3)
0 A
Cout/Cin
0
1
Wetland Area
First-order Area-based Treatment Model
C*
Kadlec and Knight first-order treatment model
Q dC/dA = -k(C-C*)
Calibrate k by pollutant
? ? ?
First-order treatment model was expanded to the P-k-C* model, which is solved by relating: 1) hydraulic loading, 2) removal rate, 3) concentration terms, and 4) hydraulic mixing
Rate Constant vs Hydraulic Loading Rate
0
500
1,000
1,500
2,000
2,500
3,000
0 50 100 150 200 250
k T(m
/yr)
HLR (cm/d)
22A 33A 22B 33B 22C 33C 22D 33D
Barrel Trendline Equation R2
Outlet 022A y=281.32e0.0115x 0.9888
Outlet 022B y=413.88e0.0108x 0.9045
Outlet 022C y=293.37e0.012x 1
Outlet 022D y=331.43e0.0134x 0.8944
Outlet 033A y=122.74e0.0283x 0.9122
Outlet 033B y=155.06e0.0272x 0.9892
Outlet 033C y=201.14e0.019x 0.9872
Outlet 033D y=64.395e0.035x 0.9215
First-Order, Area-Based Tanks-in Series Rate Constant Modeling Results
Passive Treatment Systems rarely operate as plug-flow reactors– Heterogeneous substrate,
preferential flow Passive Treatment Systems are
better characterized hydraulically as a series of well-mixed tanks (3 – 6 in series for VFW)
Barrel-study data was configured in a three tank series using the P-k-C* model to assess the variable hydraulic characteristics on overall performance
First-Order, Area-Based Tanks-in Series Rate Constant Modeling Results
1st-order, Area Based, Rate Constants (m/y)
Barrel Outlet 022a Outlet 033a Outlet 033b Outlet 033c
A 1,379 1,498 675 675
B 1,744 1,671 1,177 1,177
C 1,246 1,180 — —
D 1,547 1,115 — —
Note: Target effluent concentration was set at 4 µg/L. a Influent temperatures monitored during the summer and fall operations used in model ( set at 1). b Influent temperatures monitored during the winter operations used in model (10.5°C) ( determined to be 0.95). c Influent temperature set at 9°C in model (θ determined to be 0.96).
Secondary Parameters (Salts & Labile C):Conductivity, TDS, Alkalinity, BOD
0
500
1,000
1,500
2,000
2,500
3,000
5/7/2011 8/15/2011 11/23/2011 3/2/2012 6/10/2012
Cond
. (um
hos)
Outlet 033
Inf A B C D
0
500
1,000
1,500
2,000
2,500
5/7/2011 8/15/2011 11/23/2011 3/2/2012 6/10/2012
TDS
(mg/
L)
Outlet 033
Inf A B C D
1
10
100
1,000
5/7/2011 7/16/2011 9/24/2011 12/3/2011
BOD
(mg/
L)
Outlet 033
Inf A B C D
0
200
400
600
800
1000
1200
5/7/2011 8/15/2011 11/23/2011 3/2/2012 6/10/2012
Alk
alin
ity
(mg/
L)
Outlet 033Alkalinity
Inf A B C D
Secondary Parameters: Nutrients and Color
0.0
0.5
1.0
1.5
2.0
2.5
5/7/2011 7/16/2011 9/24/2011 12/3/2011
Nit
rate
+Nit
rite
(mg/
L)
Outlet 033
Inf A B C D
0
5
10
15
20
25
30
5/7/2011 7/16/2011 9/24/2011 12/3/2011
Phos
phor
us (m
g/L)
Outlet 033
Inf A B C D
0
10
20
30
40
50
60
70
80
90
5/7/2011 7/16/2011 9/24/2011 12/3/2011
TKN
(mg/
L)
Outlet 033
Inf A B C D
0
500
1,000
1,500
2,000
2,500
3,000
5/7/2011 7/16/2011 9/24/2011 12/3/2011
Colo
r (ηg
/L P
tCc)
Outlet 033
Inf A B C D
Secondary Parameters: Metals
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
5/7/2011 7/16/2011 9/24/2011 12/3/2011
Iron
(mg/
L)
Outlet 033
Inf A B C D
0.0
0.5
1.0
1.5
2.0
2.5
3.0
5/7/2011 7/16/2011 9/24/2011 12/3/2011
Mn
(mg/
L)
Outlet 033
Inf A B C D
Conclusions
Selenium was effectively reduced by ~90% on average in all barrels. Selenium removal was effective at all HRTs tested (12 to 24 hours). Cold temperatures (below freezing) did not affect selenium removal
rates significantly, and effluent selenium concentrations remained below discharge limits following startup.
The organic substrates tested could generate secondary byproducts:– short‐term increase in conductivity from the flushing of excess salts from the
substrate during startup– elevated oxygen demand from excess labile carbon in the effluent– suspended solids (mainly fine particulate organic matter)– excess nutrients in the substrate mixture– release of regulated metals such as iron and manganese, which might also
occur in small amounts in the substrate mixture.
Conclusions
Se removal occurs at the influent water‐substrate interface– consistent with first‐order processes– extent of selenium distribution within the substrate is dependent on the selenium load.
Substrate Se speciation indicates:– dominant mechanism is reduction of selenate to reduced forms of selenium that are weakly adsorbed
to the substrate during early stages of selenium removal– approximately >50% weakly adsorbed Se attributable to selenite– Highly immobile elemental selenium or selenosulfide account for about a quarter of the total selenium
retained– very little metal selenide was found.
Both substrate coring and water-balance mass-balance evaluation methods show selenium retention– higher water balance method suggests a higher overall selenium retention than that observed in the
substrate cores, suggesting loss to volatilization. Barrel media type B appeared to balance high treatment performance with moderate
production of byproducts. Selenium treatment BCRs have been shown to treat mine site discharges effectively
in order to achieve selenium compliance limits.
Acknowledgements
Thanks to Alpha Natural Resources
Thanks to supporting engineering and science staff at CH2M HILL
Questions