A history of successful mercury control efforts at alarge urban wastewater treatment plant, 1994‐2018Steven J. Balogh, Metropolitan Council Environmental Services, Saint Paul, MN
2018 NIMD Forum
• Serves 7 counties (3,000 square miles)
• 250 mgd on average
• 8 wastewater treatment plants
• 600 miles of interceptors
• 2+ million wastewater customers in 108 communities
• 2.6 million population served in water resources (3.6 million in 2030)
MCES Service Area and Facilities
$6 billion in existing assets
Metropolitan Wastewater Treatment Plant
sludge thickening
primarysettling
activatedsludge
finalsettling
sludgedewatering
multiple‐hearthincineration391 g/day
air‐pollutioncontrol
plantinfluent248 g/day
finaleffluent10 g/day
ash0 g/day
spent scrubber water153 g/day
return activated sludge
waste activated sludge228 g/day
air
stackemission238 g/day
GTO6 g/day
FTO10 g/day
primarysludge
187 g/dayFTS
sludgestorage
181 g/day
399 g/day
218 g/day
filtrate8 g/day
GTS
Hg mass balance‐Metro Plant 1994
770 oC
Keeping Hg out of municipal wastewater:Installing Hg amalgam separators in Metro area dental offices
0
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100
% in
stal
led,
cum
ulat
ive
1994 2003 2004 2005 2006 2007 2008100
150
200
250
m
edia
n H
g lo
ad (g
d-1
)
The Hg load in Metro Plant influent decreased with increasing cumulative amalgam separator installation.
03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 180
200
400
600Metro Plant Influent Hg Load (g d -1): 2003-2018
2003 2006 2009 2012 2015 20180.0
0.5
1.0
1.5
2.0Metro sludge cake THg (mg dry kg-1)
Metro Plant Process Schematic ‐ 2018
Metro Plant Sludge Incineration System ‐ 2018
Factors Effecting Mercury Emissions
• From previous MCES R&D testing (Hg in spent scrubber water):• Fluidized bed temperature ‐ NOx• Baghouse ash breakthrough• Carbon feed rates
• New results (Hg in stack gas):• Outlet dust level in baghouse
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1300 1350 1400 1450 1500 1550
Hg
in s
crub
ber w
ater
(ng/
L) F
BR
3
NO
x FB
R3
bedzone 1 T (F) FBR3
NOx Hg in scubber water
carbon >3 lb/hr and < 6 lb/hr
NOx interferes with Hg adsorption on activated carbon‐ Higher Hg levels in scrubber water at higher bed temperatures
Mercury Sampling ‐ EPA Method 30B
A total of 25 separate tests were performed using a mercury sampling console and probe.
Carbon sorbent traps: Section 1 ‐ collects the Hg Section 2 ‐ backup trap for Hg breakthrough
Analysis: Traps digested and analyzed using cold vapor atomic fluorescence with single gold trap amalgamation.
Method detection limit: 2 ng/DSCM
Section 1 Section 2
Heated
Dry gasmeter
PumpWater drop out
Dessicant
Dry gasmeter
PumpWater dropout
Dessicant
Sampling portflange
Dual channel heated sampling probeCarbon traps (2)
Stack gas in
Gas flow
Stack wall
Stack sampling schematic –Method 30B
2016 Incinerator Stack Testing
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350
0 20 40 60 80 100 120
Hg Co
ncen
tration (ng/DSCM)
BH Outlet Dust Level (%)
Stack gas Hg concentration vs BH outlet dust level
High Carbon (12‐13 lb/hr)
Low/Mid Carbon (3.3‐7.4 lb/hr)
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350
0 2 4 6 8 10 12 14
Hg Co
ncen
tration (ng/DSCM)
Carbon feed rate (lb/hr)
Stack gas Hg concentration vs carbon feed rate
Low/Mid BH Dust (27‐54%)High BH Dust (68‐107%)
Hg concentrations increased with increasing baghouse outlet dust levels and decreasing carbon feed rates.
Hg Concentrations in Incinerator Stack Gas
All samples collected werebelow the EPA standard fornew FBRs of 1000 ng/DSCM
Actual Hg concentrationsranged from 35 to 300ng/DSCM
Levels were much lowerthan previous reports usingless sensitive methods
Results: Comparison with previous data
sludge thickening
750 oC
primarysettling
activatedsludge
finalsettling
sludgedewatering
fluidized‐bedincineration99 g/day
air‐pollutioncontrol
plantinfluent98 g/day
finaleffluent1 g/day
ash andresiduals97 g/day
spent scrubber water2 g/day
return activated sludge625 g/day
waste activated sludge25 g/day
air
stackemission0.2 g/day
GTO2 g/day
FTO1 g/day
primarysludge
78 g/dayFTS
sludgestorage
76 g/day
100 g/day
24 g/day
centrate1 g/day
GTS
Hg mass balance‐Metro Plant 2018
Conclusions
• 25 years of effort to control Hg discharges at the Metro Planthave resulted in substantial reductions
• Hg inputs to the plant have been reduced by >60%
• Hg discharges to the Mississippi River have been reduced by >90%
• Hg emissions to the atmosphere have been reduced by >99%
2018 NIMD Forum
Thank you for your attention!
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