sulfur-assisted biological reduction of nitrate
TRANSCRIPT
a Potentially Promising Technology Sulfur-Assisted Biological Reduction of Nitrate -
Nicholas DuganWater Infrastructure Division – Biological Contaminants Branch
EPA Region 7 Science Academy Lunch & Learn September 22, 2021Office of Research and DevelopmentCenter for Environmental Solutions and Emergency Response
Motivations for nitrate treatment
• EPA nitrate maximum contaminant level (MCL) = 10 mg NO3-N/L• EPA nitrite MCL = 1.0 mg NO2-N/L• Nitrate/nitrite MCLs protective of childrens’ health → excess nitrate can
cause methemoglobinemia (“blue baby syndrome”), especially in children <= 6 months
• High levels of nitrate may be associated with increased probabilities of certain cancers, birth defects, and thyroid issues
• From 1994 to 2016:o Increasing percentage of groundwater systems exceeding nitrate MCLo Increasing percentage of exceedances are repeats
• Systems serving populations < 500 account for 82% of violations• Pennino et al. Trends in Drinking Water Nitrate Violations Across the
United States. Environmental Science and Technology. 2017:51:13450-13460
Biological nitrate removal in drinking water
• Operates under anoxic conditions (defined for this study as ≤ 1 mg/L dissolved oxygen)
• Utilizes biomass attached to granular support media• Takes advantage of indigenous micro-organisms with the capability
to engage in nitrate reduction• Heterotrophic:
o Utilizes an organic substrate as carbon source and electron donor
o For example, acetate, ethanol, or methanolo Acetate is used in drinking water applications
• Chemolithoautotrophic:o Utilizes inorganic carbon and an inorganic electron donor
Biological nitrate removal in drinking water
• Heterotrophic nitrate removal with acetate as the electron donor:o Potential advantages: Faster acclimation Faster reaction rates Easier acceptance
o Potential disadvantages: Requires regular chemical handling Rapid biomass development may require close
monitoring and possible operational adjustments (increased backwashing)
Biological nitrate removal in drinking water
• Chemolithoautotrophic nitrate removal with elemental sulfur as the electron donor:o Early discussions by: Driscoll and Bisogni, 1978 (Journal Water Pollution Control
Federation, vol. 50, no. 3, pp. 569-577) Kruithof et al, 1988 (Water Supply, vol. 6, pp. 207-217)
o Potential advantages: Reduces need for day-to-day chemical handling Slower biomass growth rate translates to lower need for
operator monitoring and intervention o Potential disadvantages: Slower acclimation Slower loading rates No known operational history in the US for drinking water Public perception
Biological nitrate removal in drinking water
• Proposed stoichiometry (Kruithof et al, 1988):
55S + 20CO2 + 50NO3- + 38H2O + 4NH4
+ →
4C5H7O2N + 25N2 + 55SO42- + 64H+
Biomass production
pH depression
Inorganic C consumption
• Proposed phosphorous requirements (Kruithof et al, 1988):
C5H7O2NP0.03
Bench-scale treatment system
Cincinnati tapwater
Activated carbonfor
de-chlorination
Nitrogen stripping
forde-aeration
N2
Pond water
formicrobial seeding
NO3, PO4addition
Sulfurcontactor
Slow sandfilter
Re-aeration
Bench-scale treatment system
Study goals
• Restart two contactors following 2-month shutdown
• Run contactors in parallel, eventually ceasing PO4 addition in System 2
• Investigate impact of increasing influent dissolved oxygen concentrations
• Investigate the efficacy of effluent polishing by aeration and slow sand filtration
Effluent NO3 & surface loading rate
Time (days)
0 50 100 150 200
Efflu
ent N
O3 c
once
ntra
tion
(mg
NO
3-N
/ L)
0
2
4
6
8
10
12
14
16
18
Surfa
ce lo
adin
g ra
te (m
/hr)
0.00
0.05
0.10
0.15
0.20
0.25
Average influent NO3 = 17.4 mg NO3-N/L
System 1 eff. NO3 (Left axis, Method 353.2)
System 2 eff. NO3 (Left axis, Method 353.2)
System 1 eff. NO3 (Left axis, ISE)
System 2 eff. NO3 (left axis, ISE)System 1 loading rate (right axis)System 2 loading rate (right axis)
NO3 (left axis)
Surface loading rate (right axis)
Slow sand filter hydraulic loading rates = 0.06 – 0.3 m/hr (AWWA Manual of Design for Slow Sand Filtration)
Effluent NO3 & influent PO4
Time (days)
0 50 100 150 200
Efflu
ent N
O3 c
once
ntra
tion
(mg
NO
3-N
/ L)
0
2
4
6
8
10
12
14
16
18
Influ
ent P
O4 (
mg
PO4-
P/L
)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Average influent NO3 = 17.4 mg NO3-N/L
NO3 (left axis)
Sys. 1 eff. NO3 (Left, Method 353.2)
Sys. 2 eff. NO3 (Left, Method 353.2)
Sys. 1 eff. NO3 (Left, ISE)
Sys. 2 eff. NO3 (left, ISE)
Sys. 1 inf. PO4 Sys. 2 inf. PO4
PO4 (right axis)
Effluent NO3 & influent dissolved O2
Time (days)
0 50 100 150 200
Efflu
ent N
O3 c
once
ntra
tion
(mg
NO
3-N
/ L)
0
2
4
6
8
10
12
14
16
18
Influ
ent d
isso
lved
oxy
gen
(mg
O2/L
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Average influent NO3 = 17.4 mg NO3-N/L
NO3 (left axis)
Sys. 1 eff. NO3 (Left, Method 353.2)
Sys. 2 eff. NO3 (Left, Method 353.2)
Sys. 1 eff. NO3 (Left, ISE)
Sys. 2 eff. NO3 (left, ISE)
Sys. 1 inf. DO (right)Sys. 2 inf. DO (right)
DO (right axis)
Effluent NO2- concentrations
System 1
Time (days)
0 50 100 150 200
Nitr
ite c
once
ntra
tion
(mg
NO
2-N
/L)
0.00
0.02
0.04
0.06
0.08
System 2
Time (days)
0 50 100 150 200
Nitr
ite c
once
ntra
tion
(mg
NO
2-N
/L)
0.00
0.02
0.04
0.06
0.08
Influent Effluent
EPA maximum contaminant level (MCL) = 1.0 mg NO2-N/L
Effluent NH4+ concentrations
Time (days)
0 50 100 150 200
Amm
oniu
m c
once
ntra
tion
(mg
NH
4+ -N/L
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
System 1System 2
Effluent may require aerobic biological polishing
Areas of gas accumulation
Elemental sulfur:bubble formation
• Contactors backwashed every 14 days• Small flow rate reductions observed over
that time period (≤ 5%)• Backwashing cleared out gas,
particulates, mitigated clumping of media
Gravel support media: no bubble formation
Other water quality parameters
ParameterChange across contactor
(+/-) Notes
Alkalinity - 28 mg CaCO3/L (-33%) -3.3 mg inorganic C/L
pH -1.4
Sulfate + 120 mg SO4/L Effluent max = 220 mg SO4/LEPA secondary MCL = 250 mg SO4/L
Hydrogen sulfide Increased – detectable by odor Removed during aeration
Slow sand filter
• Maximum surface loading rate = 0.2 m/hr (approx. 0.1 gal/min x ft2)
• Bed depth = 0.61 m (24 in)• Media effective size = 0.18 mm
(0.0071 in)• Uniformity coefficient = 1.9• Available head = 0.91 m (3 ft)
Effluent turbidity
Time (days)
0 50 100 150 200
Turb
idity
(NTU
)
0.01
0.1
1
10
NE anaerobic contactor effluentNE slow sand filter effluent
Scrapedschmutzdecke
System 1 anaerobic contactor effluentSystem 1 slow sand filter effluent
Conclusions
• Sulfur assisted biological nitrate reduction: o Reliably removes high (~ 17 mg NO3-N/L) influent
concentrations of nitrate at surface loading rates that are realistic for small systems
o 14-day backwash cycleo Produces an effluent with nitrate and nitrite concentrations
comfortably below their respective MCLso Produces an effluent with manageable concentrations of NH4
+
o Slow sand filtration produces effluent with turbidities < 0.3 NTU, at the cost of relatively rapid headloss development
Next steps
• Laboratory:o Evaluate maximum allowable influent dissolved oxygen
concentrationo Evaluate trade-offs between contact time, nitrate removal,
pH depression, sulfate, and sulfide production → pH depression and sulfate production are stoichiometrically related to nitrate removals → nitrate removals evaluated in this work are larger than those required in many systems
o Evaluate alternatives to slow-sand filtration for contactor effluent particulate polishing
o Evaluate pH adjustment methods• Pilot/field testing:
o Evaluate impact of temperatureo Run in parallel with heterotrophic nitrate removal system
Thank you!
Nick DuganWater Infrastructure DivisionBiological Contaminants [email protected]