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 Branchdugan.nicholas@epa.gov