study of disinfectant penetration in a drinking water

Study of Disinfectant Penetration in a Drinking Water Storage Tank Sediment Using Microelectrodes

Hong Liu, David G. Wahman, and Jonathan G. Pressman

November 15, 2016

National Risk Management Research Laboratory U.S. Environmental Protection Agency

Cincinnati, Ohio, United States

2016 Water Quality Technology Conference

Nitrification in Drinking Water Storage Tank Sediment

Water Storage Tanks Accumulate Sediment Over Time and Provide a Good Habitat for Bacteria Growth

2/14/2018 2016 Water Quality Technology Conference 2

Ammonia Nitrite AOB

Nitrate NOB

- Chloramine decay - Naturally occurring - Organic N

Research Motivation

Sediment accumulation causes water quality degradation issues:

Harbor biological growth

(nitrification) & Pathogens

Increase disinfectant decay

Nitrite & Nitrate

Disinfectant penetration within water storage sediments is largely uncharacterized


Overall Research Objective

Use microelectrodes to evaluate disinfectant and water quality within a drinking water tank sediment Monochloramine & free chlorine penetration profiles over time

Evaluate impact of switching disinfectants Monochloramine free chlorine monochloramine

Investigate the relationship between disinfectant penetration and microbial activities within the sediment Dissolved oxygen (DO), ammonium, nitrite, nitrate, & pH profiles over

time


Research Approach – Microelectrode Fabrication

Microelectrodes used in this research: Amperometric type:

- Dissolved oxygen (DO), monochloramine, free chlorine, and nitrite

Potentiometric type: - pH, ammonium, and nitrate

Monochloramine & Free Chlorine Microelectrodes


Gold wire for monochloramine Platinum wire for free chlorine

Combined O2 Microelectrode

Cited From Lu, R and Yu, T. 2002

Research Approach – Microelectrode Measurement

Faraday Cage

Microscope with Camera

Micromanipulator

Sediment

Influent pump Feed

water

Microelectrode

Air Table

Effluent pump Automatic Microprofiling Setup • Computer To waste • Camera Screen • Multimeter

Magnetic stir plate

Effluent line Microelectrode

Light source Reference electrode

Sediment vessel

Influent line Microscope


Disinfectant Application Scenario

• 5 cm diameter x 4 cm deep teflon cup - 2.0 cm (20,000 µm) sediment depth - 0.5 cm (5,000 µm) water depth over sediment

• 5 mL/min flowrate - 2 minute hydraulic residence time - 4 mg Cl2/L monochloramine (4:1 Cl2:N) or free chlorine - 5 mM borate buffer (pH 8.0)

Phase 1 (120 days): Monochloramine

Phase 2 (60 days): Free chlorine

Phase 3 (60 days): Monochloramine


Phase 1 Monochloramine - Monochloramine Profiles

Interface between bulk water and sediment

5

4

5 hours 6 days 3 34 days 76 days 119 days 2

1

0

-5000 -2500 0 2500 5000 7500 10000 12500 15000

Mon

ochl

oram

ine

(mg

Cl 2

/L)


Distance (µm) 2• After approximately 5 hours, a 2 mg Cl /L monochloramine concentration reached

the sediment’s surface, but after 119 days, the 2 mg Cl2/L monochloramine concentration only reached to a depth of 3,200 µm.

• No monochloramine was measurable at sediment depths greater than 6,200 µm.

Phase 1 Monochloramine – Dissolved Oxygen Profiles

10

8

6

4

6 days 34 days 76 days 120 days

2

0

-5000 -2500 0 2500 5000 7500 10000 12500 15000

Dis

solv

ed O

xyge

n (m

g/L)

Distance (µm) • DO progressed inward during the 120 day period. • 4 mg/L of DO reached 5,000 µm below interface after day 120 with 52% of the bulk DO consumed. • No DO was measurable at sediment depths greater than 12,500 µm. 2/14/2018 2016 Water Quality Technology Conference 9

Phase 1 Monochloramine – Nitrogen Profiles N

H4+ , N

O2- , a

nd N

O3- (m

g N

/L)


1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

-5000 -2500 0 2500 5000 7500 10000 12500 15000

NH4-N 79 days NH4-N 115 days NO2-N 100 days NO2-N 116 days NO3-N 79 days NO3-N 115 days

Distance (µm) • Nitrate increased with a concurrent decrease in ammonium with sediment depth.

• Minimal nitrite accumulation, indicating complete nitrification occurred in the sediment.

Phase 1 Monochloramine – Final Profile Summary

9 2.0

8

-5000 -2500 0 2500 5000 7500 10000 12500

Monochloramine Dissolved Oxygen pH Ammonium Nitrite Nitrate

Amm

oniu

m, N

itrite

, and

Nitr

ate

(mg

N/L

)

1.6 7

6

5

4

1.2

0.8 3

2 0.4 1

0 0.0

• Start of DO consumption corresponded with monochloramine decrease.

• Complete nitrification oxygen consumption corresponds.

• DO consumption continued after ammonia removal heterotrophic activity.

DO

(mg/

L), M

onoc

hlor

amin

e (m

g C

l 2 //L

), an

d pH

Distance (µm)


Phase 2 Free Chlorine – Free Chlorine Profiles

-5000 -2500 0 2500 5000

5 hours 28 days 60 days

5

4

3

2

1

0 Free

chl

orin

e (m

g C

l 2/L

)

Distance (µm) • Free chlorine penetrated the sediment very slowly, resulting in minimal penetration

within the 2-month free chlorine application period. • Only a 0.2 mg/L free chlorine concentration penetrated to a 500 µm depth after one

month, and it appears no further penetration occurred during the subsequent month.


Phase 2 Free Chlorine – Dissolved Oxygen Profiles

10 DO kept penetrating during free chlorine

8 burn period

6 7 days 26 days 62 days 4

DO profile at day 120 2 monochloramine

disinfection 0

-5000 -2500 0 2500 5000 7500 10000 12500 15000

Dis

solv

ed O

xyge

n (m

g/L)


Distance (µm) • As compared to the 4 mg/L DO at a 5,000 µm depth after 120 days of

monochloramine application, 4 mg/L of dissolved oxygen penetrated to a 6,800 µm depth after 62 days of free chlorine application.

• No DO was measurable at sediment depths greater than 13,500 µm.

Phase 2 Free Chlorine – Nitrogen Profiles

1.2

1.0 NH4-N 5 days

0.8 NH4-N 25 days NH4-N 59 days

0.6 NO2-N 25 days NO2-N 60 days

0.4 NO3-N 5 days NO3-N 25 days

0.2 NO3-N 59 days

0.0

-5000 -2500 0 2500 5000 7500 10000 12500 15000

NH

4+ , NO

2- , and

NO

3- (mg

N/L

)

Distance (µm)

• Ammonium and nitrate nitrogen continuously decreased over 60 days of free chlorine exposure.


Phase 2 Free Chlorine – Final Profile Summary

DO

(mg/

L), F

ree

chlo

rine

(mg

Cl 2/

L),

and

pH

9 1

8 Free Chlorine Dissolved Oxygen pH Ammonium Nitrite Nitrate

-5000 -2500 0 2500 5000 7500 10000 12500

0.8

0.6

0.4

7

6

5

4

3

2 0.2 1

0 0 Amm

oniu

m, N

itrite

, and

Nitr

ate

(mg

N/L

)

Distance (µm) • After a 2-month free chlorine application, both ammonium and nitrate decreased to

some extent, but the DO consumption and the continued presence of ammonium and nitrate indicated that there was still microbial activity within the sediment.

• A decrease in pH from 8 to 7.7 was seen.


Phase 3 Monochloramine - Monochloramine Profiles

0

1

2

3

4

5

9 days 22 days 43 days 55 days

-5000 -2500 0 2500 5000 7500 10000 12500 15000

) L/l 2

C g

m(e n

ami

lor

hoc

onM

Distance (µm)

• A 2 mg Cl2/L monochloramine reached to a similar depth (2,800 µm) as in Phase 1 (3,200 µm). • Monochloramine appears to penetrate further (7,500 µm) than was seen in Phase 1 (6,200 µm).


Phase 3 Monochloramine – Dissolved Oxygen Profiles

0

2

4

6

8

10

8 days 42 days 55 days

Red line represents DO profile at day 62 free chlorine application

-5000 -2500 0 2500 5000 7500 10000 12500 15000

)Lg/

m(n

geyxO

d evlossiD

Distance (µm) • Compared to the DO profile at the end of Phase 2, DO utilization increased within

the first 30 days.

• After day 42, DO penetrated further.


15000

Phase 3 Monochloramine – Nitrogen Profiles

NH4-N 8 days NH4-N 56 days NO2-N 11 days NO2-N 56 days NO3-N 8 days NO3-N 56 days

1.0

0.8

0.6

0.4

0.2

0.0 -5000 -2500 0 2500 5000 7500 10000 12500

NH

4+ , NO

2- , and

NO

3- (mg

N/L

)

Distance (µm)

• After day 56, 0.6 mg N/L of ammonium substrate accumulated in the sediment at 3,400 µmand then decreased with a concurrent increase of nitrate, implying complete nitrification had resumed.


Phase 3 Monochloramine – Final Profile Summary

9

8

7

6

5

4

3

2

1

0

1

0.8

0.6

0.4

0.2

-5000 -2500 0 2500 5000 7500 10000 12500

Monochloramine Dissolved Oxygen pH Ammonium Nitrite Nitrate

Amm

oniu

m, N

itrite

, and

Nitr

ate

(mg

N/L

)

DO

(mg/

L), M

onoc

hlor

amin

e (m

g C

l 2//L

), an

d pH

0

Distance (µm)

• Once switched back to monochloramine, ammonium substrate was brought into the sediment and nitrification resumed.

• The return to chloramination following a free chlorination period led to subsequent nitrification within a short period of time.


Conclusions

Microelectrodes were a useful tool for determining chemical variability within

a drinking water storage tank sediment.

Even with extended periods where the maximum regulatory allowed

chloramine residual was maintained in the bulk water, complete

monochloramine penetration was not obtained into the sediment and

biological activity remained.

Free chlorine progressed inward into the sediment very slowly, resulting in

minimal penetration within a 2-month free chlorine application period and

microbial activity remained.

Nitrification resumed upon a switch back to monochloramine.

Findings support periodic cleaning of sediments from water storage tanks.


Acknowledgements

This research was supported by the U.S. Environmental Protection Agency (U.S.EPA). The U.S. Environmental Protection Agency, through its Office of Research and Development, funded and managed, or partially funded and collaborated in, the research described herein. It has been subjected to the Agency’s peer and administrative review and has been approved for external publication. Any opinions expressed are those of the author(s) and do not necessarily reflect the views of the Agency, therefore, no official endorsement should be inferred. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use.


study of disinfectant penetration in a drinking water

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