A New Method for
Control of Nitrification
Tom O’Connor, H2O’C
Dr. John T. O’Connor, PE
Bart Murphy, Willmar, MN
Nitrification
microbially-mediated oxidation of ammonium ion
to nitrite, and eventually, nitrate
NH4+ +
3/2
O2
^ NO2--+ 2H + + H2O
NO2- + H2O ^
NO3--+ 2H +
Nitrifiers
Effects of Nitrification on
Water Quality
• Ammonium Ion consumed = Nitrite + Nitrate produced
• Dissolved Oxygen consumed (4.6 O: 1 N)
• Acid produced = Alkalinity consumed; pH lowered in system
• Disinfectant residuals (chlorine, chloramine) consumed by nitrite
ion ( NO2- ), a reducing agent formed as an intermediate
• Breakdown of chloramine frees up more ammonium ion, thus
enabling further nitrification
The ‘New’ Thing
sodium chlorite
NaClO2
History
1983: Hynes, R. K., and R. Knowles
Inhibition of Chemoautotrophic Nitrification by Sodium
Chlorate and Sodium Chlorite: A Reexamination.
Appl. Environ. Microbiol. 45: 1178-1182
1999: McGuire, M. J., N. Lieu and M. Pearthree (1999)
Using Chlorite Ion to Control Nitrification. J. AWWA 91:10:52-61
2000: H2O’C’s Willmar, MN distribution study
2001: H2O’C’s Morton, IL filter study
How Does it Work?
Chlorite enters cell, is converted to chlorine
dioxide due to acid conditions, resulting in
–Alteration of the cell membrane permeability
–Impairment of the cell’s enzyme and protein functions
–Nucleic acid damage
Similar concentrations of chlorite were not found to be toxic to E. coli or HPC bacteria
Distribution
System
Part One:
Willmar, MN
• pop. 18,000
• two treatment plants
• breakpoint chlorination
to remove ammonium
ion and thus inhibit
nitrification
Bart Murphy, Water Superintendent
What We Did
1. Fed sodium chlorite directly into finished water
Initial dosage:
0.60 mg ClO2- / l
10 gpd added to 3.64 MGD, SW Plant
Dist. Sys. avg. residual:
0.44 mg ClO2- / l
2. Monitored distribution system
Dosage was subsequently reduced
Analyses
Plant:
chlorine, chloramine, chlorite
by DPD method and amperometric titration
Distribution System:
ammonium, nitrite, nitrate ions
temperature, oxygen and pH
Chlorite
Analysis
• DPD method,
amperometric
titration were
undependable
• samples sent
to contract lab
F
0
P
P
P
F
F
P
P
F
P
J
H
F
P
P
H
F
P
H
F
P
H
F
P
H
F
P
J
H
F
P
H
J
F
P
H
J
F
P
H
J
F
P
J
F
P
J
F
P
J
F
P
J
F
P
J
F
P
P
15-Dec-2000
J
J
16-Oct-2000
J
J
H
B
21-Sep-2000
J
H
F
J
B
3-Aug-2000
J
B
15-Jul-2000
nitrite
H
28-Jun-2000
P
B
15-Jun-2000
nitrate
B
1-Jun-2000
F
B
26-May-2000
ammonia
B
23-May-2000
H
B
19-May-2000
Cl (total)
B
18-May-2000
J
B
17-May-2000
DO
B
16-May-2000
J
H
B
B
15-May-2000
B
8-May-2000
B
10-Nov-99
H
F
J
H
H
J
B
12-Aug-99
J
B
5-May-99
2
23-Sep-98
H
F
B
B
3-Feb-99
1 H
J
B
23-Jun-98
13
12
11
10
9 B
8
10-Mar-98
mg/l
Distribution System Trends
B
B
H
H
H
H
J
F
Sodium Chlorite, NaClO2
• Traditionally used for generation of
chlorine dioxide
• 31.25% solution
• $8 / gallon ($66/day for both of Willmar’s plants)
Cost Savings
Reduced Chlorine Use:
Cl2 Cost Savings:
600
80 lbs / day
$130 / day
ClO2- Feed (0.3 mg / l):
$66 / day
($12 / million gallons)
On a mass basis, overall chemical usage is significantly reduced
Health & Safety Benefits
Reduced THM formation
Reduced nitrite formation and chlorine and
oxygen depletion during distribution
Reduced hazards from the transport,
storage and handling of chlorine gas
Chlorite is a Regulated DBP
USEPA MCL:
1 mg ClO2- / l
USEPA MCLG: 0.8 mg ClO2- / l
100 mg ClO2- / l causes mild anemia and minor
suppression of thyroid function in laboratory animals
These effects have been found to be reversible
Part Two:
Filters
Old Filters and New Filters
DO
Cl
NO2-
DO
Cl
NO2-
9.7
8.9
0.004
9.7
8.9
0.004
_
_
_
_
_
_
1.7
0.7
0.460
9.5
7.5
0.012
Filter Cells
• cells on ends were
worse than center cells
Column
Studies
Got a better offer
What We Did
1. Ran aerated, chlorinated well water through
filters at 18 gpm per cell
2. Fed 0.5 mg ClO2-/l sodium chlorite directly
into of one of the three filter cells (Cell C)
3. Monitored individual filter cell effluents
Dilution
and Feed
• Target dosage of
0.5 mg ClO2 /l
Timeline
DAY
EVENT
1
beginning of data collection
13
sodium chlorite feed started
19
filter airwashed and backwashed
26
filter airwashed and backwashed
33
filter airwashed and backwashed
39
filter backwashed
40
filter sat idle for several hours
41
36 hours of service on filters
42
filter airwashed and backwashed
50
sodium chlorite feed stopped
53
filter returned to full service
67
started full-strength chlorite feed in Cell A
Chloramine
12
Influent
Cell A
Cell B
Cell C
10
mg/l
8
6
4
2
0
0
10
20
30
40
time (days)
50
60
70
Ammonium Ion
2.0
Influent
Cell A
Cell B
Cell C
mg/l
1.5
1.0
0.5
0.0
0
10
20
30
40
time (days)
50
60
70
Dissolved Oxygen
12
10
mg/l
8
6
Influent
4
Cell A
Cell B
2
Cell C
0
0
10
20
30
40
time (days)
50
60
70
Nitrite
0.25
Influent
Cell A
Cell B
Cell C
0.20
mg/l
0.15
0.10
0.05
0.00
0
10
20
30
40
time (days)
50
60
70
Conclusions
• We have demonstrated the effectiveness of sodium
chlorite as an inhibitor of nitrification both within
the plant and out in the distribution system
• Don’t try this at home
Tom O’Connor
[email protected]
877-22-WATER
www.h2oc.com
Sodium Chlorite, NaClO2
Technical : 31.25% solution, 25% active
Vulcan Chemicals, Birmingham, AL
55 gal. drums (565 lbs. net) $450/drum
Water Treatment:
Used for generation of chlorine dioxide
Materials Safety Data
USEPA MCL: 1 mg ClO2- / l
Ø 100 mg ClO2- / l causes mild anemia and minor
suppression of thyroid function in laboratory
animals
Effects found to be reversible
NaClO2 Dosage, Cost
Initial target dosage of 0.6 mg ClO2- / l
10 gpd added to 3.64 MGD ( Southwest Plant)
Distribution system residuals 0.44 mg ClO2- /l (USEPA
Method 300.0)
Dosage was cut in half. At a chemical cost of $8.22 per
gallon, the halved dosage would cost approximately $41
per day at the SW Plant and $25 per day at the 2 MGD
NE Plant.