Expanded
Summary
Simulated distribution nitrification: Nitrification
Index evaluation and viable AOB
Karen A. Schrantz, Jonathan G. Pressman, and David G. Wahman
http://dx.doi.org/10.5942/jawwa.2013.105.0046
Laboratory-scale annular reactors were used to evaluate the
impact of simultaneously increasing temporal influent
monochloramine (NH2Cl) concentrations (0.0 to 3.2 mg
chlorine [Cl2]/L) and chlorine-to-nitrogen mass ratios (0:1
to 3.2:1) on nitrification, allowing evaluation of the recently
proposed Nitrification Index (NI) for chloraminated
drinking water distribution system operation control.
Nitrification was severely affected with a 1.5 mg Cl2/L
NH2Cl residual, becoming undetectable when the NH2Cl
residual reached 3 mg Cl2/L. The calibrated NI was successful
in simulating nitrification occurrence. In addition, temporal
biofilm sample results demonstrated that viable ammoniaoxidizing bacteria were (1) dominated by Nitrosomonas
europaea–related species for an NH2Cl residual less than
approximately 0.4 mg Cl2/L, (2) dominated by N. oligo­
tropha–related species for an NH2Cl residual between 0.4
and 1.5 mg Cl2/L, and (3) not detected once the NH2Cl
residual was maintained at approximately 3 mg Cl2/L.
Chloramination may promote the growth of nitrifying
bacteria (i.e., ammonia-oxidizing bacteria [AOB] and
nitrite-oxidizing bacteria [NOB]) because of (1) unreacted
source water ammonia, (2) excess ammonia from chloramine formation, and/or (3) ammonia resulting from
chlor­amine decay and demand reactions. Although nitrifying bacteria are not considered human pathogens, nitrification can affect drinking water quality, resulting in
regulatory violations; therefore, nitrification control is of
practical importance. Studies have found that AOB from
chloraminated drinking water distribution systems
(CDWDS) were dominated by species related to Nitro­
somonas oligotropha, but these studies did not distinguish viable from nonviable AOB.
The Nitrification Potential Curve (NPC) conceptualizes
nitrification occurrence as a balance between AOB
growth and inactivation. Recently, the NPC was extended
to the NI by explicitly accounting for (1) pH dependence
of AOB growth, (2) AOB inhibition by trihalomethane
(THM) cometabolism, and (3) pH dependence of AOB
NH2Cl inactivation through a general acid-catalyzed
mechanism. Presumably, if calibrated to a given distribution system, the NI could provide an early warning indicator of an impending nitrification episode, allowing
operational adjustments (e.g., flushing, increasing NH2Cl
concentration, tighter controls on free ammonia addition
through chlorine-to-nitrogen [Cl2:N] mass ratio control)
to possibly prevent a nitrification episode.
This research focused on three objectives (1) advance
understanding of how to arrest an ongoing nitrification
event by measuring the response to an increase in influent
NH2Cl concentrations (0.0 to 3.2 mg Cl2/L) and Cl2:N
mass ratios (0:1 to 3.2:1) in two (A and B) annular reactors (ARs) used to simulate a point in a CDWDS; (2) use
chemical data collected during AR operation to evaluate
the NI as a control indicator of an ongoing nitrification
episode; and (3) evaluate the viable and total AOB communities at different operating conditions.
REACTOR PERFORMANCE
Nitrification was affected by the NH2Cl residual. As
the residual approached 0.1 to 0.2 mg Cl2/L, nitrite
(NO2–) accumulated in the reactors, providing evidence
that NOB were more susceptible than AOB to NH2Cl.
Maintenance of a 1.5-mg Cl2/L NH2Cl residual greatly
reduced an active nitrification episode. However, a 3.0mg Cl2/L NH2Cl residual was needed to completely
eliminate detectable nitrification. The Surface Water
Treatment Rule requires that a “detectable” disinfectant
residual be maintained in 95% of samples collected
throughout the distribution system on a monthly basis.
This research showed, however, that measurable
TOTNH3 [sum of ammonia (NH3-N) and ammonium
(NH4+-N)] oxidation occurred up to a 1.5-mg Cl2/L
NH2Cl residual. Thus, a “detectable” disinfectant residual
is not an appropriate endpoint to prevent nitrification.
This research also demonstrated that increasing only
the NH2Cl concentration and associated Cl2:N mass ratio
can arrest an existing nitrification event; however this
approach has been ineffective in practice. The likely
explanation involves at least two factors. First, pipe materials are present in actual CDWDSs that can increase
chloramine demand. Second, too much time is needed to
bring the CDWDS back into regulatory compliance.
NI EVALUATION
After calibration, the NI closely simulated active nitrification seen during reactor operation. Figure 1 summarizes
the relative contributions of each inactivation mechanism
to the total inactivation rate. During the first two operational periods, endogenous decay dominated. In the next
few operational periods, THM product toxicity accounted
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2013 © American Water Works Association
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for approximately 30% of the inactivation because of the
THM speciation swing toward bromine-substituted
THMs. Finally, as the NH2Cl concentration increased,
NH2Cl inactivation became dominant, and the importance
of endogenous decay and THM product toxicity decreased.
Overall, this analysis supports the NI concept; however,
kinetic parameters needed to be calibrated to the actual
system. In addition, small differences in pH, TOTNH3,
NH2Cl, and THM concentrations play a significant part
in the calculated NI value. Therefore, adjustment and
proper management of the NI parameters must be established for each CDWDS to apply the NI concept effectively.
Lacking the ability to fully calibrate the required parameters, the NI nevertheless provides a conceptual framework
to evaluate how changes in system conditions may impact
the likelihood of nitrification occurrence. Thus the NI can
be a valuable tool for operators to predict and take action
in preventing and/or controlling a nitrification episode in
a CDWDS when operational conditions change.
AOB COMMUNITY
AOB operational taxonomic units (OTUs) aligned into
three major clusters: sequences related to N. oligotropha
(NO), N. europaea (NE), or N. multiformis (NM). The
viable and total OTUs were generally detected together,
indicating that molecular methods based on the viable
community provided a reasonable representation of the
FIGURE 1
Fraction of total ammonia-oxidizing
bacteria inactivation rate attributed
to endogenous decay, monochloramine,
and THM cometabolism product toxicity
Endogenous decay
Inactivation by monochloramine
Inactivation by THM cometabolism product toxicity
1.0
AB
Fraction of Total Inactivation Rate
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0 0.47 0.91 1.9a* 1.9b* 1.9c* 2.3 2.7a 2.7b 2.7c 3.2
Operational Period†
A—reactor A, B—reactor B, THM—trihalomethane
*Subperiods for a given influent monochloramine concentration
†Operational periods were designated based on the influent
monochloramine concentration.
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total OTUs and vice versa. NM-related sequences were
only found in reactor A and during the initial operating
periods. Therefore, they did not persist upon NH2Cl addition and/or were outcompeted by NO- and NE-related
bacteria under these operating conditions. Viable NOrelated sequences were detected throughout operation until
detectable nitrification ceased. Viable NE-related sequences
were detected until the NH2Cl reactor concentration
reached approximately 0.4 mg Cl2/L. When viable NErelated sequences were present, they were found in relatively greater numbers than viable NO-related sequences.
The dominance of the NE cluster is contrary to other
research on AOB communities in pilot- and full-scale
CDWDSs. Overall, the dominant AOB present in previous
studies was related to NO with smaller detections of Nitro­
sospira-related bacteria. One explanation for a different
result in the current research is the starting condition,
which was complete nitrification without exposure to
NH2Cl. In actual systems, nitrification would have been
preceded by increased NH2Cl residuals, causing removal
of bacteria more susceptible to NH2Cl.
The nonreactive substratum (i.e., polycarbonate) in reactors used in this research represents a “best-case” scenario
in that the NH2Cl residual needed to control biofilm
growth was minimized. In real systems, pipe materials can
exert an NH2Cl demand and/or provide protected microenvironments (e.g., unlined cast-iron pipe) and thus
increase the dosage to maintain an acceptable residual.
CONCLUSIONS
Under controlled conditions, the presence of an NH2Cl
residual resulted in decreased NO2– oxidation at a relatively low NH2Cl residual (0.1 to 0.2 mg Cl2/L), measurable TOTNH3 oxidation at a 0.4-mg Cl2/L NH2Cl residual that persisted even at a 1.5-mg Cl2/L NH2Cl residual,
and elimination of nitrification at an approximately 3-mg
Cl2/L NH2Cl residual. A simultaneous increase in the
NH2Cl concentration and Cl2:N mass ratio arrested an
active nitrification event.
A calibrated NI simulated nitrification occurrence successfully and indicated AOB inactivation was mainly the
result of THM cometabolism product toxicity and NH2Cl
inactivation when a measurable NH2Cl residual was
maintained. Utilities may find the NI useful to provide an
early indication of nitrification control.
Viable AOB in this simulated CDWDS were dominated
by NE-related species for an NH2Cl residual less than
approximately 0.4 mg Cl2/L, by NO-related species for an
NH2Cl residual between 0.4 and 1.5 mg Cl2/L, and not
detected once the NH2Cl residual was maintained at
approximately 3 mg Cl2/L, corresponding to the absence
of detectable nitrification.
Corresponding author: David G. Wahman, environ­
mental engineer at USEPA, 26 W. Martin Luther King
Dr., Cincinnati, OH 45268; wahman.david@epa.gov.
MAY 2 0 1 3 | J OUR N A L AWWA • 1 0 5 :5 | S C H R A N T Z E T A L
2013 © American Water Works Association