Effects of Environmental Factors on Nitrification Occurrence in
Model Drinking Water Distribution Systems
Ng M.Y.
Division of Environmental Science and Engineering, National University
of Singapore
ABSTRACT
Nitrification is a microbial process whereby ammonia is sequentially oxidized to
nitrite and nitrate ions and is mediated by autotrophic bacteria. By using chloramines
as a water distribution system disinfectant that tends to release free ammonia, it
encourages the growth of both ammonia-oxidizing bacteria (AOB) and nitriteoxidizing bacteria (NOB). This water degradation may lead to biologically unstable
water that is thus unsafe for human consumption. Despite its drawbacks, it is a better
water distribution system disinfectant than chlorine as it is longer lasting and
minimizes the formation of regulated disinfection by-products (DBPs) such as
Trihalomethanes (THMs) and Haloacetic acids (HAAs). Therefore, it is pertinent to
devise control measures to minimize nitrification occurrence in the distribution
system.
INTRODUCTION
Over the years, chloramination has been integrated into the water distribution
system as distribution system disinfection. This is due to its potential in minimizing
microbial contamination and the ability to comply with the Federal regulations.
Although chloramines are not as reactive as chlorine, it serves as a longer lasting
disinfectant that is able to reach out to remote areas and minimize a bad taste and odor
produced (AWWA, 2003). Most importantly, it forms much lower levels of regulated
1
DBPs than chlorine. These DBPs such as THMs and HAAs are known to be
potentially carcinogenic and are harmful to the public health. On the other hand,
chloramines pose a number of problems. It encourages the growth of both AOB and
NOB via releasing free ammonia for nitrification to occur. As a result, there is a
potential risk of nitrification process taking place which will affect the operation and
water quality aspects. Nitrification is a microbial process whereby ammonia is
sequentially oxidized to nitrite and nitrate ions and is mediated by autotrophic bacteria
(Andrzej., 1996). Nitrification generally causes a decrease in chloramines residual,
alkalinity, and an increase in nitrates and nitrites and, microbial counts. This
degradation of water quality may lead to violation of the Safe Drinking Water Act.
Thus it is pertinent to prevent nitrification so as to provide a more stable chloramines
residual in the distribution system and biologically stable water which is safe for
human consumption. Application of free chlorine before free ammonia addition is the
most frequently used approach for chloramine formation.due to the risk of
encouraging nitrification within the treatment plant process (sedimentation basin,
filtration).
Hence, this study seeks to review briefly the relationship between operational
parameters and nitrification in lab-scale distribution system thereby, formulating
potential control strategies and proposing possible ways of predicting nitrification
potential for the local water distribution system.
EXPERIMENTAL DESIGN AND SET-UP
Biofilm reactors (Model CBR 90-1, Biosurface Technologies) are employed in the
experiments, with a retention time of 1 day and a fixed rotational speed of 50 rpm. It
is operated in parallel and its temperature is in a range of 28-31oc. Polycarbonate
coupons were used as the biofilm support media inside the reactor.
2
Three sets of experiments were set up to examine the effects of influent water pH,
chlorine to ammonia-N ratio, and monochloramine residual concentration on the
nitrification occurrence.
Table 1. Tabulated parameters to be investigated
Experiment No.: Parameters: Values to be investigated: 1 Influent pH 7.5 8.2 9 2 Cl2:N ratio 3:01 4:01 5:01 3 Monochloramine conc. < 0.3 mg/L 1.5 mg/L 2.5 mg/L In the first set of experiment, tap water was used as the influent water and, HCl or
NaOH solutions are added to achieve the targeted pH levels. Lower pH values were
not examined due to corrosion control. In the second set of experiment, ammonia-N
dose is varied at fixed chlorine dose to achieve the desired ratio for analysis.
Ammonium chloride (NH4Cl, 36 mM as NH3-N) and sodium hypochlorite (0.5% as
Cl2) solutions were used for monochloramine formation. The monochloramine level
was set at 0.3−0.5 mg/L. In the third set of experiment, 0.5% (as Cl2) NaOCl and 36
mM (as NH3-N) NH4Cl solutions are added to a dechlorinated tap water at a 5:1 of
chlorine-to-ammonia-nitrogen weight ratio. For each set of experiments, both Most
Probable Number (MPN) technique-based AOB enumeration data and heterotrophic
plate counts (HPC) levels were taken.
For the MPN technique, the nitrosomonas cells are enumerated by the 3-tube MPN
procedure. Three sets of 3 tubes were used (10mL of solution in each tube) and
inoculated with 1, 0.1 and 0.01mL of samples to be tested. After innoculation, tubes
were incubated in the dark for 21 days at 28oc. At the end of incubation period,
cultures were tested for presence of nitrite. A positive result is obtained when a deep
reddish color is observed (AWWA, 2003).
3
For the HPC method, a R2A agar is used. The agar is inoculated with a sample and
incubated for 7 days at 28oc. At the end of the incubation period, colonies of bacteria
are counted.
RESULTS AND DISCUSSION
Various results were obtained from the experiment and tabulated in a graphical
format as shown in Fig. 1 – 3. Changes in total organic carbon (TOC), ammonia-N,
nitrite-N, nitrate-N concentrations, and both AOB levels and HPC levels in the
reactors operated under the varied parameters were recorded.
Free ammonia is known to promote the growth of AOB thus when ammonia levels
are lower than normal, it is indicative of nitrification occurrence and that nutrient
levels are being depleted (Hill, N.A.). Nitrite and nitrate are products of nitrification,
hence it is the best indicators of nitrification. An increase in HPC R2A counts is
usually observed when nitrification takes place due to the presence of organic-rich
products released by AOB, which serves as a nutrient source for HPC bacteria. As
such, TOC is recorded. Lastly, AOB levels are taken into account as it mediates the
process of nitrification. Note that both bulk and biofilm solution are tested for AOB
levels and HPC levels. It is discovered that biofilms are able to encourage nitrifiation
as it is able to shield the nitrifying bacteria from disinfectant residuals (Skadesen.,
N.A.)
Experiments to evaluate the effect of influent pH on nitrification
For all reactor runs, pH level measured in the reactor effluents were consistently
lower than those in the influent water. These decreases in pH may not be indicative of
nitrification as it may be caused by low alkalinity and buffering capacity of water, which
could be associated with accelerated loss of monochloramine residual. From the data
collected, a slight but noticeable decrease in ammonia-N and increase in nitrite-N and
4
nitrate-N concentrations were observed in the reactor effluents as shown in Fig. 1b-d,
implying nitrification occurrence. Fluctuations were observed due to different
conditions, including both chemical and biological consumption. For the MPN
technique-based AOB enumeration, the nitrification potential is favored at pH 9.0 for
the bulk solution whereas for biofilm solution, it is the least favored. On the other
hand, HPC was observed to be at a high magnitude whereby bulk HPC levels
remained as 104CFU/mL while biofilm HPC remained as 102CFU/cm2.
0.30
0.20
0.01
(a)
(b)
pH 7.5
pH 8.2
pH 9.0
0.00
Change in NH3-N (mg/L)
Change in TOC (mg/L)
0.10
0.00
-0.10
-0.20
-0.30
pH 7.5
pH 8.2
pH 9.0
-0.40
-0.50
-0.01
-0.02
-0.03
-0.04
-0.60
-0.05
1
2
3
4
5
6
1
2
Time (weeks)
5
6
0.05
(c)
(d)
pH 7.5
pH 8.2
pH 9.0
0.04
0.03
0.02
0.01
0.04
Change in NO3-N (mg/L)
Change in NO2-N (mg/L)
4
Time (weeks)
0.06
0.05
3
0.03
0.02
0.01
pH 7.5
pH 8.2
pH 9.0
0.00
0.00
-0.01
-0.01
1
2
3
4
Time (weeks)
5
6
1
2
3
4
5
6
Time (weeks)
Fig. 1. Changes in (a) TOC, (b) ammonia-N, (c) nitrite-N, and (d) nitrate-N
concentrations in the reactors operated under different influent water pH.
Taking into account the results obtained, there is no observed differences in
nitrification potential between the three influent water pH levels. This can be
explained by the consistent drop in water pH to less than 7.6 within the reactor, thus
5
mask the effect of influent pH on nitrification potential. HPC growth can be further
inferred as a loss of monochloramine residual and biologically unstable water quality.
Experiments to evaluate the effect of Cl2:N ratio on nitrification
For all three reactors, it is observed that the measured monochloramine residual
remained consistently low and there is a change in the nitrogen species concentration.
This change can be interpreted as nitrification occurrence. A decrease in TOC
concentrations were also observed in the reactor effluent. For the MPN techniquebased AOB enumeration as shown in Fig. 2, it can be distinctly deduced that
nitrification potential is best at a 4:1 Cl2:N ratio. Also, both bulk and biofilm HPC
growth levels are relatively similar in all three reactors.
Fig. 2. (a) Bulk and (b) biofilm AOB levels in the reactors receiving the
chloraminated water produced with chlorine to ammonia-N weight ratios of 3:1, 4:1
and 5:1.
For the prevention of nitrification, it is pertinent to have proper control of
ammonia and chlorine dosage. This can be achieved by limiting the residual free
ammonia available for AOB to feed on. However, it is essential to note that a ratio
beyond 5:1 will lead to the formation of dichloramine thereby leading to customer
complaints about taste and odor. On the other hand, if the ratio is below 4:1, excess
free ammonia is able to enter the system leading to nitrification.
Experiments to evaluate the effect of monochloramine concentration on nitrification
6
A significant loss of monochloramine residual is observed regardless of the initial
amount of monochloramine residual present in the influent water. However, the
influent water with the lower monochloramine residual exhibits a comparable change
in ammonia-N, nitrite-N and AOB levels. In addition, there is not much deviation in
the HPC levels between the high residual reactor and the low residual reactor as
shown in Fig. 3. As such, it can only be inferred that a lower amount of
monochloramine residual present in the influent water may enhance nitrification.
Fig. 3. (a) Bulk and (b) biofilm HPC levels in the reactors operated with the influent
water having different monochloramine concentrations.
Raising the chloramine residual is effective at controlling nitrification. However,
in doing so, there will be a risk of AOB growth exceeding the AOB inactivation rate,
resulting in nitrification (Skadesen., N.A.). Furthermore, chloramines decay and
losses which is dependent on the water quality characteristic, distribution systems
demand and residence time, have to be considered too. A system with longer
residence time will experience greater decrease in chlorine residual, and be more
vulnerable to nitrification.
CONCLUSION
7
From the experiments conducted, it can be deduced that nitrification potential is
favored at chlorine to ammonia-N ratio of 4:1 and low influent monochloramine
residual. The optimal pH is unknown due to the difficulty in distinguishing the
differences in nitrification potential among the three influent water pH levels.
Therefore, more investigation is required to determine how locally applicable is the
results and whether it will be effective as a parameter to predict and control
nitrification. As nitrification is a biological phenomenon and can occur very rapidly,
but not instantaneously (Smith, N.A.), trend graphs are used to obtain a proper
interpretation of nitrification monitoring parameters.
REFERENCE
Andrzej. W., Joseph. G.J., Joseph. P.M., Lee. H.O. and Gregory. J.K. (1996),
“Occurrence of nitrification in chloraminated distribution systems”, AWWA
AWWA Research Foundation. (2003), “Ammonia From Chloramine Decay: Effects
on Distribution System Nitrification”, Chapter 5, pages 61-86.
Hill. P. H., (N.A.), “Assessment and Operational Responses to Nitrification Episodes”,
AWWA Manual M56, 1st Ed., Chapter 9, page 189-221
Smith. C.D., (N.A.), “Monitoring for Nitrification Prevention and Control”, AWWA
Manual M56, 1st Ed., Chapter 7, 129-132
Skadesen. J. and Cohen. Y.K., (N.A.), “Operational and Treatment Practices to
Prevent Nitrification”, AWWA Manual M56, 1st Ed., Chapter 8, page 151-188
8