Chinese Journal of Chemical Engineering, 16(5) 778—784 (2008)
Nitrogen Removal by Simultaneous Nitrification and Denitrification
via Nitrite in a Sequence Hybrid Biological Reactor*
WANG Jianlong (王建龙), PENG Yongzhen (彭永臻)**, WANG Shuying (王淑莹) and GAO
Yongqing (高永青)
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, China
Abstract Sequence hybrid biological reactor (SHBR) was proposed, and some key control parameters were investigated for nitrogen removal from wastewater by simultaneous nitrification and denitrification (SND) via nitrite.
SND via nitrite was achieved in SHBR by controlling demand oxygen (DO) concentration. There was a programmed decrease of the DO from 2.50 mg·L－1 to 0.30 mg·L－1, and the average nitrite accumulation rate (NAR)
was increased from 16.5% to 95.5% in 3 weeks. Subsequently, further increase in DO concentration to 1.50 mg·L－1
did not destroy the partial nitrification to nitrite. The results showed that limited air flow rate to cause oxygen deficiency in the reactor would eventually induce only nitrification to nitrite and not further to nitrate. Nitrogen removal
efficiency was increased with the increase in NAR, that is, NAR was increased from 60% to 90%, and total nitrogen
removal efficiency was increased from 68% to 85%. The SHBR could tolerate high organic loading rate (OLR),
COD and ammonia-nitrogen removal efficiency
were greater than 92% and 93.5%, respectively, and it even operated under low DO concentration (0.5 mg·L－1) and maintained high OLR (4.0 kg COD·m－3·d－1). The presence of
biofilm positively affected the activated sludge
settling capability, and sludge volume index (SVI) of activated
sludge in SHBR never hit more than 90 ml·g－1 throughout the experiments.
Keywords nitrogen removal, simultaneous nitrification and denitrification, nitrite accumulation, demand oxygen, pH
1
INTRODUCTION
Biological nitrification-denitrification is the most
commonly used process for nitrogen removal from
wastewater. Nitrification is the autotrophic oxidation
of NH +4 -N, first to NO −2 -N and then, to NO3− -N,
whereas denitrification is the heterotrophic and anoxic
conversion of NO3− -N, first to NO −2 -N and then, to
gaseous nitrogen compounds [1]. Therefore, two separate reactors are required to provide different environments for the two kinds of bacteria: nitrifiers and
denitrifiers, such as conventional anoxic/oxic process.
However, in recent years, the phenomenon of simultaneous nitrification and denitrification (SND) has
been described for various systems [2, 3]. Such processes eliminate the need of two separate reactors, thus,
simplify the treatment system. Various types of treatment units have been proposed so far to realize such a
system [4].
Some other novel nitrogen removal processes
have arisen during the last few years, such as short-cut
nitrification denitrification and anaerobic ammonium
oxidation (ANAMMOX) [5-7]. The short-cut process
is based on the fact that nitrite is an intermediary
compound in both steps: nitrification and denitrification. This approach leads to savings of oxygen during
nitrification and a reduction of organic carbon requirement during denitrification, which reduces the
costs for nitrogen removal. Investigation of the shortened nitrogen removal pathway via nitrite revealed: (1)
40% reduction of COD demand during denitrification;
(2) 63% higher rate of denitrification; (3) 300% lower
biomass yield during anaerobic growth; and (4) no
apparent nitrite toxicity effects for the microorganisms
in the reactor [8, 9]. If SND is accompanied by the inhibition of nitrite oxidized to nitrate, theoretically a
saving of organic energy of up to 40% could be realized [9]. Yoo et al. [3] achieved the SND via nitrite by
controlling the DO in an intermittently-aerated suspended reactor. Gao et al. [10] observed the phenomenon of SND via nitrite in the suspended SBR
reactor, and the maximum SND efficiency reached up
to 54.6% by controlling the DO. All these results
showed that it is feasible to realize the SND via nitrite
by controlling certain operation conditions.
The objective of this research is to extend the
conventional suspended activated sludge nitrogen removal unit for SND to activated sludge and biofilm
combined system. A sequence hybrid biological reactor nitrogen removal process utilizing SND via nitrite
was proposed, without an anoxic period, to evaluate
its performance. Some key control parameters and
their optimal values were determined in the laboratory
to analyze and investigate the relative importance of
factors affecting nitrite accumulation in the sequence
hybrid biological reactor (SHBR) reactor.
2
2.1
MATERIALS AND METHODS
Reactor system
A schematic diagram of the experimental SHBR
is shown in Fig. 1. The reactor was a column tank,
with a maximum working volume of 12 L. The reactor
was equipped with an aerator, a peristaltic pump, as
well as probes to follow variations of pH and DO
concentration. All experiments were conducted at ambient temperature (23-25°C). Mixed liquor suspended
Received 2007-10-18, accepted 2008-04-20.
* Supported by the National Key Project of Scientific and Technical Supporting Program of Ministry of Science and Technology of
China (2006BAC19B03), Academic Human Resources Development in Institutions of Higher Leading under the Jurisdiction of
Beijing Municipality, and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20060005002).
** To whom correspondence should be addressed. E-mail: [email protected]
779
Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008
ing relationship:
NAR(%) =
2.4
Figure 1 Experiment reactor
1—temperature controller; 2—ORP probe; 3—pH probe; 4—
DO probe; 5—air flow meter; 6—aerator; 7—mixture
solids (MLSS) concentration of SHBR was main－
tained approximately at 1500 mg·L 1 throughout the
experimental periods. The reactor was operated into
quasi steady state before detailed experiments (described below) commenced.
The reactor was filled with fixed media (30%, by
volume) to attach and retain ammonium oxidizers.
The characteristics of the media are shown in Table 1.
2.2
Sludge and wastewater
The seeding sludge for the SHBR was collected
from the full-scale plant of Beijing WWTP, which is a
nitrogen removal plant with a Carrousel oxygen ditch
process. The sludge was elutriated with clean water in
triplicate to remove fine suspended solids that might
interfere with the analyses. Once the reactor was
－
seeded with activated sludge (MLSS＝1500 mg·L 1),
the reactor was fed with synthetic wastewater (the
wastewater was synthesized through dilution of domestic wastewater and NH4Cl, CH3COONa, and
K2HPO4 were added). The feed composition is shown
in Table 2.
2.3
NO −2 - N
NO3− - N + NO −2 - N
× 100% .
(1)
Experimental design
The SHBR was operated in a filling (instant)-aerobic (660 min)-setting (40 min)-drawing (20
min) sequence for a period of 14 months. Daily measurements of MLSS showed little growth, which was
presumably offset by cell lyses, death and the amount
taken for the MLSS sample itself. Thus, the amount of
nitrogen assimilated into new biomass during the period of a track study was probably minor, and nitrogen
losses via assimilation were not included in the SND
nitrogen-loss calculations, as per the practices of other
SND researchers [12].
At the end of each cycle, the mixture drained
from the bottom of the reactor for setting, and 50% of
the reactor contents were decanted after the setting,
meaning that the initial feed concentrations were
halved at the start of the next cycle. Online monitoring
of DO and pH were undertaken continuously. In addition, separate biomass wasting was practiced once in
10 days to obtain the desired average SRT (10d). The
average SRT of all biomass in the SHBR was computed as per the following method [13]:
SRT =
V ( Xa + Xs )
(2)
Qe X e + Qw X w
where Xa, Xs, Xe, and Xw were the biomass (VSS)
concentrations attached to the media, in suspension,
suspended in the effluent, and in the waste biomass,
respectively. Qe and Qw were the effluent flow rate
and the time-averaged waste-flow rate, respectively.
Attached biomass in the reactor was 65.5% of the total
biomass.
3
RESULTS AND DISCUSSION
3.1 Partial nitrification by controlling the air flow
rate
Analytical methods
Samples were collected regularly from the reactor at different time intervals, the measurement of
ammonia, nitrate, nitrite, COD, alkalinity, and MLSS
was according to standard methods [11]. Nitrite accumulation rate (NAR) was computed using the followTable 1
Figure 2 shows the outline of NO −x -N ( NO −2 -N
and NO3− -N) concentrations in the effluent, NAR,
and nitrogen removal efficiency before the 308 days.
Nitrite did not accumulate in the initial 30 days, and
the system showed full nitrification to nitrate when the
Characteristics of the carrier
Material
Density
－
/g·cm 3
Compressibility
of intensity/kPa
Tensility
/%
Diameter
/mm
Ratio of
inter-space/%
Resistibility of
temperature/°C
Stability of resistant
acid and alkaline
polypropylene
0.9-0.92
41.2-49.2
3.60-3.97
1.5-0.8
90-95
80-100
stable
Table 2
－1
CODCr/mg·L
350
+
4
－1
NH -N/mg·L
65
－1
TN/mg·L
71
Synthetic feed composition
NO −x -N/mg·L
PO34− -P/mg·L
pH
0.5
8
7.4
－1
－1
－1
Alkalinity /mgCaCO3·L
250
780
Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008
－
air flow rate was controlled at 55 L·h 1 (DO＝2.5
－
－
mg·L 1). Once the air flow rate was lowered to 15L·h 1
－1
+
(DO＝0.3 mg·L ) in steps, the NH 4 -N removal efficiency was maintained above 96%. Effluent
NO −2 -N was increased approximately from 7.2 to 12.3
－
－
mg·L 1, and the NO3− -N was lower than 1.0 mg·L 1
in the subsequent 20 days, and therefore, a remarkable
improvement of NAR from approximately 60.0% to
95.4%. After an additional 3 weeks time, full inhibition of nitrite-oxidizing bacteria under low air flux
with effluent nitrate concentration lower than 1.0
－
mg·L 1 and NAR over 95% were observed. It has
been reported that a dissolved oxygen concentration
－
below 1.0 mg·L 1 favors the dominance of ammonium
oxidizers, whose growth rate was 2.56 times faster
than that of nitrite oxidizers [14, 15]. In our study, partial nitrification was successfully accomplished at an
－
DO concentration of 0.3 mg·L 1. This was also in
agreement with Wyffels et al. [16], who observed that
partial nitrification with 50% ammonium conversion
－
was attained at DO concentrations below 0.2 mg·L 1 in
a biofilm system. Similarly, Goreau et al. [17] reported
that ammonium oxidizers could grow, and that the
growth yield of Nitrosomonas sp. on the basis of cell
number per nitrite production at a DO concentration of
－
0.18 mg·L 1 was five times higher than during saturated DO conditions.
Further experiments were conducted under conditions of increased airflow to overcome the inherent
deficiency of increased reaction time associated with
low air flow rates when air flow rate was increased
－
stepwise and was first increased to 21 L·h 1 (DO＝0.5
－1
mg·L ), The NAR was decreased to 90%, and the
effluent NO −2 -N and NO3− -N concentration were
－
lower than 1 and 5.6 mg·L 1, respectively. In the subsequent 40 days, the maximum nitrogen removal efficiency was achieved during this period, and the average nitrogen removal efficiency was 85%. A stable
－
nitrite build-up at dissolved oxygen of 0.5 mg·L 1 was
also reported by Chung et al. [18], as nitrite oxidation
was restricted under low DO conditions, furthermore,
it has also been found that a DO concentration around
－
0.5 mg·L 1 was suitable for achieving a nitrification
rate equal to the denitrification rate, which would,
therefore, lead to complete SND [12]. Then, with the
－
air flow rate increasing to 39 L·h 1 stepwise, there
was little decrease of the NAR and nitrogen removal
efficiency, and little increase of the NO −2 -N and
NO3− -N in the effluent. From 275 to 307 days, with
－
the air flow rate increasing to 47 L·h 1 (DO＝2.0
－1
mg·L ), the NAR was decreased abruptly, and it was
reduced from about 75% to 25% in 2 days; the nitrogen removal efficiency was also reduced because the
ammonium oxidizers were inhibited under high DO
concentration and nitrite oxidizers becomes superior.
Wiesmann [19] suggested that at DO concentrations
－
below 1.8 mg·L 1, the specific growth rate of ammonium oxidizers is superior to nitrite oxidizers. This
result was reconfirmed in this study. With restricted
－
DO concentration lower than 1.0 mg·L 1, nitrate production was limited to a negligible amount, and the
increase in nitrite accumulation was continuous. Consequently, partial nitrification was steadily achieved in
a hybrid reactor. So, it is a remarkable and significant
observation that there was an optimal DO concentration to realize the maximum NAR and nitrogen
removal.
At a higher NAR, nitrogen removal efficiency
without affecting ammonia removal took place when
－
the air flow rate was maintained at 21 L·h 1 although
the maximum NAR was achieved when the air flow
－
rate was maintained at 15 L·h 1. Fig. 3 summarizes
these results. Each point represents stable operational
behavior for each condition. A 90% NAR was feasible
with 98% of ammonia conversion. On the basis of
stoichiometry this accumulation implies a 22.5% reduction in the oxygen needed for nitrification: 1.55
mol of oxygen per mol of ammonia nitrogen compared with 2 for complete nitrification. Furthermore,
the operation at low dissolved oxygen concentrations
enhances oxygen mass transport, because of an increase of mass transfer driving force [20]. Data from
Fig. 3 suggest that maximum nitrogen removal efficiency and high NAR were achieved when the air
Figure 2 Effect of DO on nitrogen removal
+
+
▼ air flow rate; ○ NH 4 -Nin; ■ NH 4 -Neff;
NO3− -N;
◇
NO −2 -N;
□
NH +4 -NRE; × TNRE;
▲
NAR
Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008
Figure 3 Effect of DO concentration on ammonia removal
efficiency and NAR
DO; ▼ NH +4 -NRE; ■ TNRE; ○ NAR
－
flow rate was controlled at 21 L·h 1; therefore, an air
－
flow rate of 21 L·h 1 was selected as the air flow rate
for the SHBR, during the combined operation of nitrification and denitrification steps.
3.2
DO and pH profiles under different air flow rates
During partial nitrification, significant nitrogen
removal was also observed under oxygen-limiting
condition. The low DO level (as shown in Fig. 4) during the aeration phase, especially before DO breakthrough, allowed nitrification and denitrification to
occur simultaneously, and significant nitrogen removal was observed. To date, the so-called simultaneous nitrification and denitrification (SND) phenomena are explained by three main factors [13, 21, 22]:
(1) bioreactor microenvironment developing within
the aerated bioreactor as a result of mixing pattern or
aeration style; (2) floc microenvironment developing
inside individual activated sludge floc; and (3) novel
microorganisms that have the ability to use previously
unrecognized biochemical pathways to realize nitrogen removal. It is believed that the floc microenvironment inside activated sludge floc formed by the
low DO concentration could account for the SND observed during this experiment. During most of the
781
operation time in each cycle, as described in Fig. 3,
－
the DO remained lower than 1.0 mg·L 1, and the limited oxygen supply made it possible to form the anoxic
microzone in flocs, so that denitrification could occur.
Online detection of DO and pH could be used to
better explain the formation and maintenance of partial nitrification. Fig. 4 shows the typical DO and pH
profiles under different air flow rates. Once aeration is
commenced, all the supplied oxygen was immediately
consumed, preventing the build-up of residual DO.
Once the COD was depleted, the DO was increased
abruptly, and the DO increased gradually in sequence.
Through off-line analysis it was found that the
NO −2 -N also increased temporarily after the system
change, which is represented by the jump in DO, and
this might indicate a slight suppression of the ability
of the biomass to simultaneously reduce the oxidized
forms of nitrogen ( NO3− -N, NO −2 -N) [23]. With the
increase of the air flow rate level, the DO concentration increased markedly, during most of the aeration
－
time, and DO concentration was well below 2.0 mg·L 1
before the end of nitrification. At the air flow rate of
－
47 L·h 1, DO concentration increased markedly from
－
－
about 0.5 mg·L 1 to over 2.5 mg·L 1 often in 5h, and
at the same time, chemical analytical data also showed
－
ammonium concentration to be less than 1.0 mg·L 1.
Because different air flow rates were used in the ex－
periment, the DO (given in Fig. 4) reached 3.0 mg·L 1
after aeration for over 10.5, 9.3, 7.4, and 5.8 h for an
－
air flow rate of 15, 21, 39, and 47 L·h 1, respectively.
The specific DO profile made it possible to detect
online the nitrite accumulation conditions and control
the endpoint of nitrification. From the pH profiles we
can find that: the characteristic points appeared at the
end of the nitrification under different air flow rate
levels, which are corresponding to the characteristic
points on DO profiles.
3.3
Time profiles in a typical cycle
In order to confirm that SND via nitrite apt to
happen in SHBR under low DO concentration (DO＝
Figure 4 Typical DO and pH time profiles under different air flow rates
Qair/L·h－1: ■ 21; + 28; ○ 37; △ 55
782
Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008
－
0.5 mg·L 1), the change of nitrogen composition in a
cycle was studied, and at the same time, online monitoring of pH and DO in SHBR were carried out
(Fig. 5). From 0 to 125 min aeration, the concentration
of NH +4 -N changed little, while nitrogen was removed mainly by assimilation; COD was decreased to
－
a level lower than 100 mg·L 1, and biodegradable
COD was almost used up. From the 125 th minute
onwards, DO increased gradually, whereas pH
dropped slightly, and at the same time in the initial
stage of nitrification, the concentration of NH +4 -N
decreased gradually. From 125 min to 360 min, the
NH +4 -N was removed without an increase of NO −x -N,
and the rate of denitrification was faster than nitrifica－
tion when the DO was lower than 0.5 mg·L 1. So,
during this period, the SND efficiency was maximum,
and TN removal efficiency was more than 50%. Nitrite appeared when the DO concentration was more
－
than 0.5 mg·L 1 at the 360th minute. Nitrite concentration rose steadily until the 560th minute when the
nitrification finished. Then the NAR reached the
maximum, and the pH reached the minimum. Characteristic points of B, and β appeared on the DO and pH
profile. At the 660th minute, the NO −2 -N translated
into NO3− -N completely. From the nitrogen removal
efficiency profile, we can find that the nitrogen was
removed mainly from 125th minute to 560th minute.
The characteristic point B of DO and β of pH profiles
can well show the end of the nitrification. So, we can
use pH and DO as parameters to control the aeration
length and maintain the nitrogen removal through
SND via nitrite method, at the same time avoid the
effect of excessive aeration on nitrite accumulation.
3.4
Effect of OLR on the performance of SHBR
The COD and nitrogen removal efficiency were
－
observed at different OLR with air flow rate of 21 L·h 1
(Fig. 6). In general, the percentage of COD removed
decreased linearly with the increase of OLR, whereas
the SHBR shows high OLR tolerance. Although the
COD removal efficiency of the system decreased with
increased OLR, more than 90% feed COD could be
－
－
removed up when OLR is at 4.0 kgCOD·m 3·d 1,
which indicates that this hybrid system was highly
effective. COD removal efficiency was decreased by
only 5% when OLR increased from 0.5 to 4.0
Figure 5 Cyclic study of the SND via nitrite in SHBR
TN; COD; ■ NH +4 -N; NO −2 -N; ★ NO3− -N; □ pH;
Figure 6 Impact of OLR on the SHBR
−
−
+
—— OLR; ● NO3 -N; ▲ NO 2 -N; ○ NH 4 -Neff;
● DO
TNRE; × NAR
Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008
783
Figure 7 SVI value under different air flow rates and OLR
flow rate; OLR; × SVI
○ air
－
－
kgCOD·m 3·d 1. The SHBR was capable of removing
more substrate at higher OLR with low DO. The attachment of high biomass hold-up in this hybrid reactor via the immobilization of microorganisms on the
carrier particles, contributed to such good system efficiency. In fact, the equilibrium biomass hold-up in the
reactor was strongly dependent on the OLR applied.
Nitrogen removal efficiency was increased from 85.0%
to 93.5%, with the increase of OLR and NH +4 -N removal efficiency was maintained above 98% when
－
－
OLR was increased from 0.5 to 4.0 kgCOD·m 3·d 1.
Under high OLR, the denitrifiers can maintain high
activity, at the same time the nitrifiers can grow well
on biofilms, and the NO −2 -N and NO3− -N accumulation was almost in the unoccurred state. It shows that
the nitrification rate decreased with the increase in
OLR, whereas the denitrification was increased. So
the SND efficiency was increased with the increase in
OLR. The nitrogen removal efficiency was decreased
－
－
only when the OLR was over 4.0 kgCOD·m 3·d 1, for
the decrease of the nitrification efficiency.
3.5
The sludge settle ability
Figure 7 shows the sludge volume index (SVI)
value under different air flow rates. After adding
biofilm materials into the activated sludge, the SVI
－
value decreased from 150 to 70 ml·g 1 in 8 days with
－1
the air flow rate of 55L·h . It shows that the presence
of biofilm positively affected the activated sludge settling capabilities. The SVI value was vibrating with
the change in air flow rate in the following days, but it
－
was never over 90 ml·g 1. When the air flow rate in－1
creased again to 47 L·h , the SVI value decreased to
－
－
53 ml·g 1, from approximately 90 ml·g 1 under air
－1
flow rate of 34 L·h . The sludge maintained good
settling property during the experiment even when the
－
DO was always lower than 0.5 mg·L 1. As known to
us, low DO concentration is liable to cause sludge
bulking problems. It was believed that SBR was an
ideal plug-flow reactor, and that the fed-batch opera-
tion produced a carbonaceous substrate gradient in the
aeration tank. The kinetic selection theory could be
used to best explain the good sludge stability [24].
However, if this strategy of realizing partial nitrification by low DO control was applied to other reactor
figurations, special attention should be paid to the potential problem of sludge stability.
4
CONCLUSIONS
The study focused on the feasibility of simultaneous COD and nitrogen removal by adding biofilm
carrier into a conventional SBR. The experiments
showed that high efficiency of SND via nitrite was
successfully achieved in a SHBR operated under limited DO concentration. During the operational period,
the SHBR exhibit high carbon and nutrient removal
efficiency (92% COD, 85% nitrogen). So far, all previous studies have been able to remove nitrogen in
SBR, under intermittent aeration, and at longer HRT.
The air flow rate proved to be very critical for SND
via nitrite. The optimum air flow rate for nitrogen re－
－
moval was 21 L·h 1 (DO＝0.50 mg·L 1), although the
maximum NAR (95.5%) was achieved when air flow
－
－
rate was 15 L·h 1 (DO＝0.30 mg·L 1). The SHBR can
tolerate strong OLR shock when the OLR was in－
－
creased from 0.5 to 4.0 kgCOD·m 3·d 1, only a 5%
decrease of COD removal efficiency was observed,
and the system could recover to normal treat efficiency in 3 days, whereas the nitrogen removal efficiency was increased from 85.0% to 93.5%. The
presence of biofilm positively affected the activated
sludge settling capabilities, and SVI of activated
sludge in SHBR never reached a value greater than 90
－
ml·g 1 throughout the experiment.
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