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Reactor
Xiaolan Zeng, Xuebin Hu, and Wenchuan Ding Three Gorges Reservoir Area’s Ecology and Environment Key Laboratory of Ministry of Education, Chongqing
University, Chongqing, 400045, China
Email: [email protected], [email protected], [email protected]
Dan Wu Department of Resource Exploration and Civil Engineering, Chengdu University of Technology, Chengdu, 610059,
China
Email: [email protected]
Abstract—To optimize partial nitrification process, a
modified Multi-stage Declined Aeration strategy in a
sequencing batch biofilm reactor (SBBR) was developed.
The experiments were carried out with low total organic
carbon to nitrogen (C/N) ratio (3.8-5.7) synthetic wastewater
at temperature of 29±1°C under three different aeration
modes. The results showed that oxidation of ammonia to
nitrite and simultaneous nitrification and denitrification
(SND) in a SBBR system successfully achieved under low
DO concentration condition (≈0.8 mg·L-1). Nitrite
accumulation rate (NAR) under three modes CA, SDA1 and
SDA2 were 91.1%, 88.3% and 100.0%, while SND efficiency
were 63.4% , 69.5% and 74.7%, respectively. Three-stage
Declining Aeration (SDA2 mode) was the best strategy with
a complete partial nitrification in SBBR reactor. Meanwhile,
SDA2 could save aeration flux by 6% and increase TN
removal rate by 10% in comparison to Continuous Aeration
(CA) during the aeration phase. It is speculated that
anaerobic ammonium oxidation (ANAMMOX) process
could be involved in TN removal via SND during the
aeration period in SBBR system. The results suggest that
Multi-stage Declined Aeration strategy is an alternative
aeration approach to promote partial nitrification in a
biofilm system.
Index Terms—partial nitrification, Dissolved oxygen (DO),
Sequencing batch biofilm reactor (SBBR), Multi-stage
declining aeration, Nitrite accumulation rate (NAR),
I. INTRODUCTION
A typical biological process for nitrogen removal from
wastewater includes nitrification and denitrification. In
the process of nitrification, ammonia is oxidized to nitrate
via nitrite by ammonia oxidizing bacteria (AOB) and
nitrite oxidizing bacteria (NOB) under aerobic condition.
Subsequently, this nitrate is reduced to gaseous nitrogen
by the denitrifying bacteria in the presence of sufficient
organic carbon as electron donors under oxygen
exclusion condition. However, many municipal
Manuscript received June 3, 2014; revised September 13, 2014.
wastewater treatment plants (WWTPs) in China,
especially those employed in rural area, are currently
characterized their influent water by organic carbon
deficiency and ammonia overload. The addition of
external organic matter are needed for those WWTPs to
complete the conventional denitrification process which
results in increasing of operation costs. Therefore, it is
necessary to develop cost-effective technologies for
nitrogen removal in wastewater of low total organic
carbon to nitrogen (C/N) ratio.
Short-cut nitrification and denitrification has been
observed that ammonium can be converted to nitrite
during ammoxidation by AOB, and then, the produced
nitrite in combination with free ammonium will be
transformed to gaseous nitrogen without the supply of
organic carbon, which is referred to anaerobic ammonium
oxidation (ANAMMOX). These processes theoretically
lower oxygen demand up to 25% in ammonia oxidation
and decrease 40% of organic carbon consumption for
subsequent heterotrophic denitrification in comparison
with conventional nitrogen removal process. It also
presents advantages of reducing sludge generation and
reaction time [1]. So it has been raised concerns to be
technologically feasible for low C/N ratio wastewater
treatment [2]-[4]. The crucial step in these processes
named partial nitrification is to suppress nitrite oxidation
without inhibition of ammoxidation activity, i.e., to
terminate the oxidation of ammonium to nitrite [5]. Many
factors such as dissolved oxygen (DO) concentration [6]-
[10], temperature[11], [12] and hydraulic retention time
(HRT) [13] were reported that could regulate the
accumulation of nitrite during nitrification. Aeration
control was considered to be a priority due to its
advantages of saving energy, flexible controlling mode
and broad application in different biological wastewater
treatment systems [14].
There were two kinds of aeration control strategies
commonly used for partial nitrification system. The
1
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014
2014 Engineering and Technology Publishing
Multi-Stage Declined Aeration Promoting Partial
doi: 10.12720/jolst.2.1.1-7
Nitrification in a Sequencing Biofilm Batch
[2], free ammonia (FA) concentration [9], [8], pH value
continuous aeration strategy kept aeration at a constant
airflow rate throughout the aeration period. It had been
applied to partial nitrification system in all kinds of
reactors [15]-[17]. When oxygen concentration below 0.5
mg·L-1, the nitrite oxidation rate decreased more than the
ammonia oxidation rate, thus occurred nitrite
accumulation [8]. But low DO concentration generally
not only reduced the activity of AOB but also resulted in
sludge bulking [18], [19]. The intermittent aeration
strategy switched aeration and non-aeration periodically
to create anaerobic/anoxic conditions. It currently used to
facilitate nitrogen removal [20]. But under intermittent
aeration process, a portion of nitrate was produced in
effluent so that extra organic carbon was needed for total
nitrogen removal via heterotrophic denitrification.
Based on the results of our previous work, high air flux
was needed in the early aeration stage to meet the
requirement of DO consumption for COD degradation
and ammonification by heterotrophic bacteria. Thereafter,
decreased airflow would be more beneficial to inhibit
activity of NOB and achieve high nitrite accumulation
rate (NAR) in the end of aeration. In this paper, a
modified continuous aeration mode named Multi-stage
Declined Aeration (MSDA) was proposed to improve the
performance of partial nitrification. A lab-scale
sequencing biofilm batch reactor (SBBR) was used to
investigate the performance of short-cut nitrification
denitrification under different aeration strategies: constant
aeration (CA), two-section declining aeration (SDA1) and
three-section declining aeration (SDA2). The objectives
of this study are to: (i) investigate the performance of
partial nitrification, (ii) evaluate energy consumption
under new aeration strategy comparing with traditional
continuous aeration strategy.
II. MATERIALS AND METHODS
A. Laboratory-Scale Reactor
A laboratory-scale SBBR system with an working
volume of 12.0 L was used for the study. The cylindrical
reactor was made of polyvinylchloride, with an internal
diameter of 200 mm and a height of 600 mm. The reactor
was filled with multiple layers flexible plastic fiber
carriers (30% v/v) and the theoretical surface area of
2600 m2·m
-3. For each layer, the fibrous carriers were
fixed on a plastic discs with diameter of 150 mm. The
aeration system was composed of one air pump and two
porous stone air diffusers, and the air flow rate was
regulated by an flowmeter. Three peristaltic pumps were
used. The first one pumped wastewater into the reactor
from a storage tank during the fill phase. The second one
discharged the effluent from the reactor during the draw
phase. The last one circulated the water in the tank for
mixing. The temperature in the reactor was kept at
29±1°C with an adjustable heater.
B. SBBR Operation and Synthetic Wastewater
Preparation
The duration of a complete cycle was 480 min, giving
three cycles per day. In each cycle, the reactor was
operated with instantaneous FILL, 5.5h AERATION,
1.5h MIXING, 0.5h SETTLE and 0.5h DRAW/IDLE.
Three different aeration modes were adopted during the
period of experiment. CA mode was continuous aeration
at a constant airflow rate of 12.0 L·h-1
in aeration phase
of each cycle (Fig. 1(a)). SDA1 mode was two-stage
aeration at an airflow rate of 15.6 L·h-1
lasting 1.5h with a
following airflow rate of 12.0 L·h-1
lasting 4.0h (Fig.
1(b)). SDA2 mode separated aeration phase into three
sequential stages at an airflow rate of 15.6 L·h-1
lasting
1.5h, an airflow rate of 12.0 L·h-1
lasting 2.0h and an
airflow rate of 7.2 L·h-1
lasting 2.0h, respectively (Fig.
1(c)). At the end of each cycle, 6.0 liter of treated effluent
was decanted by a peristaltic pump and the same volume
of feeding solution was pumped into the reactor at the
beginning of next cycle. The SBBR system ran
automatically by a PLC programmable logic control to
manipulate the pumps.
0 60 120 180 240 300 360 420 480
Time
(min)
Fill Draw
QL=15.6L·h-1 QL=12.0L·h-1QL=7.2L·h-1
Three-section Declined Aeration (SDA2)
Fill Draw
QL=15.6L·h-1 QL=12.0L·h-1
Two-section Declined Aeration (SDA1)
Fill DrawConstant Aeration (CA)
Aeration Non-aeration Settle Idle
QL=12.0L·h-1
(c)
(b)
(a)
Figure 1. Sequencing operation of the SBBR system
The seed sludge was collected from an aerobic tank at
the Tangjiatuo Wastewater Treatment Plant in Chongqing,
China. Before this experiment was conducted, the SBBR
system had run 120 days and undergone four stages (data
are not shown): start-up (30 days, Days 1-30), a constant
airflow rate of 12.0 L·h-1
during aeration phase (40 days,
Days 31-60), a constant airflow rate of 15.6 L·h-1
during
aeration phase (30 days, Days 61-90), a constant airflow
rate of 7.2 L·h-1
during aeration phase (30 days, Days 91-
120). The synthetic wastewater was prepared to simulate
the practical domestic wastewater by mixing the
following composition (mg·L-1
) into tap water: glucose
(63), soluble starch (37), NH4Cl (175), peptone (250),
yeast extract (50), KH2PO4 (8), Na2CO3 (350) and 1mL
of trace elements solution. The composition of trace
elements solution was based on Ruiz et al. (2003). The
main characteristics of synthetic wastewater were: COD
350.0±50.0 mg·L-1
, NH4+-N 67.5±7.5 mg·L
-1, TN 77.0
±9.0 mg·L-1
, TP 4.5±1.2 mg·L-1
, pH 8.5±0.3, C/N 3.8-
5.7.
C. Analysis Methods
NH4+-N, NO2
--N, NO3
--N and TN were measured with
a UV-VIS spectrophotometer (Beijing Persee Corp.,
China) in accordance with the standard APHA methods
[21]. COD was measured using Hach COD-kits (Hach,
22014 Engineering and Technology Publishing
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014
USA). DO and pH were measured with a JPBJ-608 DO
meter (Heng Odd Inc., China) and a PHS-25B pH meter
(Dapu Inc., China), respectively. The NAR, SND
efficiency and the TN removal rate were calculated
according to the following equations [22], [23]:
%100--
-
32
2
NNONNO
NNONAR
(1)
%100
-
-- %
)(4
ox4
oxidized
producedxidized
NNH
NNONNHSND
(2)
%1001%
initial
final
TN
TNTN
(3)
A. Performance of the SBBR System
The study lasted 45 days and separated with three of
15d period for each aeration mode. The suspended sludge
in the SBBR reactor was negligible because released flocs and detached biofilm were discharged out of the
reactor in the draw phase each day. So the effluent quality
was tested directly without clarification. The overall
performance of the reactor is shown in Fig. 2. The mean
value of measurement in last 10 day runs of each aeration
mode was calculated because the SBBR system could run
stably after aeration mode changing 5 days. Effluent
COD concentration revealed that the average COD
removal rate were 92% (CA), 91% (SDA1) and 89%
(SDA2), respectively (Fig. 2(A)). The data are similar to
other partial nitrification systems [24], [25]. The effluent
COD concentration was mostly below 40 mg·L-1
in each
mode which was satisfied by Chinese national discharged
standard of pollutants for municipal wastewater treatment
plant. As the average DO concentration during the
aeration phase under CA, SDA1 and SDA2 mode was
less than 0.8 mg·L-1
, it indicated that low DO concen-
tration in solution was sufficient to the requirement of
oxygen consumption for organic matter degradation.
Figure 2. The performances of the SBBR system.
The ammonium oxidation in SBBR system is shown in
Fig. 2 (B). NO2--N concentration in effluent was much
higher than NO3--N concentration of each aeration mode
indicating dominant partial nitrification during aeration
32014 Engineering and Technology Publishing
III. RESULTS AND DISCUSSION
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014
period. The average NAR calculated according to
equation (1) of the mode CA, SDA1 and SDA2 at the end
of aeration were 91.1%, 88.3% and 100.0%, respectively.
The results suggest fully partial nitrification could
achieve in a SBBR system under specific aeration
strategy. It could be seen in Fig. 2 (C), the average NH4+-
N removal rates of the system of three modes were 90% ,
98% and 94%, respectively. Lim et al [26] suggested that
about 11-14% of NH4+-N was removed by the process of
biological assimilation. So the most of NH4+-N removal
was carried out through ammonium oxidation. The
average TN removal rates of the SBBR system were
75.5% (CA), 75.3% (SDA1) and 79.8% (SDA2),
respectively. But it was measured that the average TN
removal rates were 69.1%, 73.5% and 76.1% at the end
of aeration phase, i.e., the most of TN removal was
completed during the aeration phase. The data
demonstrate occurrence of SND in SBBR system during
aeration which was reported by other researches [3], [28],
[28]. The mean values of SND efficiency calculated
according to equation (2) were 63.4% (CA), 69.5%
(SDA1) and 74.7% (SDA2), respectively. Given that both
ammonium and nitrite concentrations in effluent
decreased further in non-aeration (mixing) phase, it
suggest that the rest TN removal could be attributed to
anaerobic ammonium oxidation (ANAMMOX) [29] in
anoxic condition.
B. Nitrogen Conversion in a Cyclic Operation
To investigate nitrogen conversion under Multi-stage
Declined Aeration mode in the SBBR system, variations
of nitrogen species, COD and control parameters
including DO and pH in a typical cycle operation were
measured on the same day of each aeration mode. It
could be seen in Fig. 3, the NH4+-N concentration
presented its highest level after the instantaneous fill,
because NH4Cl was added in the synthetic wastewater as
a source of resolvable nitrogen. Thereafter, the NH4+-N
concentration decreased linearly and the NO2--N
concentration increased linearly similar to intermittent
aeration strategy [21]-[26]. The means of NH4+-N
oxidation rate of 15.6 L h-1
under SDA1 and SDA2 mode
were 8.7 mg·(L·h)-1
and 8.5 mg·(L·h)-1
at the first 90min,
which were higher than 5.1 mg·(L·h)-1
of CA with the
airflow rate of 12.0 L·h-1
. Then the NH4+-N oxidation rate
of SDA1 and SDA2 decreased to 5.2 mg·(L·h)-1
and 5.6
mg·(L·h)-1
closing to CA (5.1 mg·(L·h)-1
), while each of
aeration mode had the same airflow rate of 12.0 L h-1
in
the mid aeration period (90-210min). In the last of
aeration period (210-330min) under SDA2 mode, the
airflow rate dropped to 7.2 L·h-1
and the corresponding
NH4+-N oxidation rate was 3.7 mg·(L·h)
-1 which was
lower than that of the CA mode. The data suggest the
positive correlation between NH4+-N conversion
efficiency and aeration density.
The COD concentration was observed dropping to
around 60 mg·L-1
at the first 90min of aeration period in
three aeration modes and then decreased gently. It
indicates most COD degradation completed in a very
short period when the most amount of DO was consumed
at the beginning of aeration phase. Laanbroek et al. [30]
revealed that the metabolic activities of heterotrophic
bacteria AOB and NOB needed the participation of
dissolved oxygen, but AOB and NOB had no competitive
advantages for dissolved oxygen utilization with
heterotrophic bacteria in lack of dissolved oxygen
condition. In this study, SDA1 and SDA2 mode adopted
an airflow rate of 15.6 L·h-1
at the beginning 90 minutes,
which was higher than that of CA (12.0 L·h-1
) , resulting
in the higher NH4+-N oxidation rate in the beginning and
the higher overall NH4-N removal rate during aeration
period than that of CA. Therefore, it could be proved that
large airflow at the beginning of the aeration phase would
promote ammonium oxidation reactions.
Figure 3. Typical profiles of nitrogen species in different cycles
Along with NH4+-N concentration decreasing due to
nitrification, NOB obtained ample substrate of NO2--N
and advantage of DO concentration for nitrite oxidation.
It was considered that nitrification was shifted to the
support of nitrite oxidation progress from ammonium
42014 Engineering and Technology Publishing
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014
oxidation progress [31], [32]. As shown in Fig. 3, when
the airflow rate was above 12.0 L·h-1
, the nitrate was
produced via nitrite oxidation under each aeration mode.
The mode SDA1 had the highest NO3--N concentration in
effluent with the largest total volume of air input (71
L·cycle-1
versus 66 L·cycle-1
to CA and 62 L·cycle-1
to
SDA2) into the reactor during the aeration phase among
three modes. The result is in accordance with previous
studies that over aeration would lead to more NO3--N
production in solution and partial nitrification
deterioration [14]. But the airflow rate of 7.2 L·h-1
adopted by SDA2 mode at the last 120 minutes during the
aeration phase presented the nearly 100% of NAR,
namely no nitrate was produced. The result suggests that
small airflow could effectively inhibit the conversion
from NO2--N to NO3
—N, and enhance SND in this stage
which increased SND by 10% in comparison with CA
mode. Therefore, it could be concluded that small airflow
in the late of aeration period would facilitate to
development of partial nitrification.
The cyclic variations of DO concentration in SBBR
system, which is shown in Fig. 4, could reveal the oxygen
balance for organic substance degradation and nitrogen
conversion during one cycle operation. Under each mode,
the DO dropped to less than 0.1 mg·L-1
after the
instantaneous fill and increased linearly up to around 0.8
mg·L-1
in the subsequent 90 minutes of aeration, and then
the DO concentration increasing slowed down during the
rest aeration period. The DO increasing trend
synchronizing with COD concentration decreasing trend
(Fig. 3) indicates oxygen consumption was dominated by
organic substance degradation at the beginning of the
aeration phase.
Figure 4. Typical profiles of DO and pH in a complete cycle
Then, the ammonium oxidation reaction was
responsible for the primary oxygen consumption in
reactors. The solution pH decreased during the aeration
period was attributed to nitrification. It could be seen in
Fig. 3 and Fig. 4, the performances of partial nitrification
in SBBR system during aeration phase greatly depended
on variation of DO concentration of each aeration mode.
Especially in the last 120 minutes aeration period (210-
330min), the DO concentration under CA and SDA1
modes increased fast indicating oxygen supplied by
aeration excessive to oxygen consumed by nitrification.
Thus the NO3--N concentration increased in the end of
aeration under CA and SDA1 modes due to partial
nitrification deterioration. On the contrary, the DO
concentration increased more gently under SDA2 (Fig.
3C) when the airflow rate decreased to 7.2 L·h-1
resulting
in no nitrate production.
As mentioned before, SND was observed in SBBR
system under three aeration modes similar to other
reports [3]-[31]. It was because the diffusion resistance of
DO towards the inner zone of the biofilm increased with
the depth of biofilm resulting in anoxic zone existence
inside the biofilm during aeration. Münch et al. [33]
suggested DO concentration around 0.5 mg·L-1
was
suitable to achieve complete SND for SBR system. But in
this study, over 60% SND efficiency achieved under
average DO concentration around 0.8 mg·L-1
(Fig. 3).
The experimental results indicate that the suitable DO
concentration for complete SND in biofilm or attached
growth systems could be higher than that of in suspended
growth systems. The COD concentration had only slight
decrease suggested that organic substance degradation
had been completed after 90 minutes aeration (Fig. 3).
That means no sufficient organic carbon to be used for
reduction of nitrite and nitrate produced in the aeration
phase to gaseous nitrogen by heterotrophic denitrification.
Therefore, it is implied that ANAMMOX process could
be the alternative pathway for TN removal via SND
during the aeration period in SBBR system.
IV. CONCLUSION
52014 Engineering and Technology Publishing
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014
(1) Partial nitrification and SND in a SBBR system for
low C/N wastewater treatment were successfully
achieved under low DO concentration condition (≈0.8
mg·L-1
).
(2) Multi-stage Declining Aeration strategy owned
higher NH4+-N removal rates (98% to SDA1 and 94% to
SDA2) and SND efficiency (69.5% to SDA1 and 74.5%
to SDA2) in aeration phase than that of Conventional
Constant Aeration strategy (90% and 63.4% to CA).
(3) Three-stage Declining Aeration mode (SDA2) was
the best strategy for partial nitrification in SBBR reactor
with 100% NAR obtained. Meanwhile, SDA2 could save
aeration flux by 6% and increase TN removal rate by 6%
in comparison to Conventional Constant Aeration
strategy (CA).
ACKNOWLEDGMENT
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62014 Engineering and Technology Publishing
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The authors gratefully acknowledge financial support
from the Water Pollution Control and Management of
Major Special Science and Technology in China (Project
number 2012ZX07102-001-04), the Scientific and
Technical Innovation Project of Chongqing University
Graduation Foundation in China (Project number
CDJXS11210015) and the support of K.C. Wong
Education Foundation, Hong Kong.
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Wenchuan Ding was born in Chengdu, the capital of Sichuan Province, mainland of
China in 1969. He received his Ph.D degree
from Chongqing University in municipal engineering in 2007. His major field of study
was wastewater treatment and solid waste management.
He is currently a professor in the School of
Urban Construction and Environmental Engineering at Chongqing University. He has
been working in the areas of 1) biochar for wastewater treatment and contaminated lands remediation, 2) trace elements control in water and
sediment, and 3) the recycling of biosolids and other residuals derived
from agricultural, industrial, and municipal activities. Prof. Ding is China National Association of Engineering Consultants
(Registered Consulting Engineer, Specialist of Chongqing Science & Technology Consulting Center, the Chinese Institute of Certified Public
Accountants, and the member of Chongqing Science & Technology
Association.
72014 Engineering and Technology Publishing
Journal of Life Sciences and Technologies Vol. 2, No. 1, June 2014