Influence of Volume Ratio of Anoxic Zone to Aerobic
Zone on Denitrifying Phosphorus Removal in Carrousel
2000 Oxidation Ditch Process
Yang Luo1,a, Xuehong Zhang1,b, Hua Zhang 1,c, Zhoufang Guo 1,d
1
College of Environmental Science and Engineering, Guilin University of Technology,
The Guangxi Key Laboratory of Environmental Engineering, Protection and
Assessment, Guilin, 541004, China
a
email: wangyingxuan2046@126.com, bemail: zhangxuehong@x263.net, cemail:
zhanghua@glite.edu.cn, demail: zfguo110@gmail.com
Abstract：The influence of the volume ratios of anoxic zone to aerobic zone on denitrifying
phosphorus removal in the Carrousel 2000 oxidation ditch process for treating domestic sewage
was studied. The results show that when the volume ratios of anoxic zone to aerobic zone are 0.35,
0.49 and 0.64, the average NH3-N removal rates are 92.96%, 94.93% and 97.60%, the average
phosphorus removal rates are 80.22%, 86.55% and 92.30% respectively. The average COD
removal rates are also all higher than 90% under the three volume ratios. Therefore, when the
volume ratios of anoxic zone to aerobic zone continue to be increased, the internal recirculation
ratio should be adjusted, otherwise the nitrate concentration will not be enough in anoxic zone and
phosphorus will be released again.
Key words：Carrousel 2000 oxidation ditch process；volume ratio；biological phosphorus removal
Introduction
The first generation of Carrousel oxidation ditch was developed by DHV Company in 1967,
which combines the conventional oxidation ditch wastewater treatment process and the benefits
and mainly for the purpose of removal of BOD5.What’s more, the NH3-N can be removed too.
Carrousel 2000 oxidation ditch is the second generation of Carrousel oxidation ditch system。It
was invented by DHV company and its U.S. patent licensing company EMCO on the basis of
general Carrousel oxidation ditch. Carrousel 2000 oxidation ditch strengthened the common
features of nitrogen and phosphorus removal compared with the general Carrousel oxidation ditch.
An anaerobic zone and anoxic zones (denitrifying zone) [1] was added in front of an ordinary
Carrousel oxidation ditch system, in order to remove nitrogen and phosphorus.
In the process of Carrousel oxidation ditch wastewater treatment, there are three kinds of
factors impacting nitrogen and phosphorus removal: Class 1 is the environmental factors, such as
temperature, pH and DO and so on. Class 2 is the operating parameters,such as MLSS, carbon and
nitrogen ratio (C/N), load rate and so on.Class3 is running mode, that is hydraulic retention time
(HRT), sludge reflux ratio, sludge retention time (SRT), aeration rate, volume ratio [2].Current
Carrousel oxidation ditch wastewater treatment process not only need to remove organic matter,
but also must have nitrogen and phosphorus removal capabilities, which requires the various
design parameters of Carrousel oxidation ditch system can coordinate with each other to ensure
that the organic matter and nitrogen and phosphorus can be efficiently removed in the process of
biological treatment.
Therefore, how to effectively remove organic matter and nitrogen and phosphorus in depth at
the same time becomes one of the hot areas for Carrousel oxidation ditch wastewater treatment
research. We need to research the various factors of pollutant removal in Carrousel oxidation ditch
wastewater treatment process so as to develop an energy saving technology and an effective
controlling strategy. As the anoxic zone and aerobic zone volume ratio (n) have a greater impact
on the effect of denitrification and phosphorus release results of the microbial in Carrousel 2000
oxidation ditch. The test was studied in the case of pH, DO, MLSS and HRT and other factors
remain unchanged. We choose the anoxic zone and aerobic zone volume ratio (n) as the reference
factor, focusing on the effect of nitrogen and phosphorus removal under different volume ratio (n)
for Carrousel 2000 oxidation ditch system.
Materials and Methods
This is a pilot-scale study conducted in Carrousel 2000 oxidation ditch model. The Carrousel
2000 oxidation ditch device is shown in Figure 1.
Fig. 1 Schematic diagram of the Carrousel 2000 oxidation ditch system. (1) water tank; (2)
influent pump; (3) anaerobic zone; (4) anoxic zone; (5) aerobic zone of OD; (6) air pump; (7) air
diffuser; (8) mixer; (9) secondary clarifier; (10) returned activated sludge pump; (11) wasted
activated sludge pump; (12) DO probe; (13) ORP probe; (14) pH probe; (15) effluent.
The Carrousel 2000 oxidation ditch system includes three water tanks which were made of
aluminum. The three water tanks were connected by water pipe at the bottom with each other, the
volume of each water tank is 0.436 m3, and the total volume is 1.308m3. There is a flowmeter after
the influent pump so as to adjust the wastewater flowing into the anaerobic zone. The flowing
adjustment range from 0.05m3/h to 0.149m3/h. The influent pump works 24h one day and it is
continuous operation. The maximum flow rate is 0.149 m3/ h.
There are two anaerobic zones in front of the Carrousel 2000 oxidation ditch. The material is
steel, the size (L × B × H) is 0.4m × 0.4m × 0.45m. The anaerobic zone is connected by a pipeline,
single anaerobic zone can work independently. So the wastewater and the return sludge can mixed
in the anaerobic tank 1,then outflow to the oxidation ditch; or the water and return sludge mixed in
the anaerobic tank 1 and then flow to anaerobic pond 2,finally flow to the oxidation ditch.The two
anaerobic tank are both equipped with a blender, the speed is between 4060r/min and 60r/min to
prevent the sludge sedimentation in anaerobic zone(Blender Model: YN80-25, speed adjustment
range from 0 r/min to 1500r/min , Electric Co, Ltd. Shanghai Exhibition rain).
The effective volume of the Oxidation ditch is 780L, the ditch depth is 0.7m, the corridor
width is 0.12m, the water depth is 0.6m, the unilateral straight ditch length is 1.43m.The depth of
the bottom of the oxidation ditch is ± 0.000. There are two outlets: the height of the two outlet
pipe is 0.6m and 0.3m respectively. There is one inlet.The height is 0.65m.Seven separate electric
mixer are installed on the pre-anoxic zone and post-anoxic zone of the Oxidation ditch in order to
promote mixture circulating in the trench and prevent the activated sludge sediment in the local
trench.The speed of the mixer is between 150300r/min and 300r/min.The aerobic area of the
oxidation ditch using microporous aeration,three trench of the Oxidation ditch are aeration zone,
the aeration equipment is electromagnetic air compressor. The air release to the mixture through
the aeration pipe and 12 microporous aeration strip installed in the aerobic zone at the bottom of
the oxidation ditch.The most significant difference between microporous aeration and other means
is that the microporous aeration strip can produce a large amount of 1 mm in diameter tiny air
bubbles, which greatly increased the surface area of the air bubbles.When the effective volume of
the Oxidation ditch is unchanged, the increasing specific surface area can increase the total
transfer oxygen.[3]
The secondary sedimentation tank is a vertical-flow sedimentation tank and the volume is
230L settling tank using vertical flow sedimentation tank, volume 10L.There is a flowmeter after
the returned activated sludge pump so as to adjust the returned activated sludge back into the
anaerobic zone.The flowing adjustment range from 0.05m3/h to 0.149m3/h.The returned activated
sludge pump works 24h one day and it is continuous operation.The maximum flow rate is 0.149
m3/ h.
In this study, the seed sludge come from the returned activated sludge of oxidation ditch in
Qilidian wastewater treatment plant in Guilin.The wastewater come from the grit Chamber in
Qilidian wastewater treatment plant.The quality of the wastewater during the study is showed as in
Table 1.
Tab1 Characteristics of influent wastewater
mg·L-1
parameter
n=0.64
n=0.49
n=0.35
range
average
range
average
range
average
temperature
22-25
24
19-22
20
16-20
18
COD
201.06-354
273.21
165.58-342.7
239.27
157.55-250.33
200.24
BOD5
120-180
149
130-170
151
140-160
154
42.35-48.03
44.73
38-46.83
42.34
41.63-48.35
44
TP(PO4
4.60-7.94
6.01
3.4-7.78
5.83
4.15-8.22
5.84
PH
6.93-7.14
7.09
7.01-7.19
7.08
6.5-7.21
7.06
NH+
4-N
3--P)
The water temperature is between 16℃ and 25℃ during the study,the MLSS remained
unchanged at 3000mg/L,the sludge age is 10d,the sludge return ratio is 100%,the DO in the
aerobic zone is controled between 1 mg/L and 1.5mg/L,the DO in the anoxic zone is controled
below 0.5mg/L.The HRT is 12 h.
COD, MLSS, NH3-N, BOD5, TN and TP were mensurated by the national standard method[4],
pH, DO was mensurated with WTW company's portable DO/pH analyzer.The inoculated sludge
was obtained from the returned activated sludge of oxidation ditch in Qilidian wastewater
treatment plant in Guilin, The inoculated sludge was domesticated after continuous culture,so the
oxidation ditch system has a good denitrifying and phosphorus removal capabilities.After that we
began to tracking measurement.
This study was operated under the three conditions, the volume ratios of anoxic zone to
aerobic zone are 0.35，0.49 and 0.64 respectively, anoxic zone and aerobic zone volume ratio (n) is
10
100
9
90
8
80
7
70
6
60
5
50
4
40
T P(influent)
T P(effluent)
T P removal rate(%)
3
2
30
20
1
10
0
0
1
2
3
4
5
6
7
8
Experiment date(day)
a.The volume ratio is 0.35
9
10
TP removal rate(%)
TP(PO 43--P,mg/l)
0.35,0.49,0.64.Each condition needs to operated one week before collecting water samples and
testing to insure that the oxidation ditch was stable.The water quality index such as TN,
TP(PO43--P), NH+4-N, and COD,BOD5 values were analysed everyday, the sludge settlement ratio
(SV), sludge volume index (SVI) and MLSS were analysed everyday,the DO, pH,ORP and other
operating parameters were monitored continuously.
Results and Discussion
The influence of the volume ratio on phosphorus removal
In Carrousel 2000 oxidation ditch,The TP was removed by the phosphorus accumulating
organisms (PAOs) releasing phosphorus in the anaerobic zone and absorbing phosphorus in the
aerobic zone. To achieve a good phosphorus removal effect,it is necessary for PAOs to release
phosphorus fully.Sufficient organic carbon source and strict anaerobic environment are two
necessary conditions for PAOs to release phosphorus fully.
When the volume ratios were 0.35,0.49,0.64,the phosphorus removal effect of Carrousel
2000 oxidation ditch system is shown in Figure 2.The average effluent total phosphorus
concentrations were 1.19,0.79, and 0.48mg/L under the three kinds of volume ratio condition
respectively, the average removal rates were 80.22%, 86.55% and 92.30%.So we can draw a
conclution that the greater the value of the volume ratios is,the lower the effluent total phosphorus
concentrations is and the better the treatment is.We also found that when the volume ratios was
0.64,the effluent total phosphorus concentration in the anoxic zone was increased slightly, because
the influent NH+4-N concentration is low,the anoxic zone’s electron acceptor is not enough
without increasing the circulating reflux ratio,resulting releasing phosphorus in anoxic
condition.one effluent phosphorus concentration, but due to the PAOs uptaking phosphorus in
aerobic zone, the effluent total phosphorus concentration of Carrousel 2000 oxidation ditch system
is not affected.We can imagine that if we keep the influent organic carbon source and NH3-N
concentration unchanged and continue to increase the value of volume ratios, we need to improve
the internal circulation reflux ratio to make sure there are enough electron acceptor in anoxic zone.
Otherwise there will be a second phosphorus releasing phenomenon in anoxic zone.
100
9
90
8
80
7
70
6
60
5
50
4
40
3
T P(influent)
T P(effluent)
2
T P removal rate(%)
30
20
1
10
0
0
1
2
3
4
5
6
7
8
Experiment date(day)
9
TP removal rate(%)
TP(PO 43--P,mg/l)
10
10
b.The volume ratio is 0.49
100
90
80
70
60
50
40
30
20
10
0
TP removal rate(%)
TP(PO 43--P,mg/l)
10
9
8
7
6
5
4
3
2
1
0
TP(influent)
TP(effluent)
TP removal rate(%)
1
2
3
4
5
6
7
8
Experiment date(day)
9
10
c. The volume ratio is 0.64
Fig2 TP removal in Carrousel 2000 oxidation ditch process under different volume ratios
The influence of the volume ratio on NH3-N removal
The nitrification process of Oxidation ditch occurs mainly in the aerobic zone. As the long
sludge age of oxidation ditch, and the nitrification bacteria’s generation cycle is greater than the
10d, so a long sludge age is beneficial to the breeding and survival of nitrifying bacteria. When
oxygen is sufficient, the nitrification process can be very full.The NH3-N in the wastewater was
turned into nitrite and nitrate under the effect of nitrification and denitrification by nitrate bacteria
and nitrification bacteria respectively. The higher the conversion rate of NH3-N,the better the
remove effect is.
When the volume ratios were 0.35,0.49,0.64,the NH3-N removal effect of Carrousel 2000
oxidation ditch system is shown in Figure 3.
When the volume ratios were 0.35,0.49,0.64,the NH3-N removal effect of Carrousel 2000
oxidation ditch system is stable,the average effluent NH3-N concentrations were 3.1,2.14,and
1.07mg/L under the three kinds of volume ratio respectively,the average removal rates were
92.96%, 94.93% and 97.60%. So we can draw a conclution that the greater the value of the
volume ratios is, the higher the NH3-N removal rate is.
During the experiment, the dissolved oxygen(DO) of the aerobic zone was remained
bettween 2 and 3mg / L, so there was no accumulation of nitrite, and the NH3-N nitrogen removal
rate was almost reach 100%. We also found when the volume ratios were 0.49 and 0.64,the
effluent nitrate concentration in anoxic zone was between 1 and 2 mg/L, which not only improve
the total nitrogen removal rate, but also consistent with the theory of Yuan [5 ].The theory of Yuan
is to keep the nitrate concentration at the end of anoxic zone between 1 and 3mg / L，under this
conditions, the activated sludge can take full advantage of the solubility biodegradable COD for
denitrification, and reduced the COD influent into the aerobic zone. So the total nitrogen was not
only optimal controlled, but also provided sufficient electron acceptor for the anoxic
denitrification and phosphorus removal. According to the number of denitrification of NO-x (The
NO-x is refers to the NO-3 and NO-2)and phosphorus uptake volume in the anoxic reactor during
the experiment, Under the three volume ratios,the activated sludge absorption 1mg per phosphorus
consume about 0.61,0.73 and 0.82mg of NO-x in the anoxic zone,which indicating that the larger
the volume ratio is, the higher the activity of denitrifying phosphate accumulating organisms is.
100
90
80
70
60
50
40
30
20
10
0
NH3-N removal rate(%)
60
NH3-N(mg/l)
50
40
NH3-N(influent)
30
NH3-N(effluent)
20
NH3-N removal rate(%)
10
0
1
2
3
4
5
6
7
8
9
10
Experiment date(day)
a.The volume ratio is 0.35
100
90
80
70
60
50
40
30
20
10
0
NH3-N removal rate(%)
60
NH3-N(mg/l)
50
40
30
NH3-N(influent)
20
NH3-N(effluent)
NH3-N removal rate(%)
10
0
1
2
3
4
5
6
7
8
Experiment date(day)
b.The volume ratio is 0.49
9
10
100
50
80
40
60
NH3-N(influent)
30
NH3-N(effluent)
20
40
NH3-N removal rate(%)
10
20
0
0
1
2
3
4
5
6
7
8
Experiment date(day)
9
NH3-N removal rate(%)
NH3-N(mg/l)
60
10
c.The volume ratio is 0.64
Fig3 NH3-N removal in Carrousel 2000 oxidation ditch process under different volume ratios
The influence of the volume ratio on COD removal
The results showed that, although the influent COD concentration was fluctuating, the COD
removal rate was very stable. The effluent COD concentrations were 12.25 to 26.26, 7.88 to 36.76
and 3.48 to 22.38mg/L under the three kinds of volume ratios, the average effluent COD
concentrations were 19.24, 19.16 and 14.57mg/L under the three kinds of volume ratio condition
respectively. The average COD removal rates were 90.44%, 91.78% and 94.42%.In addition,
although the volume ratios are different, a large number of COD in the anaerobic zone are
converted to intracellular polymers in poly-P bacteria (PHB).The COD was removed by PHB. The
average effluent COD concentrations in the anaerobic zone were 63.42, 62.71 and 60.82mg/L
under the three kinds of volume ratios and the average COD removal rates were 81.72%, 82.93%
and 84.31%, while the average COD removal rates in anoxic zone were 6.37%, 6.89% and 7.63%,
and the larger the volume ratio is, the higher the average COD removal rates is in anoxic zone.
The reason is that increasing hydraulic retention time of anoxic zone is beneficial to denitrification
conducting, thereby the removal rate of COD was increased. As the anoxic zone and anaerobic
zone almost removed all of the COD, it can guarantee the advantages of nitrifying bacteria growth,
while reducing the amount of aeration required for aerobic zone.
400
100
90
80
70
60
50
40
30
20
10
0
COD removal rate(%)
350
COD(mg/l)
300
250
200
150
COD(influent)
COD(effluent)
COD removal rate(%)
100
50
0
1
2
3
4
5
6
7
8
Experiment date(day)
a.The volume ratio 0.35
9
10
100
90
80
70
60
50
40
30
20
10
0
350
COD(mg/l)
300
250
200
COD(influent)
COD(effluent)
COD removal rate(%)
150
100
50
0
1
2
3
4 5 6 7 8
Experiment date(day)
9
COD removal rate(%)
400
10
b.The volume ratio 0.49
400
100
90
80
70
60
50
40
30
20
10
0
COD removal rate(%)
350
COD(mg/l)
300
250
200
COD(influent)
COD(effluent)
COD removal rate(%)
150
100
50
0
1
2
3
4
5
6
7
8
Experiment date(day)
9
10
c.The volume ratio 0.64
Fig4 COD removal in Carrousel 2000 oxidation ditch process under different volume ratios
Conclusions
The results show that when the volume ratios of anoxic zone to aerobic zone are 0.35,0.49
and 0.64,the COD,NH3-N and total phosphorus removal rates were higher. The average COD
removal rates were 90.44 %,91.78% and 94.42%, the average NH3-N removal rates were 92.96%,
94.93% and 97.60%,the average total phosphorus removal rates were 80.22%,86.55% and
92.30%.The average effluent COD concentrations were 19.24,19.16 and 14.57mg/L,the average
effluent NH3-N concentrations were 3.1,2.14 and 1.07mg/L,the average effluent total phosphorus
concentrations were 1.19,0.79 and 0.48mg/L.The best volume ratio of anoxic zone and aerobic
zone is around 0.64.Then the average effluent NH3-N concentration, the average effluent total
phosphorus concentration and the average effluent COD concentration can all meet the first class
of B standard of "emission standards for urban wastewater treatment plant" (GB18918- 2002).
When the volume ratios of anoxic zone to aerobic zone were increased, the effluent water
quality of Carrousel 2000 oxidation ditch system was getting better. The increased volume ratio
can increase the hydraulic retention time of anoxic zone and the proportion of denitrifying
phosphate accumulating organisms in PAOs. So the removal rate of COD, NH3-N and TP were
increased.
The increased volume ratio is beneficial to reduce aeration energy consumption and operating
costs. The calculated results show that the aeration energy can be save 17% when the volume ratio
increased from 0.35 to 0.64.We can imagine that if we keep the influent organic carbon source and
NH3-N concentration unchanged and continue to increase the value of volume ratios, we need to
improve the internal circulation reflux ratio to make sure there are enough electron acceptor in
anoxic zone. Otherwise there will be a second phosphorus releasing phenomenon in anoxic zone.
Acknowledgements
This works has been financially supported by the National Critical Patented Projects in the
Control and Management of the Polluted Water Bodies (2008ZX07317-02-03) and Innovation
Project of Guangxi Graduate Education (2010105960814M06)
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