Comparison of operational strategies for nitrogen removal in aerobic granule sludge sequential batch reactor (AGS-SBR): A model-based evaluation

Accepted Manuscript Title: Comparison of operational strategies for nitrogen removal in aerobic granule sludge sequential batch reactor (AGS-SBR): A model-based evaluation Author: Feng Sun Yiming Lu Jun Wu PII: DOI: Reference:

S2213-3437(19)30437-3 https://doi.org/doi:10.1016/j.jece.2019.103314 JECE 103314

To appear in: Received date: Revised date: Accepted date:

22 June 2019 19 July 2019 21 July 2019

Please cite this article as: F. Sun, Y. Lu, J. Wu, Comparison of operational strategies for nitrogen removal in aerobic granule sludge sequential batch reactor (AGS-SBR): A model-based evaluation, Journal of Environmental Chemical Engineering (2019), https://doi.org/10.1016/j.jece.2019.103314 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparison of operational strategies for nitrogen removal in aerobic granule sludge sequential batch reactor (AGS-SBR): A model-based evaluation

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Feng Sun, Yiming Lu, Jun Wu*

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School of Environmental Engineering and science, Yangzhou University, 196 West Huayang

* Corresponding author: Tel. +8651487979528 (J. Wu)

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E-mail address: [email protected]

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Road, Yangzhou, Jiangsu, 225127, China

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Abstract

The nitrogen removal in the aerobic granule sludge sequential batch reactor

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(AGS-SBR) could be operated as two modes: the simultaneous nitrification and

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denitrification (SND) mode and the denitrification-nitrification reaction in serials

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mode. It is not clear which mode deliver better nitrogen removal performance. In this study, the performance of the two operation modes and their control strategies were evaluated in three operation scenarios (A, B and C) based on mathematical model simulation. The control strategy for the SND in AGS-SBR involved the dissolved oxygen (DO) concentration control, in which the DO was manipulated to optimize nitrogen removal (scenario A). The denitrification-nitrification AGS-SBR was achieved by pre-denitrification, high DO nitrification and low DO SND configuration. In the operation scenario B, the duration of high DO nitrification was manipulated to produce optimized nitrogen removal. The duration for pre-denitrification, high DO 1

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nitrification and low DO SND were fixed for the operation scenario C, without using control strategies. The results indicated that operation scenario B and C produced

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better performance than scenario A in terms of nitrogen removal. The average TN removal rate was 84.4%, 95% and 92% for the scenario A, B and C respectively. The

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DO control strategy used in scenario A could not cope with the dynamic influent

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feeding condition. The pre-denitrification, high DO nitrification and low DO SND operation scenarios demonstrated satisfactory nitrogen removal even without using

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control strategies.

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Keywords: aerobic granule sludge; feedback control; biofilm model; activated sludge

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1. Introduction

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model;

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The aerobic granule sludge (AGS) technology developed in the 90s [1] is now widely used in the domestic and industrial wastewater treatment [2, 3]. The AGS technology was usually applied in the sequential batch reactor (AGS-SBR)[4]. To maximize carbon and nitrogen removal efficiency, several control and operational strategies were developed [5, 6].

The nitrogen removal in the conventional activated sludge (CAS) plant [7] was usually available in two format: the two staged denitrification-nitrification [8] and simultaneous nitrification and denitrification (SND)[9]. The dissolved oxygen (DO) control was the main control variable for nitrogen removal in the CAS plant [10, 11]. 2

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The aeration duration or volume control was also used to optimize the nitrogen removal [12].

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In the AGS plant, the nitrogen removal also available in the two formats mentioned above. The DO control strategy developed by [13] was applied in stimulate single

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stage carbon and nitrogen removal in the SBR. This strategy utilized the deferent

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aerobic and anoxic layers within the aerobic granules for simultaneous nitrification and denitrification for nitrogen removal. The DO concentration was manipulated to

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create the stratified aerobic and anoxic layer within the granules. The authors [13]

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identified that the optimal DO concentration could be achieved by maintaining a small amount of ammonium nitrogen at the end of SBR cycle. The optimal DO for the next

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cycle was adjusted in proportion to the ammonium concentration measured at the end

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of previous cycle and was kept constant during the whole reaction cycle.

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In the other control and operational strategy used in the full scale AGS [2], the SBR reaction cycle was broken down into several stages. The initial feeding stage was served as the pre-denitrification stage, followed by the relative high DO concentration (i.e. at 2 mg O2/L) aeration, then by the low DO concentration (i.e. at 0.2 mg O2/L)

stage for SND.

The two operation strategies worked in two different principles. The DO control strategy worked in the SND principle. While the pre-denitrification – nitrification – SND serial reactions was used in the second strategy. The control variable in the first strategy is usually DO concentration, while in the second strategy, the nitrification 3

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duration is the control variable. In this study, the two operational and control strategies were examined and compared to identify the optimal strategy for carbon

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and nitrogen removal by the AGS technology. A biofilm model [14] from was used to evaluate the control strategies.

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2. Material and methods

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2.1. The biofilm model

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The biofilm model was constructed using the one dimensional multispecies biofilm

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model of Wanner and Gujer [14, 15]. The ASM (activated sludge model) style model matrix and kinetic rate equations was taken from [13]. Five particulate components

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(AOB (ammonium oxidizing bacteria), NOB (nitrite oxidizing bacteria), HET

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(heterotrophic bacteria), STO (storage products) and INT (inert biomass), and Five

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soluble components SS (soluble biodegradable COD), DO, NH4+, NO2-, and NO3were included in the model. The density for the AOB, NOB, HET, STO and INT solid species was chosen to be 350, 350, 150, 108 and 400 Kg/m3 [16]. The diameter of

granules was set at 3 mm. The porosity of the granule was fixed at 0.8. The number of granules in the reactor was chosen so that the granule volume occupied 30% of the total reactor volume. The initial dry weight fractions and their radius distribution for AOB, NOB, HET, STO and INT in the granules were derived by simulating the model for 1000 days under influent COD at 400 mg O2/L, NH4+ at 50 mg N/L, DO concentration of 5 mg O2/L, hydraulic retention time (HRT) at 8 h, and under the continuous flow. The model was implemented using the MATLAB 7.0 software[17]. 4

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The accuracy of the model was checked by comparing the simulation results from this research to the results from the Aquasim software [18].

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2.2. The control strategies

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Three operation scenarios including two control strategies (A and B) and one

operation scenario (C) without control were compared in this study. The scenario A

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was adopted from [13], in which the DO concentration in the whole 4 h reaction cycle

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was kept constant. The exact DO concentration (DO set point) was manipulated to promote maximum carbon and nitrogen removal (Fig. 1(a)). This was achieved by

measured (

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maintaining a small amount of NH4+ at the end of the reaction cycle, which was ) and compared to the NH4+ set point (

). The DO set point

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in the next reaction cycle (DOn) was determined using the PD (Proportional

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Differential) controller (equation 1). The PD controller calculated the DO set point in

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the next reaction cycle (DOp) using the proportional difference (kp) and differential

difference (kd) between the measured NH4+ concentration ( point (

) and the NH4+ set

). The NH4+ set point was chosen at 4 mg N/L. The airflow was

adjusted to achieve the bulk DO concentration calculated by the PD controller.

(1)

In the scenario B (Fig. 1(b)), the reaction cycle was divided into three stages: a pre-denitrification stage of 0.5 h (Td), followed by high DO aeration stage (Th) and the low DO aeration stage (Tl). The high DO aeration was stopped once the bulk NH4+ 5

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concentration reached the NH4+ set point value of 4 mg N/L. The low DO aeration stage (Tl) was set at (4- Td - Th) h.

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The operation scenario C was similar to the scenario B except that the duration of the three reaction stages were fixed, i.e., the pre-denitrification stage (Td) at 0.5 h, the

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high DO aeration stage (Th) at 1 h and the low DO aeration stage (Tl) at 2.5 h.

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A summary of the three operation scenarios was listed in Table -1.

Fig.1-The schematic representation of two control strategies

Table-1 Summary of the three operation scenarios

Scenario

Operation mode The DO concentration in the AGS-SBR for SND was kept

A

constant. The exact DO concentration for the next cycle was manipulated by the PD controller based on the NH4+ 6

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concentration measured at the end of the last reaction cycle Operated at pre-denitrification stage (Td) of 0.5 h, the high DO aeration stage (Th) ended when the NH4+ concentration reached

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B its set point value and the low DO aeration stage (Tl) of 4 h -

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Td - Th.

C

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Operated at pre-denitrification stage (Td) of 0.5 h, the high DO aeration stage (Th) of 1 h and the low DO aeration stage (Tl) of

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2.5 h.

2.3. The influent data

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The dry weather influent data from the COST simulation benchmark manual was used to test the three operation scenarios [19]. The influent data was available in 14 days at

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15 minutes interval (Fig.2). Considering the water quality equalization effect in the SBR, the initial NH4+ and chemical oxygen demand (COD) were averaged by taking into account of wastewater volume accumulated in 4 h. For long term (more than 14 days) simulation, the influent data was recycled, i.e. the influent data from the day 14-28 was the same as the influent data from the day 0-14.

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Influent COD (mg O 2 N/L)

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Influent NH +4 (mg N/L)

(b)

600

40 35 30 25 20 15

Orginal Average

500

400

300

200 0

2

4

6

8

10

12

14

0

2

4

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8

10

12

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Time (days)

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Time (days)

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(a)

50

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Fig.2. Initial influent NH4+ (a) and COD (b) concentration used for testing the control strategies. The original value was from [19]. The average value was the 4 h

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wastewater volume based average concentration.

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3. Results

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3.1. The simulation results of the three operation scenarios

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Fig.3 (a) showed the resulted DO concentration to reach the NH4+ set point of 4 mg

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N/L at the end of reaction cycle. The DO concentration varied between 0.5 to 0.65 mg O2/L. Fig. 3(b) showed the high DO (2 mg O2/L) reaction duration that ended when the bulk NH4+ concentration reached the same set point value. (a)

0.65

(DO (mg O 2 /L))

(b)

1.2

High DO duration (h)

0.7

0.6

0.55 0.5 0.45

1

0.8

0.6

0.4 0

2

4

6

8

10

12

14

0

Time (days)

2

4

6

8

10

12

14

Time (days)

Fig.3- (a) The derived DO concentration from operation scenario A that aimed for the 8

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NH4+ set point at the end of SBR cycles; (b) the high DO (2 mg O2/L) reaction duration that ended when the bulk NH4+ concentration reached setpoint value of 4 mg

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N/L

Fig. 4 showed the NH4+, NO2-, NO3- and TN (total nitrogen) at the end of SBR cycles

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(effluent). Despite effort in adjusting DO to maintain the NH4+ set point at the end of

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the SBR cycles, the NH4+ concentration varied between 0-8 mg N/L for the scenario A. The NH4+ concentration remained low (less than 2 mg N/L) for scenario B and C. The

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NO2- and NO3- were low for the three scenarios. The TN concentration was the lowest

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in scenario B and highest in scenario A. The average TN removal rate was 84.4%, 95% and 92% for the scenario A, B and C respectively. This suggested that the high

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DO duration control in scenario B offered the best performance in terms of nitrogen

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removal. The DO concentration control in scenario A offered the worst performance.

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6

(NO -2 (mg N/L))

4

2

0

Scenario A Scenario B Scenario C

1

0.5

0 0

5

10

15

0

Time (days)

15

(d)

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10 8

(TN (mg N/L))

4 3

6

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(NO -3 (mg N/L))

10

Time (days)

(c)

5

5

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(NH+4 (mg N/L))

(b)

1.5

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(a)

8

2

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1

4

0

0

0

5

10

15

0

5

10

15

Time (days)

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Time (days)

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Fig.4- The effluent (a) NH4+, (b)NO2-, (c) NO3- and (d) TN concentration of the three

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operation scenarios.

Fig.4 (a) suggested the DO manipulation failed to reach the stable NH4+ set point at

the end of the SBR cycles under the dynamic influent feeding condition. To examine the impact of dynamic feeding on the performance of the control strategy A. The performance under constant feeding condition was examined. The NH4+ and COD

concentration in the constant feeding were the average value of the dynamic feeding shown in the Fig.2. Fig.5 showed the simulation results of control strategy A under constant feeding condition. After a few cycles, the bulk NH4+ concentration stabilized at the setpoint value of 4 mg N/L. The result suggested that dynamic feeding 10

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condition impeded the performance of the control strategy A. (a)

0.7

(b)

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0.4

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0.5

8 6 4 2

0.3

0 0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

Time (days)

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Time (days)

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(NH+4 (mg N/L))

(DO (mg O 2 /L))

10 0.6

Fig.5- (a) The derived DO concentration from operation scenario A to achieve the

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NH4+ set point at the end of SBR cycles under constant feed condition; (b) the NH4+

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3.2. The long term performance

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concentration reached its set point value after a few cycles of operation.

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To examine long term performance of the operation scenario B and C, the simulation

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was extended to 84 days by reusing the 14 days influent data 6 times. The long term performance of the operation scenario A was not studied since it underperformed in the short term 14 days simulation.

The NH4+ concentration at the end of SBR cycles (effluent) was shown in Fig. (6). For

scenario B, the NH4+ concentration remained at ca. 1 mg N/L. While in scenario C,

the NH4+ concentration gradually increased from 0 to peak value of ca. 3 mg N/L. The peak NO2- concentration was slightly higher in scenario B than in scenario C. While the peak NO3- concentration was lower in the scenario B than in the scenario C. The average effluent TN concentration was similar in the scenario B and C. Large 11

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variation in the TN concentration was observed in the scenario C. (a)

2

1

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(NH+4 (mg N/L))

3

0 0

10

20

30

40

60

0.5

0 0

10

20

30

40

80

60

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80

70

80

70

80

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6

4

2

0 0

10

20

30

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(NO -3 (mg N/L))

50

Time (days) (c)

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1

40

50

60

d

Time (days) (d) Scenario B Scenario C

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6

4

2

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(TN (mg N/L))

70

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1.5

(NO -2 (mg N/L))

50

Time (days) (b)

0

0

10

20

30

40

50

60

Time (days)

Fig. 6 - Effluent of the long term operation of the operation scenario B and C, (a) NH4+, (b)NO2-, (c) NO3- and (d) TN concentration

3.3. Change in the biomass species radius distribution

The initial biomass species radius distribution was derived from 1000 days simulation in the continuous flow mode. The biomass distribution in the radius direction after 14 days of SBR operation was shown in Fig. 7. Compared to the initial biomass 12

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distribution, the HET (heterotrophic biomass) increased in the three operational scenarios and especially for the scenario A. The AOB and NOB decreased compared

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to the initial AOB and NOB distribution. The STO (storage product) was high in the initial biomass distribution and scenario A. In both scenario B and C, the STO was

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2000 1500 1000

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2

1

0

500

0

1000

Distance to granule center ( (c)

2000

0

m)

10000

d

1500

1500

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500

STO (mg COD/L)

0

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1000 500

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NOB (mg COD/L)

(b)

2500

AOB (mg COD/L)

HET (mg COD/L)

(a)

10 4

3

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reduced.

0

0

500

1000

Distance to granule center (

8000

500

1000

Distance to granule center ( (d)

1500

m)

Initial Distribution scenario A scenario B scenario C

6000 4000 2000 0

1500

0

m)

500

1000

Distance to granule center (

1500

m)

Fig.7- Change of biomass species distribution in the radius direction after 14 days of operation, (a) HET, (b) AOB, (c) NOB, (d) STO

4. Discussion

4.1. The control strategies for AGS-SBR

Based on mathematical biofilm model simulation, three operation scenarios for 13

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AGS-SBR were evaluated. The DO concentration control strategy used in the operation scenario A produced the worst performance in term of the TN removal. The

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performance of the DO concentration control strategy proposed by [13] was satisfactory under stable influent condition. It could not cope with the dynamic feed

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condition used this study. The three-stage operation: pre-denitrification, followed by

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high DO aeration, then by low DO aeration in the operation scenario B and C

produced satisfactory performance under dynamic feeding condition, even without the

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aeration duration control.

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Satisfactory nitrogen removal has been achieved in AGS-SBRs that were divided into several reaction stages (i.e. anoxic and aerobic) as demonstrated in the simulation

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study and several other lab-scale [20] or full-scale experiment [2]. The simulation in

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this study indicated the AGS-SBRs that divided into pre-denitrification, high DO

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aeration and low DO aeration stages could achieve satisfactory TN removal with minimum control requirement (and therefore instrumentation). This means savings in the construction cost for wastewater treatment plant (WWTP), since automation and instrumentation are a significant investment for WWTP construction [21].

Compared to the AGS-SBRs that were divided into anoxic and aerobic stages, the one-stage AGS-SBR kept DO constant during the whole reaction cycle. It offered a simper reaction structure. The DO concentration chosen in this control strategy was to make sure there was a small amount of remaining NH4+ at the end of the reaction cycles [13]. The remaining NH4+ concentration at the end of the previous reaction 14

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cycle was used feedback information to determine the DO concentration in the next cycle. Under dynamic feed condition used in this study, this strategy didn’t work. A possible solution could either be a feedforward control strategy, in which the DO

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concentration was determined from the NH4+ measurement at the beginning of the

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reaction cycle, or a combined feedforward-feedback control strategy that utilized the

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NH4+ measurement at the beginning and end of the reaction cycle.

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4.2. The impact of control strategies on biomass species

The control strategies used in this study mainly aimed to achieve maximum nitrogen

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removal. However, different operational conditions from the various control strategies could also lead to the change in the biomass species composition. Fig. 7 indicated that

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the HET was high in the operation scenario A. This was due to the constant oxygen

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supply in the operation scenario A. Therefore, HET was cultured in the aerobic

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environment. In the operation scenario B and C, ca. 75% of the reaction cycle was in the anoxic stage. The HET growth rate was lower in the anoxic condition than in the aerobic condition [22], as reflected in the lower maximum specific growth rate for HET under anoxic condition [23]. The low STO observed under the operation scenario B and C also suggested that more carbon source was available for denitrification, therefore led to better TN removal.

The low DO condition in the three operation scenarios might lead to less favorable biomass species growth, i.e. filamentous bacteria overgrowth [24, 25] and the breakdown of granules [26]. Phosphate removal was not included in the study, in the 15

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future investigation, the impact of control strategies on the growth and distribution of phosphate accumulating organisms (PAO) and glycogen accumulating organisms

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(GAO) should also be considered [16, 20, 27].

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Conclusion

The control strategies for nitrogen removal in the AGS-SBRs were evaluated based

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model simulation. The results indicated that the feedback control method to choose a

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constant DO to maintain a small amount of NH4+ at the end of the reaction cycles was not successful under the dynamic influent feeding condition. The AGS-SBRs that

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divided into pre-denitrification, high DO aeration and low DO aeration stage achieved

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Acknowledgment

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satisfactory TN removal, with or without the high DO aeration duration control.

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The study is supported by the Natural Science Foundation of China (Grant number: 51478410).

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DO control strategy for SND in aerobic granular sludge SBR was evaluated The Feedback DO control could not cope with the dynamic influent feeding condition Pre-denitrification, high DO aeration and low DO SND serial reaction mode offered better nitrogen performance

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