Rapid Start-Up of the Aerobic Granular Reactor under Low Temperature and the Nutriment Removal Performance of Granules with Different Particle Sizes
, Jing Zhang *, Yaodong Wu, Dongyue Li, Peilin Li and Kai ZhangRound 1
Reviewer 1 Report
Introduction: the introduction is well written and gives the appropriate overview of gap in the research and necessity of this research.
Materials and Methods: he experimental set-up of the SBR is well explained, as well as the characteristics of the synthetic wastewater and the methods for analysis.
Results and Discussion: the results are thoroughly presented with appropriate graphs and findings discussed along with comparison with other peer-reviewed researches.
Minor comments:
- please check the spelling of word "sedmitation" at lines 119 and 127
- check the space between SVI 30 in line 141
Author Response
Reviewer 1
- Introduction: the introduction is well written and gives the appropriate overview of gap in the research and necessity of this research.
Response: Thanks for your comment.
- Materials and Methods: the experimental set-up of the SBR is well explained, as well as the characteristics of the synthetic wastewater and the methods for analysis.
Response: Thanks for your comment.
- Results and Discussion: the results are thoroughly presented with appropriate graphs and findings discussed along with comparison with other peer-reviewed researches.
Response: Thanks for your comment.
- Please check the spelling of word "sedmitation" at lines 119 and 127.
Response: Thanks for your advice. The term of "sedmitation time" has been revised as "settling time".
- Check the space between SVI 30 in line 141.
Response: Thanks for your advice. The space between SVI 30 has been removed.
Reviewer 2 Report
Introduction: the problem statement is not clearly explained/stated in the introduction. Previous works related to this work and the knowledge gap in missing. Thus, the significance of the work is not clear.
Materials and Methods: the placement of figures/tables need to checked.
drainage ratio change to exchange ratio
inlet water change to feeding
sedimentation change settling
drainage change to decant
detail operation on how the settling time was reduced during phase 1 should be included. Was it step reduction? Duration for each settling time?
DO of 6 mg/L is considered high for PN/A. why the authors chose to maintain at this concentration?
Line 82: Characteristic of the inoculum sludge? Floccular or granular sludge?
Line 93: “the mass transfer performance of the substrate was enhanced by magnetic stirring” did the authors carry out studies without magnetic stirring?
Results and discussion
Line 199: give details of R1, R2 and R3, also for other figures.
The work done in this manuscript in good. However, the depth of the study and the discussion is doubtable. The manuscript is dealing with the DO level, and it was stated that the DO is low, but DO 6mg/L was used. Why the authors considered 6 is low. The other factor studied and stated in title is low temperature. But this is not discussion in detail in the manuscript.
Author Response
Reviewer 2
Introduction:
1.The problem statement is not clearly explained/stated in the introduction. Previous works related to this work and the knowledge gap in missing. Thus, the significance of the work is not clear.
Response: Thanks for your advice. We have revised the problem statement in the introduction.
With the increasingly serious water environment pollution problem, many countries have put forward higher requirements on the nitrogen and phosphorus discharge standards [1]. Therefore, it is necessary to upgrade the existing sewage treatment process. Moreover, carbon neutrality is a key indicator towards sustainable wastewater treatment [2]. It is important to reduce energy consumption in wastewater treatment plants and develop energy-saving technology. The traditional biological nitrogen removal was achieved through nitrification and denitrification process. Recently, the partial nitrification/anammox (PN/A) process has been developed to remove nitrogen, which requires less aeration and carbon source [3]. In the PN/A process, the ammonium was first oxidized to nitrite by ammonia-oxidizing bacteria (AOB). Then the anammox bacteria converted the ammonium and nitrite to nitrogen gas. The inhibition of nitrite-oxidizing bacteria (NOB) is one of the biggest challenge in the application of PN/A process, which needs to be further investigated.
The aerobic granular sludge (AGS) process was considered to be one of the most promising wastewater treatment technologies [4]. In the past few decades, researchers have applied the AGS process to the treatment of mainstream wastewater and industrial wastewater [5]. Aerobic granule is composed of microorganisms and extracellular polymeric substances. The AGS process has the advantages of high biomass concentration, good sedimentation performance and energy saving. The AGS is composed of multiple layers, which makes it possible to enrich aerobic bacteria, facultative bacteria and anaerobic microorganisms simultaneously. Previous studies reported that the AGS reactor achieved excellent partial nitrification performance under the conditions of high temperature and high ammonium concentration [6]. As the growth rate of AOB is greater than NOB under high operating temperature (30 ℃), NOB could be washed out by controlling short sludge age [7]. For high ammonium wastewater treatment, the NOB could be severely inhibited by high free ammonia (FA) concentration, while the AOB activity was slightly suppressed [8]. In some aeras, especially northern China, the mainstream wastewater had a low temperature and low ammonium concentration. Temperature had a significant impact on the sludge activity and microbial population structure. Under low temperature, the growth and biological activity of microorganisms were seriously inhibited [1]. Meanwhile, the excessive reproduction of filamentous bacteria and poor sludge sedimentation performance were also observed [9, 10]. According to a prior research, the AGS has been cultivated at 7℃ by inoculating cold-adapt sludge [11]. When dealing with low ammonium wastewater, low dissolved oxygen (DO) and intermittent aeration strategies were proposed to suppress the NOB activity [12, 13]. Vazquez-Padin et al. (2010) reported that stable nitrite accumulation could be achieved in the AGS reactor at room temperature (24 ℃) [14]. As described by Zhang et al. (2018), the NOB activity of the AGS reactor was successfully inhibited by operating anaerocic/aerobic mode [15]. The nitrite accumulation ratio reached 97.3% when the influent NH4+-N concentration was 39.3-78.7 mg/L. So far, the cultivation of partial nitrification AGS under low trmperature and low ammonium concentration was rarely reported. Moreover, the high mass transfer resistance of granules led to low DO concentration in AGS, which was considered as the major reason for NOB inhibition [16]. However, due to the technical limitation, the earlier research mainly analyzed the DO distribution in AGS through model fitting [17, 18]. Later, the DO distribution in granules under room temperature operation was explored with microelectrode device [16, 19]. The DO distribution in AGS was closely related to the temperature and granule size[20]. Therefore, it is necessary to investigae the impact of granule size on the partial nitrification performance. So far, the integrated relationship between the granule size, DO distribution and partial nitrification performance was rarely analyzed.
The aim of this study was to cultivate AGS for mainstream wastewater treatment under low temperature. The nutrient removal performance and sludge characteristics were investigated. Batch experiments were conducted to evaluate the substrate oxidization rate and nitrite accumulation rate (NAR) of mature AGS with different particle sizes. The DO distribution in AGS was further analyzed with microelectrode device. The results could provide important guidance for the rapid start-up of AGS reactor and performing high NAR in mainstream wastewater treatment.
Materials and Methods:
- The placement of figures/tables need to checked.
Response: Thanks for your advice. We have checked the placement of figures/tables.
- Drainage ratio change to exchange ratio; inlet water change to feeding; sedimentation change settling; drainage change to decant
Response: Thanks for your advice. The term of "drainage ratio" has been revised as "exchange ratio". The term of "inlet water" has been revised as "feeding". The term of "sedimentation" has been revised as "settling". The term of "drainage" has been revised as "decant".
- Detail operation on how the settling time was reduced during phase 1 should be included. Was it step reduction? Duration for each settling time?
Response: Thanks for your advice. In stage 1, the settling time was gradually reduced from 5 min to 1 min. The duration of each settling time was illustrated in Figure 2.
- DO of 6 is considered high for PN/A. why the authors chose to maintain at this concentration?
Response: In previous studies, low DO concentration was applied to maintain stable partial nitrification. In this study, the AOB activity could also be suppressed by low temperature. In order to improve the AOB activity under low temperature condition, high DO concentration was maintained. Besides, the high air flow rate led to a higher water shear force, which was important to the fast cultivation of AGS. Because the higher water shear force might stimulate the production of EPS. Therefore, high DO concentration was maintained in the AGS reactor.
- Line 82: Characteristic of the inoculum sludge? Floccular or granular sludge?
Response: Thanks for your advice. The inoculum sludge was floc sludge. It has been mentioned in the material and methods (section 2):
The inoculated sludge was the returned sludge (floc sludge) from the secondary sedimentation tank in Gaobeidian sewage treatment plant, Beijing, China.
- Line 93: “the mass transfer performance of the substrate was enhanced by magnetic stirring” did the authors carry out studies without magnetic stirring?
Response: Thanks for your advice. In the batch experiment, the air flow was not strong enough to keep the AGS in suspension. Therefore, the magnetic stirring was operated to keep the AGS in suspension. In the AGS reactor, the air flow rate was strong and the AGS was fully suspended without using the magnetic stirring device.
Results and discussion
- Line 199: give details of R1, R2 and R3, also for other figures.
Response: The details of R1, R2 and R3 was described in section 2.3:
At the end of stage 2, mature aerobic granules were taken out from the reactor and screened to three categories: R1 (1.0-2.0 mm), R2 (2.0-3.0 mm) and R3 (larger than 3.0 mm).
- The work done in this manuscript in good. However, the depth of the study and the discussion is doubtable. The manuscript is dealing with the DO level, and it was stated that the DO is low, but DO 6 mg/L was used. Why the authors considered 6 is low. The other factor studied and stated in title is low temperature. But this is not discussion in detail in the manuscript.
Response: Thanks for your advice.
In previous studies, low DO concentration was applied to maintain stable partial nitrification. In this study, the AOB activity could also be suppressed by low temperature. In order to improve the AOB activity under low temperature condition, high DO concentration was maintained. Besides, the high air flow rate led to a higher water shear force, which was important to the fast cultivation of AGS. Because the higher water shear force might stimulate the production of EPS. Therefore, high DO concentration was maintained in the AGS reactor.
In some aeras, especially northern China, the mainstream wastewater had a low temperature and low ammonium concentration. So far, the cultivation of partial nitrification AGS under low trmperature and low ammonium concentration was rarely reported. In this study, the AGS reactor was started-up under low temperature. The integrated relationship between the granule size, DO distribution and partial nitrification performance was analyzed.
Author Response File: 
Author Response.docx
Reviewer 3 Report
In introduction, the authors must focus on what has been done in previous works and how this paper can contribute to the missing/gaps.
Please add some citations in section 2. It is crucial which papers you followed and cited to do this work.
Please do comparison with other works available in literature and this must be put in section 3.
Please be specific for this sentence in conclusion. "The particle size of AGS had a significant impact on the NH4+-N removal rate and nitritation performance." How much the significant? x time higher or percentage.
Author Response
Reviewer 3
- In introduction, the authors must focus on what has been done in previous works and how this paper can contribute to the missing/gaps.
Response: Thanks for your advice. We have revised the problem statement in the introduction.
With the increasingly serious water environment pollution problem, many countries have put forward higher requirements on the nitrogen and phosphorus discharge standards [1]. Therefore, it is necessary to upgrade the existing sewage treatment process. Moreover, carbon neutrality is a key indicator towards sustainable wastewater treatment [2]. It is important to reduce energy consumption in wastewater treatment plants and develop energy-saving technology. The traditional biological nitrogen removal was achieved through nitrification and denitrification process. Recently, the partial nitrification/anammox (PN/A) process has been developed to remove nitrogen, which requires less aeration and carbon source [3]. In the PN/A process, the ammonium was first oxidized to nitrite by ammonia-oxidizing bacteria (AOB). Then the anammox bacteria converted the ammonium and nitrite to nitrogen gas. The inhibition of nitrite-oxidizing bacteria (NOB) is one of the biggest challenge in the application of PN/A process, which needs to be further investigated.
The aerobic granular sludge (AGS) process was considered to be one of the most promising wastewater treatment technologies [4]. In the past few decades, researchers have applied the AGS process to the treatment of mainstream wastewater and industrial wastewater [5]. Aerobic granule is composed of microorganisms and extracellular polymeric substances. The AGS process has the advantages of high biomass concentration, good sedimentation performance and energy saving. The AGS is composed of multiple layers, which makes it possible to enrich aerobic bacteria, facultative bacteria and anaerobic microorganisms simultaneously. Previous studies reported that the AGS reactor achieved excellent partial nitrification performance under the conditions of high temperature and high ammonium concentration [6]. As the growth rate of AOB is greater than NOB under high operating temperature (30 ℃), NOB could be washed out by controlling short sludge age [7]. For high ammonium wastewater treatment, the NOB could be severely inhibited by high free ammonia (FA) concentration, while the AOB activity was slightly suppressed [8]. In some aeras, especially northern China, the mainstream wastewater had a low temperature and low ammonium concentration. Temperature had a significant impact on the sludge activity and microbial population structure. Under low temperature, the growth and biological activity of microorganisms were seriously inhibited [1]. Meanwhile, the excessive reproduction of filamentous bacteria and poor sludge sedimentation performance were also observed [9, 10]. According to a prior research, the AGS has been cultivated at 7℃ by inoculating cold-adapt sludge [11]. When dealing with low ammonium wastewater, low dissolved oxygen (DO) and intermittent aeration strategies were proposed to suppress the NOB activity [12, 13]. Vazquez-Padin et al. (2010) reported that stable nitrite accumulation could be achieved in the AGS reactor at room temperature (24 ℃) [14]. As described by Zhang et al. (2018), the NOB activity of the AGS reactor was successfully inhibited by operating anaerocic/aerobic mode [15]. The nitrite accumulation ratio reached 97.3% when the influent NH4+-N concentration was 39.3-78.7 mg/L. So far, the cultivation of partial nitrification AGS under low trmperature and low ammonium concentration was rarely reported. Moreover, the high mass transfer resistance of granules led to low DO concentration in AGS, which was considered as the major reason for NOB inhibition [16]. However, due to the technical limitation, the earlier research mainly analyzed the DO distribution in AGS through model fitting [17, 18]. Later, the DO distribution in granules under room temperature operation was explored with microelectrode device [16, 19]. The DO distribution in AGS was closely related to the temperature and granule size[20]. Therefore, it is necessary to investigae the impact of granule size on the partial nitrification performance. So far, the integrated relationship between the granule size, DO distribution and partial nitrification performance was rarely analyzed.
The aim of this study was to cultivate AGS for mainstream wastewater treatment under low temperature. The nutrient removal performance and sludge characteristics were investigated. Batch experiments were conducted to evaluate the substrate oxidization rate and nitrite accumulation rate (NAR) of mature AGS with different particle sizes. The DO distribution in AGS was further analyzed with microelectrode device. The results could provide important guidance for the rapid start-up of AGS reactor and performing high NAR in mainstream wastewater treatment.
- Please add some citations in section 2. It is crucial which papers you followed and cited to do this work.
Response: Thanks for your advice. We have referred some previous studies. The corresponding studies have been cited in the introduction. In section 2, the reference of the trace elements was added in the manuscript. The reference of the batch experiment procedure was added in the manuscript.
- Please do comparison with other works available in literature and this must be put in section 3.
Response: The sludge characteristics, DO distribution in AGS and the possible NO inhibition mechanism were discussed in section 3. The corresponding literature was added and discussed.
- Please be specific for this sentence in conclusion. "The particle size of AGS had a significant impact on the NH4+-N removal rate and nitritation performance." How much the significant? x time higher or percentage.
Response: The results were described in section 3.2:
After 12 h, the NH4+-N removal efficiencies of R1, R2 and R3 were 100.0%, 73.4% and 52.7%, respectively. Meanwhile, the NARs were 36.2%, 77.2% and 94.9%, respectively.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Line 40: “high temperature and high ammonium concentration” and line 44 “low temperature and low ammonium concentration” could the authors provide a more specific value or range.
Line 43: “high free ammonia (FA) concentration” could the authors provide a more specific value or range.
Line 43: “aeras”
Line 54: “trmperature”
Below is the comment and response of the authors. Would suggest the authors to include this in the manuscript for clarity.
- DO of 6 is considered high for PN/A. why the authors chose to maintain at this concentration?
Response: In previous studies, low DO concentration was applied to maintain stable partial nitrification. In this study, the AOB activity could also be suppressed by low temperature. In order to improve the AOB activity under low temperature condition, high DO concentration was maintained. Besides, the high air flow rate led to a higher water shear force, which was important to the fast cultivation of AGS. Because the higher water shear force might stimulate the production of EPS. Therefore, high DO concentration was maintained in the AGS reactor.
Below is the comment and response of the authors. Wouls suggest the authors to include the details in the title of the figures. Figures & Tables are stand alone, should have sufficient details to understand data presented in the figures/tables
- Line 199: give details of R1, R2 and R3, also for other figures.
Response: The details of R1, R2 and R3 was described in section 2.3:
Author Response
Reviewer 2
- Line 40: “high temperature and high ammonium concentration” and line 44 “low temperature and low ammonium concentration” could the authors provide a more specific value or range.
Response: Thanks for your advice. We have addressed the specific value in the manuscript as follows:
Previous studies reported that the AGS reactor achieved excellent partial nitrification performance under the conditions of high temperature (30-35 ℃) and high ammonium concentration (>500 mg/L)
- Line 43: “high free ammonia (FA) concentration” could the authors provide a more specific value or range.
Response: Thanks for your advice. We have addressed the specific value in the manuscript as follows:
For high ammonium wastewater treatment, the NOB could be severely inhibited by high free ammonia (FA) concentration (>1 mg/L), while the AOB activity was slightly suppressed
- Line 43: “aeras”
Response: Thanks for your advice. We have made the revision in the manuscript.
- Line 54: “trmperature”
Response: Thanks for your advice. We have made the revision in the manuscript.
- Below is the comment and response of the authors. Would suggest the authors to include this in the manuscript for clarity.
1）DO of 6 mg/L is considered high for PN/A. why the authors chose to maintain at this concentration?
Response: In previous studies, low DO concentration was applied to maintain stable partial nitrification. In this study, the AOB activity could also be suppressed by low temperature. In order to improve the AOB activity under low temperature condition, high DO concentration was maintained. Besides, the high air flow rate led to a higher water shear force, which was important to the fast cultivation of AGS. Because the higher water shear force might stimulate the production of EPS. Therefore, high DO concentration was maintained in the AGS reactor.
Response: Thanks for your advice. The explanation is added to section 3.1 as folloes:
In this study, the AOB activity would be greatly suppressed by low temperature. In order to improve the AOB activity, high DO concentration (6 mg/L) was maintained. Besides, the high air flow rate led to a higher water shear force, which was important to the fast cultivation of AGS. This was because the higher water shear force could stimulate the production of extracellular polymeric substances.
- Below is the comment and response of the authors. Would suggest the authors to include the details in the title of the figures. Figures & Tables are stand alone, should have sufficient details to understand data presented in the figures/tables
Line 199: give details of R1, R2 and R3, also for other figures.
Response: Revision has been made in the manuscript.
Figure 4. Performance of R1 (1.0-2.0 mm), R2 (2.0-3.0 mm) and R3 (larger than 3.0 mm) granules in batch test.
Figure 5. DO distribution in R1 (1.0-2.0 mm), R2 (2.0-3.0 mm) and R3 (larger than 3.0 mm) granules.
Author Response File: Author Response.docx
