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Characteristics of aerobic granule and nitrogen and phosphorus removal in a SBR
Journal of Hazardous Materials 164 (2009) 1223–1227
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Characteristics of aerobic granule and nitrogen and phosphorus removal in a SBR Fen Wang, Shan Lu, Yanjie Wei, Min Ji ∗ School of Environmental Science and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 8 January 2008 Received in revised form 9 September 2008 Accepted 9 September 2008 Available online 17 September 2008 Keywords: Aerobic granular sludge Domestic wastewater Simultaneous nitrification and denitrification
a b s t r a c t The performance of a sequencing batch reactor (SBR) seeded with aerobic granular sludge was studied. The lab-scale SBR treating domestic wastewater operated at a volumetric loading rate (VLR) of 0.75–3.41 kg COD/(m3 d). The granule stability was related to the organic loading, and high loading would be favorable for granule stability. Analysis of typical cycle showed that granular sludge had good ability to simultaneously remove nitrogen and phosphorus. Most organic substances were removed at the anaerobic stage. At the aerobic stage, simultaneous nitrification and denitrification (SND) happened with phosphorus absorption. The SBR had good removal performance for organic matter and phosphate. However, the total nitrogen (TN) removal performance was ordinary, with average removal efficiency of about 52%. Batch experiments indicated that increases of influent C/N ratio and a large percentage of granule in the sludge were conducive for SND in SBR. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aerobic granular sludge technology has been widely reported in sequencing batch reactors (SBRs) [1,2]. It has the characteristics of both suspended activated sludge and biofilm. On one hand, it is evolved from flocculating sludge and is resulting in the formation of suspended microbial aggregates without any support media; On the other hand, it is similar to biofilm in mass transferring, with aerobic zone, anoxic zone and anaerobic zone along the direction of mass transfer [3], which will provide favorable environment for growth of facultative and aerobic bacteria, such as ammonia oxidizing bacteria, denitrifying phosphateaccumulating bacteria (DPB), denitrifying glycogen-accumulating bacteria, and phosphate-accumulating organisms (PAOs). Therefore, aerobic granule can remove carbon, nitrogen and phosphorus simultaneously. Recently, researchers have focused on characteristics description, formation identification and application of aerobic granule in SBR. Most researches apply synthetic wastewater (mainly acetate and glucose as carbon source), and researches applying real wastewater are few. The reported literatures include the following: dairy wastewater [4], abattoir wastewater [5], domestic wastewater [6], soybean-processing wastewater [7], brewery wastewater [8], and paper-making wastewater [9]. In this work, aerobic granular sludge cultivated using synthetic wastewater (sodium acetate as carbon source) was used to treat real domestic wastewater. Stability and characteristics of granular
∗ Corresponding author. Tel.: +86 22 27406057; fax: +86 22 27406057. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Ji). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.09.034
sludge were observed. Removal performance and influence on SND were also explored. 2. Materials and methods 2.1. SBR The SBR with an effective volume of 8 L was used. Dimensions of the unit were: inner diameter of 15 cm, height of 48 cm. The height to the diameter ratio (H/D) was 3.2. The SBR was operated at a cycle of 6 h, including 5 min of feeding, 90 min of anaerobic stirring, 240 min of aerobic reaction, 5 min of sludge settling, 10 min of effluent discharge, 10 min of idling. The volumetric exchange ratio was 70%, sludge retention time (SRT) was 20–40 d, and dissolved oxygen (DO) was 2–3 mg/L. 2.2. Characteristics of domestic wastewater and seeding granular sludge At the beginning, the synthetic wastewater was fed to the SBR, and the main components were as follows: sodium acetate of 400 mg/L, NH4 Cl-N of 35 mg/L, KH2 PO4 -P of 3.8 mg/L, MgSO4 ·7H2 O of 50 mg/L, CaCl2 of 20 mg/L, KCl of 20 mg/L. From cycle 33, real domestic wastewater from campus of Tianjin University was fed to the SBR gradually, and domestic wastewater quality was as shown in Table 1. The seeding sludge was granular sludge cultivated using synthetic wastewater [10]. It was black, looked like fine grit, and particle size was 0.8–1.5 mm. Observed from microscope, it was globular or ellipsoidal in shape, its structure was compact, and
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F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Table 1 Characteristics of domestic wastewater (mg/L). Constituent
COD
BOD5
TOC
Phosphate
NH3 -N
TN
SS
Range Average
272–1423 723
71–424 208
31–198 93
1–9 7
13–107 71
43–147 99
105–827 377
outline was clear. The settling performance was good, and SVI5 was 20–40. Mixed liquor solid concentration (MLSS) was about 3000 mg/L. 2.3. Operation strategy During the experiment, SBR operated for 705 cycles, including cycles 1–32, 40% domestic wastewater + 60% synthetic wastewater, cycles 33–132, 60% domestic wastewater + 40% tap water, cycles 133–705, 100% domestic wastewater. The period from cycle 1 to cycle 132 was called acclimatization period and the one from cycle 133 to cycle 705 was called normal running period. From cycle 649 to cycle 680, shock loading effect on SBR was investigated, and influent COD increased to about 1000 mg/L by adding sodium acetate.
size influenced biological activity of sludge. It was observed that MLVSS/MLSS ratio increased from 69% to 88% with the increase of granule size (Table 2). It could be seen from Table 2, in the range of granule particle size of 0.18–2.0 mm, MLVSS/MLSS ratio increased with particle size increasing. The result was quite similar to that of Toh [13]. It meant that when dry gravity was equal, because MLVSS/MLSS ratio in the granule was higher than the one in the flocs, granule would obtain higher biological activity than flocs. Laser particle sizer was used to determine sludge size. As for seeding granule, its particle size distributed in a narrow range with a mean size of 1114.42 m (as shown in Fig. 1a) [10]. At cycle 261, the sludge size distribution was shown in Fig. 1b. Compared with the granule using synthetic wastewater, the size distributed in a wider range with a mean size of 948.36 m, which was less than the one cultivated using synthetic wastewater. In cycle 445, a part of
2.4. Analytical methods COD, TOC, BOD5 , NH3 -N, TN, TP, MLSS, MLVSS were tested according to the methods stated in Standards Methods for the Examination of Water and Wastewater [11]. Sludge particle size distribution was determined by laser particle sizer (Malvern Master Sizer) and granulometry procedure [12]. 3. Results and discussions 3.1. Granular sludge stability At the beginning (cycles 1–80), the appearance of granule was similar to the seeding ones. It was black, looked like fine grit, and particle size was 0.8–1.5 mm. Observed from microscope, it was globular or ellipsoidal in shape, its structure was dense. After cycle 80, it became yellow, and it was spherical or elliptical in shape, its structure was dense, and particle size of granule increased after the influent was domestic wastewater completely. After cycle 360, COD volumetric loading rate decreased, a part of granules disintegrated, flocs increased, and both granules and flocs existed in the SBR. The ratio of MLVSS to MLSS described percentage of biomass in the sludge approximately. For the sludge in conventional activated sludge process treating domestic wastewater, MLVSS/MLSS ratio was about 75%. The MLVSS/MLSS ratio of granule cultivated using synthetic wastewater was 86%, it decreased to 85% at cycle 377, and it decreased to 81% at the end of the experiment. The reason for the decrease was that SS in the domestic wastewater was high, which would add inorganic substances in the sludge. However, the granule still had better biological activity than flocs. The granule Table 2 MLVSS/MLSS ratio of different particle size. Sludge particle size interval (mm)
MLVSS/MLSS (%)
<0.18 0.18–0.45 0.45–0.6 0.6–0.9 0.9–1.25 1.25–1.6 1.6–2 >2
69 80 82 86 87 87 88 88
Fig. 1. Particle size distribution of granule. (a) Size distribution of seeding granule. (b) Granule size distribution of cycle 261. (c) Granule size distribution of cycle 445.
F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Fig. 2. Effect of COD volumetric loading on sludge concentration.
granule disintegrated, and granule size distribution was shown in Fig. 1c. Because a part of granule disintegrated, the particle size distribution range was wider and mean size decreased to 617.29 m. After acclimatization, the granule existed stably for about 3 months, and then a part of granule disintegrated. It was assumed that influent organic loading was important for granule stability. The effect of influent COD volumetric loading on sludge concentration was shown in Fig. 2. When the SBR operated at high loading, COD volumetric loading was about 2.0–3.5 kg/(m3 d), sludge concentration was 5000–7000 mg/L, granular sludge concentration increased from 3000 to 5000 mg/L, and percentage of granule increased from 70% to 85%. When the SBR operated at low loading, COD volumetric loading was lower than 2.0 kg/(m3 d), sludge concentration was 2500–4000 mg/L, and sludge concentration decreased to 1500–3000 mg/L, and percentage of granule in the sludge decreased to 50%. From the above, it was inferred that high influent COD volumetric loading rate was favorable for granule stability, increasing of sludge concentration and sludge particle size. 3.2. COD, nitrogen and phosphorus removal performance The SBR operated for 6 months and 705 cycles in total, including cycles 1–132 of acclimatization period and cycles 133–705 of normal running period. The removal performances of COD, TOC, NH3 -N, TN and phosphate were investigated. COD, TN and phosphate in the effluent were shown in Fig. 3. The following would explain the removal performance.
Fig. 3. COD, TN and phosphate in the effluent.
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At the beginning, COD removal efficiency was 80%. From cycles 133–417, the influent COD ranged from 516 to 1423 mg/L, and COD removal efficiency was 73–91%, and average removal efficiency was 83%. From cycle 418 to cycle 645, influent COD was low, which was only 273–490 mg/L, and removal efficiency was 64–89%. The BOD5 /COD ratio in the effluent was 0.02–0.21, and COD in the effluent was mainly composed of difficult-biodegradable organic matter. Influent TOC fluctuated much (as shown in Table 1), however, TOC in the effluent was less than 30 mg/L. During influent shock loading period, SBR could adapt itself to high loading, and good removal performance was obtained. But when loading decreased again, it had bad impact on TOC removal efficiency. Similarly, each decrease of influent TOC was correlated with the decrease of TOC removal efficiency, which indicated that the SBR was bad at the adaptation of low loading, and was more suitable for running at high loading. Simultaneous nitrogen and phosphorus removal in granular SBR was based on carbon circulation, and accomplished by metabolism of intracellular storage substances. The TOC removal efficiency at the anaerobic stage was important for simultaneous nitrogen and phosphorus removal performance. At the beginning (cycles 1–13), TOC removal efficiency at the anaerobic stage was close to the SBR fed with synthetic wastewater, and it reached 70%. After that in acclimatization period, TOC removal efficiency at the anaerobic stage decreased to about 50%. In normal running period, TOC removal efficiency at the anaerobic stage was about 60% and most of TOC was removed at the anaerobic stage. It was similar to other nitrogen and phosphorus removal researches [5]. TOC was mainly removed during the anaerobic stage, and microorganisms stored TOC as poly--hydroxybutyrate (PHB) under anaerobic condition for later use during the aerobic stage [14]. For phosphorus removal, normal running period was divided into two parts, good performance stage for cycles 133–397 and bad performance stage for cycles 417–645. During good performance stage, phosphate in the influent was 3–10 mg/L, phosphate in the effluent was lower than 0.50 mg/L, and average removal efficiency was more than 95%. During bad performance stage, from cycle 417 then on, phosphorus removal performance of SBR deteriorated quickly, and ineffective phosphorus releasing happened. To solve the problem, the sludge was taken out and put in the clean water for a day to decrease phosphorus content in the sludge. At cycle 645, the phosphorus removal efficiency recovered as 69%. During shock loading experiment, organic substances in the influent was high, and removal efficiency reached 75–96%. However, after loading decreased, removal efficiency decreased to about 60%. Ammonia nitrogen in the influent was high, however, removal performance of NH3 -N was good. Influent NH3 -N was 13–107 mg/L, and average removal efficiency was 92%, which indicated that the granular SBR had good performance at nitrification. The reason for good performance was that most of the organic substances had been removed at the anaerobic stage, and autotrophic bacteria could grow easily at the aerobic stage, which resulted in good removal performance of NH3 -N. TN in the influent was 43.06–146.67 mg/L and average removal efficiency was 47%. Removal performance for TN was ordinary. The reason was that C/N ratio in the influent was too low, TOC/TN was only 0.42–2.26, which was lower than theoretical value of 2.86 [15]. TN removal capacity under current condition had reached the limits, if TN removal performance wanted to be improved, carbon source should be added to the SBR at the aerobic stage. Time profiles of TOC, phosphate and nitrogen at cycle 245 were shown in Fig. 4. From Fig. 4, it could be seen that TOC was removed during the anaerobic stage, with removal efficiency of 61.30%. Phosphate was released at the anaerobic stage, and anaerobic phosphate
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F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Fig. 4. TOC, phosphate, NH3 -N and TN removal profiles at typical cycle (cycle 245).
release quantity was 43 mg/L. SND and phosphate absorption happened at the aerobic stage. Phosphate absorption finished within 1 h, absorption rate reached 5.24 mg PO4 3− -P/(L h), and phosphate concentration at the end of the aerobic stage was only 0.10 mg/L. NH3 -N was mainly removed at the aerobic stage. With the absorption of phosphate at the aerobic stage, SND happened, and TN removal efficiency at the aerobic stage reached 51%. 3.3. Analysis of nitrogen removal For the nitrogen removal performance was ordinary, the granule of normal running period at the end of cycle were gained to carry out aerobic SND experiment to find the reasons. Experiment parameters were: aeration time was 5 h, sludge concentration was about 4000 mg/L, DO was 2.0–3.0 mg/L. The effects of C/N ratio, organic loading and percent of granule on SND were observed. To justify the effect of C/N ratio on SND, batch experiment described as following was carried out. Sodium acetate as carbon source in the influent was controlled at 150 mg/L, NH4 Cl-N was added to change C/N ratio (TOC/N) at 2.0 and 3.0. The results were shown in Table 3. With the decrease of C/N ratio, nitrification rate of granule increased, while nitrification efficiency decreased. It showed that sudden increase of NH3 -N loading had shock effect on nitrification efficiency. Nitrification rate increased with the increase of NH3 -N concentration in the influent. TN removal efficiency at the aerobic stage went up with the increase of C/N ratio, while denitrification rate decreased. There were two reasons for decrease of TN removal efficiency. One was that C/N ratio decreased and carbon source for denitrification was lack, which resulted in incomplete denitrification, NO3 -N in the effluent was high and TN increased. The other was that NH3 -N in the influent went up, nitrification was not complete, which resulted in NH3 -N in the influent increased and TN increased. To explore the effect of organic loading on SND, batch experiment described as following was carried out. NH3 -N in the influent
Fig. 5. Effect of organic loading on SND.
was controlled at about 100 mg/L, sodium acetate was added to increase TOC and organic loading of the influent. Running parameters were as same as SBR. Experiment results were shown in Fig. 5. As indicated in Fig. 5, removal efficiency of NH3 -N was more than 90% as influent organic loading increasing. Removal performance of TOC and TN improved as organic loading increasing. TOC removal efficiency improved from 71% to 87%, and TN removal efficiency improved from 31% to 61%. The improvement was owed to the increase of organic loading, which would add carbon source for denitrification. The influence of organic loading on SND also indicated that the granular SBR was more suitable to run at high organic loading.
Table 3 Characteristic of SND for granule at different C/N ratios. Item +
Nitrification rate (mg NH4 -N/(L h)) Nitrification efficiency (%) Denitrification rate (mg NOx − -N/(L h)) TN removal efficiency (%) SND efficiencya (%)
TOC/N = 2.0
TOC/N = 3.0
12.82 90.11 6.51 45.76 50.78
9.37 97.86 5.74 59.91 61.22
a Note: SND efficiency = (NH3 -N removal quantity at the aerobic stageaccumulated NOx − -N at the end of aerobic stage)/NH3 -N removal quantity at the aerobic stage.
Fig. 6. Effect of different sludges on SND.
F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
To investigate the effect of granule on SND, the sludge at the end of cycle was gained from the reactor, and it was divided into two parts. One part was undisturbed, and was mixture of granules and flocs, in which, granule with particle size of more than 0.45 mm was about 50%. While the other part was sieved by 0.45 mm screen and only the granule with particle size of more than 0.45 mm was obtained. Two parts of sludges were put into batch reactors with effective volume of 1.0 L, the same domestic wastewater was treated, and operation parameters were as same as SBR. Sludge concentration was controlled at about 4000 mg/L. Results were shown in Fig. 6. As shown in Fig. 6, the increase of granule percent in the sludge could increase SND. The granule with particle size of more than 0.45 mm almost had the same NH3 -N removal efficiency (more than 98%) as the mixed sludge. However, compared with the mixed sludge, the granule with particle size of more than 0.45 mm could improve the TOC removal efficiency from 68% to 86%, improve TN removal efficiency from 53% to 75%, and SND efficiency from 30% to 68%. There was no obvious NO2 -N and NO3 -N accumulation in granular SBR. Therefore, to obtain efficient and stable nitrogen removal performance, granule should dominate in the sludge. 4. Conclusions (1) High influent COD volumetric loading is favorable for granule stability and increase of sludge concentration and particle size. In addition, high organic loading is conducive for SND. Therefore, granule SBR is more suitable for operation at high organic loading. (2) The granule SBR treating real domestic wastewater operated for 6 months. Although COD, NH3 -N and TP in the influent fluctuates much, the SBR maintains stable removal performances for COD, TOC, phosphate, NH3 -N and TN, and average removal efficiencies are 80%, 70%, 71%, 92%, and 47%, respectively. The SBR has a certain shock loading resistance ability. (3) Analysis of typical cycle shows that granule has good ability of simultaneous nitrogen and phosphorus removal performance. Most of organic substances are absorbed and phosphate releasing happens at the anaerobic stage. At the aerobic stage, SND happens with phosphorus absorption. The phosphate concen-
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tration at the end of the aerobic stage is only 0.10 mg/L, and aerobic removal efficiency of NH3 -N and TN is 95% and 51%, respectively. (4) Increases of influent C/N ratio and a large percentage of granule in the sludge are favorable for SND of SBR. References [1] E. Morgenroth, T. Sherden, M.C.M. van Loosdrecht, J.J. Heijnen, P.A. Wilderer, Aerobic granular sludge in a sequencing batch reactor, Water Res. 31 (1997) 3191–3194. [2] J.J. Beun, M.C.M. van Loosdrecht, J.J. Heijnen, Aerobic granulation in a sequencing batch airlift reactor, Water Res. 36 (2002) 702–712. [3] M.K. de Kreuk, J.J. Heijnen, M.C.M. van Loosdrecht, Simultaneous COD, nitrogen, and phosphate removal by aerobic granular sludge, Biotechnol. Bioeng. 90 (2005) 761–769. [4] B. Arrojo, A. Mosquera-Corral, J.M. Garrido, et al., Aerobic granulation with industrial wastewater in sequencing batch reactors, Water Res. 38 (2004) 3389–3399. [5] D.P. Cassidy, E. Belia, Nitrogen and phosphorus removal from an abattoir wastewater in a SBR with aerobic granular sludge, Water Res. 39 (2005) 4817–4823. [6] M.K. De Kreuk, M.C.M. van Loosdrecht, Formation of aerobic granules with domestic sewage, J. Environ. Eng. 132 (2006) 694–697. [7] K.Z. Su, H.Q. Yu, Formation and characterization of aerobic granules in a sequencing batch reactor treating soybean-processing wastewater, Environ. Sci. Technol. 39 (2005) 2818–2827. [8] S.G. Wang, X.W. Liu, W.X. Gong, et al., Aerobic granulation with brewery wastewater in a sequencing batch reactor, Bioresour. Technol. 98 (2007) 2142–2147. [9] H.L. Wang, G.L. Yu, G.S. Liu, F. Pan, A new way to cultivate aerobic granules in the process of papermaking wastewater treatment, Biochem. Eng. J. 28 (2006) 99–103. [10] J.F. Wang, Nitrogen and Phosphorous Removal of Aerobic Granules and Granules Membrane Bioreactor (in Chinese), Dissertation, Tianjin University, Tianjin, 2006. [11] APHA, Standards Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Assoc., Washington, DC, USA, 1998. [12] A. Laguna, A. Ouattara, R.O. Gonzalez, et al., A simple and low cost technique for determining the granulometry of upflow anaerobic sludge blanket reactor sludge, Water Sci. Technol. 40 (1999) 1–8. [13] B.Y.P. Moy, J.H. Tay, S.K. Toh, et al., High organic loading influences the physical characteristics of aerobic sludge granules, Lett. Appl. Microbiol. 34 (2002) 407–412. [14] X. Wang, et al., Anaerobic fast-absorption of organics under alternatively anaerobic-aerobic conditions (in Chinese), J. Agro-Environ. Sci. 24 (2005) 322–327. [15] J.Y. Sun, Treatment Technology and Application for Nitrogen Removal in Wastewater, Chemical Industry Press, Beijing, 2003 (in Chinese).
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Characteristics of aerobic granule and nitrogen and phosphorus removal in a SBR Fen Wang, Shan Lu, Yanjie Wei, Min Ji ∗ School of Environmental Science and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 8 January 2008 Received in revised form 9 September 2008 Accepted 9 September 2008 Available online 17 September 2008 Keywords: Aerobic granular sludge Domestic wastewater Simultaneous nitrification and denitrification
a b s t r a c t The performance of a sequencing batch reactor (SBR) seeded with aerobic granular sludge was studied. The lab-scale SBR treating domestic wastewater operated at a volumetric loading rate (VLR) of 0.75–3.41 kg COD/(m3 d). The granule stability was related to the organic loading, and high loading would be favorable for granule stability. Analysis of typical cycle showed that granular sludge had good ability to simultaneously remove nitrogen and phosphorus. Most organic substances were removed at the anaerobic stage. At the aerobic stage, simultaneous nitrification and denitrification (SND) happened with phosphorus absorption. The SBR had good removal performance for organic matter and phosphate. However, the total nitrogen (TN) removal performance was ordinary, with average removal efficiency of about 52%. Batch experiments indicated that increases of influent C/N ratio and a large percentage of granule in the sludge were conducive for SND in SBR. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aerobic granular sludge technology has been widely reported in sequencing batch reactors (SBRs) [1,2]. It has the characteristics of both suspended activated sludge and biofilm. On one hand, it is evolved from flocculating sludge and is resulting in the formation of suspended microbial aggregates without any support media; On the other hand, it is similar to biofilm in mass transferring, with aerobic zone, anoxic zone and anaerobic zone along the direction of mass transfer [3], which will provide favorable environment for growth of facultative and aerobic bacteria, such as ammonia oxidizing bacteria, denitrifying phosphateaccumulating bacteria (DPB), denitrifying glycogen-accumulating bacteria, and phosphate-accumulating organisms (PAOs). Therefore, aerobic granule can remove carbon, nitrogen and phosphorus simultaneously. Recently, researchers have focused on characteristics description, formation identification and application of aerobic granule in SBR. Most researches apply synthetic wastewater (mainly acetate and glucose as carbon source), and researches applying real wastewater are few. The reported literatures include the following: dairy wastewater [4], abattoir wastewater [5], domestic wastewater [6], soybean-processing wastewater [7], brewery wastewater [8], and paper-making wastewater [9]. In this work, aerobic granular sludge cultivated using synthetic wastewater (sodium acetate as carbon source) was used to treat real domestic wastewater. Stability and characteristics of granular
∗ Corresponding author. Tel.: +86 22 27406057; fax: +86 22 27406057. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Ji). 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.09.034
sludge were observed. Removal performance and influence on SND were also explored. 2. Materials and methods 2.1. SBR The SBR with an effective volume of 8 L was used. Dimensions of the unit were: inner diameter of 15 cm, height of 48 cm. The height to the diameter ratio (H/D) was 3.2. The SBR was operated at a cycle of 6 h, including 5 min of feeding, 90 min of anaerobic stirring, 240 min of aerobic reaction, 5 min of sludge settling, 10 min of effluent discharge, 10 min of idling. The volumetric exchange ratio was 70%, sludge retention time (SRT) was 20–40 d, and dissolved oxygen (DO) was 2–3 mg/L. 2.2. Characteristics of domestic wastewater and seeding granular sludge At the beginning, the synthetic wastewater was fed to the SBR, and the main components were as follows: sodium acetate of 400 mg/L, NH4 Cl-N of 35 mg/L, KH2 PO4 -P of 3.8 mg/L, MgSO4 ·7H2 O of 50 mg/L, CaCl2 of 20 mg/L, KCl of 20 mg/L. From cycle 33, real domestic wastewater from campus of Tianjin University was fed to the SBR gradually, and domestic wastewater quality was as shown in Table 1. The seeding sludge was granular sludge cultivated using synthetic wastewater [10]. It was black, looked like fine grit, and particle size was 0.8–1.5 mm. Observed from microscope, it was globular or ellipsoidal in shape, its structure was compact, and
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F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Table 1 Characteristics of domestic wastewater (mg/L). Constituent
COD
BOD5
TOC
Phosphate
NH3 -N
TN
SS
Range Average
272–1423 723
71–424 208
31–198 93
1–9 7
13–107 71
43–147 99
105–827 377
outline was clear. The settling performance was good, and SVI5 was 20–40. Mixed liquor solid concentration (MLSS) was about 3000 mg/L. 2.3. Operation strategy During the experiment, SBR operated for 705 cycles, including cycles 1–32, 40% domestic wastewater + 60% synthetic wastewater, cycles 33–132, 60% domestic wastewater + 40% tap water, cycles 133–705, 100% domestic wastewater. The period from cycle 1 to cycle 132 was called acclimatization period and the one from cycle 133 to cycle 705 was called normal running period. From cycle 649 to cycle 680, shock loading effect on SBR was investigated, and influent COD increased to about 1000 mg/L by adding sodium acetate.
size influenced biological activity of sludge. It was observed that MLVSS/MLSS ratio increased from 69% to 88% with the increase of granule size (Table 2). It could be seen from Table 2, in the range of granule particle size of 0.18–2.0 mm, MLVSS/MLSS ratio increased with particle size increasing. The result was quite similar to that of Toh [13]. It meant that when dry gravity was equal, because MLVSS/MLSS ratio in the granule was higher than the one in the flocs, granule would obtain higher biological activity than flocs. Laser particle sizer was used to determine sludge size. As for seeding granule, its particle size distributed in a narrow range with a mean size of 1114.42 m (as shown in Fig. 1a) [10]. At cycle 261, the sludge size distribution was shown in Fig. 1b. Compared with the granule using synthetic wastewater, the size distributed in a wider range with a mean size of 948.36 m, which was less than the one cultivated using synthetic wastewater. In cycle 445, a part of
2.4. Analytical methods COD, TOC, BOD5 , NH3 -N, TN, TP, MLSS, MLVSS were tested according to the methods stated in Standards Methods for the Examination of Water and Wastewater [11]. Sludge particle size distribution was determined by laser particle sizer (Malvern Master Sizer) and granulometry procedure [12]. 3. Results and discussions 3.1. Granular sludge stability At the beginning (cycles 1–80), the appearance of granule was similar to the seeding ones. It was black, looked like fine grit, and particle size was 0.8–1.5 mm. Observed from microscope, it was globular or ellipsoidal in shape, its structure was dense. After cycle 80, it became yellow, and it was spherical or elliptical in shape, its structure was dense, and particle size of granule increased after the influent was domestic wastewater completely. After cycle 360, COD volumetric loading rate decreased, a part of granules disintegrated, flocs increased, and both granules and flocs existed in the SBR. The ratio of MLVSS to MLSS described percentage of biomass in the sludge approximately. For the sludge in conventional activated sludge process treating domestic wastewater, MLVSS/MLSS ratio was about 75%. The MLVSS/MLSS ratio of granule cultivated using synthetic wastewater was 86%, it decreased to 85% at cycle 377, and it decreased to 81% at the end of the experiment. The reason for the decrease was that SS in the domestic wastewater was high, which would add inorganic substances in the sludge. However, the granule still had better biological activity than flocs. The granule Table 2 MLVSS/MLSS ratio of different particle size. Sludge particle size interval (mm)
MLVSS/MLSS (%)
<0.18 0.18–0.45 0.45–0.6 0.6–0.9 0.9–1.25 1.25–1.6 1.6–2 >2
69 80 82 86 87 87 88 88
Fig. 1. Particle size distribution of granule. (a) Size distribution of seeding granule. (b) Granule size distribution of cycle 261. (c) Granule size distribution of cycle 445.
F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Fig. 2. Effect of COD volumetric loading on sludge concentration.
granule disintegrated, and granule size distribution was shown in Fig. 1c. Because a part of granule disintegrated, the particle size distribution range was wider and mean size decreased to 617.29 m. After acclimatization, the granule existed stably for about 3 months, and then a part of granule disintegrated. It was assumed that influent organic loading was important for granule stability. The effect of influent COD volumetric loading on sludge concentration was shown in Fig. 2. When the SBR operated at high loading, COD volumetric loading was about 2.0–3.5 kg/(m3 d), sludge concentration was 5000–7000 mg/L, granular sludge concentration increased from 3000 to 5000 mg/L, and percentage of granule increased from 70% to 85%. When the SBR operated at low loading, COD volumetric loading was lower than 2.0 kg/(m3 d), sludge concentration was 2500–4000 mg/L, and sludge concentration decreased to 1500–3000 mg/L, and percentage of granule in the sludge decreased to 50%. From the above, it was inferred that high influent COD volumetric loading rate was favorable for granule stability, increasing of sludge concentration and sludge particle size. 3.2. COD, nitrogen and phosphorus removal performance The SBR operated for 6 months and 705 cycles in total, including cycles 1–132 of acclimatization period and cycles 133–705 of normal running period. The removal performances of COD, TOC, NH3 -N, TN and phosphate were investigated. COD, TN and phosphate in the effluent were shown in Fig. 3. The following would explain the removal performance.
Fig. 3. COD, TN and phosphate in the effluent.
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At the beginning, COD removal efficiency was 80%. From cycles 133–417, the influent COD ranged from 516 to 1423 mg/L, and COD removal efficiency was 73–91%, and average removal efficiency was 83%. From cycle 418 to cycle 645, influent COD was low, which was only 273–490 mg/L, and removal efficiency was 64–89%. The BOD5 /COD ratio in the effluent was 0.02–0.21, and COD in the effluent was mainly composed of difficult-biodegradable organic matter. Influent TOC fluctuated much (as shown in Table 1), however, TOC in the effluent was less than 30 mg/L. During influent shock loading period, SBR could adapt itself to high loading, and good removal performance was obtained. But when loading decreased again, it had bad impact on TOC removal efficiency. Similarly, each decrease of influent TOC was correlated with the decrease of TOC removal efficiency, which indicated that the SBR was bad at the adaptation of low loading, and was more suitable for running at high loading. Simultaneous nitrogen and phosphorus removal in granular SBR was based on carbon circulation, and accomplished by metabolism of intracellular storage substances. The TOC removal efficiency at the anaerobic stage was important for simultaneous nitrogen and phosphorus removal performance. At the beginning (cycles 1–13), TOC removal efficiency at the anaerobic stage was close to the SBR fed with synthetic wastewater, and it reached 70%. After that in acclimatization period, TOC removal efficiency at the anaerobic stage decreased to about 50%. In normal running period, TOC removal efficiency at the anaerobic stage was about 60% and most of TOC was removed at the anaerobic stage. It was similar to other nitrogen and phosphorus removal researches [5]. TOC was mainly removed during the anaerobic stage, and microorganisms stored TOC as poly--hydroxybutyrate (PHB) under anaerobic condition for later use during the aerobic stage [14]. For phosphorus removal, normal running period was divided into two parts, good performance stage for cycles 133–397 and bad performance stage for cycles 417–645. During good performance stage, phosphate in the influent was 3–10 mg/L, phosphate in the effluent was lower than 0.50 mg/L, and average removal efficiency was more than 95%. During bad performance stage, from cycle 417 then on, phosphorus removal performance of SBR deteriorated quickly, and ineffective phosphorus releasing happened. To solve the problem, the sludge was taken out and put in the clean water for a day to decrease phosphorus content in the sludge. At cycle 645, the phosphorus removal efficiency recovered as 69%. During shock loading experiment, organic substances in the influent was high, and removal efficiency reached 75–96%. However, after loading decreased, removal efficiency decreased to about 60%. Ammonia nitrogen in the influent was high, however, removal performance of NH3 -N was good. Influent NH3 -N was 13–107 mg/L, and average removal efficiency was 92%, which indicated that the granular SBR had good performance at nitrification. The reason for good performance was that most of the organic substances had been removed at the anaerobic stage, and autotrophic bacteria could grow easily at the aerobic stage, which resulted in good removal performance of NH3 -N. TN in the influent was 43.06–146.67 mg/L and average removal efficiency was 47%. Removal performance for TN was ordinary. The reason was that C/N ratio in the influent was too low, TOC/TN was only 0.42–2.26, which was lower than theoretical value of 2.86 [15]. TN removal capacity under current condition had reached the limits, if TN removal performance wanted to be improved, carbon source should be added to the SBR at the aerobic stage. Time profiles of TOC, phosphate and nitrogen at cycle 245 were shown in Fig. 4. From Fig. 4, it could be seen that TOC was removed during the anaerobic stage, with removal efficiency of 61.30%. Phosphate was released at the anaerobic stage, and anaerobic phosphate
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F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
Fig. 4. TOC, phosphate, NH3 -N and TN removal profiles at typical cycle (cycle 245).
release quantity was 43 mg/L. SND and phosphate absorption happened at the aerobic stage. Phosphate absorption finished within 1 h, absorption rate reached 5.24 mg PO4 3− -P/(L h), and phosphate concentration at the end of the aerobic stage was only 0.10 mg/L. NH3 -N was mainly removed at the aerobic stage. With the absorption of phosphate at the aerobic stage, SND happened, and TN removal efficiency at the aerobic stage reached 51%. 3.3. Analysis of nitrogen removal For the nitrogen removal performance was ordinary, the granule of normal running period at the end of cycle were gained to carry out aerobic SND experiment to find the reasons. Experiment parameters were: aeration time was 5 h, sludge concentration was about 4000 mg/L, DO was 2.0–3.0 mg/L. The effects of C/N ratio, organic loading and percent of granule on SND were observed. To justify the effect of C/N ratio on SND, batch experiment described as following was carried out. Sodium acetate as carbon source in the influent was controlled at 150 mg/L, NH4 Cl-N was added to change C/N ratio (TOC/N) at 2.0 and 3.0. The results were shown in Table 3. With the decrease of C/N ratio, nitrification rate of granule increased, while nitrification efficiency decreased. It showed that sudden increase of NH3 -N loading had shock effect on nitrification efficiency. Nitrification rate increased with the increase of NH3 -N concentration in the influent. TN removal efficiency at the aerobic stage went up with the increase of C/N ratio, while denitrification rate decreased. There were two reasons for decrease of TN removal efficiency. One was that C/N ratio decreased and carbon source for denitrification was lack, which resulted in incomplete denitrification, NO3 -N in the effluent was high and TN increased. The other was that NH3 -N in the influent went up, nitrification was not complete, which resulted in NH3 -N in the influent increased and TN increased. To explore the effect of organic loading on SND, batch experiment described as following was carried out. NH3 -N in the influent
Fig. 5. Effect of organic loading on SND.
was controlled at about 100 mg/L, sodium acetate was added to increase TOC and organic loading of the influent. Running parameters were as same as SBR. Experiment results were shown in Fig. 5. As indicated in Fig. 5, removal efficiency of NH3 -N was more than 90% as influent organic loading increasing. Removal performance of TOC and TN improved as organic loading increasing. TOC removal efficiency improved from 71% to 87%, and TN removal efficiency improved from 31% to 61%. The improvement was owed to the increase of organic loading, which would add carbon source for denitrification. The influence of organic loading on SND also indicated that the granular SBR was more suitable to run at high organic loading.
Table 3 Characteristic of SND for granule at different C/N ratios. Item +
Nitrification rate (mg NH4 -N/(L h)) Nitrification efficiency (%) Denitrification rate (mg NOx − -N/(L h)) TN removal efficiency (%) SND efficiencya (%)
TOC/N = 2.0
TOC/N = 3.0
12.82 90.11 6.51 45.76 50.78
9.37 97.86 5.74 59.91 61.22
a Note: SND efficiency = (NH3 -N removal quantity at the aerobic stageaccumulated NOx − -N at the end of aerobic stage)/NH3 -N removal quantity at the aerobic stage.
Fig. 6. Effect of different sludges on SND.
F. Wang et al. / Journal of Hazardous Materials 164 (2009) 1223–1227
To investigate the effect of granule on SND, the sludge at the end of cycle was gained from the reactor, and it was divided into two parts. One part was undisturbed, and was mixture of granules and flocs, in which, granule with particle size of more than 0.45 mm was about 50%. While the other part was sieved by 0.45 mm screen and only the granule with particle size of more than 0.45 mm was obtained. Two parts of sludges were put into batch reactors with effective volume of 1.0 L, the same domestic wastewater was treated, and operation parameters were as same as SBR. Sludge concentration was controlled at about 4000 mg/L. Results were shown in Fig. 6. As shown in Fig. 6, the increase of granule percent in the sludge could increase SND. The granule with particle size of more than 0.45 mm almost had the same NH3 -N removal efficiency (more than 98%) as the mixed sludge. However, compared with the mixed sludge, the granule with particle size of more than 0.45 mm could improve the TOC removal efficiency from 68% to 86%, improve TN removal efficiency from 53% to 75%, and SND efficiency from 30% to 68%. There was no obvious NO2 -N and NO3 -N accumulation in granular SBR. Therefore, to obtain efficient and stable nitrogen removal performance, granule should dominate in the sludge. 4. Conclusions (1) High influent COD volumetric loading is favorable for granule stability and increase of sludge concentration and particle size. In addition, high organic loading is conducive for SND. Therefore, granule SBR is more suitable for operation at high organic loading. (2) The granule SBR treating real domestic wastewater operated for 6 months. Although COD, NH3 -N and TP in the influent fluctuates much, the SBR maintains stable removal performances for COD, TOC, phosphate, NH3 -N and TN, and average removal efficiencies are 80%, 70%, 71%, 92%, and 47%, respectively. The SBR has a certain shock loading resistance ability. (3) Analysis of typical cycle shows that granule has good ability of simultaneous nitrogen and phosphorus removal performance. Most of organic substances are absorbed and phosphate releasing happens at the anaerobic stage. At the aerobic stage, SND happens with phosphorus absorption. The phosphate concen-
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tration at the end of the aerobic stage is only 0.10 mg/L, and aerobic removal efficiency of NH3 -N and TN is 95% and 51%, respectively. (4) Increases of influent C/N ratio and a large percentage of granule in the sludge are favorable for SND of SBR. References [1] E. Morgenroth, T. Sherden, M.C.M. van Loosdrecht, J.J. Heijnen, P.A. Wilderer, Aerobic granular sludge in a sequencing batch reactor, Water Res. 31 (1997) 3191–3194. [2] J.J. Beun, M.C.M. van Loosdrecht, J.J. Heijnen, Aerobic granulation in a sequencing batch airlift reactor, Water Res. 36 (2002) 702–712. [3] M.K. de Kreuk, J.J. Heijnen, M.C.M. van Loosdrecht, Simultaneous COD, nitrogen, and phosphate removal by aerobic granular sludge, Biotechnol. Bioeng. 90 (2005) 761–769. [4] B. Arrojo, A. Mosquera-Corral, J.M. Garrido, et al., Aerobic granulation with industrial wastewater in sequencing batch reactors, Water Res. 38 (2004) 3389–3399. [5] D.P. Cassidy, E. Belia, Nitrogen and phosphorus removal from an abattoir wastewater in a SBR with aerobic granular sludge, Water Res. 39 (2005) 4817–4823. [6] M.K. De Kreuk, M.C.M. van Loosdrecht, Formation of aerobic granules with domestic sewage, J. Environ. Eng. 132 (2006) 694–697. [7] K.Z. Su, H.Q. Yu, Formation and characterization of aerobic granules in a sequencing batch reactor treating soybean-processing wastewater, Environ. Sci. Technol. 39 (2005) 2818–2827. [8] S.G. Wang, X.W. Liu, W.X. Gong, et al., Aerobic granulation with brewery wastewater in a sequencing batch reactor, Bioresour. Technol. 98 (2007) 2142–2147. [9] H.L. Wang, G.L. Yu, G.S. Liu, F. Pan, A new way to cultivate aerobic granules in the process of papermaking wastewater treatment, Biochem. Eng. J. 28 (2006) 99–103. [10] J.F. Wang, Nitrogen and Phosphorous Removal of Aerobic Granules and Granules Membrane Bioreactor (in Chinese), Dissertation, Tianjin University, Tianjin, 2006. [11] APHA, Standards Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Assoc., Washington, DC, USA, 1998. [12] A. Laguna, A. Ouattara, R.O. Gonzalez, et al., A simple and low cost technique for determining the granulometry of upflow anaerobic sludge blanket reactor sludge, Water Sci. Technol. 40 (1999) 1–8. [13] B.Y.P. Moy, J.H. Tay, S.K. Toh, et al., High organic loading influences the physical characteristics of aerobic sludge granules, Lett. Appl. Microbiol. 34 (2002) 407–412. [14] X. Wang, et al., Anaerobic fast-absorption of organics under alternatively anaerobic-aerobic conditions (in Chinese), J. Agro-Environ. Sci. 24 (2005) 322–327. [15] J.Y. Sun, Treatment Technology and Application for Nitrogen Removal in Wastewater, Chemical Industry Press, Beijing, 2003 (in Chinese).