References

1. J.R. Bidwell, C. Becker, S. Hensley, R. Stark, M.T. Meyer, Occurrence of organic wastewater and other contaminants in cave streams in Northeastern Oklahoma and Northwestern Arkansas, Arch. Environ. Contam. Toxicol., 58 (2010) 286–298.
2. M. Heidari, M. Kazemipour, B. Bina, A. Ebrahimi, M. Ansari, M. Ghasemian, M.M. Amin, A qualitative survey of five antibiotics in a water treatment plant in central Plateau of Iran, J. Environ. Public Health, 2013 (2013) 1–9.
3. J. Qiu, Double threat for Tibet, Nature, 512 (2014) 240–241.
4. R. Yang, S. Zhang, X. Li, D. Luo, C. Jing, Dechloranes in lichens from the southeast Tibetan Plateau: evidence of long-range atmospheric transport, Chemosphere, 144 (2016) 446–451.
5. G. Andreottola, E. Damiani, P. Foladori, P. Nardelli, M. Ragazzi, Treatment of mountain refuge wastewater by fixed and moving bed biofilm systems, Water Sci. Technol., 48 (2003) 169–177.
6. A. Albuzio, C. Lubian, R. Parolin, R. Balsamo, I. Camerin, P. Valerio, Wastewater from a mountain village treated with a constructed wetland, Desal.Wat. Treat., 1 (2009) 232–236.
7. J. Harada, T. Inoue, K. Kato, N. Uraie, H. Sakuragi, Performance evaluation of hybrid treatment wetland for six years of operation in cold climate, Environ. Sci. Pollut. Res., 22 (2015) 12861–12869.
8. E.M. Symonds, M.E. Verbyla, J.O. Lukasik, R.C. Kafle, M. Breitbart, J.R. Mihelcic, A case study of enteric virus removal and insights into the associated risk of water reuse for two wastewater treatment pond systems in Bolivia, Water Res., 65 (2014) 257–270.
9. M. Juanico, H. Weinberg, N. Soto, Process design of waste stabilization ponds at high altitude in Bolivia, Water Sci. Technol., 42 (2000) 307–313.
10. A. Kajenthira, A. Siddiqi, L.D. Anadon, A new case for promoting wastewater reuse in Saudi Arabia: bringing energy into the water equation, J. Environ. Manage., 102 (2012) 184–192.
11. J. Zabalaga, G. Amy, E. von Munch., Evaluation of agricultural reuse practices and relevant guidelines for the Alba Rancho WWTP (primary and secondary facultative ponds) in Cochabamba, Water Sci. Technol., 55 (2007) 469–475.
12. D. Fang, G. Zhao, X. Xu, Q. Zhang, Q. Shen, Z. Fang, L. Huang, F. Ji, Microbial community structures and functions of wastewater treatment systems in plateau and cold regions, Bioresour. Technol., 249 (2017) 684–693.
13. L. Niu, Y. Li, P. Wang, W. Zhang, C. Wang, Q. Wang, Understanding the Linkage between elevation and the activated-sludge bacterial community along a 3,600-meter elevation gradient in China, Appl. Environ. Microbiol., 81 (2015) 6567–6576.
14. J. Xu, P.F. Wang, Y. Li, L.H. Niu, Performance and characterization of the microbial community structures in the activated sludge from wastewater treatment plant at high altitudes in Tibet of China, Desal.Wat. Treat., 106 (2018) 108–115.
15. B. Wu, J.Q. Tian, C.M. Bai, M.C. Xiang, J.Z. Sun, X.Z. Liu, The biogeography of fungal communities in wetland sediments along the Changjiang River and other sites in China, ISME J., 7 (2013) 1299–1309.
16. U. Wiesmann, I.S. Choi, E.M. Dombrowski, Fundamentals of Biological Wastewater Treatment, Wiley-VCH, 2006.
17. J.C.P. Leslie Grady, G.T. Daigger, N.G. Love, C.D.M. Filipe, Biological Wastewater Treatment, CRC Press, 2011.
18. S. He, L. Ding, K. Li, H. Hu, L. Ye, H. Ren, Comparative study of activated sludge with different individual nitrogen sources at a low temperature: effluent dissolved organic nitrogen compositions, metagenomic and microbial community, Bioresour. Technol., 247 (2018) 915–923.
19. B.C. Jiang, F. Ma, L. Wei, J.B. Guo, A. Li, Operational conditions and improvement methods of municipal wastewater treatment plants under low temperatures in northern China, in: Z.Y. Du, X.B. Sun (Eds.) Environment Materials and Environment Management Pts 1-32010, pp. 651-+.
20. L. Niu, Y. Li, P. Wang, W. Zhang, C. Wang, W. Cai, L. Wang, Altitude-scale variation in nitrogen-removal bacterial communities from municipal wastewater treatment plants distributed along a 3600-m altitudinal gradient in China, Sci. Total Environ., 559 (2016) 38–44.
21. A. Karkman, K. Mattila, M. Tamminen, M. Virta, Cold temperature decreases bacterial species richness in nitrogenremoving bioreactors treating inorganic mine waters, Biotechnol. Bioeng., 108 (2011) 2876–2883.
22. G. Knowles, A.L. Downing, M.J. Barrett, Determination of kinetic constants for nitrifying bacteria in mixed culture, with the aid of an electronic computer, J. Gen. Microbiol., 38 (1965) 263–278.
23. D.J. Kim, D.I. Lee, J. Keller, Effect of temperature and free ammonia on nitrification and nitrite accumulation in landfill leachate and analysis of its nitrifying bacterial community by FISH, Bioresour. Technol., 97 (2006) 459–468.
24. A. Rodriguez-Caballero, S. Hallin, C. Pahlson, M. Odlare, E. Dahlquist, Ammonia oxidizing bacterial community composition and process performance in wastewater treatment plants under low temperature conditions, Water Sci. Technol., 65 (2012) 197–204.
25. J.J. Park, I.G. Byun, S.R. Park, T.J. Park, Nitrifying bacterial communities and its activities in aerobic biofilm reactors under different temperature conditions, Korean J. Chem. Eng., 25 (2008) 1448–1455.
26. S. Knoop, S. Kunst, Influence of temperature and sludge loading on activated sludge settling, especially on Microthrix parvicella, Water Sci. Technol., 37 (1998) 27–35.
27. M. Miazga-Rodriguez, S. Han, B. Yakiwchuk, K. Wei, C. English, S. Bourn, S. Bohnert, L.Y. Stein, Enhancing nitrification at low temperature with zeolite in a mining operations retention pond, Front. Microbiol, 3 (2012).
28. W.W. Li, H.L. Zhang, G.P. Sheng, H.Q. Yu, Roles of extracellular polymeric substances in enhanced biological phosphorus removal process, Water Res., 86 (2015) 85–95.
29. C. Helmer, S. Kunst, Low temperature effects on phosphorus release and uptake by microorganisms in EBPR plants, Water Sci. Technol., 37 (1998) 531–539.
30. M.H. Gerardi, Viscous Bulking or Zoogloeal Growth, Settleability Problems and Loss of Solids in the Activated Sludge Process, John Wiley & Sons, Inc.2003.
31. L. Huang, L.K. Ju, Sludge settling and online NAD(P)H fluorescence profiles in wastewater treatment bioreactors operated at low dissolved oxygen concentrations, Water Res., 41 (2007) 1877–1884.
32. A. Raszka, M. Chorvatova, J. Wanner, The role and significance of extracellular polymers in activated sludge. Part I: literature review, Acta. Hydroch. Hydrob., 34 (2006) 411–424.
33. B. Mishukov, E. Smirnova, Optimisation of wastewater treatment for safety in St Petersburg, Russia, P. Inst. Civil Eng - Water Manage., 170 (2017) 184–197.
34. R. Delatolla, N. Tufenkji, Y. Comeau, A. Gadbois, D. Lamarre, D. Berk, Investigation of laboratory-scale and pilot-scale attached growth ammonia removal kinetics at cold temperature and low influent carbon, Water Qual. Res. J. Can., 45 (2010) 427–436.
35. S. McGovarin, T. Sultana, C. Metcalfe, Biological responses in brook trout (Salvelinus fontinalis) caged downstream from municipal wastewater treatment plants in the Credit River, ON, Canada, B. Environ. Contam. Tox., 100 (2018) 106–111.
36. A. Gonzalez-Martinez, B. Munoz-Palazon, A. Rodriguez-Sanchez, P. Maza-Marquez, A. Mikola, J. Gonzalez-Lopez, R. Vahala, Start-up and operation of an aerobic granular sludge system under low working temperature inoculated with coldadapted activated sludge from Finland, Bioresour. Technol., 239 (2017) 180–189.
37. A. Kruglova, M. Krakstrom, M. Riska, A. Mikola, P. Rantanen, R. Vahala, L. Kronberg, Comparative study of emerging micropollutants removal by aerobic activated sludge of large laboratory-scale membrane bioreactors and sequencing batch reactors under low-temperature conditions, Bioresour. Technol., 214 (2016) 81–88.
38. H. Satoh, Y. Nakamura, H. Ono, S. Okabe, Effect of oxygen concentration on nitrification and denitrification in single activated sludge flocs, Biotechnol. Bioeng., 83 (2003) 604–607.
39. J. Guo, L. Zhang, W. Chen, F. Ma, H. Liu, Y. Tian, The regulation and control strategies of a sequencing batch reactor for simultaneous nitrification and denitrification at different temperatures, Bioresour. Technol., 133 (2013) 59–67.
40. C.M. Fitzgerald, P. Comejo, J.Z. Oshlag, D.R. Noguera, Ammonia-oxidizing microbial communities in reactors with efficient nitrification at low-dissolved oxygen, Water Res., 70 (2015) 38–51.
41. N.A. Keene, S.R. Reusser, M.J. Scarborough, A.L. Grooms, M. Seib, J. Santo Domingo, D.R. Noguera, Pilot plant demonstration of stable and efficient high rate biological nutrient removal with low dissolved oxygen conditions, Water Res., 121 (2017) 72–85.
42. A. Mosquera-Corral, M.K. de Kreuk, J.J. Heijnen, M.C.M. van Loosdrecht, Effects of oxygen concentration on N-removal in an aerobic granular sludge reactor, Water Res., 39 (2005) 2676–2686.
43. M. Carvalheira, A. Oehmen, G. Carvalho, M. Eusebio, M.A.M. Reis, The impact of aeration on the competition between polyphosphate accumulating organisms and glycogen accumulating organisms, Water Res., 66 (2014) 296–307.
44. M. Yang, P.D. Sun, R.Y. Wang, J.Y. Han, J.Q. Wang, Y.Q. Song, J. Cai, X.D. Tang, Simulation and optimization of ammonia removal at low temperature for a double channel oxidation ditch based on fully coupled activated sludge model (FCASM): a full-scale study, Bioresour. Technol., 143 (2013) 538–548.
45. F. Guo, T. Zhang, Profiling bulking and foaming bacteria in activated sludge by high throughput sequencing, Water Res., 46 (2012) 2772–2782.
46. B. Xie, X.C. Dai, Y.T. Xu, Cause and pre-alarm control of bulking and foaming by Microthrix parvicella - A case study in triple oxidation ditch at a wastewater treatment plant, J Hazard. Mater., 143 (2007) 184–191.
47. W.W. Eckenfelder, Industrial Water Pollution Control, McGraw-Hill, Singapore, 2000.
48. Z. Ma, X.H. Wen, F. Zhao, Y. Xia, X. Huang, D. Waite, J. Guan, Effect of temperature variation on membrane fouling and microbial community structure in membrane bioreactor, Bioresour. Technol., 133 (2013) 462–468.
49. Y. Liu, Q.S. Liu, Causes and control of filamentous growth in aerobic granular sludge sequencing batch reactors, Biotechnol. Adv., 24 (2006) 115–127.
50. C. Kragelund, C. Levantesi, A. Borger, K. Thelen, D. Eikelboom, V. Tandoi, Y.H. Kong, J. van der Waarde, J. Krooneman, S. Rossetti, T.R. Thomsen, P.H. Nielsen, Identity, abundance and ecophysiology of filamentous Chloroflexi species present in activated sludge treatment plants, FEMS Microbiol. Ecol., 59 (2007) 671–682.
51. A.M.P. Martins, J.J. Heijnen, M.C.M. van Loosdrecht, Effect of dissolved oxygen concentration on sludge settleability, Appl. Microbiol. Biot., 62 (2003) 586–593.
52. H. Meng, K. Li, M. Nie, J.R. Wan, Z.X. Quan, C.M. Fang, J.K. Chen, J.D. Gu, B. Li, Responses of bacterial and fungal communities to an elevation gradient in a subtropical montane forest of China, Appl. Microbiol. Biot., 97 (2013) 2219–2230.
53. L. Niu, Y. Li, P. Wang, W. Zhang, C. Wang, Q. Wang, Elevational characteristics of the archaeal community in full-scale activated sludge wastewater treatment plants at a 3660-meter elevational scale, Water Sci. Technol., (2017).
54. D.R. Nemergut, E.K. Costello, M. Hamady, C. Lozupone, L. Jiang, S.K. Schmidt, N. Fierer, A.R. Townsend, C.C. Cleveland, L. Stanish, R. Knight, Global patterns in the biogeography of bacterial taxa, Environ. Microbiol., 13 (2011) 135–144.
55. L. Zinger, L.A. Amaral-Zettler, J.A. Fuhrman, M.C. Horner- Devine, S.M. Huse, D.B. Welch, J.B. Martiny, M. Sogin, A. Boetius, A. Ramette, Global patterns of bacterial beta-diversity in seafloor and seawater ecosystems, PLoS One, 6 (2011) e24570.
56. N. Fierer, R.B. Jackson, The diversity and biogeography of soil bacterial communities, Proc. Natl. Acad. Sci. USA., 103 (2006) 626–631.
57. Y. Yang, Y. Gao, S. Wang, D. Xu, H. Yu, L. Wu, Q. Lin, Y. Hu, X. Li, Z. He, Y. Deng, J. Zhou, The microbial gene diversity along an elevation gradient of the Tibetan grassland, ISME J., 8 (2014) 430–440.
58. T. Zhang, M.F. Shao, L. Ye, 454 Pyrosequencing reveals bacterial diversity of activated sludge from 14 sewage treatment plants, ISME J., 6 (2012) 1137–1147.
59. A. Vikram, D. Lipus, K. Bibby, Produced Water Exposure Alters Bacterial Response to Biocides, Environ. Sci. Technol., 48 (2014) 13001–13009.
60. X.H. Wang, M. Hu, X. Yu, X.H. Wen, K. Ding, Pyrosequencing analysis of bacterial diversity in 14 wastewater treatment systems in China, Appl. Environ. Microbiol., 78 (2012) 7042–7047.
61. A. Cydzik-Kwiatkowska, M. Zielinska, Bacterial communities in full-scale wastewater treatment systems, World J. Microbiol. Biotechnol., 32 (2016).
62. F.M. Ibarbalz, E.L.M. Figuerola, L. Erijman, Industrial activated sludge exhibit unique bacterial community composition at high taxonomic ranks, Water Res., 47 (2013) 3854–3864.
63. E. Rodriguez, P.A. Garcia-Encina, A.J.M. Stams, F. Maphosa, D.Z. Sousa, Meta-omics approaches to understand and improve wastewater treatment systems, Rev. Environ. Sci. Bio., 14 (2015) 385–406.
64. A. Valentin-Vargas, G. Toro-Labrador, A.A. Massol-Deya, Bacterial community dynamics in full-scale activated sludge bioreactors: operational and ecological factors driving community assembly and performance, Plos One, 7 (2012) e42524.
65. X.H. Wang, Y. Xia, X.H. Wen, Y.F. Yang, J.Z. Zhou, Microbial community functional structures in wastewater treatment plants as characterized by GeoChip, Plos One, 9 (2014).
66. S.Q. Xia, L.A. Duan, Y.H. Song, J.X. Li, Y.M. Piceno, G.L. Andersen, L. Alvarez-Cohen, I. Moreno-Andrade, C.L. Huang, S.W. Hermanowicz, Bacterial community structure in geographically distributed biological wastewater treatment reactors, Environ. Sci. Technol., 44 (2010) 7391–7396.
67. B. Young, R. Delatolla, K. Kennedy, E. Laflamme, A. Stintzi, Low temperature MBBR nitrification: Microbiome analysis, Water Res., 111 (2017) 224–233.
68. A. Kruglova, A. Gonzalez-Martinez, M. Krakstrom, A. Mikola, R. Vahala, Bacterial diversity and population shifts driven by spotlight wastewater micropollutants in low-temperature highly nitrifying activated sludge, Sci. Total Environ., 605 (2017) 291–299.
69. J.A. Bryant, C. Lamanna, H. Morlon, A.J. Kerkhoff, B.J. Enquist, J.L. Green, Colloquium paper: microbes on mountainsides: contrasting elevational patterns of bacterial and plant diversity, P. Natl. Acad. Sci. USA., 105 (2008) 11505–11511.
70. C.E. Nelson, C.A. Carlson, Differential response of highelevation planktonic bacterial community structure and metabolism to experimental nutrient enrichment, PLoS One, 6 (2011) e18320.
71. L.M. Zhang, M. Wang, J.I. Prosser, Y.M. Zheng, J.Z. He, Altitude ammonia-oxidizing bacteria and archaea in soils of Mount Everest, FEMS Microbiol.Ecol., 70 (2009) 52–61.
72. L. Wilhelm, K. Besemer, L. Fragner, H. Peter, W. Weckwerth, T.J. Battin, Altitudinal patterns of diversity and functional traits of metabolically active microorganisms in stream biofilms, ISME J., 9 (2015) 2454–2464.
73. D. Singh, K. Takahashi, J.M. Adams, Elevational patterns in archaeal diversity on Mt. Fuji, PLoS One, 7 (2012) e44494.
74. C.J. Hayden, J.M. Beman, Microbial diversity and community structure along a lake elevation gradient in Yosemite National Park, California, USA, Environ. Microbiol., 18 (2016) 1782–1791.
75. M. Hu, X.H. Wang, X.H. Wen, Y. Xia, Microbial community structures in different wastewater treatment plants as revealed by 454-pyrosequencing analysis, Bioresour. Technol., 117 (2012) 72–79.
76. X.H. Wang, M. Hu, Y. Xia, X.H. Wen, K. Ding, Pyrosequencing analysis of bacterial diversity in 14 wastewater treatment systems in China, Appl. Environ. Microbiol, 78 (2012) 7042–7047.
77. Z. Zhou, W. Qiao, C. Xing, Y. An, X. Shen, W. Ren, L.-M. Jiang, L. Wang, Microbial community structure of anoxic– oxic-settling-anaerobic sludge reduction process revealed by 454-pyrosequencing, Chem. Eng. J., 266 (2015) 249–257.
78. K.A. Fimlaid, A. Shen, Diverse mechanisms regulate sporulation sigma factor activity in the Firmicutes, Curr. Opin. Microbiol., 24 (2015) 88–95.
79. L.H. Niu, Y. Li, L.L. Xu, P.F. Wang, W.L. Zhang, C. Wang, W. Cai, L.Q. Wang, Ignored fungal community in activated sludge wastewater treatment plants: diversity and altitudinal characteristics, Environ. Sci. Pollut. Res., 24 (2017) 4185–4193.
80. S. Siripong, B.E. Rittmann, Diversity study of nitrifying bacteria in full-scale municipal wastewater treatment plants, Water Res., 41 (2007) 1110–1120.
81. Z.H. Huang, P.B. Gedalanga, P. Asvapathanagul, B.H. Olson, Influence of physicochemical and operational parameters on Nitrobacter and Nitrospira communities in an aerobic activated sludge bioreactor, Water Res., 44 (2010) 4351–4358.
82. R. Lebrero, A.C. Gondim, R. Perez, P.A. Garcia-Encina, R. Munoz, Comparative assessment of a biofilter, a biotrickling filter and a hollow fiber membrane bioreactor for odor treatment in wastewater treatment plants, Water Res., 49 (2014) 339–350.
83. A. Kruglova, P. Ahlgren, N. Korhonen, P. Rantanen, A. Mikola, R. Vahala, Biodegradation of ibuprofen, diclofenac and carbamazepine in nitrifying activated sludge under 12 degrees C temperature conditions, Sci. Total Environ., 499 (2014) 394–401.
84. L. Wu, J. Wang, X. Liu, Enhanced nitrogen removal under low-temperature and high-load conditions by optimization of the operating modes and control parameters in the CAST system for municipal wastewater, Desal. Wat. Treat., 53 (2015) 1683–1698.
85. S.M. Hocaoglu, G. Insel, E.U. Cokgor, D. Orhon, Effect of low dissolved oxygen on simultaneous nitrification and denitrification in a membrane bioreactor treating black water, Bioresour. Technol., 102 (2011) 4333–4340.
86. E. Torresi, M.E. Casas, F. Polesel, B.G. Plosz, M. Christensson, K. Bester, Impact of external carbon dose on the removal of micropollutants using methanol and ethanol in postdenitrifying Moving Bed Biofilm Reactors, Water Res., 108 (2017) 95–105.
87. J.H. Guo, Y.Z. Peng, C.Y. Peng, S.Y. Wang, Y. Chen, H.J. Huang, Z.R. Sun, Energy saving achieved by limited filamentous bulking sludge under low dissolved oxygen, Bioresour. Technol., 101 (2010) 1120–1126.
88. M. Bellucci, I.D. Ofiteru, D.W. Graham, I.M. Head, T.P. Curtis, Low-dissolved-oxygen nitrifying systems exploit ammoniaoxidizing bacteria with unusually high yields, Appl. Environ. Microbiol., 77 (2011) 7787–7796.
89. S. Zheng, H. Li, C. Cui, An upflow microaerobic sludge blanket reactor operating at high organic loading and low dissolved oxygen levels, Biotechnol. Lett., 33 (2011) 693–697.
90. H.D. Park, D.R. Noguera, Evaluating the effect of dissolved oxygen on ammonia-oxidizing bacterial communities in activated sludge, Water Res., 38 (2004) 3275–3286.
91. Z. Zhou, X.L. Shen, L.M. Jiang, Z.C. Wu, Z.W. Wang, W.C. Ren, D.L. Hu, Modeling of multimode anaerobic/anoxic/aerobic wastewater treatment process at low temperature for process optimization, Chem. Eng. J., 281 (2015) 644–650.
92. T.L.G. Hendrickx, C. Kampman, G. Zeeman, H. Temmink, Z. Hu, B. Kartal, C.J.N. Buisman, High specific activity for anammox bacteria enriched from activated sludge at 10 degrees C, Bioresour. Technol., 163 (2014) 214–221.
93. Q. Yang, Y. Peng, X. Liu, W. Zeng, T. Mino, H. Satoh, Nitrogen removal via nitrite from municipal wastewater at low temperatures using real-time control to optimize nitrifying communities, Environ. Sci. Technol., 41 (2007) 8159–8164.
94. Y. Hao, X.G. Jiang, Q. Tian, A.Y. Chen, B.L. Ma, Isolation and identification of Nitrobacteria adapted to low temperature, Adv. Mater. Res-Switz., 518–523 (2012) 406–410.
95. J.B. Guo, J.H. Wang, D. Cui, L. Wang, F. Ma, C.C. Chang, J.X. Yang, Application of bioaugmentation in the rapid start-up and stable operation of biological processes for municipal wastewater treatment at low temperatures, Bioresour. Technol., 101 (2010) 6622–6629.
96. Q. Zhang, G. Yang, L. Zhang, Z. Zhang, G. Tian, R. Jin, Bioaugmentation as a useful strategy for performance enhancement in biological wastewater treatment undergoing different stresses: application and mechanisms, Crit. Revi. Env. Sci. Tec., 47 (2017) 1877–1899.
97. M.A. Head, J.A. Oleszkiewicz, Bioaugmentation for nitrification at cold temperatures, Water Res., 38 (2004) 523–530.
98. F. Persson, C. Suarez, M. Hermansson, E. Plaza, R. Sultana, B.M. Wilen, Community structure of partial nitritation-anammox biofilms at decreasing substrate concentrations and low temperature, Microb. Biotechnol., 10 (2017) 761–772.
99. E.M. Gilbert, S. Agrawal, S.M. Karst, H. Horn, P.H. Nielsen, S. Lackner, Low temperature partial nitritation/anammox in a moving bed biofilm reactor treating low strength wastewater, Environ. Sci. Technol., 48 (2014) 8784–8792.
100. A. Wang, X. Yang, F. Ye, Study on bio-denitrification in wastewater treatment plant under low temperature in South China, Water Wastewater Eng., 35 (2009) 28–33.
101. H. Fan, L. Qi, G. Liu, Y. Zhang, Q. Fan, H. Wang, Aeration optimization through operation at low dissolved oxygen concentrations: evaluation of oxygen mass transfer dynamics in different activated sludge systems, J. Environ. Sci., 55 (2017) 224–235.
102. H. Fan, X. Liu, H. Wang, Y. Han, L. Qi, H. Wang, Oxygen transfer dynamics and activated sludge floc structure under different sludge retention times at low dissolved oxygen concentrations, Chemosphere, 169 (2017), 586–595.