Applied Microbiology and Biotechnology

, Volume 102, Issue 15, pp 6383–6392 | Cite as

Sludge flotation, its causes and control in granular sludge upflow reactors

  • Bo Wang
  • Di WuEmail author
  • XiaoLei Zhang
  • Hamish R. Mackey
  • Guang-Hao Chen


Sludge flotation is a commonly reported and long-standing issue hindering not only the widespread implementation of upflow anaerobic sludge bed (UASB)-type bioreactors in wastewater treatment but also the development of novel anaerobic/anoxic treatment processes such as anammox, partial denitrification, and biological sulfate reduction. This review attempts to address the instability of UASB-type bioreactors due to sludge flotation. Possible causes of sludge flotation are classified into intrinsic and extrinsic ones. Extrinsic causes include substrate overloading, inappropriate carbon source, overloading of proteins or oils, insufficient reactor mixing, a low temperature, and a low pH. These unfavorable extrinsic conditions can lead to unexpected intrinsic changes in sludge granules, including high gas production, formation of hollow space inside the granules, filamentous bacterial overgrowth, inappropriate production of extracellular polymeric substances, and development of an adhesive granule surface. These intrinsic changes can increase the flotation potential of sludge through reducing the granule density and promoting gas entrapment. To control the sludge flotation problem, both preventive and corrective strategies are summarized. Preventive strategies include maintaining a temperature of 30–35 °C and a pH of 7–9, preventing substrate overloading, providing sufficient nutrients and multiple carbon sources in the influent, applying pre-acidification, and enhancing reactor mixing. If the causes of a sludge flotation incident cannot be identified quickly, corrective strategies including breaking up floating granules and dosing with chemicals such as Fe2+ and surfactants can be applied to suppress the flotation problem.


Sludge flotation Solid-liquid separation Anaerobic treatment Upflow anaerobic sludge bed (UASB) 


Funding information

This study was supported by the Hong Kong Innovation and Technology Commission (grant no. ITC-CNERC14EG03) and the National Natural Science Foundation of China (grant no. 51638005).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Supplementary material

253_2018_9131_MOESM1_ESM.pdf (243 kb)
ESM 1 (PDF 242 kb)


  1. Alphenaar PA (1994) Anaerobic granular sludge: characterization and factors its functioning. PhD dissertation, Wageningen Agricultural University, Wageningen, The NetherlandsGoogle Scholar
  2. Al-Shamrani AA, James A, Xiao H (2002) Destabilization of oil water emulsions and separation by dissolved air flotation. Wat Res 36:1503–1512. CrossRefGoogle Scholar
  3. Angenent LT, Sung S (2001) Development of anaerobic migrating blanket reactor (AMBR), a novel anaerobic treatment system. Wat Res 35:1739–1747. CrossRefGoogle Scholar
  4. Bae BU, Shin HS, Paik BC, Chung JC (1995) Re-activation characteristics of preserved anaerobic granular sludges. Bioresour Technol 53:231–235. CrossRefGoogle Scholar
  5. Batstone DJ, Keller J (2001) Variation of bulk properties of anaerobic granules with wastewater type. Wat Res 35:1723–1729. CrossRefGoogle Scholar
  6. de Beer D, O’Flaharty V, Thaveesri J, Lens P, Verstraete W (1996) Distribution of extracellular polysaccharides and flotation of anaerobic sludge. Appl Microbiol Biotechnol 46:197–201. CrossRefGoogle Scholar
  7. Blaszczyk R, Gardner D, Kosaric N (1994) Response and recovery of anaerobic granules from shock loading. Wat Res 28:675–680. CrossRefGoogle Scholar
  8. Borzacconi L, Ottonello G, Castelló E, Pelaez H, Gazzola A, Viñas M (1999) Denitrification in a carbon and nitrogen removal system for leachate treatment: performance of a upflow sludge blanket (UASB) reactor. Wat Sci Tech 40:142–151. CrossRefGoogle Scholar
  9. Campos JL, Val del Río A, Pedrouso A, Raux P, Giustinianovichc EA, Mosquera-Corral A (2017) Granular biomass floatation: a simple kinetic/stoichiometric explanation. Chem Eng J 311:63–71. CrossRefGoogle Scholar
  10. Cao S, Li B, Du R, Ren N, Peng Y (2016) Nitrite production in a partial denitrifying upflow sludge bed (UASB) reactor equipped with gas automatic circulation (GAC). Wat Res 90:309–316. CrossRefGoogle Scholar
  11. Chen J, Ji Q, Zheng P, Chen T, Wang C, Mahmood Q (2010) Floatation and control of granular sludge in a high-rate anammox reactor. Wat Res 44:3321–3328. CrossRefGoogle Scholar
  12. Chen H, Ma C, Yang G, Wang H, Yu Z, Jin R (2014) Floatation of flocculent and granular sludge in a high-loaded anammox reactor. Bioresour Technol 169:409–415. CrossRefPubMedGoogle Scholar
  13. Corsaro MM, Lanzetta R, Parrilli E, Parrilli M, Tutino ML, Ummarino S (2004) Influence of growth temperature on lipid and phosphate contents of surface polysaccharides from the antarctic bacterium Pseudoalteromonas haloplanktis TAC 125. J Bacteriol 186:29–34. CrossRefPubMedPubMedCentralGoogle Scholar
  14. Cuervo-López FM, Martinez F, Gutiérrez-Rojas M, Noyola RA, Gómez J (1999) Effect of nitrogen loading rate and carbon source on denitrification and sludge settleability in upflow anaerobic sludge blanket (UASB) reactors. Wat Sci Technol 40:123–130. CrossRefGoogle Scholar
  15. Daffonchio D, Thaveesri J, Verstraete W (1995) Contact angle measurement and cell hydrophobicity of granular sludge from upflow anaerobic sludge bed reactors. Appl Environ Microbiol 61:3676–3680 PubMedPubMedCentralGoogle Scholar
  16. Dapena-Mora A, Campos JL, Mosquera-Corral A, Jetten MSM, Méndez R (2004) Stability of the anammox process in a gas-lift reactor and a SBR. J Biotechnol 110:159–170. CrossRefPubMedGoogle Scholar
  17. Dogsa I, Kriechbaum M, Stopar D, Laggner P (2005) Structure of bacterial extracellular polymeric substances at different pH values as determined by SAXS. Biophys J 89:2711–2720. CrossRefPubMedPubMedCentralGoogle Scholar
  18. Fang HHP, Chui HK, Li YY, Chen T (1994) Performance and granule characteristics of UASB process treating wastewater with hydrolysed proteins. Wat Sci Technol 30:55–63 CrossRefGoogle Scholar
  19. Franco A, Roca E, Lema JM (2006) Granulation in high-load denitrifying upflow sludge bed (UASB) pulsed reactors. Wat Res 40:871–880. CrossRefGoogle Scholar
  20. Gauglitz PA, Mahoney LA, Mendoza DP, Miller MC (1994) Mechanisms of bubble retention. PNL-10120, Pacific Northwest Laboratory, Richland, Washington.
  21. van Haandel AC, Lettinga G (1994) Anaerobic sewage treatment. A practical guide for regions with a hot climate, John Wiley and Sons Ltd, ChichesterGoogle Scholar
  22. Halalsheh M, Koppes J, Den Elzen J, Zeeman G, Fayyad M, Lettinga G (2005) Effect of SRT and temperature on biological conversions and the related scum-forming potential. Wat Res 39:2475–2482. CrossRefGoogle Scholar
  23. Hao T, Wei L, Lu H, Chui H, Mackey HR, van Loosdrecht MC, Chen GH (2013) Characterization of sulfate-reducing granular sludge in the SANI® process. Wat Res 47:7042–7052. CrossRefGoogle Scholar
  24. Hao T, Luo J, Wei L, Mackey HR, Liu R, Morito GR, Chen GH (2015) Physicochemical and biological characterization of long-term operated sulfate reducing granular sludge in the SANI® process. Wat Res 71:74–84. CrossRefGoogle Scholar
  25. Hendriksen HH, Ahring BK (1996) Integrated removal of nitrate and carbon in an upflow anaerobic sludge blanket reactor (UASB) reactor: operating performance. Wat Res 30:1451–1458. CrossRefGoogle Scholar
  26. Hickey RF, Wu WM, Veiga MC, Jones R (1991) Startup, operation, monitoring and control of high-rate anaerobic treatment systems. Wat Sci Technol 24:207–255 CrossRefGoogle Scholar
  27. Hou X, Liu S, Zhang Z (2015) Role of extracellular polymeric substance in determining the high aggregation ability of anammox sludge. Water Res 75:51–62. CrossRefPubMedGoogle Scholar
  28. Hulshoff Pol LW, de Castro Lopes SI, Lettinga G, Lens PNL (2004) Anaerobic sludge granulation. Wat Res 38:1376–1389. CrossRefGoogle Scholar
  29. Ives KJ (1984) The scientific basis of flotation. NATO Advanced Science Institute Series. Martinus Nijhoff Publishers, BostonGoogle Scholar
  30. Jeganathan J, Nakhla G, Bassi A (2006) Long-term performance of high-rate anaerobic reactors for the treatment of oily wastewater. Environ Sci Technol 40:6466–6472. CrossRefPubMedGoogle Scholar
  31. Jeganathan J, Nakhla G, Bassi A (2007) Oily wastewater treatment using a novel hybrid PBR–UASB system. Chemosphere 67:1492–1501. CrossRefPubMedGoogle Scholar
  32. Jorand F, Boué-Bigne F, Block JC, Urbain V (1998) Hydrophobic/hydrophilic properties of activated sludge exopolymeric substances. Wat Sci Technol 37:307–315 CrossRefGoogle Scholar
  33. Kuenen JG (2008) Anammox bacteria: from discovery to application. Nat Rev Microbiol 6:320–326. CrossRefPubMedGoogle Scholar
  34. Leitão RC, van Haandel AC, Zeeman G, Lettinga G (2006) The effects of operational and environmental variations on anaerobic wastewater treatment systems: a review. Bioresour Technol 97:1105–1118. CrossRefPubMedGoogle Scholar
  35. Lens PNL, Kuenen JG (2001) The biological sulfur cycle: novel opportunities for environmental biotechnology. Wat Sci Technol 44:57–66 CrossRefGoogle Scholar
  36. Lettinga G, Hulshoff Pol LW (1991) UASB-process design for various types of wastewaters. Wat Sci Technol 24:87–107 CrossRefGoogle Scholar
  37. Li J, Hu B, Zheng P, Qaisar M, Mei L (2008) Filamentous granular sludge bulking in a laboratory scale UASB reactor. Bioresour Technol 99:3431–3438. CrossRefPubMedGoogle Scholar
  38. Li W, Zheng P, Ji J, Zhang M, Guo J, Zhang J, Abbas G (2014) Floatation of granular sludge and its mechanism: a key approach for high-rate denitrifying reactor. Bioresour Technol 152:414–419. CrossRefPubMedGoogle Scholar
  39. Liao BQ, Allen DG, Droppo IG, Leppard GG, Liss SN (2001) Surface properties of sludge and their role in bioflocculation and settleability. Wat Res 35:339–350. CrossRefGoogle Scholar
  40. Liu Y, Tay J (2002) The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge. Wat Res 36:1653–1665. CrossRefGoogle Scholar
  41. Liu YQ, Liu Y, Tay JH (2004) The effects of extracellular polymeric substances on the formation and stability of biogranules. Appl Microbiol Biotechnol 65:143–148. CrossRefPubMedGoogle Scholar
  42. Lu HF, Zheng P, Ji QX, Zhang HT, Ji JY, Wang L, Chen JW (2012) The structure density and settlability of anammox granular sludge in high-rate reactors. Bioresour Technol 123:312–317. CrossRefPubMedGoogle Scholar
  43. Lu X, Zhen G, Estrada AL, Chen M, Ni J, Hojo T, Kubota K, Li YY (2015) Operation performance and granule characterization of upflow anaerobic sludge blanket (UASB) reactor treating wastewater with starch as the sole carbon source. Bioresour Technol 180:264–273. CrossRefPubMedGoogle Scholar
  44. Martins AMP, Pagilla K, Heijnen JJ, van Loosdrecht MCM (2004) Filamentous bulking sludge—a critical review. Wat Res 38:793–817. CrossRefGoogle Scholar
  45. Marx JG, Carpenter SD, Deming JW (2009) Production of cryoprotectant extracellular polysaccharide substances (EPS) by the marine psychrophilic bacterium Colwellia psychrerythraea strain 34H under extreme conditions. Can J Microbiol 55:63–72. CrossRefPubMedGoogle Scholar
  46. Moosbrugger RE, Loewenthal RE, Marais GR (1990) Pelletisation in a UASB system with protein (casein) as substrate. Wat SA 16:171–178 Google Scholar
  47. Mozzi F, de Giori GS, Oliver G, de Valdez GF (1996) Exopolysaccharide production by Lactobacillus casei under controlled pH. Biotechnol Lett 91:846–852. CrossRefGoogle Scholar
  48. Nguyen HT, Razafindralambo H, Blecker C, N’Yapo C, Thonart P, Delvigne F (2014) Stochastic exposure to sub-lethal high temperature enhances exopolysaccharides (EPS) excretion and improves Bifidobacterium bifidum cell survival to freeze–drying. Biochem Eng J 88:85–94. CrossRefGoogle Scholar
  49. Overmeire A, Lens PNL, Verstraete W (1994) Mass transfer limitation of sulfate in methanogenic aggregates. Biotechnol Bioeng 44:387–391. CrossRefPubMedGoogle Scholar
  50. Pereboom JHF (1994) Size distribution model for methanogenic granules from full scale UASB and IC reactors. Wat Sci Technol 30:211–221 CrossRefGoogle Scholar
  51. Pereboom JHF, Vereijken TLFM (1994) Methanogenic granule development in full scale internal circulation reactors. Wat Sci Technol 30:9–21 CrossRefGoogle Scholar
  52. Pevere A, Guibaud G, Van Hullebusch ED, Boughzala W, Lens PNL (2007) Effect of Na+ and Ca2+ on the aggregation properties of sieved anaerobic granular sludge. Coll and Surf A: Physicochem and Engin Asp 306:142–149. CrossRefGoogle Scholar
  53. Rinzema A, Alphenaar A, Lettinga G (1989) The effect of lauric acid shock loads on the biological and physical performance of granular sludge in UASB reactors digesting acetate. J Chem Technol and Biotech 46:257–266. CrossRefGoogle Scholar
  54. Seghezzo L, Zeeman G, van Lier JB, Hamelers HVM, Lettinga G (1998) A review: the anaerobic treatment of sewage in UASB and EGSB reactors. Bioresour Technol 65:175–190. CrossRefGoogle Scholar
  55. Torino MI, Tarranto MP, Sesma F, de Valdez GF (2008) Heterofermentative pattern and exopolysaccharide production by Lactobacillus helveticus ATCC 15807 in response to environmental pH. J Appl Microbiol 91:846–852. CrossRefGoogle Scholar
  56. Tsui TH, Chen L, Hao TW, Chen GH (2016) A super high-rate sulfidogenic system for saline sewage treatment. Water Res 104:147–155. CrossRefPubMedGoogle Scholar
  57. Vanderhaegen B, Ysebaert E, Favere K, van Wambeke M, Peeters T, Pánic V, Vandenlangenbergh V, Verstraete W (1992) Acidogenesis in relation to in-reactor granule yield. Wat Sci Technol 25:21–30 CrossRefGoogle Scholar
  58. Veiga MC, Jain MK, Wu W, Hollingsworth RI, Zeikus JG (1997) Composition and role of extracellular polymers in methanogenic granules. Appl Microbiol Biotechnol 63:403–407 Google Scholar
  59. Vidal G, Carvalho A, Mendez R, Lema JM (2000) Influence of the content in fats and proteins on the anaerobic biodegradability of dairy wastewaters. Bioresour Technol 74:231–239. CrossRefGoogle Scholar
  60. Visser A, Beeksma I, Van der Zee F, Stams AJM, Lettinga G (1993) Anaerobic degradation of volatile fatty acids at different sulphate concentrations. Appl Microbiol Biotechnol 40:549–556. CrossRefGoogle Scholar
  61. Wang S, Liang Z (2016) Acetate-triggered granular sludge floatation in methanogenic bioreactors. Wat Res 107:93–101. CrossRefGoogle Scholar
  62. Wang LL, Wang LF, Ren XM, Ye XD, Li WW, Yuan SJ, Sun M, Sheng GP, Yu HQ, Wang XK (2012) pH dependence of structure and surface properties of microbial EPS. Environ Sci Technol 46:737–744. CrossRefPubMedGoogle Scholar
  63. Wang F, Jin X, Yang S, Liu Y, Chen X (2013) A control strategy for promoting the stability of denitrifying granular sludge in upflow sludge blankets. Environ Technol 35(1):52–59.
  64. Wang B, Wu D, Ekama GA, Huang H, Lu H, Chen GH (2017) Optimizing mixing mode and intensity to prevent sludge flotation in sulfidogenic anaerobic sludge bed reactors. Water Res 122:481–491. CrossRefPubMedGoogle Scholar
  65. Wang B, Wu D, Ekama GA, Tsui TH, Jiang F, Chen GH (2018) Characterization of a new continuous gas-mixing sulfidogenic anaerobic bioreactor: hydrodynamics and sludge granulation. Water Res 135:251–261. CrossRefPubMedGoogle Scholar
  66. Wu D, Ekama GA, Chui HK, Wang B, Cui YX, Hao TW, van Loosdrecht MCM, Chen GH (2016) Large-scale demonstration of the sulfate reduction autotrophic denitrification nitrification integrated (SANI®) process in saline sewage treatment. Water Res 100:496–507. CrossRefPubMedGoogle Scholar
  67. Yoda M, Nishimura S (1997) Controlling granular sludge floatation in UASB reactors. Wat Sci Technol 36:165–173. CrossRefGoogle Scholar
  68. Yu HQ, Fang HH, Tay JH (2000) Effects of Fe2+ on sludge granulation in upflow anaerobic sludge blanket reactors. Wat Sci Technol 41:199–205 CrossRefGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Bo Wang
    • 1
  • Di Wu
    • 1
    • 2
    Email author
  • XiaoLei Zhang
    • 3
  • Hamish R. Mackey
    • 4
  • Guang-Hao Chen
    • 1
    • 2
  1. 1.Department of Civil and Environmental Engineering, Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution, and Water Technology CenterThe Hong Kong University of Science and TechnologyHong KongChina
  2. 2.Wastewater Treatment Laboratory, FYT Graduate SchoolThe Hong Kong University of Science and TechnologyGuangzhouChina
  3. 3.Department of Civil and Environmental EngineeringHarbin Institute of Technology (Shenzhen)ShenzhenChina
  4. 4.Division of Sustainable Development, College of Science and EngineeringHamad Bin Khalifa UniversityDohaQatar

Personalised recommendations