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Aerobic Granulation

Successful wastewater treatment depends upon the selection of metabolically capable microorganisms and the efficient separation of those organisms from the treated effluent. Much research has focused on reducing the settling time required for activated sludge by forming dense flocks or by using biofilm reactors. Biogranules are a kind of condensed biofilm formed through self-immobilization. Biogranulation can be classified as aerobic or anaerobic granulation. These granules are dense microbial consortia packed with different bacterial species and typically contain millions of organisms per gram of biomass. 

Formation of anaerobic granules has been extensively studied and is probably best recognized in the upflow anaerobic sludge blanket (UASB) reactor. Anaerobic granulation technology has already been applied in many wastewater treatment plants (Alves et al., 2000). In granular sludge reactors, the anaerobic granules with high density settle rapidly, which reduces the separation time of the treated effluent from the biomass. On the other hand, anaerobic granular sludge technology is suitable for high-strength wastewater treatment. However, the anaerobic granulation technology has some drawbacks, such as a long start-up period, a relatively high operation temperature and unsuitability for low strength organic wastewater and nutrient removal. In order to overcome those weaknesses, research has been devoted to the development of aerobic granulation technology.

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Mechanism of aerobic granulation

The mechanism of microbial aerobic granulation is still a topic of considerable discussion, owing to the complexity of aerobic granulation. Currently there are three mechanisms proposed to explain the biological assemblies. Based on experiments in an aerobic upflow sludge blanket (AUSB), Mishima and Nakamura (1991) hypothesized that, similar to the anaerobic granulation process, filamentous bacteria tangled with each other to form aerobic granules. Beun et al. (1999) proposed a model in which aerobic granulation could be started with fungi. As shown in Fig 2-9 (Beun et al., 1999), fungi easily formed pellets, which settled very fast and could be retained in the reactor. When the pellets grew up to a diameter of 5-6 mm, they lysed, probably due to oxygen limitations in the inner part of the pellets. Then, the pellets broke up and only the colonies that were dense enough could settle. The colonies eventually grew out to form new granules. 

Tay et al. (2001) proposed the model of gradual formation of aerobic granules from seed sludge to compact aggregates, then to granular sludge and finally to mature granules with the sequential operation proceeding. During the granulation process, a significant shift in the microbial diversity was demonstrated by amplified ribosomal DNA restriction analysis (ARDRA) (Yi et al., 2003). Comparing with the initial stage and death stage of aerobic granules, the bacterial community of mature granules showed the least diversity in terms of richness, evenness and Shannon-Wiener index. Some specific microorganisms were suggested to play an important role in the development of aerobic granules.

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Fig. 1 Proposed mechanism of aerobic granulation in SBR (Beun et al., 1999)

Recently, Liu et al. (2004) proposed that hydraulic pressure is a decisive parameter in the formation of biogranulation, and cell hydrophobicity contributes to the formation of granules. In addition, the formation of aerobic granules was the result of cooperative effects among different functional groups, as well as the interaction between these functional groups and the environment. It has been disclosed that bacteria can sense a large number of environment signals and process this information into specific transcriptional responses in recent molecular studies. Quorum sensing is one example of social behavior in bacteria, as signal exchange among individual cells allows the entire population to choose an optimal way of interacting with the environment. Intercellular communication and multi-cell coordination are known to contribute to the organization of bacteria into spatial structures (Liu et al., 2004). So it is reasonable to assume that interaction of both environmental signals and signals between individual cells play an important role in aerobic granules formation and spatial structure organization.

Characteristics of Aerobic Granules

Aerobic granular sludge is completely different from floc like sludge. Characteristic of aerobic granular sludge are described as follows (Etterer and Wilderer, 2001; Moy et al., 2002; Tay et al., 2003; Zheng et al., 2005)

  • Round and regular shape with a clear and smooth outer surface;
  • Dense and compact microbial structure;
  • Large enough to be visible as separate entities in the mixed liquor during mixing and settling phase;
  • Excellent settleability to ensure a fast and easy liquid-solid separation;
  • High biomass retention;
  • Ability to withstand high organic loading rates;
  • Ability to resist the toxicity of recalcitrant chemicals and heavy metals in wastewater.

Application of Aerobic Granulation Technology

To treat high-strength organic wastewater efficiently, a high biomass concentration and high microbial degradation rate are expected for biological systems. In aerobic granulation SBRs, a high biomass concentration of 6.0 to 12.0 g l-1 has been accumulated in reactors because of the compact and dense structure of the granules (Tay et al., 2002). The feasibility of applying aerobic granulation technology for the treatment of high-strength organic wastewaters was demonstrated by Moy et al. (2002). Aerobic granules were able to sustain a maximum organic loading rate of 15.0 kg COD m-3 day with glucose as substrate, while removing more than 92% of the COD. The potential of this technology to treat industrial wastewater has been studied by Arrojo et al. (2004) who operated two reactors fed with industrial wastewater produced in a laboratory for analysis of dairy products (Total COD: 1500–3000 mg/L; soluble COD: 300–1500 mg/L; total nitrogen: 50–200 mg/L). These authors applied organic and nitrogen loading rates up to 7 g COD/(L·d) and 0.7 g N/(L·d) obtaining removal efficiencies of 80%.

With their compact structure and high degradation efficiency, aerobic granules show excellent ability in degradation of toxic compounds, such as phenol (Jiang et al., 2002). For an influent phenol concentration of 500 mg l-1, a stable effluent phenol concentration of less than 0.2 mg l-1 was achieved in an aerobic granular sludge reactor (Jiang et al., 2004). Aerobic granules possess high tolerance to phenol because much of the biomass in the granules is not exposed to the same high concentration as present in the wastewater. 

Aerobic granules may prove powerful bioagents for removing other inhibitory and toxic organic compounds from high strength industrial wastewaters. Recent studies also demonstrated that aerobic granules can be used for phosphate and ammonia removal (Dulekgurgen et al., 2003; Tsuneda et al., 2003) 

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References

Alves, M., Cavaleiro, A. J., Ferreira, E. C., Amaral, A. L., Mota, M., da Motta, M., Vivier, H., Pons, M. N. (2000), “Characterization by image analysis of anaerobic sludge under shock conditions”, Water Science and Technology, 41(12), 207-214.
Arrojo B., Mosquera-Corral A., Garrido J.M. and Méndez R. (2004) Aerobic granulation with industrial wastewater in sequencing batch reactors. Water Research, Vol. 38, Nos. 14-15, pp. 3389 – 3399.              Beun, J. J., Hendriks, A., Van Loosdrecht, M. C. M., Morgenroth, E., Wilderer, P. A. & Heijnen, J. J. (1999), “Aerobic granulation in a sequencing batch reactor”, Water Research, 33(10), 2283-2290.
Dulekgurgen, E., Ovez, S., Artan, N., Orhon, D. (2003), “Enhanced biological phosphate removal by granular sludge in a sequencing reactor”, Biotechnology Letters, 25(9), 687-693.
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Jiang, H. L., Tay, J. H. & Tay, S. T. L. (2004), “Changes in structure, activity and metabolism of aerobic granules as a microbial response to high phenol loading”, Applied Microbiology and Biotechnology, 63(5), 602-608.
Mishima, K. & Nakamura, M. (1991), “Self-immobilization of aerobic activated sludge - A pilot study of the Aerobic Upflow Sludge Blanket Process in municipal sewage treatment”, Water Science and Technology, 23(4-6), 981-990.
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Tay, J. H., Liu, Q. S. & Liu, Y. (2001), “The effects of shear force on the formation, structure and metabolism of aerobic granules”, Applied Microbiology and Biotechnology, 57(1-2), 227-233.
Tay, J. H., Liu, Q. S. & Liu, Y. (2002), “Aerobic granulation in sequential sludge blanket reactor”, Water Science and Technology, 46(4-5), 13-18.
Tay, J. H., Pan, S., Tay, S. T. L., Ivanov, V. & Liu, Y. (2003), “The effect of organic loading rate on the aerobic granulation: the development of shear force theory”, Water Science and Technology, 47(11), 235-240.
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Zheng, Y. M., Yu, H. Q. & Sheng, G. P. (2005), “Physical and chemical characteristics of granular activated sludge from a sequencing batch airlift reactor”, Process Biochemistry, 40(2), 645-650.

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This article was written by Zeng Ping of the Chinese Research Academy of Environmental Sciences.

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