Aerobic acceleration of granulation (Liu et al. 2016;

Aerobic granular sludge (AGS) is a hot topic due to its several
advantages, such as excellent anti-shock loading capacity, avoidance from
sludge bulking, and high biomass retention (Ionescu et al. 2015; Liu et al.
2010; Pronk et al. 2015; Song et al. 2015). AGS is a specific biofilm structure
that forms axiolitic or spherical structure by gradual granulation and
self-immobilization (Adav et al. 2008a; Wang et al. 2012). In 1990s, AGS was
first found in the upflow sludge blanket reactor and later on it was observed
that quorum sensing plays a crucial role in AGS formation. As granulation
changes, bacterial accumulation and surface characteristics such as
hydrophobicity and charge also modifies. The quorum sensing molecules have a
direct impact on the granules formation which is evident from the presence of
more sensing molecule in mature molecule than small granules Xiong and Liu
(2010) and Liu et al. (2016). Valle et al. 2004 shows that AHLs induce the
microorganisms granulation and biofilm formation. AGS formation is
significantly influenced by the synthesis of EPS and biosurfactants (Schuster
and Greenberg 2006) which is based on sensing molecules. Hence, the structure
of granules can be enhanced by appropriate regulation of quorum sensing (Frederick
et al. 2011; Tan et al. 2014).

Aerobic granulation increases the yields of signal molecules, and
Zhang and Tay (2015) noted that the rate of biofilm synthesis is about 109
greater than that in the initial stage. Thus, signal molecules are essential to
the maturity of aerobic granules. However, signal molecules still induce the
adhesion growth and accumulation of suspended bacteria into mature granules,
contributing to the maintenance of the stable granule structure and the
acceleration of granulation (Liu et al. 2016; West et al. 2006). In addition,
the increase in QS potency in the start-up stage evidently advances AGS
formation (Ren et al. 2010). It is believed that autoinducer-2 (AI-2) and
tyrosine are the key molecules for biofilm formation and aerobic granulation
process. As the regulators for EPS production, they can lay the basis for the
maintenance of aerobic granular sludge (Xiong and Liu 2012). The operation
conditions were more complicated in fullscale wastewater treatment process,
therefore, the relationship between molecules contents and EPS
production should be investigated in the perspective of alternating starvation
time and organic loading rates (OLRs), which is beneficial for flocculent
sludge to transform into granular sludge and in turn preserve their integrity.
Liu noted that the EPS content was positively correlated with AI-2 level in
aerobic granulation process and the increase of AI-2 can result in moderately
high secretion of large molecular weight EPS during long starvation time (Liu
et al. 2016; Garnier et al. 2005). The similar deduction was claimed under
variable OLRs conditions, and the experimental results by Sun suggested that
AI-2 could enhance bacterial adhesiveness by regulating bacteria to secrete
certain EPS, which finally led to the formation of aerobic granular sludge (Sun
et al. 2016; Lv et al. 2014). Based on sensing the variation of AI-2 content, a
possible mechanism of aerobic granulation at alternating OLR strategy was
elucidated in
Fig. 3. (1) AI-2 production by an intensified selection pressure; (2) key gene
transcription by AI-2 and EPS secretion; and (3) bacteria gradually aggregate
and become mature due to the enhanced hydrophobicity and adhesion capability.
Moreover, the dosage of Ltyrosine can obviously reduce the formation time of aerobic
granules by more than 30 days, and the
enrichment of QS L-tyrosine relating bacteria genera and the stable production
of EPS suggested the positive contribution of L-tyrosine to aerobic granulation
and granules stability (Luo et al. 2015).