text: Granular sludge biomass is developed in sequencing batch reactors (SBR) and without carrier materials. These systems fulfil most of the requirements for their formation as:
  : Feast – Famine regime: short feeding periods must be selected to create feast and famine periods (Beun et al. 1999), characterized by the presence or absence of organic matter in the liquid media, respectively. With this feeding strategy the selection of the appropriate micro-organisms to form granules is achieved. When the substrate concentration in the bulk liquid is high, the granule-former organisms can store the organic matter in form of poly-β-hydroxybutyrate to be consumed in the famine period, giving an advantage over filamentous organisms. When an anaerobic feeding is applied this factor is enhanced, minimising the importance of short settling time and higher hydrodynamic forces.
  : Short settling time:  This hydraulic selection pressure on the microbial community allows the retention granular biomass inside the reactor while flocculent biomass is washed-out. (Qin et al. 2004)
  : Hydrodynamic shear force : Evidences show that the application of high shear forces favours the formation of aerobic granules and the physical granule integrity. It was found that aerobic granules could be formed only above a threshold shear force value in terms of superficial upflow air velocity above 1.2 cm/s in a column SBR, and more regular, rounder, and more compact aerobic granules were developed at high hydrodynamic shear forces (Tay et al., 2001 ).
  Granular activated sludge is also developed in flow-through reactors using the Hybrid Activated Sludge (HYBACS) process, comprising an attached-growth reactor with short retention time upstream of a suspended growth reactor. The attached bacteria in the first reactor, known as a SMART unit, are exposed to a constant high COD, triggering the expression of high concentrations of hydrolytic enzymes in the EPS layer around the bacteria. The accelerated hydrolysis liberates soluble readily-degradable COD which promotes the formation of granular activated sludge.