The biological treatment of wastewater in the sewage treatment plant is often accomplished using conventional activated sludge systems. These systems generally require large surface areas for treatment and biomass separation units due to the generally poor settling properties of the sludge. Aerobic granules are a type of sludge that can self-immobilize flocs and microorganisms into spherical and strong compact structures. The advantages of aerobic granular sludge are excellent settleability, high biomass retention, simultaneous nutrient removal and tolerance to toxicity. Recent studies show that aerobic granular sludge treatment could be a potentially good method to treat high strength wastewaters with nutrients, toxic substances.
The aerobic granular sludge usually is cultivated in SBR (sequencing batch reactor) and applied successfully as a wastewater treatment for high strength wastewater, toxic wastewater and domestic wastewater. Compared with conventional aerobic granular processes for COD removal, current research focuses more on simultaneous nutrient removal, particularly COD, phosphorus and nitrogen, under pressure conditions, such as high salinity or thermophilic condition.
In recent years, new technologies have been developed to improve settleability. The use of aerobic granular sludge technology is one of them.
Context
Proponents of aerobic granular sludge technology claim "it will play an important role as an innovative technology alternative to the present activated sludge process in industrial and municipal wastewater treatment in the near future" and that it "can be readily established and profitably used in activated sludge plants". However, in 2011 it was characterised as "not yet established as a large-scale application ... with limited and unpublished full-scale applications for municipal wastewater treatment."
Aerobic granular biomass
The following definition differentiates an aerobic granule from a simple floc with relatively good settling properties and came out of discussions which took place at the 1st IWA-Workshop Aerobic Granular Sludge in Munich (2004):
Formation of aerobic granules
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.
Advantages
The development of biomass in the form of aerobic granules is being studied for its application in the removal of organic matter, nitrogen and phosphorus compounds from wastewater. Aerobic granules in an aerobic SBR present several advantages compared to conventional activated sludge process such as:
Stability and flexibility: the SBR system can be adapted to fluctuating conditions with the ability to withstand shock and toxic loadings