Factors affecting aerobic granule characteristics and stabilityand stability

Once the operational conditions are determined to produce aerobic granular biomass, it is still important to take into account the potential effect of other parameters on the properties and stability of the aerobic granules. These parameters include the substrate composition, the organic loading rate and concentration, EPS formation, presence of divalent cations, dissolved oxygen (DO) concentration and hydrodynamic shear forces.

The aerobic granulation versus activated sludge 101

6.2.3.1 Substrate composition

Aerobic granules can be produced in SBR systems when the readily biodegradable organic matter content of the treated wastewater is larger than 75 mg COD/L (Mosquera-Corral et al. 2011). Lower concentrations do not provide the minimum concentration gradient conditions for the establishment of the required feast-famine regime. Taking into account that the aerobic granules are a special kind of ‘biofilm’, the knowledge obtained regarding the effect of the organic matter composition on the biofilm formation and characteristics could be applied in the case of the aerobic granule formation. In this sense, previous works on biofilms have demonstrated the relationship between the degree of reduction of the substrate and the biofilm density (Mosquera-Corral et  al. 2003). For example, acetate results in high maximum biomass growth rates (μmax = 9.6 d⁻¹) and produce less dense biofilms (18 g VSS/Lbiofilm) than methanol resulting in slow maximum biomass growth rates (μmax = 5.26 d⁻¹) and high biofilm density (80 g VSS/Lbiofilm). In the case of aerobic granules it is necessary to take into account that the organic compounds present in the wastewater are first transformed into storage compounds (PHA), during the feast period, and these are latter used for growth during the famine period. In these conditions, slow growing microorganisms are developed because the biomass growth rate using an intracellular polymer (PHA) is slower than using a simple extracellular compound. Furthermore, some authors have proposed to operate SBR systems to select for Poly-P accumulating bacteria (PAO) and glycogen accumulation organisms (GAO) which are known to grow at slow rates (de Kreuk & van Loosdrecht, 2004).

From a microscopic point of view, previous works have also indicated that substrates containing carbohydrates favour the uncontrolled growth of filamentous bacteria (Schwarzenbeck et al. 2005), while acetate is more suitable to produce aerobic granules. In general, simple substrates are better than complex ones because are easy to store by the biomass. Aerobic granules have been successfully cultivated on a wide variety of substrates (Figure 6.3) including glucose, acetate, ethanol, peptone, phenol and industrial wastewater (Morgenroth et al. 1997; Beun et al. 1999; Yi et al. 2003;

Arrojo et al. 2004; Schwarzenbeck et al. 2004b; de Kreuk & van Loosdrecht, 2006;

Sun et al. 2006; Figueroa et al. 2008; Ho et al. 2010; Val del Río et al. 2012a).

Figure 6.3 Aerobic granules formed in SBRs fed with acetate solution (a), pig manure (b) and effluent from the seafood industry (c). The bar in the images corresponds to 2 mm.

6.2.3.2 Organic loading rate

The properties of the aerobic granules depend on the organic loading rate (Tay et al.

2004; Adav et al. 2010). High organic loading rates result in the production of less dense and resistant granular biomass (Moy et al. 2002). This can be related to the fact that there is a limit to the storage rate of the biomass. Furthermore, high organic loading rates are related to limitations of dissolved oxygen concentration which are known to promote the development of filamentous organisms and the deterioration of the physical properties of the aggregates (Mosquera-Corral et al. 2005a).

The organic load treated is directly related to the biomass concentration in the reactor. In aerobic granular systems the biomass concentration ranges normally from 4 to 15 g VSS/L (Beun et al. 1999; Di Iaconi et al. 2004; Val del Río et al. 2012a).

These values are higher than those conventionally achieved in activated sludge reactors of 1–2 g VSS/L. The systems can operate under an organic loading rate of up to 19 g COD/(L ⋅ d) on such a high level of biomass concentration (Adav et al. 2010).

6.2.3.3 Exopolymeric substances

EPS produced in the granules include different kinds of compounds in variable amounts comprising proteins, polysaccharides, lipids, glycoproteins, humic-like substances, nucleic acids, among others. These compounds are produced under different conditions such as substrate or oxygen limiting conditions, ionic strength, temperatures of operation and so on. There are conflicting results reported in literature regarding the function of the EPS on the formation of aerobic granules (de Kreuk et al. 2005b). It is believed that one of the EPS functions is to act as a ‘glue’ among the microorganisms present in an aggregate due to its physical properties (Tay et al. 2001a). Moreover, according to previous research, the EPS content increases with granulation, there are differences in loosely bound and tightly bound EPS and, within the granule structure, insoluble versus soluble polysaccharide gradients occur.

The polysaccharides have been identified as the major compounds forming the granular EPS and the gel-like structures responsible for bacterial granule formation (Lin et  al. 2010). Furthermore, two different exoplysaccharides have been identified by isolation techniques as the responsible for the gel-forming matrix of the granules: alginate-like polysaccharide (Lin et al. 2010) and granulan (Seviour et al. 2011). At the moment it is still not clear which EPS is responsible for the formation of aerobic granules and, even new compounds could arise as important components of these structures (Seviour et al. 2012).

6.2.3.4 Presence of divalent cations

From research works there is evidence that divalent cations as Ca²⁺ and Mg²⁺

attach to negatively charged groups present on the bacteria surfaces and to the EPS molecules promoting the aerobic granulation process. Jiang et al. (2003) reported

The aerobic granulation versus activated sludge 103 that the addition of Ca²⁺ accelerated the aerobic granulation process. In the case of other cations, such as Mg²⁺, it was demonstrated that the presence of this ion in the feeding media enhanced the sludge granulation process in sequencing batch reactors (Li et  al. 2009). The external addition of these ions should not be an option, since they are often already present in the wastewater and may have a positive role during the granulation process.

6.2.3.5 Dissolved oxygen concentration

In aerobic reactors the dissolved oxygen (DO) concentration is an important parameter and must be kept at sufficient high levels to allow the different biological processes to occur (Mosquera-Corral et al. 2005a), and simultaneously, avert the growth of filamentous which decrease the granule density.

In systems where the mixing is performed pneumatically, the DO supply is also directly linked to the degree of mixing. For this reason, modification of the hydrodynamic conditions of the system by reducing the supplied air flow will affect the quality of the formed granules (Tay et al. 2001b). However, this fact is not always taken into account.

6.2.3.6 Hydrodynamic shear forces

Evidence shows that application of high shear forces favour the formation of more compact and dense aerobic granules (Shin et al. 1992; Tay et al. 2001b; Liu & Tay, 2006; Di Iaconi et al. 2006). Tay et al. (2001a, c) reported that the production of extracellular polysaccharides, and, consequently, the stability of aerobic granules is closely associated to the applied shear forces. The collisions between the granules provoke the detachment of weakly attached materials from the surface of the aggregates helping to maintain their high densities and smooth surfaces. The detached solid parts are removed through the effluent of the reactor.