Anaerobic digester: Types and functionality

Anaerobic digesters can be categorized according to two main criteria: 1) whether the biomass is fixed to a surface, i.e. attached or mobilised to support growth or can mix freely with the reactor liquid (wastewater), i.e. suspended growth; and 2) by the organic loading rate, i.e. the influent mass rate of chemical oxygen demand required per unit volume. Some examples of anaerobic digesters are (Shannon et al, 2002):

 Batch system anaerobic digester,

 Continuous stirred-tank reactor (CSTR),

 Expanded granular sludge bed digestion (EGSB), and

 Up-flow anaerobic sludge blanket digestion (UASB).

2.7.1. Upward-flow anaerobic sludge blanket reactor (UASB)

Anaerobic granular sludge bed technology refers to a reactor concept in which a high rate of anaerobic treatment can be achieved. The concept was initiated with an upward flow anaerobic sludge blanket (UASB) reactor (Chan, 2009). The UASB operates in three distinct phases: liquid, solid and gas phases. The liquid phase is whereby the wastewater is being treated, while the solid phase is the sludge or biomass present in the reactor. The gas phase consists of the biogas formed during the anaerobic digestion process (Caixeta et al., 2002). The wastewater flow is directed upward through an anaerobic sludge bed whereby the sludge comes into contact with the organic matter in the wastewater. The sludge bed is composed of microorganisms that naturally form granulates of 0.5 to 2 mm (Chan, 2009), and that have a high sedimentation velocity and thus resist upward movement by pneumatic forces, which prevents a biomass wash-out from the system even at high hydraulic loading rates, and therefore results in low HRT. The resulting organic matter anaerobic degradation process is responsible for the production of biogas containing CH4 and CO2. The upward

motion of the generated biogas causes hydraulic turbulence that provides pneumatic mixing. At the top of the reactor, the sludge and gas are separated in a three-way phase separator, i.e. a biogas-liquid-solid separator. The three-phase separator consists of a biogas cap with a settler unit being situated above the biogas collection port. Below, the opening of the biogas cap, baffles are used to divert the biogas to the gas-port opening (Chan, 2009). Figure 2.4 illustrates the set-up of the UASB.

Figure 2-4: Schematic diagram of a UASB (Adopted from Chan, 2009)

The success of a UASB depends on the pre-treatment of the wastewater prior to anaerobic treatment to reduce fats and suspended solids. It is important to apply suitable influent up- flow velocity, i.e. surface speed, in UASB to minimise sludge washout. A suitable average up-flow velocity is between 0.5 – 0.7 m/h which is also dependant on reactor volume and configuration. Overall, UASB reactors have been demonstrated to be robust (Seghezzo et al., 1998), with experiments showing COD removal rates >60% for most wastewater from different industries (von Sperling et al., 2001). However, like all other bioreactors, the UASB has some disadvantages.

2.7.1.1. Disadvantages of UASB

The successful operation of a UASB is largely dependent on overcoming the following disadvantages (Chan et al., 2009):

 Accumulation of suspended solids and FOG,

 Reduction in methanogenic activity biomass washout, and

 Periodic re-inoculation.

2.7.2. Expanded granular sludge bed reactor (EGSB)

An EGSB is a variant/modified design of the UASB concept, which includes a recirculation stream for the reactor (Seghezzo et al., 1998) in order to improve dissolved organic matter biomass contact (von Sperling and de Lemos Chernicharo, 2017). The achievement of biomass contact of an expanded granular sludge bed with a high up-flow velocity, i.e. >4 m/h, which was determined to improve the reactor performance and hydraulic mixing, when compared to the UASB (von Sperling and de Lemos Chernicharo, 2005).

The distinguishing feature of the EGSB is that a higher rate of flow velocity can be implemented (Beddow, 2010), particularly for systems with a height/diameter ratio of >20 (von Sperling and de Lemos Chernicharo, 2005). The increased flux permits partial anaerobic bed expansion, i.e. fluidization of the granular sludge bed, resulting in improved wastewater-sludge contact, while enhancing the segregation of inactive suspended particles from the sludge bed into a wash-out port. The EGSB design is appropriate for low strength wastewater having 1 - 2 g soluble COD/L that contains inert or poor/partially biodegradable SS which should not be allowed to accumulate in the sludge bed (Chan, 2009). Figure 2.5 illustrates the EGSB described.

Operationally, the EGSB has demonstrated better performance than the UASB; however, a UASB can handle high strength wastewater when compared to the EGSB, which can only handle low soluble COD containing wastewater (Chan 2009). According to Zhang et al. (2008), the EGSB can achieve a reported 91% COD removal for an HRT of 48h with the feed content containing 80 g soluble COD/L, which is larger than the maximum cited in Chan (2009). To demonstrate versatility, the EGSB design concept has been applied in many other treatment plants, i.e. breweries, starch processors, molasses producers, as well as in domestic and municipal WWTP (Seghezzo et al., 1998; Zhang et al., 2008). Research reports on EGSB are rare, but based on reports of the UASB, the EGSB can be assessed for its suitability to treat PSW, a focus of this study. Furthermore, EGSB has been proven to be suitable for the treatment of low strength wastewater, i.e. dilute PSW at ambient temperatures (Chu et al., 2003). Like the UASB, an EGSB has some disadvantages.

2.7.2.1. Advantages of an EGSB

The advantages below listed are some of the operational attributes for the EGSB (Seghezzo et at., 1998; Chu et al., 2005; Chan et al., 2009; Saravanan and Sreekrishnan 2006; van Haandel and van der Lubbe, 2007):

 Effective removal of soluble pollutants,

 The design can be optimised to treat high strength organic wastewater up to an OLR of 30 kg COD/m3d,

 Minimal accumulation of flocculating/excess sludge,

 The recycle can be used to alter the concentration of the influent supplied to the unit,

 Higher biogas production, pneumatic mixing, up-flow velocities (thus treatment capacity), and a small plant footprint,

 Expanded sludge bed, resulting in improved organic matter-biomass contact and

 Active sludge remains granular, with excellent settleability characteristic, which effectively provides operational longevity.

However, with such high up-flow velocities some disadvantages associated with such an operational strategy are not abnormal.

2.7.2.2. Disadvantages of an EGSB

Due to the high up-flow velocities, there are numerous disadvantages associated with the EGSB (van Haandel and van der Lubbe, 2007), which include:

 Reduced ability to remove particulate organic matter due to high up-flow velocities,

 The Suspended Solids are not retained by the granular bed thus exit with the effluent to downstream units, and

 High sludge washout when granule activity is reduced.