Attachment and adhesion force between biogas bubbles and anaerobic granular sludge in the up-flow anaerobic sludge blanket

Highlights

• The interaction of AGS and biogas bubbles was studied for the first time.

• The attachment and adhesion force of CH₄ on AGSp were higher than that of AGSb.

• The surface hydrophobicity and functional groups of AGS affected their interaction.

Abstract

The performance of the up-flow anaerobic sludge blanket (UASB) is significantly governed by the hydrodynamics of the reactor. Though the influence of hydrodynamics on mass transfer, granular size distribution, and biogas production was well studied, the interaction between biogas bubbles and anaerobic granular sludge (AGS) is poorly understood. This study used the impinging-jet technique and bubble probe atomic force microscope (AFM) to investigate the attachment and adhesion force between biogas bubbles (CH₄ and CO₂) and AGS. The fluxes of normalized CH₄ or CO₂ bubble-attachment on two kinds of AGS were directly affected by gas velocity and decreased with an increase in the Reynolds number ranged from 40 to 140. The bubble-attachment had a positive linear relationship with the contact angles, ratio of exopolymeric protein and polysaccharide, and hydrophilic functional groups of AGS. A bubble probe AFM was used to explore the adhesion force between a single bubble and AGS. The results indicated that the adhesion force between the bubbles and the two kinds of AGS also decreased with increasing approach velocity. Overall, these results contribute to a new insight into the understanding of interaction between biogas bubbles and AGS in UASB reactors.

Introduction

Over the past decades, anaerobic digestion has been widely used in the management of wastes and wastewaters as it reduces organic pollutants and produces renewable energy (Nguyen and Khanal, 2018). Moreover, it has undergone significant improvement due to the use of bio-reactors that are based on granular sludge (Zhang et al., 2012). Among them, up-flow anaerobic sludge blanket (UASB) is one of the most widely used high-rate anaerobic reactors for sewage treatment (Lettinga, 2001). The performance of a UASB is significantly influenced by the mixing of wastewater with anaerobic granular sludge (AGS), which is governed by the hydrodynamics of the reactor (Kundu et al., 2014). The characteristics of hydrodynamics in a UASB have been studied extensively under different conditions (Batstone et al., 2005; Peña et al., 2006; Ren et al., 2009). In previous papers, granular settlement in UASB was considered as Stokes flow, having a Reynolds number lower than 1 (Gupta and Gupta, 2005; Laguna et al., 1999). However, further research indicated that the Reynolds number of AGS in the typical UASB reactor fell in an intermediate flow regime category, with the Reynolds number between 1 and 500 (Tassew et al., 2019). Computational fluid dynamics (CFD) with Eulerian-Eulerian model has often been used to characterize and optimize the hydrodynamics in a UASB (Wei et al., 2019). However, the gas phase in this model is considered as a continuum, similar to the liquid phase. Though the influence of hydrodynamic conditions on mass transfer, AGS size distribution, and biogas production are well studied (Afridi et al., 2017; Wu et al., 2012a, 2012b), little is known about the interaction between biogas bubbles and AGS.

AGS has pores, channels, and cavities, which are conducive to the transport of nutrients, metabolites, and biogas bubbles (Jiang et al., 2016). Jiang et al. (2016) proposed that the micro-bubbles of biogas inside AGS diffused toward large-size channels to form an observable bubble on AGS. At a certain time, the bubble detaches from the surface of AGS to form a crater. In addition, extracellular polymeric substances (EPS) that are located outside AGS govern the mass transfer of substances and products (Sheng et al., 2010). As biogas bubbles must pass through the EPS layer before detaching from AGS into the liquid phase, EPS might play a crucial role in the interaction between biogas bubbles and AGS. Hence, the interaction between gas phase and solid phase, especially the single bubble and AGS, has not been described suitably.

The impinging-jet technique (Fig. 1A) was initially used in the deposition of latex particles on glass surfaces (Dabroś and van de Ven, 1983; Dabros and Vandeven, 1987). Then it was successfully used to describe the particle and submicron colloidal particles deposition (Dijt et al., 1990; Sanders et al., 1995). Furthermore, Yang et al. (2003) measured the attachment of bubble flux onto a solid surface in an impinging-jet stagnation flow, indicating that both electrolyte concentration and hydrodynamic flow were key factors for the attachment flux. Hence, the impinging-jet technique might provide direct information on the attachment between biogas bubbles and AGS.

Even though both impinging-jet technique and CFD can describe the hydrodynamics of UASB, there is a difficulty to directly observe the interaction forces between biogas bubbles and AGS at a single-particle level. Atomic force microscope (AFM), a powerful tool for sensitive force measurements, has been widely used to quantitatively measure the interaction forces between a bubble and the surface of solids or cells (Cui et al., 2016; Ditscherlein et al., 2018; Xie et al., 2015). It has led to further understanding of many physical and biological processes, such as mining, froth flotation, and fermentation (Liu et al., 2019; Wang et al., 2009; Zhang et al., 2017). The bubble probe AFM technique, where a gas bubble anchored on a tipless cantilever, was applied as a force probe to measure the interaction forces between a bubble and substrates (Tabor et al., 2011a, 2011b; Vakarelski et al., 2008; Xie et al., 2015). With the help of this technique, previous studies investigated the interaction of a bubble and particles under different conditions (Cui et al., 2017; Xie et al., 2015). Furthermore, Nguyen et al. (2003, 2004) indicated that the repulsive force between a bubble and a hydrophilic particle increased with increasing approach velocities of AFM tips. This bubble probe AFM technique with different approach velocities might provide a new insight into the interaction between a biogas bubble and AGS, which has important benefits on the optimization of hydrodynamics in UASB.

For the first time, the interaction between AGS and biogas bubble is investigated in this study both by impinging-jet technique and bubble probe AFM technique. The attachment between biogas bubbles (CO₂ and CH₄) and AGS at different Reynolds numbers was studied using impinging-jet technique. In addition, the adhesion force between a biogas bubble (CO₂ and CH₄) and AGS was also studied by bubble probe AFM at different approach velocities. These results may contribute to a better and new insight into the understanding of surface interaction between biogas bubbles and AGS in UASB.