The formation mechanism of aerobic granular sludge in wastewater biological treatment

In granular sludge, aerobic granular sludge (AGS) has advantages such as smooth surface, high density, good settling performance, ability to maintain high biomass, and withstand high organic loads. M. Pronk et al. pointed out that the total energy consumption of the aerobic granular sludge system is 13.9 kW·h ，which is lower than the average energy consumption level of traditional activated sludge plants in the Netherlands by 58%~63% ，and its effluent water quality can reach or even exceed that of traditional activated sludge processes. The volume required for the aerobic granular sludge system is also lower than that of existing conventional activated sludge devices by 33% 左右，reducing both energy consumption and civil engineering costs.

Formation mechanism of aerobic granular sludge

      The formation of aerobic granular sludge is a complex process involving numerous factors, including microbial action, as well as physical and chemical actions. Domestic and foreign scholars have conducted long-term research on the formation of aerobic granular sludge, mainly forming the following theories.

1 、 Microbial self-aggregation principle

      Self-aggregation is a microbial aggregation phenomenon that spontaneously occurs under appropriate conditions. Studies have shown that the formation of aerobic granular sludge is a gradual process of gradual densification and aggregation of seed sludge, forming granular sludge through various influences. It is determined by many factors such as hydraulic shear force and pH ，whether the granules can eventually form a stable structure.

2 、 Filamentous bacteria hypothesis

      In the cultivation process of aerobic granular sludge, the microorganisms in the inoculated sludge are mainly dominated by filamentous bacteria. The types of granular sludge cultivated in the reactor are different, and the role of filamentous bacteria in the process of granule formation is also different.

      Studies have obtained the formation structure of filamentous bacteria in granular sludge by crushing different colors of granular sludge. Aerobic granular sludge appears yellow, black and white in different stages of the reactor. 3 Different colored granules, different granular sludge have different proportions of strains and morphological structures. In general, filamentous bacteria play an important role in the formation and stability of aerobic granular sludge.

3 、 Cell surface hydrophobicity hypothesis

      According to thermodynamic theory, the increase in cell surface hydrophobicity will reduce the excess Gibbs free energy on the cell surface, thereby increasing the interaction between cells to form a dense and stable structure.

      Studies have shown that in 3 the formation process of aerobic granular sludge, the hydrophobicity of sludge increased from 39% to 73% ，thus proving that cell surface hydrophobicity is an important affinity for cell self-aggregation and adhesion, playing a key role in the formation of aerobic granular sludge. Hydrophobicity is of great significance to the interaction between cells, which may cause the initial self-stability of microorganisms and further bind bacteria tightly together.

4 、 Selection pressure driven hypothesis

      Studies have shown that controlling the settling time to control the selection pressure is a decisive factor in the formation of aerobic granular sludge in a sequencing batch reactor ( SBR ) . Shortening the settling time helps to wash out the flocculent sludge with poor settling performance, resulting in a relatively strong selection pressure and promoting the formation of aerobic granular sludge.

      Within a certain range, increasing the selection pressure will lead to an increase in the particle size of aerobic granular sludge. Shortening the settling time can significantly improve the yield of cell polysaccharides, cell surface hydrophobicity and microbial activity, which is conducive to the formation of aerobic granular sludge. Control and in-depth research on selection pressure will help to better understand the formation mechanism of aerobic granular sludge.

5 、 Extracellular polymer hypothesis

      Extracellular polymeric substances ( EPS ) are macromolecular organic substances secreted by microorganisms on the cell surface under certain suitable conditions. Autoinducer ( autoinducer ， AI ) (signal molecule) is formed and released, which can be detected by bacteria in quorum sensing ( Quorum sensing ， QS ). QS It is a phenomenon of bacteria surviving and adapting in a constantly changing environment, through QS ，bacteria can monitor population density and activate gene expression for bacterial growth.

      Studies have shown that loosely attached EPS around aerobic granular sludge is an important factor in the granulation process, mainly determined by the proteins in it.

EPS formation depends on the operation mode and environment in the reactor, and controlling the relevant parameters is conducive to EPS appropriate production, thus forming stable aerobic granular sludge. According to the degree of binding, EPS can be divided into soluble EPS （ soluble EPS ， SEPS ）and bound EPS （ bond EPS ， BEPS ）， BEPS and is further divided into loosely bound EPS （ loosely bond EPS ， LEPS ）and tightly bound EPS （ tightly bond EPS ， TEPS ）。

6 、Stages of Hypothesis Formation

      The hypothesis of aerobic granular sludge formation is divided into 4 stages, each stage influenced by different forces or substances, promoting the gradual formation of granular sludge from the inoculated sludge.

      In the first stage, the physical movement between bacteria on the surface of the inoculated sludge promotes granulation, such as hydrodynamic and diffusion forces;

      In the second stage, various physical, chemical and biological attractions maintain stable connections between the solid cell surface and multiple cells, such as van der Waals forces, chemical bonds and cell membrane fusion;

      In the third stage, microorganisms promote the maturation of aggregated bacteria, EPS the production and growth of microbial flora occur during this stage;

      In the fourth stage, a stable three-dimensional structure is formed through hydrodynamic shear force. This formation mechanism is a relatively comprehensive theory of granular sludge formation, but due to the mutual influence of various factors, it is still difficult to completely cover the entire formation process of aerobic granular sludge.