## 3 Results and Discussion  
### Surface Morphology  
At the initial stage of start-up, images of activated sludge growing in mABR were collected on the 12 th day of operation at an OLR of 1 kg-COD/m\({}^{3}\) d (Figure 2a). It was found that most of the sludge in the four compartments was flocculent and scattered, and a small  
Figure 1: Schematic diagram of the experimental apparatus. amount of smooth and fine granular sludge was brought in from the anaerobic treatment tower during inoculum. As the reactor start-up, the dispersed sludge flocs gradually formed flocculating constituents. The analysis was that the number of microorganisms in the activated sludge gradually increased, making the bacteria collide with each other and adsorb to the solid surface. In this period, the growth rate of activated sludge was very slow because the inoculated sludge was initially dispersed, and the flocculent sludge accounted for a large proportion. Therefore, in the process of operation, the water flow should be ensured to have a slow upward flow rate to avoid sludge loss caused by a too fast flow rate. The images captured of the activated sludge on the 25 th day (OLR = 1.3 kg-COD/m\({}^{3}\) d) of start-up showed that the sludge flocs in the first two compartments started to turn white, while in the latter two compartments, they gradually deepened, with black flocculent sludge dominating (Figure 2b). This was mainly due to the environmental conditions of the anterior compartments being suitable for the growth of acidogenic bacteria, while the rear compartments were suitable for the growth of methanogens [22]. Later in this phase it was found that the four compartments had preliminary formed large specific gravity particles as the core of sludge granulation, which reflected that the new granular sludge with a rough surface and an irregular shape appeared in the flocculent sludge of the 1 st compartment, and the core bodies were formed in the sludge flocs of the last three compartments. The Images captured of the activated sludge on the 35 th day (OLR = 2.0 kg-COD/m\({}^{3}\) d) of start-up showed that the proportion of granular sludge in the 1 st compartment was the largest among the four compartments because it was in the most anterior part of the reactor with adequate microbial nutrition and bearing a much higher organic load than the other three compartments (Figure 2c). In addition, the fly ash added in the 1 st compartment at the beginning of the start-up could serve as the core carrier of sludge granulation and play a skeleton role in the formation of granular sludge. At the same time, the adsorption of  
Figure 2: Captured images of the sludge in four compartments on (**a**) the 12 th day; (**b**) the 25 th day; (**c**) the 35 th day; (**d**) the 45 th day during start-up (10\(\times\)). fly ash also created the most primitive conditions for the enrichment of sludge flocs and the formation of zoogloeas [3]. In general, the proportion of granular sludge in the 2 nd compartment was slightly smaller than that in the 1 st compartment, and a small amount of fly ash was discovered in the 2 nd compartment because the speed of the water flow increased and the disturbance of gas production was enhanced. Part of the fly ash in the 1 st compartment was carried into the next compartment. The proportion of granular sludge in the 3 nd compartment was smaller than that in the first two compartments, and the organic polymer PAM added to this compartment could strengthen the adhesion between the bacteria and promote the rapid aggregation of microorganisms under the traction of external force, which was reflected in part of the granular sludge surface, which appeared distinctly viscous, and many fine particles adhered to each other and formed clumps. To a certain extent, these conditions could reduce the contact area between the sludge and wastewater, but this phenomenon gradually disappeared with the shear action of water flow.