## 3 Results and Discussion
### Particle Size Distribution  
Since the granular sludge began to appear to different degrees in four compartments at the 3 rd phase of the start-up, the particle size distribution of granular sludge on the 35 th and 45 th day was analyzed. As shown in Figure 3a, the highest degree of granulation was found in the 1 st compartment of the reactor after 35 d of acclimation. The particle size of granular sludge mainly was concentrated in 0.5-1.0 mm with an average value of 1.2 mm, among which the particle size greater than 1.0 mm accounted for 25.8% of the total granular sludge in the 1 st compartment. The granular sludge with a particle size of 0.5-1.0 mm accounted for the highest proportion, and the particle size greater than 1.0 mm accounted for 16.7% of the total granular sludge in the 2 nd compartment, which was much less than that in the 1 st compartment, therefore, the average particle size was only 0.9 mm. The particle size of granular sludge decreased significantly from the 3 rd compartment. The proportion of granular sludge with a particle size less than 0.5 mm in the 3 rd compartment and the 4 th compartment reached 71.4% and 82.5%, respectively, and the average particle size was 0.6 mm and 0.4 mm, respectively. From the above, it could be concluded that when the OLR was low, due to the limited nutrients available to the microorganisms, organic matter was first decomposed in the front compartments, while the bacteria in the back compartments could only obtain a small amount of nutrients and grow slowly. Therefore, the particle size of the granular sludge in each compartment showed a law of change from large to small along the direction of the water flow. The granular sludge with good precipitation performance was formed in four compartments after 45 d of acclimation. As shown in Figure 3b, the particle size of granular sludge was mainly concentrated in 1.0-2.0 mm with an average value of 1.9 mm, among which the particle size greater than 3.0 mm accounted for 10.7% of the total granular sludge in the 1 st compartment. The granular sludge with a particle size less than 1.0 mm accounted for the highest proportion and a particle size greater than 3.0 mm accounted for 4.5% of the total granular sludge in the 2 nd compartment; the average particle size was only 1.2 mm, which was less than that in the 1 st compartment. The particle size of the granular sludge was mainly concentrated in 1.0-2.0 mm with an average value of 1.6 mm, among which the particle size greater than 3.0 mm accounted for 7.9% of the total granular sludge in the 3 rd compartment. The quantity of granular sludge with a large particle size in the 3 rd compartment was less than that in the 1 st compartment, so even though the concentration range of particle sizes was similar, the average value was quite different. The granular sludge with a particle size less than 1.0 mm accounted for the majority of the granular sludge and a particle size greater than 3.0 mm accounted for only 3.5% of the total granular sludge in the 4 th compartment; the average particle size was 0.9 mm. The particle size of the granular sludge in the 4 th compartment was the smallest among the four compartments, both in terms of the concentration range and the average value. From the above, it could be concluded that when the OLR was high, the particle size of the granular sludge in the 3 rd compartment began to exceed that in the 2 nd compartment and was second only to the 1 st compartment.