The Mechanism And Characteristics Of Granular Sludge Formed In Biological Phosphorus Removal System

Aerobic biogranulation is a promising wastewater treatment technology as it not only possesses the advantages of activated sludge and biofilm technologies, but also avoids their shortcomings. It has excellent settleability, high biomass concentration, ability to withstand shocking loadings, tolerance to toxicity, simple process, compact reactor structure and needless of carriers. Usually, the cultivation of aerobic granular sludge needs short settling time, high aeration, high hight to diameter (H/D) ratio and so on. These conditions make the cultivation and the practical application of aerobic granular sludge technology difficult. This study carried out in accordance with the formation of granules in enhanced biological phosphorus removal (EBPR) system which doesn't possess those cultivation conditions. The formation process, the characteristics and the mechanism of this granules were studied to explain the phenomenon and provide a new thought to the cultivation of aerobic granules.With the same running conditions, SBR(H/D=40:15), 40min settling time, 120L/h aeration ratio and other running parameters the same, there was only activated floc sludge in the reactor when the SBR run in O/A biological nitrogen removal state. The average SVI was 138.9mL/g in this state. While when the SBR changed to A/O enhanced biological phosphorus removal state, the settleability of the activated sludge improved, and with the performance of the phosphorus removal become better, the floc sludge assembled to granules. The number of granules in the reactor manifold and the granules grew rapidly with the stable operation of the SBR. The average biomass concentration was 4800mg/L, SVI was 25mL/g, and the average diameter of the granules was 0.8mm when the granules become maturation.The granules have clear un-regular outline, cauliflower or honeycomb shape, relatively compact structure, high specific gravity (the specific gravity was 1.0767) with excellent settleability (the settling velocity was 12m/h , large amount of pores favoring the substrate transport, high mechanical strength, low water ratio. As the granules have mass transfer resistance, its SOUR fell slightly compared with the flocs.During the study of the formation of the granules, it was found that the EPS didn't have obvious changes, that is, the amount and the compose of the EPS did't have much relationship with the formation of the granules. While the sticky filamentous EPS played an important role in the formation of the granules, they worked as bridging agent and backbone of the granules. And the natural formation of the granules in this EBPR system was contributed to the release of the PO₄^3- in the anaerobic stage, as the present of PO₄^3-,Ca²⁺,Mg²⁺,NH₄⁺ and CO₃^2- in the reactor could form sedimentations, and these tiny sedimentation particles might work as nuclei of granules. What's more, it was found that they were positively charged which would neutralize the negatively charges of the cells, and then reduce the electrostatic repulsion, promoting the microorganisms to accumulate together to form granules.At the second stage of this study, the SBR also run in A/O mode with the other operating parameters the same as the EBPR system, just reducing the pH of the influent. As a result, the system had a bad phosphorus removal performance, as GAOs accumulated in the system. It was surprised that granules also presented in this system, though the granules in this system were a little smaller and their formation and maturation process were a little slower than the EBPR system. The analysis results showed that the formation of the granules had something to do with the A/O operating mode of the SBR, as the A/O operating mode could cause a anaerobic (feast)– aerobic (famine) condition in the system. Facing such conditions, the microorganisms changed their metabolism process, and secreted more extracellar protein in the famine stage, which stimulated the formation of granules. In addition, the amount of extracellar polysaccharides continued to increase to work as the backbone of the granules. At the same time, it was found that the granules formation had no obvious relation to the hydrophobicity of the cells.Thereby, it is believed that the formation of granules in the EBPR or GAOs accumulation systems is largely related to the A/O operating mode of the SBR. And the present of positively charged particles in the EBPR system further enhanced and speed up the granules formation process.