Effect of Organic Loading Rate on Aerobic Granulation. II: Characteristics of Aerobic Granules

Four sequential aerobic sludge blanket reactors, Reactors $\mathrm{R1}$, $\mathrm{R2}$, $\mathrm{R3}$, and $\mathrm{R4}$, were operated at organic loading rates (OLRs) of 1, 2, 4, and $8\mathrm{kg}$ chemical oxygen demand $\left(\mathrm{COD}\right)/m^3\mathrm{day}$, respectively. Aerobic granules were not detected at the low OLRs in $\mathrm{R1}$ and $\mathrm{R2}$. Aerobic granules first appeared on Day 14 in Reactor $\mathrm{R3}$, operating at a moderate OLR of $4\mathrm{kg}$ $\mathrm{COD}/m^3\mathrm{day}$. Aerobic granules were initially observed on Day 18 in $\mathrm{R4}$, operating at the highest OLR tested of $8\mathrm{kg}$ $\mathrm{COD}/m^3\mathrm{day}$. These granules were unstable and disintegrated within 2 weeks after their first appearance. Under the OLR of $4\mathrm{kg}$ $\mathrm{COD}/m^3\mathrm{day}$, the process of aerobic granulation could be clearly divided into three phases of acclimation, multiplication, and maturation, with specific granular growth rates $\left({\nu }_{\phi }\right)$ of 0.1081, $-0.0064$, and $-0.0008{\mathrm{day}}^{-1}$, respectively. The values of ${\nu }_{\phi }$ became smaller with time, and indicated that the aerobic granules had stabilized. Compared to the looser and more amorphous flocs, the compact granules in Reactor $\mathrm{R3}$ possessed a higher specific gravity of 1.064, a higher strength with an integrated coefficient of 99.5%, a higher cell surface hydrophobicity of 75%, and a higher ratio of polysaccharides (PS) to proteins (PN) at $5.0\mathrm{mg}$ PS per mg PN.