Settling velocity of algal-bacterial granular sludge studied in the sequence batch reactor

Granule settling velocity is critical to biosolids-liquid separation. The settling velocity of algal-bacterial granules versus the diameter of granules was examined as a function of granule diameter in the sequence batch reactor in eleven days of cultivation. Despite granules having an apparently compact and rounded shape and considerable scatter of the data, the results never showed granules are homogeneously dense packed structures in the range of 10 μm to 1.0 mm in diameter. The settling velocity increases with granule size. The estimated fractal dimension was always less than two. The result was interpreted as forming heterogeneous compact layered structures in the shell from the early stage of cultivation rather than rearrangement from a bulk floc into a dense structure. Additional analysis for the cultivation of 150 days confirmed the considerable growth of size and fluffy hair. However, the settleability of small fragments remained relatively the same as heterogeneous compact structures. [Display omitted] •Close observation of the settling velocity of ABGS was carried out.•The velocity data of ABGS was confirmed to have a large scatter.•The data is not influenced by oxygen conditions.•The estimated fractal dimension of ABGS always shows less than two.