Determining the mechanisms for aerobic granulation from mixed seed of floccular and crushed granules in activated sludge wastewater treatment

Aerobic granulation is a novel and promising technology for wastewater treatment. However, long start-up periods required for the development of granules from floccular sludge, and the loss of biomass in this period leading to poor nutrient removal performance are key challenges. In a recent study the addition of crushed granules to a floccular sludge significantly reduced the start-up period, and also maintained the nutrient removal performance during granulation. In this study, we examined the mechanisms responsible for the fast granulation from a mixture of floccular and granular sludges. Fluorescent microbead particles (4 μm diameter) were successfully applied to differentially label the surfaces of floccular and crushed granular aggregates. Labelled flocs and crushed granules were added to a laboratory scale wastewater treatment reactor, and the granule formation process was monitored using confocal laser scanning microscopy over an 80 day period. Flocs were observed to attach to the surface of the seeding granules, resulting in reduced biomass washout during granulation. This mechanism not only reduces the granulation period, but also maintains the nutrient removal performance of the reactor. The results indicate that the granules acted as nuclei for floccular particle attachment, which accelerated granule formation.

Graphical abstractOutline of our study using crushed granules and floccular biofilm that are fluorescently labelled and some hypothetical outcomes of granule formation mechanisms. Step A. The surface of flocs and granules are labelled with fluorescent microbeads (flocs red, granules green) and are mixed and used to seed a reactor. Step B. Larger granules form during the start-up period and the patterns of beads on the outside and within the granule are assessed to understand the mechanism of formation. Our results using this novel approach support hypothesis 1, that the crushed granules act as nuclei where the flocs attach.Figure optionsDownload full-size imageDownload high-quality image (142 K)Download as PowerPoint slideHighlights► Fluorescence microbeads approach was used to monitor aerobic granule formation. ► Adding crushed granules in the seeding floccular sludge enhanced granulation process. ► Flocs attachment in the crushed granules aid to retain more biomass during granules formation. ► Granules acted as nuclei for floccular particle attachment, which accelerated granule formation.