Conventional activated sludge (CAS) and sludge obtained using hydro-cyclone selective wasting were compared at a full-scale water resources recovery facility. The tested sludge, containing around 30–50% of aerobic granules, showed enhanced settleability with low and stable sludge volume index (SVI) compared to CAS sludge which suffered recurrent filamentous bulking. Further in-depth batch settling tests were carried out using a 40 cm diameter column fitted with ultrasonic transducers to monitor both sludge blanket height and vertical velocity profiles. Hindered settling and compression parameters were calibrated from the experiment for latter modelling use. Test sludge displayed more than doubled settling velocities compared to CAS, with hindered settling velocities remaining >3 m·h−1 even at high solids concentrations of 6.85 g·L−1. The compression regime was attained at much higher critical concentration for the test sludge. It also displayed enhanced thickening properties, with concentrations obtained after 30 min of settling being 20.9 and 8.5 g·L−1 respectively for test andcontrol sludge. This allows for a substantial reduction of recirculation rates in practice. These results open perspectives in optimizing existing plant operation as well as clarifier design and modelling using densified sludge.

  • Hydrocyclone selective wasting helped to control activated sludge seasonal bulking and to stabilize dSVI below 50 mL·g−1 TSS.

  • Hindered settling and compression model parameters were calibrated.

  • Densified sludge settles faster at high concentrations, suggesting possibilities to increase clarifier load and biomass inventory.

  • Densified sludge stable settling parameters open perspectives for improved design.

Graphical Abstract

Graphical Abstract
Graphical Abstract
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Supplementary data