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Updated: May 24, 2026

Measuring the Shape and Size of Activated Sludge Particles Immobilized in Agar with an Open Source Software Pipeline
Published on: January 30, 2019
The relationship between mixed-liquor particle size and solids retention time in the activated sludge process
Licheng Chan1, Shao-Yuan Leu, Diego Rosso
1Civil and Environmental Engineering Department, University of California, Los Angeles, CA 90095, USA.
Abstract:
Particle size distribution (PSD) analysis was used to evaluate the quality of mixed liquors collected from different activated sludge process modifications (i.e., conventional activated sludge, modified Ludzack-Ettinger, high-purity oxygen, step-anoxic, and oxidation ditch). An experiment protocol was developed to define the allowable sample holding time and provide representative and repeatable results. Samples of 26 treatment plants, with a total of 37 samples, were tested. A new indicator, called mean particle size (MPS), was introduced to describe the integrated mean particle size. The results of MPSs of three cut-off sizes (0.5 to 50, 100, and 200 microm) showed that the average size of mixed-liquor biosolids increased with increasing solids retention time (SRT), and the number of particles in the sedimentation supernatant decreased with increasing SRT. Particle deflocculation occurred after excessive sample holding time, and analysis within 12 hours generally eliminated sample holding problems. The results provide a methodology using PSD for characterizing mixed-liquor biosolids.
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