The relationship between mixed-liquor particle size and solids retention time in the activated sludge process

Licheng Chan, Shao Yuan Leu, Diego Rosso, Michael K. Stenstrom

Research output: Journal article publicationJournal articleAcademic researchpeer-review

12 Citations (Scopus)


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 μm) 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.
Original languageEnglish
Pages (from-to)2178-2186
Number of pages9
JournalWater Environment Research
Issue number12
Publication statusPublished - 1 Jan 2011
Externally publishedYes


  • Particle size distribution
  • Solids retention time
  • Wastewater treatment

ASJC Scopus subject areas

  • Environmental Chemistry
  • Water Science and Technology

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