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The Relationship Between Mixed-Liquor Particle Size and Solids Retention Time in the Activated Sludge Process

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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.

Keywords: particle size distribution; solids retention time; wastewater treatment

Document Type: Research Article


Affiliations: 1: *Civil and Environmental Engineering Department, University of California, Los Angeles, CA, 90095, U.S.A.; 2:  Civil and Environmental Engineering Department, University of California, Irvine, California

Publication date: 2011-12-01

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  • Water Environment Research® (WER®) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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