19.2.1 Activated Sludge Samples

Activated sludge samples were taken from the laboratory-controlled sequencing batch reactors (SBRs) fed a synthetic wastewater containing glucose and inorganic salts. The SBRs were operated at different SRTs (4 to 20 days). Details of the SBR system are given by Liao et al.15

19.2.2 Contact Angle Measurement

Sludge samples collected from the SBRs were first washed with deionized distilled water twice using a centrifuge at 2000 x g for 5 min each time. Then the washed sludge samples were dispersed by a Vortex mixer and deposited on a membrane filter (Black MSI Microsep*, 0.45 ^m) under 400 mmHg vacuum. The sludge cake was filtered until moist and there were no signs of excess water that could be sucked. Contact angle of deionized distilled water on sludge cakes was measured on a partially hydrated sludge cake using the axisymmetric drop shape analysis-contact diameter (ADSA-CD) technique.20,21

19.2.3 Liquid Surface Tension

Surface tension of the treated effluent was determined by a CENCO tension-meter (Sigma Chemical Co., MO) equipped with a 6 cm diameter platinum ring at ambient temperature (21 ± 2°C). Prior to surface tension measurement, the effluent was centrifuged at 15,000 x g for 15 min at 4°C to remove colloidal particles.

19.2.4 Effluent Suspended Solids

The flocculating ability of activated sludge was evaluated by determining the ESS after 40 min settling of mixed liquor in the SBRs. The mixed liquor suspended solids at different SRTs were maintained at the same level (2000 ± 150 mg/l). The measurement of ESS was in accordance with Standard Methods.22

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