The initial fish weight was 500 g and the average feeding ratewas 2% of body weight. Fecal waste samples were collected anddiluted similar to the first experimental period. Total solids con-tent of the samples was determined according to APHA 2540 B(APHA, 1998). Each day of measurements, the diluted feces fromone tank were divided in three subsamples of 100 ml and soni-cated for either 0, 4 or 16 min. Between 3–10 mL of sample wereused directly in the OUR test, depending on the sample degrad-ability and the activity of the sludge in the OUR test. The sampleswere stored on ice at all times to prevent degradation. We usedthe OUR test to measure the effects of sonication on short-termCOD bioavailability. The sludge necessary for the OUR tests wasobtained from an upflow sludge blanket denitrification reactor,which has been in stable operation for a year with catfish underFig