SUMMARY: To enumerate the bacteria present in activated sludge it is necessary to release them undamaged from the flocs. Samples of diluted activated sludge were sonicated for periods ranging from 20 to 240 s at five different intensities. During sonication, floc disruption was followed by measuring the absorbance of treated samples at 400 and 610 nm, energy input by recording changes in temperature and the release of viable organisms by plating techniques.

The release of bacteria depended on the intensity and duration of sonication. Most viable bacteria were recovered after 80 to 100 s at a power output of 26 J s. Patterns of recovery of viable bacteria from 10 different activated sludges were of two types. Comparable recovery of bacteria from identical samples showed that two instruments could be successfully compared using physical criteria only.

After sonication the number of bacteria recovered depended on the isolation medium. More bacteria were recovered from Casitone glycerol yeast extract agar than from Tryptone glucose beef and yeast extract vitamins agar (TGEVA) for five out of six sludges, and more were recovered from minimal pyruvate than from TGEVA for three out of six sludges.


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