A series of batch and semi-continuous laboratory scale aerobic reactors was evaluated, with emphasis on the microbiological aspects. Batch treatment of waste activated sludge (COD 10,000 mg/l) was carried out over a period of eight weeks in three separate reactors at 5, 10 and 20°C. A semi-continuous unit operating at 5°C was investigated at a loading rate of 0.64 kg VSS/m3/day. Analyses performed included pH, O2 uptake rate, total solids, suspended solids, volatile suspended solids and COD determinations. The microbiological tests included total plate counts (20°C) and psychrotrophic counts (l°C).

Results indicated that long (impractical) detention times were required to aerobically treat waste activated sludge at low temperatures. At temperatures supposedly favourable to psychrotrophic growth, it was found that the proportion of psychrotrophic bacteria could not develop to a significant level and remained at less that 1% of the total viable bacterial population, regardless of the mode of operation or the detention time. This is in contrast to the finding that approximately 54% of the viable bacteria in the aeration tank of a large conventional activated sludge plant, operating at a mixed liquor temperature of 13°C, were psychrotrophic. It is apparent that psychrotrophic bacteria played an insignificant role in the degradation of waste activated sludge at low temperatures, (5 and 10°C) and that biological oxidation was performed primarily be mesophilic bacteria. The study indicated that under the experimental conditions employed, the amount of available substrate present was one of the major regulating factors in the development or selection of psychrotrophic bacteria.

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