Activated sludge constitutes a widely used inoculum source in biodegradability tests, due principally to its microbial diversity and ubiquity. In ready biodegradability tests, the composition and the quantity of this type of inoculum are the major factors which affect the reproducibility of results. Experiments with dodecylbenzene sulfonate were carried out to evaluate the impact of the non-controlled inoculum on test response. At low inoculation levels, cellular growth and soluble residual carbon accumulation were favoured and low mineralization levels were obtained. A procedure for inoculum preparation was proposed: it consists in a pre-treatment of activated sludge, collected in different municipal wastewater treatment plants, by centrifugation (500 g for 4 minutes) which enabled the supernatant cellular density (CFU concentration) to be more uniform than with the conventional decantation procedure (30 min).

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