Laboratory batch and sequencing-batch reactors and closed respirometry techniques were used to study the response of activated sludge to non-ionic synthetic surfactants. Two laboratory-grade and three commercial surfactant mixtures used in the textile industry (alkyl- and alkylphenol-ethoxylate types) were tested. Respirometric profiles during degradation experiments were complemented with titrimetric surfactant measurements and TOC determinations, representing primary and ultimate biodegradation, respectively. Though non-acclimatized sludge was apparently not inhibited by the surfactant products, total degradation took at least 20 hours with S0/X0 values in the 0.24–0.86 range. Respiration rate profiles (respirograms) showed several peaks, suggesting stepwise primary-ultimate degradation of the surfactant molecules. Acclimatization shortened degradation times and produced repirograms with a single peak. Biomass adaptation, response to load increases and memory loss was faster for primary than for ultimate degradation. Alkyl ethoxylate-adapted biomass was apparently more capable of adaptation to alkylphenol ethoxylate than the inverse.

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