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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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The biodegradation of surfactants is a frequent and complex problem in domestic and industrial wastewater treatment processes. In addition to the resulting metabolites being sometimes refractory, the complete biodegradation of many of the most employed non-ionic surfactants requires long hydraulic retention times and the presence of specialised bacterial consortia. Preliminary acclimatisation tests highlighted the importance of the sludge acclimatisation state to a specific surfactant substrate for biotreatment efficiency. This paper reports on studies aimed at quantifying activated sludge acclimatisation and memory retention levels when subjected to changes in the type of surfactant included in the feed. Several transitions were tested, namely from an alkylphenol ethoxylate to a linear alkyl ethoxylate and the reverse, and between alkyl ethoxylates with different hydrophobic and hydrophilic molecular chain lengths. The kinetic results showed that sludge activation and memory loss were more dynamic for primary biodegradation It was found that the sludge was harder to adapt to alkylphenol ethoxylate than to alkyl ethoxylate. The former also apparently introduced an inhibitory effect, resulting in very slow degradation kinetics when imposed to alkyl ethoxylate acclimatised sludge. When replacing an alkyl ethoxylate with another surfactant of the same family, a longer ethoxylate chain reduced the degradation rates. This effect was further enhanced by simultaneously increasing the hydrophobic chain length of the substrate. The acclimatisation kinetic after the replacement of an alkyl ethoxylate by a longer counterpart was slower than the reverse case, and memory was also more easily lost.