The fate of linear alcohol ethoxylates during activated sludge sewage treatment

Model continuous activated sludge (CAS) plants (Husmann units) were used to study the fate of two commercial, alcohol ethoxylate (AE) surfactants duri ng aerobic sewage treatment. The surfactants were produced by the ethoxylat ion of an essentially linear C12-15 alcohol (NEODOL 25((TM))) with an avera ge of 7 (C12-15EO7) or 3 (C12-15EO3) moles of ethylene oxide (EO). Recent a nalytical developments made it possible to measure levels of AE that includ ed the free alcohol and EO1 oligomers across the CAS system, from the influ ent feed, on the activated sludge, through to the effluent. Measured concen trations of AE (as C12-15EO0-20) in the synthetic sewage feeds to the test CAS plants lay in the range 11-13 mg/l. During stable operation at 20 degre esC, an average of 5 mug/l AE were present in the C12-15EO7 CAS plant efflu ent, giving a removal (bioelimination) of > 99.9%. When levels of AE on the sludge, and polyethylene glycols (PEGs - an expected biodegradation interm ediate) in the effluent and on the sludge were also taken into account, bio degradation was considered to be responsible for > 98.7% of the observed re moval. During operation at a winter temperature (10 degreesC), an average o f 26 mug/l AE were present in the C12-15EO7 CAS plant effluent, giving a re moval of 99.8%. Biodegradation was estimated to be responsible for > 97.2% of the observed removal. During operation at 20 degreesC, an average of 7 m ug/l AE were present in the C12-15EO3 CAS plant effluent, giving a removal of > 99.9%. No analysis for PEG was performed in this case but the low leve l of AE on the sludge (0.2 mg/g dry solids) suggested that biodegradation w as responsible for most of the observed removal. Neither surfactant had any adverse effect on the sewage treatment efficiencies of the CAS plants in t erms of dissolved organic carbon (DOC) removal, nitrification or biomass le vels. (C) 2001 Shell Global Solutions (UK) Ltd. License to publish granted to Elsevier Science Ltd.