The outcome of a 21-month monitoring study on anionic surfactants (AS) at five (27-70 ML/d) up-flow anaerobic sludge blanket (UASB) based sewage treatment plants (STPs) is described. The average removals of AS were around 8-30%. Appreciable concentrations of AS were being discharged to the watercourse (average 4.30 mg/L; range 3.60-4.91 mg/L). On an average dried sludge contained 1452 mg AS/kg dry weight. Mass balance at three STPs indicated that, AS load of the order of 5-17% and approximate to 12% is removed by adsorption in UASB reactors and polishing ponds (PP) respectively Biodegradation of AS under anaerobic conditions in UASB reactors and PP does not seem to take place. in the sludge stream, appreciable biodegradation (approximate to 46%) of adsorbed AS under aerobic conditions on the sludge drying beds takes place. If influent AS mass flux is normalized to 100 units, than average of approximate to 74 and approximate to 7 units are discharged with treated effluent and dried sludge respectively, while 12 and 6 units are adsorbed/settled in PP and aerobically biodegrade on sludge drying beds respectively At two STPs (34 and 56 ML/d), the filterable fluxes in UASBR increased so that the mass balance could not be computed. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All fights reserved.