Behavior of fluorescent whitening agents during sewage treatment

Fluorescent whitening agents (FWAs) are contained in most modern laundry detergents and are thus discharged in substantial quantities with household wastewater. To determine the mass flows and fate of the predominant detergent FWAs, a field study was conducted at a full-scale mechanical–biological sewage treatment plant at Zürich-Glatt, Switzerland. Samples of wastewater (raw sewage, and primary and secondary effluent) and sludge (raw, activated, and anaerobically-digested sludge) were collected during a 10-day period. The concentration of FWAs in water samples was determined using solid-phase extraction with C18 disks and high-performance liquid chromatography (HPLC). The concentration of FWAs in sludge was determined by supercritical fluid extraction and HPLC. The removal of FWAs from wastewater during sewage treatment varied significantly (53–98%) between different FWAs. Removal was due to adsorption to primary and activated sludge and the observed extent of removal was consistent with the individual sorption behavior of FWAs. No evidence for biodegradation of FWAs was found during the aerobic biological treatment of municipal wastewater with activated sludge and during anaerobic–mesophilic digestion of sewage sludge. Average annual discharge of FWAs to Swiss surface water is approximately 20 t (22% of the consumed FWAs) while FWA disposal to farmland associated with sewage sludge is 11 t/y (12%), estimated based on the extent of removal from sewage and on a previously reported survey of FWA concentrations in sewage sludge. The discharge of FWAs to surface water leads to a projected average FWA concentration in surface waters of 0.5 μg/l, which is well below toxic levels for these compounds.