Occurrence of Linear Alkylbenzene Sulfonates Homologues in Sludge Stabilization Treatments

Linear alkylbenzene sulfonates (LAS) are one of the organic pollutants of most concern in sewage sludge due to their widespread occurrence in domestic sewage. In this work, the occurrence of LAS was assessed in 15 wastewater treatment plants (WWTPs), with different sludge stabilization treatments, from September 2023 to March 2024. Samples were analyzed by ultrasound-assisted extraction and LC-MS/MS. In primary sludge, LAS homologues displayed the typical fingerprint of laundry detergents, suggesting these products are a primary source in influent wastewater. There was no clear correlation between the population served and the LAS concentrations in the studied WWTPs. The highest concentrations of LAS (sum of the homologues C10–C13) were found in anaerobic lagoons, followed by aerobically (6438 mg/kg) and anaerobically digested (5521 mg/kg) sludge. The lower levels were observed in composted sludge (215 mg/kg). 100% of the composted samples showed concentrations lower than 2600 mg/kg (concentration limit currently proposed by the EU for LAS), while these percentages were reduced to 25 and 13% in the case of aerobically and anaerobically digested sludges. These results showed that composting could be an effective method for ensuring compliance with a future EU Directive on sludge application to the soil.