Influence of the type of treated sludge on LAS degradation after soil application: Risk assessment and considerations under the EU Directive 86/278/EEC

In 2000, the European Commission issued a draft document to regulate the application of sludges from wastewater treatment plants to soil, proposing concentration limits for certain organic compounds, including Linear Alkylbenzene Sulfonates (LAS). A concentration limit of 2600 mg/kg dry matter (dm) for LAS was set, although requirements vary across EU Member States. The aim of this work was to assess the influence of different types of treated sludge (anaerobically-digested, aerobically-digested, dehydrated, lagoon, and compost) on LAS degradation after soil application and to conduct an environmental risk assessment, considering the provisions of EU Directive 86/278/EEC. Laboratory experiments showed LAS concentrations in treated sludges ranged from 236 mg/kg dm in compost to 11,302 mg/kg dm in lagoon sludge. After soil amendment, LAS concentrations in soil ranged from less than 1.5 mg/kg to 66 mg/kg dm, and after three weeks, they ranged from 0.7 to 19 mg/kg dm. LAS degraded rapidly, with half-lives (DT₅₀) from 2 days (dehydrated sludge) to 13 days (anaerobically-digested sludge and compost). Degradation followed a double-exponential decay model, with faster degradation in the solution phase compared to the sorbed phase. DT₅₀ values for LAS increased with chain length for anaerobically- and aerobically-digested sludges but decreased for dehydrated sludge. The type of sludge influenced LAS degradation due to microbial populations and LAS sorption. LAS posed no ecological risk when applied via treated sludge, except for lagoon sludge, which posed a risk for up to seven days. Composting was the most recommended treatment for sludge prior to land application.