Influence of Anaerobic Digestion with Pretreatment on the Phytotoxicity of Sewage Sludge

The aim of this study is to evaluate the influence of anaerobic digestion with pretreatment on the phytotoxicity of sewage sludge. The phytotoxicity was evaluated on sewage sludge (SS) and biosolids that came from conventional anaerobic digestion (CAD) and anaerobic digestion with a pretreatment by sequential ultrasound and low-thermal hydrolysis, called advance anaerobic digestion (AAD). To compare the phytotoxicity, eight elutriate concentrations (0.5–100% v/v) from SS, CAD, and AAD were studied on three testing plants: Lactuca sativa, Raphanus sativus, and Triticum aestivum. The percentages of seed germination inhibition, root elongation, and germination index (GI) were evaluated. GI is an phytotoxicity indicator that combines seed germination and root growth, therefore reflecting a more complete estimation of toxicity. Phytotoxicity assays showed that SS, CAD, and AAD elutriates have a beneficial effect on R. sativus. Similar results were observed for T. aestivum for CAD and AAD, with GI values up to 80% in both biosolids. Only for SS, moderate toxicity was observed in T. aestivum. Moreover, L. sativa showed GI values below 50% for SS and CAD, which reflected high toxicity. Only for AAD, no presence of phytotoxic substances was observed in L. sativa. This study concluded that biosolids from AAD improved the plants’ development with a GI above 78% with respect to biosolids from SS and CAD and reduced the phytotoxicity of sewage biosolid.