Impact of sewage sludge treatment processes on the removal of the endocrine disrupters nonylphenol ethoxylates

Several treatment processes of mixed sludge naturally contaminated with nonylphenol ethoxylates (NPE) were compared in order to evaluate their efficiency for the removal of these endocrine disrupters. Anaerobic and aerobic treatments were carried out in continuous stirred tank reactors, operated separately or combined together, at mesophilic and thermophilic temperatures and with or without ozone post-treatment. Anaerobic mesophilic removal of NPE consisted of complete removal of nonylphenol diethoxylate, incomplete removal of nonylphenol monoethoxylate and non stoechiometric production of nonylphenol, with consequently a NPE removal of 25%. At thermophilic temperature, anaerobic digestion led to an increase of the total solids removal efficiency, while improving NPE degradation (30%). Under thermophilic aerobic condition, the three compounds were removed simultaneously with a NPE removal efficiency higher than under anaerobic condition (39%). This removal is always well correlated to the total solids removal meaning that bioavailability remains the main limiting factor. Combination of either thermophilic aerobic–mesophilic anaerobic or mesophilic anaerobic-ozonation treatments enhanced the NPE removal by comparison to single systems (45% and 48%, respectively). These results confirm the high potential of existing and up-grading sewage sludge treatments to degrade such refractory and aged compounds.