Nonylphenol(poly)ethoxylates (NPnEOs) have caused great concern over the last few decades due to their lipophilic, hydrophobic, and toxic properties. NPnEOs can be degraded to a certain extent in wastewater treatment plants; however since they accumulate in sludge, understanding their influence on digester performance and their fate become important, especially in regards to land application of biosolids. In this respect, the biotransformation of nonylphenol diethoxylate (NP2EO), an accumulative metabolite of NPnEOs, into degradation products was monitored in NP2EO-amended lab-scale semi continuous anaerobic digesters. Determination and quantification of the degradation products, nonylphenol monoethoxylate (NP1EO), nonylphenol (NP) and nonylphenoxy acetic acid (NP1EC), was performed by gas chromatography-mass spectrometry (GC/MS). Results revealed that 90% of NP2EO was eliminated from the system within approximately 52 days following the spike. As biodegradation of NP2EO progressed, both NP1EO and NP accumulated in the sludge samples. NP1EC was not detected in any of the digesters. Methane production, solids reduction and COD removal were not affected by the spike of NP2EO or by the presence of biodegradation products (NP1EO and NP) in the digester system. The persistence and accumulation of NP and NP1EO verifies concerns about the potential danger for land application of biosolids. (C) 2016 Elsevier Ltd. All rights reserved.