Engineered nanomaterials (ENMs) are commonly used in commercial products nowadays. As a result of increasing demand and consumption, a greater amount of nanoparticle (NP) containing products eventually reach wastewater treatment plants, incineration plants and landfills. Anaerobic digestion (AD) processes are extensively used for stabilization of sewage sludge and treatment of wastewater. The impacts of NPs, such as silver (Ag), zinc oxide (ZnO), titanium dioxide (TiO2), and copper oxide (CuO) within activated sludge (AS) wastewater treatment systems have recently been discussed in literature. In addition, some studies also cover the effects of these NPs on AD of sewage sludge. However, there exists relatively little information about the impacts and behaviors of inorganic metal oxide NPs on anaerobic wastewater treatment systems and anaerobic waste stabilization phase in landfills during conversion of organic matter into biogas. Furthermore, in some cases, different conclusions are drawn from studies investigating the impacts of NPs on the microbial activity during AD of sewage sludge as well. Generally, it can be concluded that metal oxide NPs such as CuO, ZnO, CeO2 and in some cases Ag, exhibit system perturbation during AD of sludge and wastewater, due to release of metal ions from NPs with respect to different environmental conditions. (C) 2015 Elsevier Ltd. All rights reserved.