Forward osmosis (FO) is a concentration process based on the natural phenomena of osmosis. It is nowadays considered a breakthrough technology that can be potentially used for concentrating suspensions and solutions, including wastes such as sewage. The diluted nature of sewage restricts the treatment technologies that can be applied. Then, sewage concentration by FO could represent a habilitating technology enabling the application of a wider range of treatment alternatives. This research studied sewage and pre-filtered sewage concentration by a FO. Thin-film-composite (TFC) and cellulose triacetate (CTA) FO membranes were considered. Results showed that TFC membranes presented higher fluxes than CTA, when operated with clean water and sewage. Filtration experiments were done using sewage and filtered sewage as feeds. Even though differences in formed fouling layers were identified by scanning electron microscopy, observed fluxes were similar. Energy dispersive X-ray spectroscopy was done to study the elemental composition of the fouling layers. Results revealed nitrogen contents in the range 10-13%, suggesting a relevant role of protein. At tested conditions, TFC membrane showed little apparent ammonia rejection due to permeation to the draw solution. Ammonia permeation was independent of applied osmotic potential, and was mainly related with the ammonia concentration difference through the membrane. Results show that concentration of sewage by forward osmosis would be technically feasible, and its application could potentially improve energy recovery from sewage. However, main drawback that needs to overcome is a low water-flux, which translates into high membrane requirements. (C) 2016 Elsevier B.V. All rights reserved.