In the usual wastewater treatment schemes the application of membranes follows a biological process. However, ultrafiltration of untreated wastewater produces permeate free of particles and bacteria in one single step and could represent the starting point for new water-reuse concepts. This thesis focuses on crossflow ultrafiltration of raw municipal wastewater, which produces a sanitised water rich in nutrients (nitrogen, phosphorous and potassium) and dissolved organics. In a newly built facility, systematic data sets are generated while varying feed wastewater (untreated sewage, effluent of primary clarifier, coagulated sewage) and fundamental operating parameters (TMP, crossflow velocity, backflush cycle). The effects are evaluated in terms of filtration characteristics (flux decline and resistance increase) with respect to elapsed time and produced volume. In addition, mathematical modelling (blocking laws) is applied, with the purpose of supporting the interpretation of results by generating comparable parameters. Results show that at the appropriate operating conditions fouling is controlled and stable operation is achieved. At TMP = 0.3 bar and crossflow velocity = 1.5-2 m/s, a loose (dynamic) cake is formed on the membrane surface, allowing gross flux values above 100 L/(m2h) and protecting the membrane from other irreversible forms of fouling. The effects of sedimentation and coagulation on the process performance appear of minor importance, whereas the interpolation of flux curves with the cake filtration model appear a useful tool for the selection of the appropriate operating conditions. A preliminary cost estimate indicates that where the (re)use of water and nutrients is possible or necessary, the application of direct ultrafiltration may prove economically attractive.