The objective of the membrane technology presented here is the production of nitrogen concentrates from pretreated swine manure. This paper reports on the effect of osmotic pressure and substrate resistance on transmembrane flux during the concentration of prefiltered swine (PFS) manure by reverse osmosis (RO) membranes. The PFS manure at various concentration levels was filtered by four highly selective polyamide RO membranes with maximum allowable pressures ranging from 41 to 83 bar. The osmotic pressure created by the PFS manure on the RO membranes fitted a second-order equation with respect to manure conductivity or total ammonia-nitrogen (TAN), indicating that the rate of increase in osmotic pressure accelerated as manure was being concentrated. Average osmotic pressure increased by a factor of 6.8, from 5.4 to 36.6 bar, as TAN was increased 5.6 times, from 1.6 g to 9.2 g/l. Substrate-related resistance, which has been attributed to specific membrane-solute interactions even in the absence of flow, tended to increase as PFS manure concentration increased. However, reduction in transmembrane flux during manure concentration was mainly due to increase in osmotic pressure. If the objective of the technology is to concentrate manure in small volumes with high nitrogen concentrations, RO systems have to be equipped with membranes that are able to sustain high applied pressures, because the decrease in flow due to increased osmotic pressure along membrane elements will be substantial. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.