Efficiency of two MBR systems (microfiltration and ultrafiltration) at producing reusable effluent was evaluated. The ultrafiltration MBR (0.034 mu m pore size) maintained recirculation sludge flow at six times the influent flow, and membranes were backwashed every 5 minutes. The microfiltration MBR (0.4 mu m pore size) maintained recirculation sludge flow at four times the influent flow, and membrane-relax was applied after the production phase. Permeates of both systems were shown to be suitable for unrestricted reuse (based on Spanish reuse guidelines); however their colour and aerobic bacteria concentration may lead to their rejection for certain purposes. The capacity of the MBR systems to eliminate organic matter depends on the biological activity in the active sludge, and it is not affected by the type of membrane. The concentration of coliphages is one of the differentiating characteristics between the two systems in terms of the quality of the permeate. This concentration was higher in the microfiltration system, due to the larger pore size, a fact which also explained the higher concentration of aerobic bacteria and the different particle size distribution. Reuse of effluent from ultrafiltration MBR systems is more safety in view of its microbiological and physicochemical quality. (C) 2012 Elsevier B.V. All rights reserved.