The present study was conducted for thickening of activated sludge by continuous electro-flotation (EF) process. The effects of some key factors such as initial pH, current density, operating time, electrode type (stainless steel and graphite) and operation conditions on the sludge thickening by determine of sludge volume reduction (SVR) and sludge solid concentration (SSC) and as well as removal of chemical oxygen demand (COD), total solids (TS), total suspended solids (TSS) and color were investigated. The results showed that the process has a good efficiency. The highest amount of SVR efficiency (89.3%) and SSC (38 g L-1) were achieved at current density of 8 mA cm(-2) in 15 min for stainless steel. Moreover, as surface/volume ratio increased, better thickening happened because increases both mass transfer and electro-generation of O-2 and H-2 at the surface of electrodes in low applied current density. Accordingly, consumed electrical energy was 0.15-1 kW h m(-3). Although suitable cell design is entirely essential, the use of chemicals and temperature increase are not effective. Consequently, EF is a comparatively appropriate process for thickening; in the water separated from the process, the amounts pertaining to COD, TS, TSS and color were respectively 112, 1601, 140 mg L-1 and 5 TCU which are useable for subsequent different consumptions. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.