The research reported here is based on a flat membrane demo-model demonstrating flow through the spacer between two membranes. The main objectives of the investigation were to measure velocity and estimate the magnitude of mixing index (MI) within a unit cell of the turbulence promoter net. The control of mixing intensity next to the membrane walls enables controlling parameters such as flux, product concentration and the local mass transfer coefficient. The particle image velocimetry (PIV) method was used for velocity measurements and MI estimations. The results show that flow direction changes occur near the spacer rods mainly due to the existence of an obstacle that the flow had to bypass. Significant mixing intensity can be found at higher flow rates than those used regularly in membrane processes. A simple, steady state two-dimensional flow analysis confirmed the measurements' main findings. It was also shown that the spacer between the membranes may induce membrane fouling close to the filaments. (c) 2005 Elsevier B.V. All rights reserved.