Many WWTPs are reaching their maximum treatment and clarification capacities, and solutions must be found to upgrade those existing plants. One relevant approach is to replace conventional primary settlers by micro-sieves, in order to recover reactor volume that can then be used for BOD and nutrient removal. Micro-sieves have been developped over the past few years and are characterised by a mesh size of 10 to 500 µm. Micro-screens are extremely compact technologies and can capture between 20-65% of solids and up to 40 % of the BOD, thus offering a relevant alternative over conventional primary settler. Different micro-sieving technologies are being developped: drum-screen, rotating-belt filter, in combination or not with coagulation/flocculation.