Since the mid nineties the MBR technology has been introduced into municipal wastewater treatment in Europe. First wwtps went in operation performing a conventional mechanical treatment without any advanced mechanical pre-treatment unit. After a short operation period braids were hanging between the membranes and hence module sludging was observed. Thus, mechanical pre-treatment was upgraded introducing sieves primarily to reduce hairs and fibrous substances. Today, however, knowledge is still relatively poor concerning effectiveness and treatment efficiency of a large number of aggregates that had been brought to market, varying in gap size, gap geometry and others. This paper describes the fundamentals of sieving, gives an overview of mechanical pre-treatment on municipal MBR plants and finally presents the results of investigations quantifying and comparing the removal efficiency of different types of sieves.