The Activated Sludge (AS) Process is old technology but is still widely adopted worldwide for its convenience and simplicity: an impressive number (many hundred of thousands) of this kind of system are in operation.Occasionally, problems such as bulking and foaming occur, causing regulation violations and large investment is often required immediately to control them. For this reason, an intense research effort has been made during the last few decades to face these problems, and this report details the work undertaken by the IWA Specialist Group on Activated Sludge Population Dynamic.This Scientific and Technical Report describes the main reasons for the growth of the different filamentous microorganisms in activated sludge, and the biological molecular tools available today for the identification of the main biomass components. The wide range of experiences around the world is documented and the methods to avoid the proliferation of these organisms are presented and critically reviewed.Activated Sludge Separation Problems consists of seven chapters, presenting up-to-date technical and scientific aspects of these processes.Specific control methods for some filamentous bacteria that in the past were the plague of AS systems have been identified and adequately tested. Remedial actions are listed for bacteria such as Sphaerotilus natans, Type 1701, Thiothrix spp., Thiothrix eikelboomii (the former Type 021 N), and even for the "King" filamentous bacteria causing problems of foaming and bulking: Microthrix parvicella.Activated Sludge Separation Problems gives a picture of the full-scale approaches made worldwide to control bulking and foaming dysfunctions, with c.100 pages dedicated to experiences in thirteen countries worldwide over the last ten years.New developments in the biomolecular microscopy tools used to identify filamentous bacteria are also described in detail, such as FISH (Fluorescence in Situ Hybrydization), facilitating observance of the protocol that the proper identification of the microorganisms responsible for AS process dysfunctions is in fact the key to implementing the correct control action.Activated Sludge Separation Problems is a compendium of present knowledge on microbial populations operating in activated sludge plants, and covers:- methods to characterize activated sludge flocs, the main filamentous bacteria and important microbial populations such as nitrifiers and Poly-P bacteria.- the design and management criteria for selectors, promising solution for bulking control.- basic methods of clarifier design.- modelling of bulking phenomena.This Scientific and Technical Report will be a valuable source of information for all those concerned with activated sludge separation including both plant operators and academic researchers. The information contained here will help wastewater treatment plant operators to save money through:- optimising clarifier operating conditions.- identifying the microorganisms responsible for dysfunctions.- reducing coagulant and toxic compound addition by preventive action.- properly planning a control strategy.