The main objective of many activated sludge plants treating wastewater from the pulp and paper industry is to remove COD only. These plants are often designed as high-load aerobic systems without any microbial selector system. As a consequence the sludge settling properties are normally poor due to fast growing filamentous microorganisms, which severely reduce the treatment capacity and the effluent quality.

Implementation of selectors, in which the substrate concentration and the metabolic pathways can be manipulated, has in many cases reduced the bulking sludge problems in activated sludge systems. An example of a successful upgrading of a Danish pulp industry wastewater treatment plant with an anoxic selector is presented. the use of a novel technique to investigate the in situ physiology of filamentous microorganisms is discussed. It is concluded that a successful application of selectors relies on detailed knowledge about: a) physiology and substrate requirement of the filamentous microorganisms, b) wastewater composition and c) substrate removal kinetics in the selector system.

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