Bulking and scum formation (scumming) are common phenomena in activated sludge plants. Four main problem groups with filamentous microorganisms with different growth requirements are distinguished: (1) “Aerobic Bulking”, (2) “Scumming due to Actinomycetes”, (3) “Low F/M Bulking and Scumming” and (4) “Bulking due to Sulphide Oxidising Bacteria”. Various wastewater fractions, such as readily biodegradable substrate, surfactants, hydrophilic and lipophilic slowly-biodegradable substrate, inoculated filamentous microorganisms and sulphide, strongly influence the biocenosis composition. Apart from wastewater composition, operating conditions, such as solids retention time, reactor configuration, mechanical stress by recirculation pumps and insufficient oxygen supply, are often decisive for the occurrence of a bulking or scumming problem, as well. A predictive scheme for the possible occurrence of the four main problem groups is presented for different combinations of wastewater compositions, operating conditions and reactor design. All four problem groups can occur in aerobic nitrifying systems. In non-nitrifying systems, only “Aerobic Bulking” and “Bulking due to Sulphide Oxidising Bacteria” are expected, depending on wastewater composition and operating conditions. In systems with a large anoxic mass fraction, “Low F/M Bulking and Scumming” is typically the only problem group. Since wastewater fractions can hardly be manipulated in practice, at least operating conditions and reactor design should be optimised in order to obtain a better performance of the plants.

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