Bulking and foaming phenomena in activated sludge wastewater treatment plants are in most cases related to the abundance of filamentous microorganisms. Quantifying these microorganisms should be a preliminary stage in their control. In this paper, the simplicity of quantifying them based on the intersection method is demonstrated, by redescribing the theory and applying a new improved protocol; new data of interest are also provided. The improved method allows us to use it with stained smears, including epifluorescence techniques. The error that could be made, when considering the distribution of filamentous bacteria in fresh microscope preparations in two dimensions rather than three is negligible. The effect of the different types of filamentous microorganisms on the settleability was also studied. The effect of the total extended filament length on the sludge settleability was shown to depend on the type of filamentous organism and how it aggregates. When these groups of filamentous organisms are found in small aggregations and there is an increase in the number of filamentous organisms, the sludge volume index (SVI) increases proportionally to the filament length. However, when aggregation increases, the impact on the SVI is significantly lower.

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