Filament content threshold for activated sludge bulking: artifact or reality?

An often-noted feature of activated sludge settleability is the existence of a threshold content of filamentous bacteria, below which filaments have a relatively small effect on settleability, and above which settleability is much more sensitive to filament content. This concept has historically been based on an apparent inflection point in settleability/filament content curves. However, this was demonstrated to be an artifact of data presentation on semi-logarithmic scales. Analyses of several published data sets indicated that there was little or no evidence of a threshold effect when data were presented on linear scales, particularly around the 150 mL/g sludge volume index (SVI) "threshold". Rather, there appears to be a generally continuous (and often nearly linear) relationship between filament content and SVI, suggesting that the threshold effect, as commonly perceived, is not accurate. A modified empirical model of filament content and settleability was also presented.