Monitoring filamentous bulking in activated sludge systems fed by synthetic or municipal wastewater.

The stability with respect to filamentous bulking of two activated sludge fully-aerobic systems, one with a completely mixed tank and one with a channel reactor, fed either by a synthetic wastewater or by a primary-settled municipal wastewater, of variable composition and flow rate, has been investigated. The morphological characteristics of the biomass in terms of floc size and roughness and of filamentous bacteria abundance have been monitored by image analysis. Severe bulking was only observed in the well-mixed tank fed at a constant flow rate by synthetic substrate of constant concentration, when the channel reactor fed in a similar manner was fully stable. Variations of biomass characteristics as well as of settling properties were observed on both systems fed with the real wastewater, but these events were related to the characteristics of the wastewater, as similar changes were observed on the full-scale plant fed with the same substrate. In any case, automated image analysis was an efficient way to monitor in detail the fate of the activated sludge at pilot and full scale.