Expanding the activated sludge model no.1 to describe filamentous bulking : The filamentous model

Several wastewater treatment plants (WWTPs) worldwide have documented the occurrence of filamentous bulking in full-scale systems despite the efforts made for filamentous bulking control. The Activated Sludge Models (ASM) can neither describe nor predict filamentous bulking at WWTPs. This research aims to expand the ASM No. 1 to be able to describe filamentous bulking sludge and to model the effects of incorporating an aerobic selector on filamentous bulking. Four theories (hydrolysis of slowly biodegradable organics theory, kinetic selection theory, substrate diffusion limitation theory, and filamentous backbone theory) were combined to expand the ASM1. The results showed that this combination was successful to distinguish between the substrate uptake by filamentous organisms and by floc forming organisms. Moreover, the concentrations of filamentous and floc forming organisms inside the reactor were converted to a “filamentous score” that predicted the outcome of filamentous bulking. Filamentous bulking would occur if the filamentous score was higher than 3, in a range of 1–6. As a case study, the Fuhais WWTP in Jordan was modelled using the expanded-ASM1 “filamentous model” and the filamentous score of 4.2 was in accordance to the visually observed bulking. However, when an aerobic selector with 3 compartments would be added before the aeration tank, the filamentous score decreased to 1.5. The selector changed the hydraulic behaviour from a completely mixed mode to a plug flow mode, which created a substrate gradient in the model, making the floc forming organisms to outcompete the filamentous organisms. Additional experimental results are required to further calibrate and validate the filamentous model.