Feedback control system for filtration optimisation based on a simple fouling model dynamically applied to membrane bioreactors

Abstract

Increasing permeate flux whilst maintaining energy demands at reasonable levels is of great interest for mem- brane bioreactors (MBRs) widespread application. One of the main issues for this is to develop effective fouling control strategies under dynamic conditions. This study proposes a filtration length-based feedback control system for optimal filtration performance. A pilot-scale tertiary MBR was used for the control system validation. The control permitted the adjustment of the permeate flux and the final allowable transmembrane pressure at real-time, in order to obtain the desirable filtration length within a filtration cycle. A flux-step procedure was used to develop the model-based control law. The control system was tested by short-term tests under different initial permeate fluxes (30–60 L/h m2 ), transmembrane pressure increment factors (15–25 Pa) and sludge characteristics (1.8–3.5 mg/L of biopolymeric clusters), revealing a short transient response and a negligible steady-state error. The potential of this control system was reinforced during long-term operation at unsteady organic load conditions. At pre-fixed filtration time of 5 min, continuous operation at supra-critical fluxes (47.1–56.8 L/h m 2 ) was maintained at low specific aeration demands (SAD pnet= 6.1–8.2 Nm 3 air/m3 permeate ). The control strategy designed in this study achieved better reversible fouling control than the conventional one (fixed permeate fluxes) which in turn led to a considerable lower residual fouling rate.