Fouling control using critical, threshold and limiting fluxes concepts for cross-flow NF of a complex matrix: Membrane BioReactor effluent

Abstract

The optimization of permeate flux is a particularly interesting strategy to control fouling and, as a consequence, enhance productivity for nanofiltration (NF) processes. Critical flux, threshold flux and limiting flux theories represent significant advance in this strategy. The aim of this study was to apply these concepts to achieve fouling control during NF of a real complex matrix: hospital wastewater after Membrane BioReactor treatment (MBR permeate). At low pressure (3bar) no flux decline was observed, revealing no fouling conditions. By applying a range of transmembrane pressure and using the square-wave method, the critical flux for irreversibility (70Lh−1m−2) and the corresponding critical pressure (3.4bar) were then determined for the NF process in this complex matrix. Above these critical conditions, irreversible fouling started to occur. The threshold pressure and related flux (transition points between low and high fouling regions) were then searched by critical flux data conversion. Our results suggest, even if an exact value for the threshold pressure could not be determined, that it could be located in the range 3.4–10bar. Operating in this pressure range should lead to acceptable fouling rate and flux decline.During filtrations conducted above the critical flux in the MBR effluent, two stable fluxes behaviours were observed indicating that different fouling stages occur: pseudo stable flux was 67Lh−1m−2 at 5bar, whereas 33Lh−1m−2 at 10–35bar. It can be then confirmed that a limiting flux occurred in this system, the value of which 33Lh−1m−2 is rather lower than that of the critical flux. This flux behaviour was proved to be related to a severe fouling in the pressure range 10–35bar due to a combined effect of colloidal silica and organics fouling and calcium phosphate scaling. The early fouling stage at 5bar was expected to be solely related to colloidal silica and organics accumulation. To characterise this change in fouling behaviour, a method allowing the estimation of the permeability before scaling was proposed. The combination of the permeability before scaling and critical flux has enabled a NF working diagram to be drawn from which the fouling stage for a given transmembrane pressure and corresponding permeate flux was able to be determined.