Abstract
Biofilm formation in membrane systems negatively impacts the filtration system performances. This study evaluated how biofilm compression driven by permeate flow increases the hydraulic resistance and leads to reduction in permeate flux. We analysed the effect of biofilm compression on hydraulic resistance and permeate flux through computational models supported by experimental data. Biofilms with homogeneous surface structure were subjected to step-wise changes in flux and transmembrane pressure during compression and relaxation tests. Biofilm thickness under applied forces was measured non-invasively in-situ using optical coherence tomography (OCT). A numerical model of poroelasticity, which couples water flow through the biofilm with biofilm mechanics, was developed to correlate the structural deformation with biofilm hydraulics (permeability and resistance). The computational model enabled extracting mechanical and hydrological parameters corresponding to the experimental data. Homogeneous biofilms under elevated compression forces experienced a significant reduction in thickness while only a slight increase in resistance was observed. This shows that hydraulic resistance of homogeneous biofilms was affected more by permeability decrease due to pore closure than by a decrease in thickness. Both viscoelastic and elastoplastic models could describe well the permanent biofilm deformation. However, for biofilms under study, a simpler elastic model could also be used due to the small irreversible deformations. The elastic moduli fitting the measured data were in agreement with other reported values for biofilm under compression. Biofilm stiffening under larger flow-driven compression forces was observed and described numerically by correlating inversely the elastic modulus with biofilm porosity. The importance of this newly developed method lies in estimation of accurate biofilm mechanical parameters to be used in numerical models for both membrane filtration system and biofouling cleaning strategies. Such model can ultimately be used to identify optimal operating conditions for membrane system subjected to biofouling.
Highlights
In the recent decades, membrane processes have become established technologies in drinking water and wastewater treatment
The biofilm hydraulic resistance depends on biofilm average thickness and permeability
An increase in biofilm hydraulic resistance decreases in turn the permeate flux
Summary
Membrane processes have become established technologies in drinking water and wastewater treatment. It has been reported that water permeation through biofilms can cause a structural re-arrangement, increasing biofilm density (Casey, 2007), decreasing the porosity and eventually increasing the hydraulic resistance (Derlon et al, 2016; Dreszer et al, 2013; Valladares Linares et al, 2015). The biofilm compression caused by permeation would lead to an elevated mechanical strength, with biofilms more difficult to be removed during the cleaning. It appears important to relate the biofilm hydraulic resistance both with the applied hydrodynamic conditions and with the biofilm mechanical behaviour. A better understanding of the link between the hydrodynamic conditions, the biofilm mechanical properties and its resulting hydraulic resistance is required to properly define operating strategies for membrane processes. An optimal operation of membrane systems should aim at maximizing permeate flux while minimizing compression of the biofilms
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