Abstract
Colloidal fouling is one of the main reasons for the reduced efficiency of membrane-based water desalination processes. The synchrony of several resistance mechanisms like hydraulic, osmotic, and electro-kinetic as well as numerous coupling effects complicate the analysis of their individual contributions to the fouling extent. A new measuring approach using a dead-end filtration test-cell allows exactly this, irrespective of any simultaneously occurring concentration polarization phenomena. First results show that the hydraulic resistance of a fully developed colloidal layer is not exclusively determined by the physicochemical properties of its constituents but seems to be strongly dependent on the specific way of its formation (e.g. ionic strength prevailing during layer build-up or filtration sequence of different particle sizes). This time-dependent effect is largely irreversible and therefore most likely due to persistent changes in fouling layer structure. A minor reversible ionic strength effect could also be demonstrated. The extent of this effect is identical irrespective of whether the ionic strength is increased or decreased. Results further indicate that commonly applied models like the Kozeny–Carman equation are lacking a size-dependent parameter that causes a disproportionate decrease of colloidal fouling layer resistance with decreasing foulant particle size.
Highlights
The deposition of water constituents on the membrane surface in solid or gel-like form usually has the effect that a fluid passing the membrane at a constant rate experiences a higher pressure drop than in an equivalent case without fouling
Between the individual filtration steps constant flux was maintained by filtering Particle- and salt-free water (PW) until a steady state in hydraulic resistance was achieved
This finding is most possibly due to the well-known effect that the particle layers that are immediately adjacent to the membrane surface usually create a significantly higher hydraulic resistance than the more distant layers (Yim & Kim ; Park et al ; Mendret et al ; Taheri et al )
Summary
The deposition of water constituents on the membrane surface in solid or gel-like form (fouling) usually has the effect that a fluid passing the membrane at a constant rate experiences a higher pressure drop than in an equivalent case without fouling This additional pressure loss significantly influences the total energetic efficiency of the filtration process and is attributed to molecular friction in the stationary fouling layer. The models that are commonly applied to describe colloidal fouling layer resistance are usually based on concepts that have been developed for macroscopic (particulate) systems Most of these models only work within the scope of the Darcy equation and incorporate the significant structural parameters of the respectively described layers. For suspended layers and macromolecular gels that lack a defined pore structure mostly cell models like
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