Characterization of Sustainable Flux Of Flocculated Mixtures. Cross-flow micro-filtration experiment set-up

Characterization of Sustainable Flux 

Of Flocculated Mixtures

R.Zhang^1,a, D.E. Wiley^1,b*, A.G. Fane^1,d 

¹UNESCO Centre for Membrane Science and Technology,

School of Chemical Engineeering and Industrial Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

Abstract

Cross-flow micro-filtration has been applied in water and wastewater treatment for a long time. In this work, the effect of operating parameters and feed properties on the permeate flux was studied. The experimental results showed that temperature had the most significant effects both at the start and throughout the filtration process. All of the five parameters studied pH, transmembrane pressure, cross-flow velocity, temperature and ionic strength had a significant effect on steady-state flux.

1.  Introduction

Membrane filtration processes have been used in water and wastewater treatment for a long time. High effluent quality and small footprint make membrane processes competitive with conventional treatment processes. The applications have been limited by membrane fouling which leads to permeate flux drop and an increase of operating cost due to the subsequent requirement for frequent membrane cleaning. Field et al [1] proposed a critical flux concept for cross-flow (CF) micro-filtration (MF). They defined the critical flux as the flux below which no membrane fouling occurs. Laboratory experiments [2, 3] demonstrated low fouling operation below critical flux. However, even below critical flux, a very slow increase in transmembrane pressure often occurs which may eventually result in flux decline and fouling.

From an industrial perspective, it is desirable to operate at the highest achievable steady state flux for the longest period of time. Such conditions would be called sustainable flux operation. By understanding membrane fouling mechanisms and the influence of feed properties and operating conditions on filtration performance, it should be possible to design and operate a filtration process under sustainable flux.

It is known [4-6] that both feed properties and operating conditions affect  filtration behaviour. Cross-flow velocity (CFV), transmembrane (TMP) and operating temperature (T) are very important factors known to affect the filtration performance. This work focused on the effects for flocculated particulate suspensions.

For particulates, the filtration flux increases with increasing cross-flow velocity due to the increasing wall shear which enhances back diffusion and limits cake growth [7] [8]. ______ 

*Corresponding author

However, flux only increases with increasing cross-flow velocity until a limiting velocity. Eventually, at point is reached where the large particles start to deposit, causing the flux to decline [9].

Transmembrane pressure (TMP) has conflicting effects on filtration performance of particulate systems because increasing TMP can cause an increase in both the permeability and the resistance of a fouling layer. When the increase in the permeability is higher than that in resistance, the steady state flux increases with increasing TMP [10]. Under the reverse conditions, filtration rate decreases with increasing TMP [11].

In particulate filtration, increasing temperature usually causes an increase of flux.

Filtration flux increases because the solubility of particulates increases with temperature [12]. 

There are other solution parameters that can also affect the filtration behaviour of particulate mixtures. For large macromolecules, such as proteins, pH affects the solubility. Normally, proteins form a single-phase solution in water. However, close to the isoelectric point, proteins tend to precipitate out of solution so their solubility declines. Under these conditions, the behaviour is more similar to that observed for particulate systems, than for dissolved solutes. The flux of proteins is therefore lowest at the isoelectric point [6].