Hydrophilisation of polyethersulfone (PES) based membrane is often achieved by addition of polyvinylpyrrolidone (PVP) leading to a physical blend of the two polymers. This paper shows that the most commonly used membrane for UF in dairy industry is a PES/PVP based one. Nevertheless if hydrophilisation limits the organic fouling, PVP is also the Achilles heel of these membranes. It is particularly true when membranes are exposed to hypochlorite as it is the case for cleaning/disinfection steps. Evidencing the disappearance of PVP from a pristine PES/PVP membrane can be easily achieved by FTIR-ATR analyses. But when one wants to study the ageing of a membrane used in UF it gets more complicated: regardless of the cleaning efficiency the membrane always remains fouled by some proteins. As both PVP and proteins own chemical bounds leading to absorption at the same wavenumber in FTIR-ATR, it thereby prevents the easy highlighting of the PVP degradation. The aim of this paper is to propose a simple treatment of raw FTIR-ATR spectra to dissociate these two contributions, allowing consequently the study of the degradation of a fouled membrane. Then the procedure is applied to a real case study on a spiral membrane. 2 1. Highlights Ͳ Ͳ Ğ|ŝĚĞŶĐŝŶŐ W^ͬWsW ŵĞŵďƌĂŶĞ ĚĞŐƌĂĚĂƚŝŽŶ ďLJ &d/Z ĚĞƐƉŝƚĞƐ ƉƌĞƐĞŶĐĞ ŽĨ ƉƌŽƚĞŝŶ ĨŽƵůŝŶŐ Ͳ Ͳ Ͳ ŵĞƚŚŽĚŽůŽŐLJ ŽĨ ƚƌĞĂƚŵĞŶƚ ŽĨ &d/Z ƐƉĞĐƚƌĂ ƚŽ ƌĞ|ĞĂů ŵĞŵďƌĂŶĞ ĚĞŐƌĂĚĂƚŝŽŶ ĚƵĞ ƚŽ EĂKů Ͳ Ͳ Ͳ ĂƉƉůŝĐĂƚŝŽŶ ƚŽ ŵĂƉƉŝŶŐ ŽĨ WsW ĚĞŐƌĂĚĂƚŝŽŶ ĚƵĞ ƚŽ EĂKů ŝŶ Ă ƐƉŝƌĂů ŵĞŵďƌĂŶĞ Ͳ Ͳ Ͳ ĂƉƉůŝĐĂƚŝŽŶ ƚŽ ŵĂƉƉŝŶŐ ŽĨ W^ ĚĞŐƌĂĚĂƚŝŽŶ ĚƵĞ ƚŽ EĂKů ŝŶ Ă ƐƉŝƌĂů ŵĞŵďƌĂŶĞ
Skim milk ultrafiltration is worldwide used in dairy. Its management at industrial scale is based on the permeate flux mastering, the first bottleneck, and consequently on the nature of the overall fouling which is not yet fully understood with respect to the membrane lifespan. The second bottleneck is the mastering and control of the cleaning efficiency aiming at the full removal of the initial irreversible fouling mainly made of proteins. In this study, we have deliberately chemically aged several spiral wound membrane elements by filtering NaOCl solution (400 ppm in total free chlorine at pH 8.0 and 50°C) up to a cumulative chlorine dose of 2,800 ppm.d. The critical/threshold and limiting fluxes have been proven to increase with the membrane ageing together with a decrease in their related transmembrane pressure. This suggest that, at industrial scale, UF might be managed with respect to the chlorine dose received by the membrane during its service life. SEM and Raman spectroscopy have been used to localise the initial irreversible fouling whereas ATR-FTIR was the tool to quantify proteins on membrane. Membrane cleaning was achieved by a non-oxidative alkaline detergent removing the hereafter called "removable fouling" whereas a "residual fouling" remained on/in the membrane. However, the following NaOCl treatment (achieved for disinfection purpose at industrial scale) also acts both as cleaner (polishing the alkaline step by removing part of the residual fouling) and as degradation agent toward the membrane. Unambiguously NaOCl induced a significant decrease in the membrane intrinsic hydraulic resistance associated with an increase of the irreversible fouling, either initial or residual, build during the following skim milk UF. This paper shows how degradation and fouling can transiently compensate each other and lead to the impression of an initial water flux recovery. This phenomenon can lead to misinterpretation of the membrane cleanliness in the specific case of aged membranes. Indeed, for a received chlorine dose close to 400 ppm.d at pH= 8.0, an irreversible residual fouling can remain, strongly anchored in/on the membrane with a water flux recovery after cleaning only 15% less than that of the pristine membrane. This residual fouling favours the rapid build-up of more and more irreversible fouling during the next skim milk filtration steps. However, with respect to the accuracy on flux measurement, the amplitude of the problem will be underestimated, preventing from getting rid of the first and crucial accumulated layers and consequently leading to an irreversible accelerated degrading process. Relationships have been found between the received chlorine dose and the aged membrane resistance and the residual irreversible fouling that could be used for management at industrial scale.
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