Fouling of dairy components on hydrophobic polytetrafluoroethylene (PTFE) membranes for membrane distillation, Journal of Membrane Science, http://dx.doi.org/10.1016/j. memsci. 2013.03.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThis study investigates fouling of membranes during membrane distillation (MD) of two model dairy feeds -skim milk and whey, as well as their major single components. Every MD experiment was conducted for 20 hours at 54 °C feed inlet temperature and 5 °C permeate inlet temperature using PTFE membranes. Performance was assessed in terms of throughput (flux) and retention efficiency.Skim milk flux was found to be lower but stable over time compared to whey. The study using single components as well as combinations thereof revealed that fouling was primarily driven by proteins and calcium, but only in combination. Lactose also played a role to a lesser extent in the protein/membrane interactions, possibly due to preferential hydration, but did not interact with the membrane polymer directly. However lactose was found to deposit once an anchor point to the membrane was established by other components. Skim milk showed strong adhesion from its principle proteins, caseins; however salts were needed to form a thick and dense cake layer. Caseins seem to form a layer on the membrane surface that prevents other components from interacting with the membrane polymer. Whey proteins, on the other hand, deposited to a lesser extent. In general, membrane distillation was found to be a process that generates high quality water with retention of all tested components >99% while simultaneously concentrating whey or skim milk.

Membrane distillation is an emerging membrane process based on evaporation of a volatile solvent. One of its often stated advantages is the low flux sensitivity toward concentration of the processed fluid, in contrast to reverse osmosis. In the present paper, we looked at 2 high-solids applications of the dairy industry: skim milk and whey. Performance was assessed under various hydrodynamic conditions to investigate the feasibility of fouling mitigation by changing the operating parameters and to compare performance to widespread membrane filtration processes. Whereas filtration processes are hydraulic pressure driven, membrane distillation uses vapor pressure from heat to drive separation and, therefore, operating parameters have a different bearing on the process. Experimental and calculated results identified factors influencing heat and mass transfer under various operating conditions using polytetrafluoroethylene flat-sheet membranes. Linear velocity was found to influence performance during skim milk processing but not during whey processing. Lower feed and higher permeate temperature was found to reduce fouling in the processing of both dairy solutions. Concentration of skim milk and whey by membrane distillation has potential, as it showed high rejection (>99%) of all dairy components and can operate using low electrical energy and pressures (<10 kPa). At higher cross-flow velocities (around 0.141 m/s), fluxes were comparable to those found with reverse osmosis, achieving a sustainable flux of approximately 12 kg/h·m(2) for skim milk of 20% dry matter concentration and approximately 20 kg/h·m(2) after 18 h of operation with whey at 20% dry matter concentration.

This study reports on fouling mechanisms of skim milk and whey during membrane distillation (MD) using polytetrafluoroethylene (PTFE) membranes. Structural and elemental changes along the fouling layer from the anchor point at the membrane to the top surface of the fouling layer have been investigated using synchrotron IR micro-spectroscopy and electron microscopy with associated energy dispersive X-ray spectroscopy (EDS). Initial adhesion of single components on a membrane representing a PTFE surface was observed in-situ utilizing reflectometry. Whey components were found to penetrate into the membrane matrix while skim milk fouling remained on top of the membrane. Whey proteins had weaker attractive interaction with the membrane and adhesion depended more on the presence of phosphorus near the membrane surface and throughout to establish the fouling layer. This work has given detailed insight into the fouling mechanisms of MD membranes in major dairy streams, essential for maintaining membrane distillation operational for acceptable times, therewith allowing further development of this emerging technology. 2 IntroductionMembrane distillation (MD) is a thermally driven membrane process and relies on a highly hydrophobic porous membrane to maintain a liquid-vapour interface. Common membrane materials for MD are polypropylene (PP), polyvinylidene fluoride (PVDF) and PTFE [1,2]. The highest performing membrane material for MD is PTFE due to its high hydrophobicity, chemical inertness and open porous structure [3]. Fouling in the MD process is different to that observed in pressure driven processes such as RO. The low operating pressure used in MD may potentially lead to a less compact, more easily removed, fouling layer. Also, since only volatile compounds pass through the membrane pores, the potential for in-pore fouling is minimized in MD applications. Studies of MD processes have, however, revealed that penetration of foulants into the membrane can occur in some instances [4]. There is a need for a better understanding how dairy components interact with MD membranes and accumulate at the membrane surface. This understanding may allow better control of performance of membrane distillation via better mitigation of fouling.The high hydrophobicity of MD membranes can result in the establishment of hydrophobic interactions between the membrane and any solutes that have hydrophobic components, such as proteins and fats. While hydrophilic coatings may be a possible avenue to reduce the fouling that results from these hydrophobic interactions [5][6][7], simple uncoated membranes have advantages in terms of lower cost and can be easier to manage over time as there is no requirement to maintain a specialised surface coating.There are numerous studies on fouling phenomena occurring with dairy components [8][9][10][11][12][13], however little can be found on the actual mechanisms behind the fouling. Most studies focus on membrane performance, not investigating kinetics behind fouling phenomena observed. In cases wh...

Membrane distillation (MD) was applied for the concentration of a range of dairy streams, such as whole milk, skim milk and whey. MD of a pure lactose solution was also investigated. Direct contact MD (DCMD) mode experiments were carried out in continuous concentration mode, keeping the warm feed/retentate and cold permeate stream temperatures at 54 °C and 5 °C respectively. Performance in terms of flux and retention was assessed. The flux was found to decrease with an increase of dry-matter concentration in the feed. Retention of dissolved solids was found to be close to 100% and independent of the dry-matter concentration in the feed. Fourier Transform Infrared Spectroscopy (FTIR) of the fouled membranes confirms organics being present in the fouling layer.

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