Effect of temperature on organic fouling and cleaning efficiency of nanofiltration membranes for loch water treatment

Abstract

We investigated the effect of solution temperature on organic fouling and cleaning of polypiperazine nanofiltration (NF) membranes. Fouling experiments were conducted in the temperature range 5 °C ≤ T ≤ 15 °C, characteristic of surface waters in northern latitudes, such as Scottish lochs. Results of laboratory-scale fouling experiments using alginate, a polysaccharide constituent of extracellular polymeric substances, showed a moderate increase in flux loss at 15 °C (41 %) compared to 5 °C (36 %). Analysis of the fouling experiments using a series-resistance model showed that the greater extent of fouling with rising temperature stems from a monotonic increase in water permeance with T. Interfacial property characterisation provided further insight into the effect of temperature on fouling determinants such as membrane hydrophobicity, nanoscale roughness, surface charge, and surface forces. No T-dependence of surface roughness was found. Similarly, we found that water contact angle is invariant within the temperature range investigated, suggesting that the observed fouling behaviour is not due to modulation of hydrophobic interactions. Conversely, colloidal-probe force spectroscopy (CPFS) measurements showed that adhesion forces become stronger with rising temperature. Further analysis showed that repulsive forces – which oppose colloidal particle deposition – become weaker with rising temperature, consistent with fouling trends. Determination of the membrane zeta potential showed that surface charge is invariant over the range of temperatures investigated, suggesting that repulsive forces have a non-electrostatic (likely steric) origin. Additionally, we assessed the effect of temperature on physical and chemical cleaning efficiency, comparing cleaning at the same temperature as the fouling experiments (5 °C ≤ T ≤ 15 °C) with cleaning at 30 °C. We observed that physical cleaning, consisting of water circulation over the fouled membrane surface, is insufficient to remove the foulant layers. On the other hand, chemical cleaning is able to restore 95 % of the original membrane permeance at 10 °C, but only 74 % at 5 °C.