Direct Nanofiltration of Surface Water

Abstract

In this thesis, direct application of hollow fiber nanofiltration on surface water is suggested as an efficacious method for surface water treatment. This thesis was carried out as a collaborative effort between Lenntech B.V. and TU Delft. The current research aims to assess the performance of these membranes as a potential solution for surface water treatment and to gain a meaningful understanding of rejection performance and fouling tendencies of these modules through lab-scale experimentation of a low MWCO hollow fiber membrane. Direct application of nanofiltration on surface water was carried out on a lab-scale using the dNF-40 hollow fiber nanofiltration membrane supplied by NXFiltration B.V. Enschede. This membrane is fabricated using a technique called Layer-by-Layer (LbL) polyelectrolyte deposition which consists of an assembly of alternatingly deposited polycationic and polyanionic nanolayers on a polyethersulfone (PES) ultrafiltration support. The dNF-40 membrane is negatively charged at neutral pH. The main objective of the research was to determine the effectiveness of the dNF40 membrane for surface water treatment in terms of three key performance parameters viz. rejection, membrane fouling and concentration polarization. Membrane characterization was carried out by measuring the pure water permeability, Molecular Weight Cut Off (MWCO) of the membrane and rejection of single salt solutions. The pure water permeability of a pristine membrane was 1.53×10^{-14} m. The MWCO was measured using PEG filtration method and was found to be 200 Da. The membrane performance is limited in terms of the flux due to concentration polarization. CP factor was measured experimentally and compared with Sherwood analytical model. Since the flow through the fibers in laminar, high cross-flow velocities are required to reduce CP are high (< 0.5 m/s) due to which hydraulic pressure losses along the feed channel are high. A pressure drop of 0.2 bar was measured for a pristine membrane at a cross-flow velocity of 0.5 m/s. Filtration experiments were carried out with two kinds of surface waters: Delft Schie water and Biesbosch reservoir water. The influence of flux and cross-flow velocity on the rejection of ions were investigated. The removal of Natural Organic Matter (NOM)in both surface waters was between 80 and 85%. The rejection of divalent cations viz. Ca2+and Mg2+was higher at low system recoveries (upto 30%) but a severe drop in rejection was observed at higher recoveries (80%). The final permeate collected at 80% recovery contained 37 mg/L of Ca2+and less than 1 mg/L of NOM. 98%rejection of SO42- was observed irrespective of the feed composition and operating conditions. The dNF-40 membrane exhibited high fouling-resistance during surface water filtration showing no mass transfer coefficient(MTC) decline during 6-hour experimental cycles with surface water. To test for fouling fractions of surface water,additional tests were carried out with model foulant solutions including sodium alginate, humic acid and bovine serum albumin with varying foulant concentrations and ionic strengths; of the three, alginate fouling was most severe in terms of flux decline. Irreversible fouling was observed during the alginate filtration tests. Fiber-blocking was also observed during alginate filtration due to aggregation of alginate and Ca2+. Chemical cleaning with 200 ppm NaOCl solution at pH 12 completely re-stored the permeability. The results presented in this thesis demonstrate that the dNF-40 hollow fiber membrane with the LbL structure can treat surface water with-out pre-treatment. These membranes are ideal for applications such as production of drinking water where partial removal of hardness and complete removal of organic matter is required.