Climate Change Adaption of Waterworks for Browning Surface Waters : Nano- and Ultrafiltration Membrane Applications for Drinking Water Treatment

Abstract: Natural organic matter (NOM) is found in all surface, ground, and soil waters. During recent decades, reports worldwide show a continuing increase in the color and NOM of the surface water, which has an adverse effect on drinking water purification. For several practical and hygienic reasons, the presence of NOM is undesirable in drinking water. Various technologies have been proposed for NOM removal with varying degrees of success. Membranes are an exceptional barrier to particles, turbidity, microorganisms, and disinfectant by-product (DBP) precursors from drinking water. Significant improvements in membrane technology, design, and materials during the past three decades, has made the drinking water production processes nowadays more effective and environmentally friendly. Based on the research trend, it can be seen that membrane applications for NOM removal will play a more prominent role in the future.This dissertation presents results for selective NOM removal by ultrafiltration (UF) and nanofiltration (NF) membrane configurations at two Swedish surface water treatment plants (WTPs). Extensive field testing with several customized and fully-automated pilot plants were carried out for evaluation of different membrane concepts for NOM removal. Modelling of membrane performance and retention behavior of hollow-fiber (HF) NF membranes using a solution-diffusion approach and two-dimensional axisymmetric computational fluid dynamics (CFD), provided accurate prediction of NOM removal. For this reason, it was used for full-scale process design and feasibility studies. Direct NF filtration of surface water indicated stable membrane performance and high average NOM removal rates (UV254: 91.6–94.7 %; TOC: 82–92.2 %). The NF membrane retention was highest for biopolymers (>95% removal), followed by Humic Substances (HS) (>80% removal). Combined coagulation/UF processes removed approximately 30% of the DOC, and 50% of the HS and biopolymers. However, building blocks (BB) and low molecular weight acids (LMWacids) remained unchanged compared with the NOM composition of the feed and permeate. For optimization of hybrid membrane processes, an automatic coagulant dosing system based on online measurements was successfully applied.Autopsy results of HF-NF membranes after twelve months of continuous pilot operation with pretreated feed water (coagulation and rapid sand filtration), showed no substantial changes of the aged membranes from different module sections compared to virgin membranes. Rigorous analysis of the nanomechanical properties of HF-UF membranes harvested after 12–14 months pilot operation showed that the detected change in nanomechanical characteristics did not result in considerable impact on the macroscopic properties of the membranes. Comprehensive UF pilot trials resulted in improvements of the two-stage full-scale UF facility during the design phase and commissioning at Kvarnagården WTP.