Optimizing Hollow Fibre Nanofiltration for Organic Matter Rich Lake Water

Over the years, various technologies have been utilized for Natural Organic Matter (NOM) removal with varying degrees of success. Conventional treatment methods comprising of coagulation, flocculation, sedimentation, or filtration are widely used to remove NOM. An alternative to these conventional methods is to use spiral wound membranes. These membranes tend to remove too much hardness whilst being ineffective in disinfection. They also have a low tolerance to chlorine and thus, have limited chemical cleaning options. In this study, we investigated how an alternative and new innovative filtration concept, based on capillary NF membranes from modified polyethersulfone (PES), may be used to treat soft but humus-rich surface waters. Comprehensive performance tests, with a fully automated membrane pilot equipped with a full-scale sized test module (40 m2 membrane surface), were conducted at WTP Görvälnverket, which is operated by the water utility Norrvatten, providing drinking water from Mälaren (SUVA = 2.7–3.3, TOC = 7.0–10.0 mg·L−1) for about 500,000 people in the northern part of the Swedish capital of Stockholm. The removal of both UV and DOC was modeled using a solution diffusion approach. The optimized parameters allow deducing optimal operation conditions with respect to energy, water consumption, and permeate water quality. Optimal cross flow velocity was determined to be 0.75 m·s−1 at 80% recovery and a flux of 12–18 L·m−2·h−1. Under these conditions, 80% of the UV, 75% of the Humic Substances (MW = 600) and 70% of TOC were removed (from 8 to below 2 mg·L−1). A higher cross flow velocity led to marginal improvement (+2%) while both higher and lower membrane fluxes degraded permeate water quality. Apparent optimized diffusion coefficients for UV and TOC were around 1.2–2.4 × 10−10·m2·s−1 and were similar to values found in the literature. Due to their higher diffusion coefficients and higher permeability coefficient, only 40% of the low molecular weight acids (MW = 300–400) were retained. Approximately 30%–40% of the low molecular weight acids in the permeate can be further removed using GAC post NF. The resulting energy consumption of a hypothetical four-stage design, at average operating temperature of 5.73 ◦C, was calculated to be around 0.6 kWh·m−3 produced water.