Long-term performance and fouling analysis of full-scale direct nanofiltration (NF) installations treating anoxic groundwater

•Anoxic feed water enables stable long term NF performance at 4 full-scale plants.•Anoxic conditions prevent rapid iron and biological fouling.•Direct NF: 10 μm cartridge filtration and antiscalant dosage only.•Cleaning once per year is sufficient to maintain stable NF performance.•Slow performance decline by composite fouling (biological and (in)organic).

Long-term performance and fouling behavior of four full-scale nanofiltration (NF) plants, treating anoxic groundwater at 80% recovery for drinking water production, were characterized and compared with oxic NF and reverse osmosis systems. Plant operating times varied between 6 and 10 years and pretreatment was limited to 10 µm pore size cartridge filtration and antiscalant dosage (2–2.5 mg L−1) only. Membrane performance parameters normalized pressure drop (NPD), normalized specific water permeability (Kw) and salt retention generally were found stable over extended periods of operation (>6 months). Standard acid–base cleanings (once per year or less) were found to be sufficient to maintain satisfying operation during direct NF of the described iron rich (≤8.4 mg L−1) anoxic groundwaters.Extensive autopsies of eight NF membrane elements, which had been in service since the plant startup (6–10 years), were performed to characterize and quantify the material accumulated in the membrane elements. Investigations using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), total organic carbon (TOC) and adenosine triphosphate (ATP) measurements revealed a complex mixture of organic, biological and inorganic materials. The fouling layers that developed during half to one year of operation without chemical cleaning were very thin (<2 µm). Most bio(organic) accumulates were found in the lead elements of the installations while inorganic precipitates/deposits (aluminosilicates and iron(II)sulfides) were found in all autopsied membrane elements.The high solubility of reduced metal ions and the very slow biofilm development under anoxic conditions prevented rapid fouling during direct NF of the studied groundwaters. When compared to oxic NF and RO systems in general (e.g. aerated ground waters or surface waters), the operation and performance of the described anoxic installations (with minimal pretreatment) can be described as very stable.