Removal of natural organic matter from two types of humic ground waters by nanofiltration

Abstract

The efficiency of nano filtration (NF) in producing drinking water from two types of humic ground waters was studied on site at a pilot scale in Denmark. At one site, the natural organic matter (NOM) consisted almost entirely of humic acids with a concentration of 20–22mg C/l, a broad molecular weight distribution and a large fraction of high molecular weight NOM. At the other site, the initial NOM concentration was 7–11mg C/l with 60% of the NOM as fulvic acids and a narrow molecular weight distribution around 2000D. A spiral wound NF membrane with a nominal molecular weight cut-off (MWCO) of 200–500D and a feed spacer of 0.75mm in diamond configuration demonstrated superior NOM removal for both waters. Membrane productivity decline was observed due to the accumulation of NOM on the membrane surface or the precipitation of inorganic salts such as calcite (CaCO3), dependent upon NOM and inorganic water chemistry types. Accumulated NOM was removed with an alkaline detergent recovering the initial membrane productivity by 90%, whereas the inorganic precipitation was removed with an acidic detergent followed by the alkaline detergent, recovering the initial productivity completely. A spiral wound NF membrane with a nominal MWCO of 180D and a feed spacer of 1.25mm in parallel configuration exhibited the same product water quality, but a higher and more stable membrane productivity than the first membrane. High cross flow and low operating pressure reduced the membrane productivity decline. Nanofiltration of humic ground water resulted in efficient removal of NOM and a stable productivity, if differences in NOM and inorganic raw water composition were considered in designing the nanofiltration process.