Comparison and optimization of removal of natural organic carbon from raw water with ultrafiltration in pilot scale experiments

Abstract: The removal of natural organic matter (NOM) from Swedish surface waters has become a large challenge for drinking water treatment plants (DWTP), as NOM content has been rising in surface waters all over Northern Europe. In the last decade, membrane technologies, often combined with coagulation, have become more and more common. The DWTP Norrvatten in Stockholm treats water from lake Mälaren with conventional coagulation and sedimentation. However, they are currently testing an online coagulation-ultrafiltration system in a large pilot plant container at Görvälnverket in Stockholm to find out whether it can serve as a future technology in times of increasing NOM levels in Mälaren and increasing need for drinking water. In this study, a compact table-scale water treatment pilot plant working with a hollow-fibre ultrafiltration membrane was used to investigate and optimize the removal of NOM from raw water. NOM removal was quantified using online sensor detection of FDOM and via DOC grab samples. It was documented that the removal of NOM with the small-scale pilot plant is highly comparable to the results from the large container-scale pilot plant at Norrvatten. Therefore, it is a valuable tool to conduct fast, easy, and cheap pretests in the future. Optimal coagulation conditions across an ultrafiltration membrane with polyaluminium chloride were identified (6 mg/l and pH=6) and three different raw waters of varying quality and NOM content were tested using high-frequency multiparameter analysis, spectrofluoresence and total organic carbon analysis. Removal efficiencies of up to 57% fluorescent dissolved organic matter could be achieved. However, raw waters with NOM contents over 15 mg/l will remain a challenge with this treatment technology, because the remaining NOM contents are higher (7,6 mg/l) than the legal threshold (ca. 5mg/l).