Abstract
Membrane hybrid processes—coagulation coupled with ultrafiltration (UF)—have become a common method to comply with the legal, chemical, and microbiological requirements for drinking water. The main advantages of integrating coagulation with membrane filtration are the enhanced removal of natural organic matter (NOM) and reduced membrane fouling. With in-line coagulation, coagulants are patched into the feed stream directly prior to the membrane process, without removing the coagulated solids. Compared with conventional coagulation/sedimentation, in-line coagulation/membrane reduces the treatment time and footprint. Coagulant dosing could be challenging in raw water of varying quality; however, with relatively stable specific ultraviolet absorbance (SUVA), dosing can be controlled. Recent studies indicate that UV absorbance correlates well with humic substances (HS), the major fraction to be removed during coagulation. This paper describes and evaluates a 30-month UF pilot trial on the surface water of Lake Neden (Sweden), providing drinking water to 60,000 residents. In this study, automatic coagulant dosing based on online measurement was successfully applied. Online sensor data were used to identify the current optimal aluminium coagulation conditions (0.5–0.7 mg L−1) and the potential boundaries (0.9–1.2 mg L−1) for efficient future (2040) NOM removal. The potential increase in NOM could affect the Al dose and drinking water quality significantly within 20 years, should the current trends in dissolved organic carbon (DOC) prevail. UV absorbance, the freshness index, and liquid chromatography-organic carbon detection (LC-OCD) measurements were used to optimise the process. Careful cross-calibration of raw and filtered samples is recommended when using online sensor data for process optimisation, even in low-turbidity water (formazin nephelometric unit (FNU) < 5).
Highlights
In the late 1980s, an increase of natural organic matter (NOM) concentration was first reported in Swedish surface waters as a link between increased amount of humic substances (HS) and the darkening of Swedish lakes [1]
Ultrafiltration with and without coagulant dosage was compared with hollow-fibre NF
Dead-end filtration with raw water resulted in rapid increase of trans-membrane pressure (TMP)
Summary
In the late 1980s, an increase of natural organic matter (NOM) concentration was first reported in Swedish surface waters as a link between increased amount of humic substances (HS) and the darkening of Swedish lakes [1]. The presence of NOM could have severe effects on drinking water quality and its treatment processes. These problems include (i) negative effects on water quality relevant to colour, taste, and odour; (ii) increased disinfectant dose requirements, which in turn result in potential harmful disinfection by-product (DBP) production [8]; (iii) promoted biological growth in the distribution system; and (iv) increased levels of complex heavy metals and adsorbed organic pollutants [9]
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