Upgrading coagulation with hollow-fibre nanofiltration for improved organic matter removal during surface water treatment

Abstract

Rising organic matter concentrations in surface waters in many Nordic countries require current drinking water treatment processes to be adapted. Accordingly, the use of a novel nanofiltration (NF) membrane was studied during a nine month period in pilot scale at a large drinking water treatment plant in Stockholm, Sweden. A chemically resistant hollow-fibre NF membrane was fed with full scale process water from a rapid sand filter after aluminum sulfate coagulation. The combined coagulation and NF process removed more than 90% of the incoming lake water dissolved organic carbon (DOC) (8.7 mg C L(-1)), and 96% of the absorbance at 254 nm (A254) (0.28 cm(-1) incoming absorbance). Including granulated active carbon GAC) filter, the complete pilot plant treatment process we observed decreases in DOC concentration (8.7-0.5 mg C L(-1)), SUVA (3.1-1.7 mg(-1) L m(-1)), and the average nominal molecular mass (670-440 Da). Meanwhile, water hardness was practically unaffected (<20% reduction). Humic substances (HS) and biopolymers were almost completely eliminated (6510-140 and 260 to 10 μg C L(-1) respectively) and low molecular weight (LMW) neutrals decreased substantially (880-190 μg C L(-1)). Differential excitation emission matrices (EEMs), which illustrate the removal of fluorescing organic matter (FDOM) over a range of excitation and emission wavelengths, demonstrate that coagulation removed 35 ± 2% of protein-like material and 65 ± 2% of longer emission wavelength, humic-like FDOM. The subsequent NF treatment was somewhat less selective but still preferentially targeted humic-like FDOM (83 ± 1%) to a larger extent than protein-like material (66 ± 3%). The high selectivity of organic matter during coagulation compared to NF separation was confirmed from analyses with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), and liquid chromatography with organic carbon detection (LC-OCD), as coagulation exclusively targeted oxidized organic matter components while NF removed both chemically reduced and oxidized components. DOC removal and change in DOC character in the GAC filters showed marked differences with slower saturation and more pronounced shifts in DOC character using NF as pre-treatment. Fluorescence derived parameters showed a similar decrease over time of GAC performance for the first 150 days but also indicated ongoing change of DOM character in the post NF GAC filtrate over time even after LC-OCD indicated steady state with respect to outgoing carbon. During our trial iron concentrations were low (<30 ppb) and thus A254 could be directly related to the concentration of HS (R(2) = 0.9). The fluorescence derived freshness index (β:α) proved to be an excellent variable for estimating the fraction of HS present in all samples. Given the recommended limit of 4 mg L(-1) for chemical oxygen demand (COD) for Swedish drinking water, coagulation will need to be supplemented with one or more treatment steps irrespective whether climate change will lead to drier or wetter conditions in order to mai...

Combined Coagulation and Ultrafiltration Process to Counteract Increasing NOM in Brown Surface Water

“…This indicator is superior to SUVA, as it has higher precision and requires only one measurement compared with the two parameters for SUVA. This finding confirmed the results obtained by Köhler et al [17] on the use of the freshness index.…”

“…This technique facilitates the subdivision of organic matter into six major subfractions: biopolymers, HS, building blocks, low-molecular-weight acids, low-molecular-weight neutrals, and hydrophobic organic carbon. Detailed information on the LC-OCD technique is available from the following studies [17,27,28].…”

“…Previously established indices were calculated from the corrected EEMs-namely humification index (HIX), fluorescence index (FI), and freshness index (β:α) according to Cory and McKnight [29]; Ohno [30]; and Parlanti et al [31]. The freshness index (β:α) has been shown to be particularly valuable for the characterisation of coagulation [7,17].…”

The Preparation of Composite NF Membrane by Interfacial Polymerization Between Sulfanilic Acid and Trimesoyl Chloride

“…Charged NF membrane has different rejection for different ions because of Donnan effect. So NF membrane has a good application prospect in many fields, such as drinking water treatment [1,2] , wastewater treatment and reuse [3,4] , dye purification [5,6] , food product [7,8] , etc.…”

Feasibility Study of Advanced NOM-Reduction by Hollow Fiber Ultrafiltration and Nanofiltration at a Swedish Surface Water Treatment Plant

“…In addition, the module can be backwashed, a procedure that destroys traditional spiral-wound nanofilters. A new type of hollow-fiber nanofilter (HFNF) has been developed for improved removal of organic matter, safe backwashing and chemical cleaning, and with no requirement for pretreatment other than screening [23][24][25].…”