Nanofiltration membrane fouling by conventionally treated Water Science and Technology: Supply surface water R. Liikanen*, H. Kiuru*, T. Tuhkanen and M. Nyström*** * Laboratory of Environmental Engineering, Helsinki University of Technology, PO Box 5200, FIN-02015 HUT, Finland (E-mail: riina.liikanen@hut.fi; heikki.kiuru@hut.fi) ** Environmental Engineering and Biotechnology, Tampere University of Technology, FIN 33101, Finland (E-mail: tuula.tuhkanen@tut.fi) *** Laboratory of Membrane Technology and Technical Polymer Chemistry, Lappeenranta University of Technology, FIN 53581, Finland (E-mail: marianne.nystrom@lut.fi) Abstract Nanofiltration is a very effective technique for improving the removal of trace organics after a conventional chemical water treatment train. However, the fouling of the membranes decreases the applicability of the process, and thus, an understanding and control of membrane fouling are crucial for a Vol 3 No 5–6 pp 183–190 more widespread use of nanofiltration in water treatment. The fouling of different nanofiltration membranes by pre-treated surface waters was investigated in a laboratory-scale filtration unit in this study. The results indicate that the traditional chemical treatment does not remove membrane foulants from the surface water. No correlation was found between the feed water constituents and nanofiltration performance, but most feed water components are expected to interact in membrane fouling. Actually, the performance of the nanofiltration process was more related to membrane than to feed water characteristics. Keywords Fouling; nanofiltration; surface water treatment © IWA Publishing 2003 Introduction Nanofiltration (NF) is one of the most promising techniques for improving the removal of trace organics in drinking water treatment. However, the fouling of the membranes decreases the applicability of the process. Thus, an understanding and the control of mem- brane fouling are crucial for a more widespread use of NF in water treatment. Efficient control of NF membrane fouling generally requires pre-treatment to lower the fouling potential of the feed water. In surface water applications, microfiltration or ultra- filtration (UF) has proved to be a very efficient pre-treatment for NF processes (Chellam et al., 1997), but a conventional chemical treatment also improves the NF performance (Ventresque et al., 2000). In an existing surface water treatment plant the present chemical water treatment process is a presumable option for pre-treatment. A wide spectrum of feed water constituents contributes to membrane fouling. These include dissolved and macromolecular organic compounds, sparingly soluble inorganic compounds, colloidal and suspended particles and micro-organisms. Especially dissolved naturally occurring organic substances (natural organic matter, NOM) are considered important foulants in membrane filtration of natural waters. NF membrane fouling is controlled by an interaction between permeation drag, electro- static repulsion and hydrophobic attraction. For solution chemistries typical of natural source waters or pre-treated surface waters, permeation drag under normal operating con- ditions plays a more significant role than membrane foulant electrostatic interaction, and may ultimately control the rate and extent of colloidal fouling (Zhu and Elimeleh, 1997). Thus, the optimisation of the process parameters is a crucial factor in fouling mitigation. The higher the permeation, the more fouling material is in contact with the membrane sur- face, and the higher is the flux decline. 183 The fouling of different NF membranes by pre-treated surface waters was investigated in a laboratory-scale filtration unit in this study. The objective was to identify the compo- nents or characteristics in the chemically treated feed waters that foul the membranes, and to determine the characteristics of the foulants. In addition, the effects of fouling on the membrane characteristics were evaluated. R. Liikanen Materials and methods Pilot plant and membranes The study was carried out in a laboratory-scale membrane filtration unit. Three cross-flow et al. flat-sheet modules were run parallel using the same feed water. The membrane area in each module was 46.0 cm2. The membranes were Desal-5 DL, NF255 and NF270. The surface layer of all the tested membranes is made of polypiperazine amide and all the membranes are categorised as tight NF membranes with cut off values around 300 g/mol. Feed waters Six different feed waters for the pilot process were collected from five surface water treat- ment plants and one artificial ground water treatment plant. Ninety litres of each sample was collected after a traditional water treatment train, but before post-treatment. The details of the treatment process in each studied plant are summarised in Table 1. Operation Virgin membranes were rinsed with reverse osmosis filtered and ion-exchanged water (RO-water). Then the membranes were pressurised at 20 bar for 15 minutes to wet them thoroughly before measuring the initial pure water flux (PWF). The PWF was measured under the actual process operation conditions. Then the membranes were pre-cleaned (0.1% Ultrasil 10, 30 minutes), rinsed with RO-water, and the PWF was measured again. The pilot process was operated in a retentate circulation mode until 60 litres of permeate was produced as a sum of all modules. Fresh feed water was added to the feed stream to compensate the permeate removal from the process. The temperature of the feed water was kept constant at 20°C. Cross-flow velocity (CFV) was constant at 0.65 m/s and the net driving pressure (NDP) was 10 bar. The NDP, feed water temperature, retentate flow and permeate flows were monitored and the data was collected continuously by a computer. At the end of the run, the membranes were rinsed with RO-water for 15 minutes. Then the PWF of the fouled membranes was measured. Table 1 Pre-treatment trains of the studied feed waters Feedwater Pretreatment train Espoo Coagulation with ferric sulfide, coagulation pH adjusted with lime, flocculation, flota- tion, sand filtration Kotka Artificial groundwater Pietarsaari Coagulation with ferric chloride sulfite, coagulation pH adjusted with lime, floccula- tion, flotation, pH adjustment with lime, sand filtration Raisio-Naantali Coagulation with polyaluminium chloride, coagulation pH adjusted with lime, cationic coagulation aid, flocculation, sedimentation, pH adjustment with lime, sand filtration Tampere Alkalinity adjustment with CO2 and pH adjustment with lime, coagulation with alum, flocculation, flotation, disinfection with ClO2, activated carbon filtration Turku Alkalinity adjustment with CO2 and pH adjustment with lime, coagulation with ferric chloride sulfite, flocculation, sedimentation, addition of powdered activated carbon and lime, coagulation with ferric chloride sulfite, coagulation pH adjusted with NaOH, cationic coagulation aid, flocculation, flotation, disinfection with a combination of Cl2, ClO , and NaClO , sand filtration 184 2 2 Evaluation of the performance of the feed waters The flux of the membranes was evaluated with RO-water as PWF and with the tested feed waters as operational flux (FWF) during the run. The fluxes were normalised to standard operating conditions (T = 20°C and NDP = 10 bar). The percentage changes in the mem- brane flux (flux change = FC) at different phases of the tests were calculated as described in Figure 1. The following parameters were rated in order of superiority to rank the different R. Liikanen feed waters: FCfouled, FCfeed, FCoper. The ranks were summed separately for all the tested membranes and finally as a whole for the overall rating of the feed waters. Water analysis et al. The feed and permeate waters were characterised by pH, conductivity, alkalinity, hardness, total organic carbon (TOC), UV254, anions, cations and the molar mass distribution of organic matter (excluding anions and cations for permeates). TOC was measured by the Shimatzu TOC-5000A analysator by combustion – non-dispersive infrared gas analysis method. The organic matter hydrophobicity was evaluated by the SUVA value (Edzwald and Tobiason, 1998). The organic matter molar mass distributions were analysed by high- performance size exclusion chromatography (Hewlett-Packard HPLC 1100). Anions were analysed by ion chromatography and cations by inductively coupled plasma atomic emission spectrometry, or by mass spectrometry. Membrane and foulant analysis The hydrophobicity, functional groups, elementary composition and visual appearance of the virgin, pre-cleaned and fouled membranes were characterised. The membrane hydrophobicity was measured as contact angle (CA) by the drop method. The functional groups were analysed using the Fourier transform infrared (FTIR) (Perkin-Elmer 2000) apparatus. Scanning electron microscopy (SEM) – elementary analysis (EDS) (JEOL JSM- 5800) were used to analyse the membrane elementary composition and visual appearance. The elementary composition of the NF270 membranes was also studied using an X-ray photoelectron spectroscopy (XPS) analysator (AXIS 165). FCin target phase = ( Fluxtarget phase - Fluxcompared phase) / Fluxcompared phase * 100% Flux FCpc FCfouled FCfeed PWFv FCoper PWFpc Average feed water FWF flux after 5 hours of PWFf operation 5 h 10 l of Run permeate time filtrated = Figure 1 Schematic presentation of the flux change (FC) calculations of the tests. PWFv virgin membrane = = = PWF, PWFpc PWF of pre-cleaned membrane, PWFf PWF of fouled membrane. FCpc PWFpc in rela- = = tion to PWFv,