Threshold Concentration of Easily Assimilable Organic Carbon in Feedwater for Biofouling of Spiral-Wound Membranes W. A. M. HIJNEN,* ,† D. BIRAUD, ‡ E. R. CORNELISSEN, † AND D. VAN DER KOOIJ † KWR Watercycle Research Institute, PO Box 1072, 3430 BB Nieuwegein, The Netherlands, and Ecole Nationale Supe ´rieure de Chimie de Mulhouse, 3 rue Alfred Werner, 68200 Mulhouse, France Received January 6, 2009. Revised manuscript received April 10, 2009. Accepted April 16, 2009. One of the major impediments in the application of spiral- wound membranes in water treatment or desalination is clogging of the feed channel by biofouling which is induced by nutrients in the feedwater. Organic carbon is, under most conditions, limiting the microbial growth. The objective of this study is to assess the relationship between the concentration of an easily assimilable organic compound such as acetate in the feedwater and the pressure drop increase in the feed channel. For this purpose the membrane fouling simulator (MFS) was used as a model for the feed channel of a spiral-wound membrane. This MFS unit was supplied with drinking water enriched with acetate at concentrations ranging from 1 to 1000 μgC · L -1 . The pressure drop (PD) in the feed channel increased at all tested concentrations but not with the blank. The PD increase could be described by a first order process based on theoretical considerations concerning biofilm formation rate and porosity decline. The relationship between the first order fouling rate constant R f and the acetate concentration is described with a saturation function corresponding with the growth kinetics of bacteria. Under the applied conditions the maximum R f (0.555 d -1 ) was reached at 25 μg acetate-C · L -1 and the half saturation constant k f was estimated at 15 μg acetate- C · L -1 . This value is higher than k s values for suspended bacteria grown on acetate, which is attributed to substrate limited growth conditions in the biofilm. The threshold concentration for biofouling of the feed channel is about 1 μg acetate-C · L -1 . Introduction The use of high pressure spiral-wound membranes, nano- filtration (NF), or reverse osmosis (RO) has increased rapidly over the past decade (1). Major fields of current application are (i) desalination of seawater, (ii) reclamation of wastewater, (iii) production of industrial waters, and (iv) treatment of surface water or groundwater for drinking water production. Biofouling is one of the major impediments in operation of these membranes, causing clogging of the feed channel and fouling of the membrane surface with increased pressure drop and decreased flux as results. The evident effect of attached microbial growth (biofilms) on the pressure drop in spiral-wound membranes is well documented in the literature (2, 3). Recently the effect of biofilms on the mass transfer coefficient (MTC) has been demonstrated and related to the occurrence of cake-enhanced osmotic pressure (4, 5). The different mechanisms involved in biofouling, viz. bacterial adhesion, growth, and sloughing and aspects such as biofilm composition and structure have been reviewed extensively (2, 3). Only a few studies reported on the relationship between the nutrient level in the feedwater and biofouling. Under most environmental conditions, organic carbon compounds are the growth limiting nutrients for biomass production (assimilation) and energy supply. A wide range of organic compounds, e.g., carboxylic acids, amino acids, proteins, and carbohydrates promote bacterial growth in the aquatic environment. The affinity of bacteria for these substances can be high which is demonstrated by a low half saturation constant k s derived from the Monod growth kinetics. For aquatic bacteria grown on acetate, benzoate, oxalate, oleate, and starch at 15 °C in drinking water k s- values ranging from 0.4 to 15.4 μgC · L -1 have been established (6, 7). In spiral-wound membranes with turbulent flow the substrate concentration in the bulk water is the most important growth- determining parameter. Biological clogging of the feed spacer of a spiral-wound membrane at acetate concentrations below 0.1 mg C · L -1 was demonstrated with a model system, the membrane fouling simulator (MFS 8, 9). In these studies the lowest acetate concentrations tested (25 μgC · L -1 ) is much higher than the k s values mentioned above. Studies of the effects of low substrate levels on biofouling do not seem available. Hence, the threshold concentration of easily assimilable organic compounds such as acetate to induce biological clogging of the feed channel and the kinetics of the biological clogging process are not clear yet. The objective of the current study was to assess the relationship between the concentration of acetate in the feedwater as a model for easily assimilable organic com- pounds, and the rate of clogging of the feed channel caused by biofouling under well-defined conditions. Extended acetate dosing studies showed that (i) biological clogging follows the kinetics of biofilm formation and (ii) the threshold concentration for biofouling in the feed channel is in the order of a few micrograms. Materials and Methods Model Membrane System. The membrane fouling simulator MFS as presented by Vrouwenvelder et al. (8) was used for the experiments. The MFS consists of a stainless steel housing divided in an upper and lower part with a sheet of a membrane fixed in between. To simulate the feed channel of a spiral-wound membrane in the upper and lower part of this MFS, a feed spacer (height, width, and length of 0.08, 4, and 20 cm, respectively) is placed on top of the membrane and a product spacer of similar size below the membrane. The MFS was operated without a vertical flux (permeate production). Under the turbulent flow conditions in the feed channel of the MFS mass transfer of acetate in a laminar layer is of minor importance and vertical fluxes are usually e10% of the total cross-flow flux. The feedwater is introduced into the feed spacer channel at a flow of 4.44 × 10 -6 m 3 · s -1 and a cross-flow velocity of 0.14 m · s -1 . A “virgin” nanofil- tration membrane (Trisep 4040-TS80-TSF) was used in the MFS as the standard membrane (feed spacer height and filament thickness 0.066 and 0.034 cm; porosity ε of 0.92). The normalized pressure drop or initial NPD o (kPa) of a * Corresponding author fax: +31(0)306061165; e-mail: wim.hijnen@ kwrwater.nl. † KWR Watercycle Research Institute. ‡ Ecole Nationale Supe ´rieure de Chimie de Mulhouse. Environ. Sci. Technol. 2009, 43, 4890–4895 4890 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 13, 2009 10.1021/es900037x CCC: $40.75  2009 American Chemical Society Published on Web 05/21/2009