Influence of natural organic matter (NOM) fouling on the removal of ...

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the removal of pharmaceuticals by nanofiltration and activated carbon ..... Proceedings of AWWA Water Quality & Technology Conference-2006 Denver. S.G.J..
S.G.J. Heijman*,***, A.R.D. Verliefde*,**, E.R. Cornelissen***, G. Amy**** and J.C. van Dijk*,*** *Department of Sanitary Engineering, Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands (E-mail: [email protected] ) **Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, University of Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium ***Kiwa Water Research, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands ****UNESCO-IHE Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands Abstract The influence of natural organic matter (NOM) fouling on the rejection of micro pollutants in nanofiltration (NF) and the adsorption during granular activated carbon (GAC) filtration is investigated for a diverse group of pharmaceuticals. These pharmaceuticals were first spiked in feed waters filtered through a virgin NF-membrane and a corresponding NOM fouled membrane. The differences in rejection were never larger than 5%. The rejection of the negatively charged molecules on the fouled membrane slightly decreased and the rejection of the positively charged molecules slightly increased. These trends can be explained by a decrease of the repulsion of negative compounds and a decrease of the attraction of positive compounds by the fouled membrane. The sequential combination of NF and GAC was very effective in removing all types of pharmaceuticals. Compounds not well rejected by nanofiltration were more readily adsorbed by the GAC. Keywords Activated carbon filtration; fouling; nanofiltration; pharmaceuticals; rejection

Introduction

In general, three major solute-membrane interactions affecting rejection efficiency of organic pollutants in nanofiltration (NF) can be distinguished: steric hindrance (sieving effect), electrostatic repulsion (charge effect) and hydrophobic-hydrophobic/adsorptive interactions. These solute-membrane interactions are determined by solute properties (e.g., molecular weight/size, charge, and hydrophobicity (e.g. expressed by log KOW values)), membrane properties (e.g., molecular weight cutoff (MWCO)/pore size, surface charge (zeta-potential), and hydrophobicity (contact angle)), operating conditions (e.g., pressure, flux, and recovery) and feed water composition (e.g. pH, temperature, DOC, inorganic matrix). An important challenge still consists in developing models that convey a fundamental understanding and a simple quantification of the governing phenomena, based on readily available knowledge about both solute and membrane properties (Bellona et al., 2004). Steric hindrance is mainly determined by the size of the solute and the size of the membranes pores. This phenomenon generally leads to a typical S-shaped rejection curve as a function of the molecular weight: solutes with a molecular weight higher than the molecular weight cut-off (molecular weight of a component that is retained by 90%) of the membrane are well rejected, while solutes with molecular weight lower than the molecular weight cut-off can easily permeate through the membrane. Hydrophobic-hydrophobic interactions between a membrane and solutes and adsorption of hydrophobic solutes onto a membrane are important factors in the rejection of uncharged organic micro pollutants. Hydrophobicity of organic molecules is usually doi: 10.2166/ws.2007.131

Water Science & Technology: Water Supply Vol 7 No 4 pp 17–23 Q IWA Publishing 2007

Influence of natural organic matter (NOM) fouling on the removal of pharmaceuticals by nanofiltration and activated carbon filtration

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quantified as the logarithm of the octanol-water partition coefficient, log KOW. Molecules with log KOW . 2 are generally classified as hydrophobic. Experiments have also shown (Braeken et al., 2005) that hydrophilic molecules are better rejected compared to hydrophobic molecules of similar molecular weight. This might be explained by hydration of hydrophilic molecules. Another explanation may lie in the hydrophobichydrophobic interactions between a hydrophobic membrane and hydrophobic solutes. Previous research shows that the place of the S-curves are depending on the log Kow of the compound. So a different cut-off value can be defined for molecules with different polarity (Verliefde et al., 2006). Electrostatic interactions occur between a charged membrane and charged solutes. Most commercially available membranes possess a surface that is negatively charged when the membrane is immersed in a water matrix, because of the dissociation of functional groups (mostly carboxylic acids and sulfonic acids) on the membrane surface. These functional groups can also be found inside the membrane pores, so the first few molecular layers of the membrane pores are also negatively charged. The charge of a solute at a given pH can be calculated if the pKa (the acid dissociation constant) of the solute is known (Bellona et al., 2005). When a negatively charged solute approaches the negatively charged membrane surface, repulsive charge interactions prohibit the solute from passing the membrane and the solute is well rejected, regardless of the solute size (Kimura et al., 2003). Less work has been done on the rejection of positively charged solutes by negatively charged membranes. It can be theoretically expected that attractive forces are at work between positively charged solutes and the negatively charged membrane surface. Small positively charged organics (smaller than the membrane pores) can be attracted towards the membrane surface and can even be accelerated into the membrane pores because of the negative charges in the pores. Therefore, they easily pass through the membrane by diffusion and low rejection values can be expected (Verliefde et al., 2005). This will lead to an increased concentration of positively charged solutes in the vicinity of the membrane surface (an additional concentration polarisation layer). The higher concentration polarisation will eventually lead to lower observed rejection values. With a NF membrane, as opposed to a reverse osmosis (RO) membrane of a number of micropollutants are not removed completely. In previous research the benefits of the combination of nanofiltration and granular activated carbon filtration have been investigated (Heijman, 1998, 1999). A GAC-filtration step following a NF step offers several advantages. 1. The NF rejects polar micropollutants to a large extent, but is less effective in removing non polar micropollutants. The non polar micropollutants passing the membrane are readily adsorbed by the activated carbon. Due to strong hydrophobichydrophobic interaction the adsorption capacity of activated carbon is large for these molecules. 2. The NOM-molecules which are responsible for competition, pre-loading and poreblocking of the GAC are removed by the NF, resulting in prolonged run times of the GAC columns. This result has been demonstrated in rapid small scale column tests (RSSCT). (Heijman, 1999). 3. The empty bed contact time (EBCT) for the GAC can be much shorter, resulting in lower investment costs for the activated carbon. This has been shown in a pilot experiment. A EBCT of 3 minutes was feasible with very long run times. (Heijman, 1998).

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In this way the activated carbon is complementary to the nanofiltration and, because of its more efficient use, will only lead to a small increase of the treatment costs.

Experimental setup

S.G.J. Heijman et al.

In this research, experimental procedures are established for bench-scale testing with a single spiral wound element. A Trisep TS-80 Membrane is characterized before use with a 500 ppm MgSO4 solution. Also contact angle and zeta-potential measurements are performed to assess hydrophobicity and charge of the membrane surface, respectively. In a first experimental step, rejections of selected pharmaceuticals are determined on clean membranes, using water from the water treatment plant Weesperkarspel in Amsterdam (coagulated surface water from the Rhine River). The selected pharmaceuticals are summarised in Table 1, along with several solute parameters. The pharmaceuticals are spiked in concentrations of about 2 mg/L. In a second experimental step, the membranes are fouled under controlled conditions: the permeate is discharged, while the concentrate is recycled to the feed vessel, until a feed water recovery of 75% is reached. At a recovery of 75%, both concentrate and permeate are recycled to the feed vessel and the membrane is fouled under these conditions for two weeks. The fouling protocol is carried out with untreated Weesperkarspel-water, containing both NOM and suspended solids: also calcium is added to the feed vessel to a total concentration of 200 mg Ca2 þ /l in order to enhance NOM-fouling (Cornelissen et al., 2005). In the next experiment, the rejection experiments with the pharmaceuticals is repeated with the fouled membranes, without cleaning them first. In this way, the influence of the fouling on the rejection of pharmaceuticals can be determined. After the fouling run, the combined influence of both NOM and colloidal fouling (which is closest to the practical situation) on the rejection is determined. In the final experiment the recovery of a small pilot installation was put at 80% on one membrane element. The feed water (surface water from the Schie in Delft) was pretreated with ultrafiltration (UF). The high recovery was obtained by recycling the concentrate directly to the feed. The system was cooled in order to prevent a temperature increase in the membrane system. The concentration of the pharmaceuticals was measured in the feed and the permeate. The permeate was further treated with a small column filled with GAC. Also the concentration of the pharmaceuticals in the effluent of Table 1 Selected pharmaceuticals for the experiments Compound

MW (g/mol)

log KOW (2)

Charge at feed water pH

Terbutaline Salbutamol Pindolol Propanolol Atenolol Metoprolol Sotalol Clenbuterol

225 239 248 259 266 267 272 277

0.90 0.64 1.75 3.48 0.16 1.88 0.24 2.00

þ þ þ þ þ þ þ þ

Phenazon/antipyrine Aminopyrine Carbamazepine Cyclophosphamide Pentoxyfilline

188 231 236 261 278

0.38 1.00 2.45 0.63 0.29

neutral neutral neutral neutral neutral

Ibuprofen Clofibric acid Fenoprofen Gemfibrozil Ketoprofen Diclofenac Bezafibrate

205 215 242 250 254 296 362

3.97 2.57 3.90 4.77 3.12 4.51 4.25

2 2 2 2 2 2 2

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Figure 1 Rejections of selected pharmaceuticals on the clean membrane

the GAC-column was measured. The EBCT was 3 minutes and the compounds were spiked to the feed of the nanofiltration for three days in order to avoid influences of the adsorption in the membranes on the rejection. Results and discussion

Figure 1 shows the rejections of the selected pharmaceuticals by a clean Trisep TS-80 membrane. The effects of steric hindrance is not very clear from this graph because there is no clear relation between the molecular weight and the rejection. However, the effect of charge interactions on the pharmaceutical rejections is very clear. Negatively charged pharmaceuticals exhibit consistently high rejections due to charge repulsion’s between the pharmaceuticals and the negatively charged membrane surface, whereas rejections for positively charged compounds are lower than can be expected based on their molecular weight. This is probably due to charge attractions with the negatively charged membrane surface, leading to a concentration increase of positively charged compounds at the membrane surface (conceptually, an additional concentration polarisation). Rejection values for uncharged (neutral) compounds are in-between the rejection values for positively and negatively charged compounds. In Figure 2 the flux decline under constant pressure is shown, inferring an increase in resistance of the membrane. The increase in resistance is attributed to NOM-fouling because the recovery is low and there are no scaling conditions. Biofouling is not probable during an experimental period of only two weeks. The NOM-fouling is enhanced by the addition of calcium ions, this mechanism has been shown in earlier research (Cornelissen et al., 2005; Heijman et al., 2005). The fouling protocol shows an approximately 20% loss in permeability. The rejection results graph for the fouled membrane (not shown here) were very similar to those in Figure 1. There was a small difference in the rejection of some pharmaceuticals but the differences were always less than 5%. In Figure 3 these differences are plotted for those pharmaceuticals which were measured properly in both experiments. The overall

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Figure 2 Fouling experiment with addition of calcium ions (5 bar and 208C)

Figure 3 Difference in rejection (%) for positive, negative and uncharged pharmaceuticals

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S.G.J. Heijman et al. Figure 4 Rejection of selected pharmaceuticals at a recovery of 80%

impression is that the rejection of the negative compounds is decreased probably because the negative charge of the membrane is decreased by the foulant and the repulsion of negative molecules is lowered. The neutral molecules are not, on average not affected by the fouling and the rejection of the positive molecules is increased due to the decreased attraction of these molecules with the membrane surface. The results of this fouling experiment are in line with the theory that the repulsion of molecules increases the rejection and the attraction of molecules decreases the rejection. In Figure 4 the rejection is shown for the experiment at 80% recovery. All (overall) rejections are lower in this experiment. This is because the concentrations in the feed spacer of membrane element are increased compared to the feed of the installation due to the internal recycle flow. The trends for negative, positive and neutral pharmaceuticals are the same as in the experiments at lower recoveries. For some of the positive pharmaceuticals the overall rejection of the experiment is only 30 to 40% and an additional treatment step is necessary. In Figure 5 the rejection of both the NF as well as the activated carbon is shown. It is clear that the compounds which are poorly removed by the nanofiltration (e.g. pindolol; propanolol; atenolol; metoprolol and sotalol) are well removed by the

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Figure 5 Removal of pharmaceuticals in an NF-GAC treatment combination

Conclusions

† The fouling by natural organic matter changes the rejection maximal 5%. † The rejection of negatively charged pharmaceuticals is decreased for a fouled membrane because the repulsion between compound and membrane surface is decreased. † The rejection of positively charged pharmaceuticals is increased for a fouled membrane because the attraction between the compound and the membrane surface is decreased. † The rejection of neutral pharmaceuticals is (on average) not changed † Nanofiltration and activated carbon filtration are complementary in removing organic micro pollutants. Compounds not well rejected by nanofiltration are more readily adsorbed by the GAC † If the GAC follows the nanofiltration the GAC is more efficient because almost all of the NOM is removed from the feed water.

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activated carbon. This is due to the fact that NF readily removes polar compounds while GAC is very efficient in removing apolar compounds. Of course this is only a very short experiment. The rejection of the nanofiltration will not change substantially in time, but an activated carbon column can show a breakthrough of the target compound in the effluent if the column becomes saturated. So even if the removal is sufficient in the short term, the run time of the activated carbon column can be such that the treatment step may be expensive due to frequent regeneration of the GAC. The efficiency of the removal of a compound can only been shown in a much longer spiking experiment.

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