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Improved Natural Organic Matter Removal By Adsorption-ultrafiltration System

Congress: 2015
Author(s): Feryal Akbal, Dilek Gimi&#;
Ondokuz Mayıs University, Engineering Faculty, Environmental Engineering Department 1, Sinop University, Construction and Technical Department2

Keyword(s): Sub-theme 1: Water supply and demand,
AbstractIntroduction

Natural organic matter (NOM) is a precursor to carcinogenic disinfection byproducts and causes complexation with metals and hydrophobic synthetic compounds. NOM cannot be readily rejected during ultrafiltration (UF) since UF membranes have relatively large membrane pores compared to the molecular size of NOM. Furthermore, NOM is well known as a major contributor to membrane fouling because it clogs membrane pores or forms a cake layer on the membrane surface. Thus, NOM is a barrier to the extensive application of the UF process in advanced water systems. Humic acid (HA), a major hydrophobic fraction of dissolved NOM (DOM), is usually considered to be responsible for severe membrane fouling. HA consists of anionic macromolecules having a wide range of molecular weights. These macromolecules are comprised of both aromatic and aliphatic components with primarily carboxylic (60-90%) and phenolic functional groups in aquatic environments. Compared to other fractions of DOM, HA is found to have the largest impact on membrane fouling because it causes extensive pore adsorption (Heo et al., 2012).

Ultrafiltration (UF) has become increasingly popular as one of the most reliable processes for the removal of particulate matter (e.g., turbidity and viruses) in water treatment. However, the inevitable problem of fouling causes a reduced production capacity of the membrane plant, which requires frequent cleaning or replacement of the membranes leading to higher operating costs (Cui and Choo, 2013). Various treatment methods have been employed to mitigate NOM fouling. Adsorption is an efficient technology for NOM removal, therefore it has been widely adopted as pretreatment for UF to enhance the performance of UF process (Li et al., 2014).

The objective of this study is to investigate the feasibility of granular activated carbon (GAC) contributing to membrane fouling control, and to evaluate potential mechanisms for HA removal in a hybrid system of UF in the presence of GAC. The rejection of HA and flux decline in GAC/UF system were investigated.

Methods/Materials

Commercially sourced humic acid sodium salt used in all experiments was purchased from Aldrich (Aldrich Co. Ltd., USA). Humic acid solutions were prepared at a concentration of 1000 mg.L-1 by adding 1 g humic acid into distilled deionized water and dissolved using the ultrasonic sonication bath in order to provide homogenous dissolution. Stock solution of humic acid was stored in amber glass bottles and protected from sunlight. Humic acid solution at 30 mg.L-1 concentration was prepared from the stock solution by dilution then used in all the experiments. GAC was obtained from Merck (catalog no: 102514). GAC were thoroughly washed with distilled deionized water, dried at 105 °C for 6 h before use in adsorption-ultrafiltration experiments.

The membrane used in UF experiments was the Sarcaton Slice 200 ultrafiltration membrane that has a MWCO of 50 kDa and an effective surface area of 200 cm2. In the hybrid adsorption/UF system feed water was first pumped at a flow rate of 3 mL min-1 into the GAC column and the collected column effluent was then supplied to the UF unit at a fixed pressure of 1 bar. Water samples were analyzed for DOC content on Teledyne Tekmar Apollo 9000 combustion TOC analyzer, which uses combustion at 750 °C to oxidize the carbon. The UV254 absorbance measurements were performed in accordance with standard methods on a Heגios- γ UV/VIS spectrophotometer at a wavelength of 254 nm, with a 1 cm quartz cell, and the SUVA254 was calculated as the ratio between the UV absorbance and the DOC (expressed in L.mg−1 m−1). The color of humic acid samples will be characterized by UV-vis spectra at 436 nm (Color436).

Results and Discussion

Bench-scale UF experiments were performed to evaluate the performance of both GAC/UF (with GAC) and the UF process alone (without GAC) in terms of flux decline profile and HA rejection. Ultrafiltration alone removed 62% of DOC and 78% of UV254 at 1 bar and 72% of DOC and 81% of UV254 at 2 bars, respectively. The GAC/UF exhibited increased rejection trends as a result of GAC binding site adsorption of HA during time-dependent fouling runs. The observed rejections of HA as DOC, UV254, Color436 and SUVA were 78, 95, 99, and 78% in GAC/UF and 62, 78, 86 and 42% in UF alone at applied pressure of 1 bar, respectively. Rejections measured with UV254 are in the range of 78-95%, which is higher than rejection measured by DOC. This indicates selectivity towards the rejection of UV-absorbing molecules, which also tend to be larger and more aromatic.

Membrane fouling was assessed by determining the reduction in filtration flux that occurred during the 30 min filtration experiment where the system was operated under constant pressure. During the filtration experiments membrane flux gradually declined from 315 to 216 L.m2h-1 for UF and from 396 to 285 L.m2h-1 for GAC/UF process due to fouling taking place at the membrane surface and interior.

Conclusion

GAC/UF clearly presented improved rejection trends compared to UF alone. The UF process alone showed lower DOC rejection, 62%, while the GAC/UF rejection was 78%. The GAC/UF system showed approximately 25% more HA removal than the UF process alone because of HA adsorption onto GAC. The UF process alone showed a larger flux decline compared with the GAC/UF. The use of GAC column could be a viable option for the practical application to membrane plants for water treatment.

Li, K., Liang, H., Qu, F., Shao, S., Yu, H. Han, Z-S., Du, X., Li, G. (2014) Control of natural organic matter fouling of ultrafiltration membrane by adsorption pretreatment: Comparison of mesoporous adsorbent resin and powdered activated carbon. Journal of Membrane Science 471, 94-102.

Heo, J., Kim, H., Her, N., Lee, S., Park, Y-G., Yoon, Y. (2012) Natural organic matter removal in single-walled carbon nanotubes-ultrafiltration membrane systems. Desalination 298, 75-84.

Cui, X., Choo, K-H. (2013) Granular iron oxide adsorbents to control natural organic matter and membrane fouling in ultrafiltration water treatment. Water Research 47, 4227-4237.

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