A hybrid photocatalysis–ultrafiltration continuous process for humic acids degradation S.I. Patsios, V.C. Sarasidis, A.J. Karabelas ⇑ Chemical Process and Energy Resources Institute (CPERI), Centre for Research and Technology-Hellas (CERTH), P.O. Box 60361, GR 570 01 Thermi, Thessaloniki, Greece article info Article history: Received 9 October 2012 Received in revised form 26 November 2012 Accepted 27 November 2012 Available online 8 December 2012 Keywords: Photocatalytic Membrane Reactor (PMR) Hybrid photocatalysis–ultrafiltration process Humic acids degradation Disinfection by-products formation control abstract The successful operation of a hybrid photocatalysis–membrane separation process (in a laboratory-scale pilot system) is demonstrated for degradation of Humic Acids (HAs), which are typical refractory compo- nents of Natural Organic Matter (NOM). The Photocatalytic Membrane Reactor (PMR) employing an Ultrafiltration (UF) submerged module was operated in continuous mode with TiO 2 catalyst concentra- tion 0.75 g/L and UV-A radiant flux 15.1 J/s, treating feed water with HA concentrations 5.0 and 10.0 mg/L. Experiments carried out within a sufficiently broad pH range (approx. 3.5–7.0) exhibited sat- isfactory HA mineralization rates and rather high HA mineralization efficiencies; a near optimum pH at approx. 5.5 was identified. The overall process removal efficiency (comprising both HA oxidation and UF membrane rejection) was even higher, as a result of the synergistic effects of photocatalytic oxidation and membrane filtration. The degradation of HA aromatic rings, that diminishes the Disinfection By-Products (DBPs) formation potential, was also very high; the specific UV-A 254 removal efficiency was greater than 95% in almost all cases studied. Concerning membrane filtration performance, the periodic backwashing employed, combined with the moderate membrane flux, effectively controlled membrane fouling thus permitting stable continuous operation with no wastewater stream. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Humic substances, a major fraction of Natural Organic Matter (NOM) present in freshwater sources, are a primary target of water treatment processes even though themselves are not considered pollutants [1–3]. Negative effects of humic substances in potable water include undesirable colour and taste, absorption and con- centration of organic pollutants, and biochemical decomposition in the water distribution systems [4,5]. Moreover, evidence that NOM can act as a precursor for the formation of Disinfection By- Products (DBPs), during the commonly performed chlorination of drinking water, and the potentially severe health effects associated with DBP, have led several countries in taking measures to ensure control of DBP and their precursors (NOM) [6]. Advanced Oxidation Processes (AOPs) have emerged as an attractive alternative for simultaneous removal and destruction of NOM [7–9] in comparison to conventional treatment technolo- gies (e.g. coagulation, ion exchange, membrane filtration and acti- vated carbon adsorption) that solely separate NOM from the water stream and transfer it to another phase or stream, thus necessitat- ing subsequent treatment and/or disposal [7]. Among AOP, hetero- geneous photocatalysis employing semi-conductor catalysts has a widely demonstrated efficiency for degrading a broad range of organics, mainly at experimental scale [8–11]. TiO 2 , which is the catalyst of choice in most of the semi-conductor photocatalysis re- search studies [12], may be either suspended in the reaction mix- ture (suspension-type) or fixed on a carrier material. The latter mode of catalyst application has some inherent disadvantages, such as the relatively small surface area-to-volume ratio and the reduced UV-A light utilization efficiency [13,14]. In parallel, large scale application of suspended TiO 2 photocatalytic water treat- ment is constrained by technical challenges mainly related to the effective recovery and extended use of the catalyst particles in a continuous process [9]. A promising approach for separation and reuse of suspended TiO 2 is the Photocatalytic Membrane Reactor (PMR) concept [14– 16], involving the coupling of photocatalysis with a membrane process [17,18]. Apart from suspension-type photocatalytic reac- tors coupled with various membranes types (i.e. Microfiltration – MF, Ultrafiltration – UF and Nanofiltration – NF), the term PMR also encompasses systems with photocatalytic particles fixed on the membrane surface (i.e. photocatalytically active membranes), which are not dealt with in this paper. The synergistic effects of photocatalysis and membrane separation has drawn the attention of numerous researchers; their results have been reviewed by Ollis [19], Molinari and Palmisano [20], Augugliaro et al. [21], Kochko- dan et al. [17], Loddo et al. [18], Mozia [14], and Zhang et al. [22]. 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.11.033 ⇑ Corresponding author. Tel.: +30 2310 498181/2; fax: +30 2310 498189. E-mail address: karabaj@cperi.certh.gr (A.J. Karabelas). Separation and Purification Technology 104 (2013) 333–341 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur