Study of membrane fouling using synthetic model solutions in UF and NF processes
Highlights
► The application of membrane processes to textile effluents is limited by membrane fouling phenomena; especially if the pretreatment is not very exhausted. ► A deep study about the relation between the membrane processes (ultrafiltration and nanofiltration) and fouling has been carried out. ► Fouling was measured as filtration resistances. ► The membrane with 1 kDa of MWCO was less prone to fouling. ► For all tested membranes no influence of osmotic pressure generated by the dyes and salts on filtration resistances was found.
Introduction
Textile industry is one of the oldest and primmest water polluters in the world. Due to their high volume and nature complexity, textile effluents have traditionally been object of big interest for the water treatment research.
These industries mainly require a large consumption of fresh water and dyes which have acquired notoriety as hazardous substances, since dyes consist of highly structured molecules with potential toxicity for organisms and high persistence in the environment [1], [2], thus not allowing their easy natural biological degradation.
In general, approximately 20–40% of the input dye remains in the dye-baths [3]. Their concentrations range varies from 10 to 1000 mg L−1 [4], [5]. This fact makes the treatment of textile wastewaters a difficult task requiring the examination of each factory operating conditions individually for the determination of the best treatment methodology. In addition, the fluctuating compounds concentrations and flow rates make the conventional treatments quite insufficient for the treatment of textile wastewaters, especially for color removal [6].
A special attention was given to the application of membrane processes, specifically ultrafiltration (UF) [7], [8] and nanofiltration (NF) [9], [10], [11], [12], [13], [14], to remove dyes. However, the main drawback of this technology is the fouling. Caniço-Gomes et al. [15] found that the main factor causing flux decline in textile solutions was the dye adsorption. This result has been reinforced by other authors [16], [17]. Although there are many works about textile wastewaters, no many of them have been performed studying AZO dyes, its fouling phenomena and its effect on membrane behaviour.
In this work, UF and NF membranes were tested to treat synthetic reactive dyes solutions (using black, blue and red Everzol AZO dyes). The filtration resistances were estimated in order to evaluate the membranes fouling. Besides, for each dye solution, normalized flux and salt, COD and color retentions were also determined to compare the membranes behaviour.
Section snippets
Membranes tested
In this work, two UF and two NF membranes were tested. The UF membranes were 10 kDa and 1 kDa from Rhodia Orelis and Synder Filtration, respectively. The NF membranes (NF-200 and NF-270) were provided by Dow Chemical. Table 1 collects main membranes characteristics according to manufacturers and literature data. Also this table includes some parameters determined by the authors according to the procedures describes in a previous work [18].
UF membranes are mainly made of polyethersulfone material
Model solutions characterization
The main characteristics of the synthetic model solutions are shown in Table 2. It can be seen that the conductivity values varied between 1600 and 2400 μS cm−1. The differences between pH, COD and color are negligible.
Effect of TMP on fouling phenomena
Pure water fluxes are represented in Fig. 3, Fig. 4, Fig. 5 show membrane flux with permeation time for UF and NF membranes. In all cases, the steady state was reached around 10800 s. As it can be observed in Fig. 4, 10 kDa membrane showed a moderate rapid initial permeate flux
Conclusions
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1 kDa membrane performs as a NF membrane considering Rm, Rs and Rs/Rt ratio values obtained.
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NF and 1 kDa membrane fouling, evaluated in terms of filtration resistance and normalized flux, are not affected by TMP. However, the higher the TMP, the higher the fouling in 10 kDa membrane.
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NF membranes are less prone to fouling than UF ones. However, a UF membrane with tighter cut-off (i.e. 1 kDa) reduces the fouling trend even more than the NF ones.
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The influence of applied TMP on color and COD retention
Acknowledgements
This work was supported by Ministerio de Asuntos Exteriores y Cooperación of Spain (project ref: A/024821/09). We also thank the technicians from the Microscope Services from UPV for their support in SEM analysis.
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