Estudio de asociación: caso/control

1.2.1

Membrane classes

There are two classes of membranes: isotropic and anisotropic membranes. Isotropic membranes are characterised by a chemically and structurally homogeneous composi-

tion. Microporous membranes, nonporous dense films, and electrically charged mem-

branes are typical examples of isotropic membrane (Baker 2004).

In contrast, anisotropic membranes are homogeneous in chemical composition (Loeb- Sourirajan membranes) with varying pore sizes and porosity across the membrane thick- ness or chemically and structurally heterogeneous (composite membranes) as described by Lee et al. (2016). The latter one exist as thin-film, coated films, and self-assembled structures. Composite membranes basically consist of a thin surface layer supported by a much thicker porous structure, the (mechanical) support layer. In some cases, a non-

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woven backing layer provides additional mechanical support. These two or three struc- tural parts of thin-film composite (TFC) membranes are mostly made of different poly- meric materials. For example, the NF270 by Dow Filmtec (Minneapolis, MN, USA) consists of a polyamide active (PA) skin or surface layer (as thin as 20 nm) based on a polysulfone supporting (approx. 10 times the thickness of the skin layer) and a polyester backing layer (approx. a few µm thick) (Nghiem et al. 2005b, Semião and Schäfer 2013). The thin film or active skin layer exclusively determines the separation of solutes and permeation rates, whereas the porous supporting layer(s) solely provide mechanical support to the skin layer (Fujioka et al. 2015b, Lee et al. 2016). This structural design of TFC membranes leads to a high permeability/flux. According to Fujioka et al. (2015) TFC membranes are widely applied in NF/RO and have become the industry standard.

1.2.2

Membrane materials

Membranes can basically be made of three types of material: organic, inorganic or an inorganic-organic hybrid material. Looking at the published studies on TrOCs removal by NF/RO in water/wastewater treatment applications conducted between 2014 and 2016 the clear dominance of polymeric membranes, particularly polyamide membranes, is ob- vious as can be seen from Figure 1-1. There were no recent studies on hybrid membranes in the context of TrOCs removal so these type will not be described in detail.

Figure 1-1: Numbers of membranes studied as a function of membrane material (active skin layer) between 2014 and 2016 (total number of membranes tested = 90)

79

6

2

2

1

Polyamide

Polyethersulfone

Cellulose Acetate

Polyvinyl Alcohol

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1.2.3

Organic membrane materials

Essentially all organic membranes are made of polymeric materials. This is due to the fact that polymers offer a wide range of structure and properties. Typical polymeric materials for industrially established filtration membranes include cellulose acetates (CA), cellulose nitrate (CN), polyacrylonitrile (PAN), polyamide, polycarbonates (PC), polyetherimides (PEIs), polyimide (PI), polysulfone (PSU), polyethersulfones (PESs), cross-linked polyether, polypropylene (PP), polyvinylidene fluoride (PVDF), and polyvinyl alcohol (PVA). These membrane materials represent the first generation of polymeric materials and to date are most widely used in membrane applications (Lee et al. 2016, Ulbricht 2006).

1.2.3.1

Polyamide membranes

Supporting the abovementioned dominance of polyamide membranes it is reported that polyamide or polyamide-derivative RO membranes are used in most if not all water rec- lamation plants (Fujioka et al. 2012, Shenvi et al. 2015). This can be partly attributed to several favourable properties of polyamide membranes such as high permeate flux, ade- quate salt rejection and the opportunity to operate in a wide pH range (Lee et al. 2011). On the downside, polyamide membranes lack a sufficient resilience to strong oxidizing agents like free chlorine, which is used to control biofouling: This may lead to a chlorine- induced membrane degradation (Glater et al. 1994, Simon et al. 2009). However, the “membrane-friendly” surrogate anti-biofouling agent, monochloramine, is a precursor for the formation of disinfection by-products (e.g. N-Nitrosodimethylamine, NDMA).

1.2.3.2

Cellulose acetate membranes

Cellulose acetate and cellulose triacetate (CTA) have re-emerged as membrane material for RO especially for feed waters with high fouling potential. This is due to the chlorine resistance (up to 1 ppm) so therefore chlorine can be used to suppress the biofouling on the membrane surface (Shenvi et al. 2015). Cellulose acetate-based membranes exhibit a significantly lower surface charge than polyamide based RO membranes. Consequently, the role of electrostatic repulsion of charged TrOCs is less significant compared to poly- amide RO membranes (Fujioka et al. 2015a). However, the knowledge regarding the re- jection of TrOCs by cellulose acetate membranes in RO applications remains very scarce

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due to the lack of respective studies. The abovementioned chlorine resistance of CA/CTA membranes is still accompanied by a disadvantage, namely a limited permeate flux (Konagaya et al. 2001). Another disadvantage of CA/CTA membranes is the very narrow tolerable operational pH range (4 to 6) (Shenvi et al. 2015), because outside this range CA membranes are prone to hydrolysis both in acidic and alkaline conditions (Vos et al. 1966).

1.2.4

Inorganic membrane materials

On the other hand, inorganic membranes have recently attracted a lot of interest due to their favourable material properties such as thermal, chemical, and mechanical robust- ness as well as their reusability. Membrane treatment of wastewater is often subject to high fouling potential and consequently requires chemical cleaning. That offers new op- portunities for the application of, for example, ceramic membranes that show greater fouling-resistance (Lee et al. 2016), since they are described as hydrophilic membranes with lower hydrogen bonding potential than polymeric membranes (Childress and Elimelech 1996). The greater chemical stability/resilience and the longer life of ceramic membranes compared to current polymeric membranes make them an interesting op- tion for e.g. drinking water applications and provide reliability and safety (Fujioka et al. 2014a, Lee et al. 2016). Loose ceramic membranes (MWCO > 500 Da) have been commer- cially available for more than a decade. Very recent developments in ceramic materials and manufacturing technologies facilitated the production of much tighter ceramic mem-

branes, e.g. MWCO = 200 Da (Fujioka et al. 2014a). Furthermore, TiO2 as an important ma-

terial for the manufacture of ceramics is considered a nontoxic, readily available, and in- expensive material (Lee et al. 2016). On the other hand, Fujioka et al. (2014) state the low permeability, relatively high capital cost and the limited availability of low MWCO mem- branes as limiting factors for a wider use of ceramic membranes.

Membrane materials currently developed usually feature nanocrystalline structures, in-

cluding porous ceramics (e.g. Al2O3, TiO2, ZrO2, ZnO, and SiO2,), composites containing two

or more materials (e.g. TiO2–SiO2, TiO2–ZrO2, and Al2O3–SiC), and various nanoparticle

composites (e.g. Ag–TiO2, Zn–CeO2, and zeolites(DeFriend et al. 2003, Kumar et al. 2014,

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