Effluent from a wastewater treatment plant was filtered (flow rate 11 m3/(m2.h)) over nanofibre membranes in the presence of UV-light. The results on the effluent filtration with non-functionalised PA-6 and TiO2 functionalised nanofibre membranes were analysed by
measuring turbidity, biological activity (total ATP) and interpretation of UV-VIS spectra (A436)
for detection of humic acids.
Figure 8.4 presents turbidity, decrease in biological activity and A436 removal and, indicating
a reduction on all parameters. Filtration with nanofibre membranes has a positive effect on the effluent quality. After filtration with non-functionalised PA-6 membranes, turbidity was 69% lower and biological activity decreased to 76% of the original effluent values. Also the effect of filtration on humic acids is shown in Figure 8.4 . Non-functionalised PA-6 membranes remove 44% of the humic acids.
When comparing with the results of Ma et al. (2010) on filtration of surface water (pore size 0.80 µm), the results are comparable with filtration performed in this study on non- functionalised PA-6 membranes. Ma et al. (2010) obtained 42% humic acid removal by filtration alone which is comparable to the 44% humic acid removal obtained in this study. Since nanofibres have a very high clean water flux (see chapter 4) a lower trans-membrane pressure is needed to obtain the same water flux, giving benefit to the novel nanofibre membranes developed in this study, with similar humic acid removal.
To obtain photodegradation effect, higher contact times and as such lower fluxes are needed. This was also demonstrated in the contact tests done in this chapter where 4 hours of illumination were needed to obtain a 84% degradation of WWTP effluent and 67% degradation of higher loaded 60 mg/l humic acids. In studies performed by Zhang et al. (2008b), Ma et al. (2010) and (Bai et al. 2012) fluxes between 450 and 1000 l/(m².h) in cross- flow set-with TiO2 loaded membranes were used compared to 11000 l/(m².h) during this
test.
From the results obtained in this chapter on filtration with non-functionalised PA-6 membranes, it can be concluded that nanofibre membranes can be used as a high-flux effluent filtration technique for example for recuperation of water. The functionalisation with TiO2 could give an extra positive aspect for a possible use in a cleaning phase if
appropriate contact times are applied. However, this still needs to be tested.
Figure 8.4: Removal of turbidity, biological activity (expressed as ATP) and humic acid removal A436 after effluent filtration by nanofibre membranes. All samples were pre-treated with sand filtration. Results express removal compared
to the original effluent values.
8.4 Conclusions
In this study the photocatalytic activity and filtration performance of nanofibre membranes functionalised with TiO2 was demonstrated with different types of wastewater. WWTP
effluent filtration with nanofibre membranes improved water quality as reduction in turbidity (69%), humic acids (44%) and bacterial activity (76%) was observed.
Contact experiments showed extra removal on humic acids and S. aureus. Humic acid removal of 83% of the WWTP effluent was obtained after 2 hours of illumination using a post-functionalised commercial TiO2 membrane which was the best performing membrane.
Further, 60% degradation after 2 hours of illumination was observed with a higher loaded commercial 60 mg/l humic acid solution.
The inactivation of S. aureus with TiO2-functionalised nanofibres plus UV light is higher than
with UV photodegradation alone. Nanofibre membranes functionalised with TiO2 give a 4 –
4.5 log10/100ml or a 99.99% removal of S. aureus after 6 hours of UV illumination.
The oxidation of humic acids and bacteria suggest the potential of a TiO2 functionalised
nanofibres membrane as a high-flux anti-fouling membrane. However, this still needs to be tested.
In a normal filtration set-up, contact time is too low to see an effect of functionalisation on the membranes, but functionalisation proves its utility when the nanofibres are under UV- illumination for 4 hours, reducing bacteria and humic acids and thus contributing to anti- fouling abilities.
Also, filtration with non-functionalised PA-6 nanofibre membranes can be used as a high-flux effluent filtration technique.
9
Structure changes and water filtration
properties of polyamide nanofibre membranes
During this work it was noticed that sometimes results for clean water permeability were not reproducible, especially when nanofibre membranes were produced, stored and tested on different occasions. The storage of nanofibres seemed to alter membranes properties. Care was taken during all previous experiments to avoid different storage conditions. This however shows the need to study this effect in more detail as to offer a solution for long term behaviour and stability.
In this chapter the nanofibre membranes were treated under different circumstances, simulating the diverse conditions in which a membrane could be stored and to simulate their use in water filtration systems. Under all these different conditions, experiments were done on the fibre morphology (SEM pictures, dimensional changes) and the membrane properties (tensile strength, clean water flux, bacterial removal). Also the benefit of a heat-treatment was demonstrated.
Daels, N., Van Hulle, S.W.H. and De Clerck, K. Structure changes and water filtration properties of polyamide nanofibre membranes. In preparation.
9.1 Introduction
Nanofibre membranes have a unique and porous structure resulting in a very high water flux. The nanofibres are randomly organised, because the jet of the polymer is bending during the electrospinning production process. The overall fibre configuration has a large influence on the membrane pore structure as was shown in different studies focusing on modelling of fibre orientation (Pradhan et al. 2013, Soltani et al. 2014). Previous chapters showed its excellent properties and potential use in water treatment technologies. For the long-term use of the nanofibres, it is essential to know how to keep their excellent properties in different environmental conditions. Water filtration systems sometimes need to be taken offline, due to some mechanical failure or because there is a temporary reduced need for water treatment. During this offline period, membrane elements can either be stored in or out of the system but today it is generally recommended to keep the membranes in wet conditions (CSMfilter 2014, DOW 2014, GEwater 2014). For these novel nanofibre membranes, it is essential to test possible variations in morphology of the nanofibre membrane due to the storage as this may affect its filtration performance.
The structure of a material is immediately related to its properties (Page 2000). The nanofibre membranes as used in this work, are made of a polyamide, PA-6. In general, polyamides are hydrophilic. PA-6 absorbs water, which acts as a plasticizer by lowering the glass transition temperature (Utracki and Jamieson 2011). Above the glass transition temperature of the polymer, the polymer chains are more mobile (Callister and Rethwisch 2011) which could affect the membranes structure and thus its filtration properties. Also, water is known to affect the tensile strength in polyamides (Utracki and Jamieson 2011). This will influence the behaviour of the PA-6 nanofibre membranes during their use in water treatment. A possible action is a heat-treatment, causing the glass transition temperature of PA-6 to rise which lowers the effect of water as a plasticizer. A heat-treatment already showed some advantages in the prevention of “layered fouling” (Bilad et al. 2011b). It could be used to produce self-supporting membranes without the need for a non-woven support (Gopal et al. 2006) and is commonly used to reinforce membranes (Bilad et al. 2011a, Bilad et al. 2011b, Liu et al. 2011, Tsai et al. 2005). In a heat-treatment, membranes are exposed to a temperature just below their melting point. The heat-treatment will increase dimensional stability during further use as long as the temperature is below the heat- treatment temperature.
In order to obtain a stable water filter the nanofibre membrane needs to keep its properties and thus its structure. To investigate possible changes in the structure of the nanofibre membranes, several conditions were simulated in which the nanofibre membranes could possibly end up when using the membranes in water treatment systems. As such the aim of this chapter is to study structure and filtration properties (tensile strength, clean water permeability, pathogen removal efficiency) of electrospun PA-6 nanofibre membranes during storage (one month) in wet, dry and mixed environments. Initially nanofibres are spun onto a substrate. Storage of the nanofibre membranes on this substrate was compared to storage of membranes that were used as stand-alone membranes. Since a heat-treatment is expected to improve the stability of the nanofibre structure this was tested in this chapter simultaneously with non-heat-treated membranes. Also the influence of grammage on the filtration properties was investigated.