De los cruces bivariables y trivariables

Another possible application for nanofibre microfiltration membranes is their use in membrane bioreactors as a separator to filter the treated and cleaned water from the biological sludge.

As such, the nanofibre membrane was applied in a MBR. The biology in the MBR was performing very well. Online registration of the dissolved oxygen showed a good bacterial activity throughout the process. Bacteria that metabolise normally, consume dissolved oxygen in the process. The mixed liquor suspended solids (MLSS) increased from 1.0 g/l to 3.9 g/l and the mixed liquor volatile suspended solids (MLVSS) enlarged from 0.6 g/l to 3.2 g/l during the 59 days operation. MBR’s are typically operated at MLSS concentrations of between 7 and 15 g/l. In this study, there was made use of MLSS of 1-4 g/l to avoid membrane rupture which was originated by delamination of the membrane.

Other important parameters on the biological activity that were monitored in this study, are the removal of NH4+, COD, TSS, NO3- and turbidity. The results are

illustrated in Table 4.6 and showed a very good removal of turbidity (99.3%), TSS (99.7%), COD (94.8%) and NH4+(94.0%). Only total nitrogen removal (NH4+ and NO3-)

was not satisfying due to an insufficient denitrification. Several measures were taken to improve the denitrification. At first the recycle ratio of the reactor was increased which resulted in a minor enhancement. Lowering the DO in the reactor (2 - 3 mg/l) seemed not to have any influence at all.

Table 4.6: Removal of turbidity, TSS, COD, NH4+ and total nitrogen Parameter Removal (%) Turbidity 99.3 COD 94.8 NH4+ 94.0 TSS 99.7 Total nitrogen 59.8

Initial tests showed that the membrane was not strong enough to resist the pressure during filtration when TMP increased due to the fouling on the membrane. At first only a 5 day working period could be guaranteed without membrane rupture. Changes were made to increase the tensile strength of the membrane. The membrane was found to be less affected by pressure when it was thicker (the relation between membrane density and flux can be found in chapter 9). After this increase in thickness of the membrane a working period of at least 59 days was possible (until the operation was stopped) and the effluent had a good quality (Table 4.1). During this 59 days there was no membrane rupture but there were some fouling problems.

The online registration of TMP and flux gave an idea of the fouling on the membrane. In Figure 4.5 this registration is illustrated during the first days of membrane operation with 3 back-wash cycles, removing the reversible fouling. After these first few days, irreversible fouling increased very fast, with a rapid decline in flux as result (see Figure 4.4).

Figure 4.5 shows the increase in TMP until the set point (0.4 bar) and the activation of the back-flush (27 l/m².h) until the TMP drops to 0 bar. The flux is controlled at 30 l/m2.h.bar. In case of membrane rupture there was no increase of the TMP. Therefore the TMP curve was a good indicator for the condition of the membrane. An earlier study showed that in case of a flat sheet membrane (Kubota) the backwashing frequency was 1/60 min (Yang et al. 2006). In the case of the nanofibre membrane presented here this frequency was 2/60 min due to the high fouling on the membrane (Figure 4.5).

Figure 4.5: Registration of TMP(bar) and flux(l/m2.h) with regular back-flush at the set point (0.4 bar)

Although this back-flush with permeate removed the reversible fouling, a persistent yellow layer could not be removed. This irreversible fouling caused a swift decay of the membrane flux after only 7 days (Figure 4.4) and was clearly visible in the SEM pictures taken after 14 days of operation. This “layered” fouling (Figure 4.6) of electrospun nanofibres was also observed by Aussawasathien et al. (2008). Pressure- changes on the nanofibre membrane causes deformation of the pores. This was already seen in the section in this chapter on the bacterial filtration: with supportive membrane a higher bacterial removal was obtained due to the pores that were less deformed. Consequentially opening and closing of the pores can cause “layered” clogging of electrospun nanofibres (Aussawasathien et al. 2008). Minimization of this fouling requires further research on operational conditions and membrane optimization. This will be done in chapter 5.

Figure 4.6: Origin of layered fouling. A: original nanofibre membrane. B: nanofibre membrane when the pores could open under filtration pressure if no supportive layer is used on the permeate side of the membrane. Fouling particles can enter easily and are trapped inside the nanofibrous structure if the pores close (adapted

to Bilad et al. (2011b)).

4.7 Cleaning of the membrane

As explained in chapter 2, cleaning the membrane with demineralised water is not sufficient to restore the flux at original level since this only removes the reversible part of fouling. Irreversible membrane fouling needs to be cleaned by use of chemicals (Kimura et al. 2004, Lim and Bai 2003). In Figure 4.7 the effect of different cleaning strategies on the removal of fouling is demonstrated by SEM pictures. Two cleaning methods were used: 30 min 0.5% NaOCl and 30 min 0,2% HCl and secondly 12 hours 0.5% NaOCl and 12 hours 0.2% HCl. In chapter 2 there is described how manufacturers use 2 hours of soaking in chemicals.

Even with the chemical cleaning procedure, the original flux could not be restored and after the cleaning phase the same decay of the flux could be observed (Figure 4.4). Further membrane development and additional measures such as membrane performance enhancement products (www.nalco.com) urge to be used and will be tested and discussed in next chapter 5.

It can be seen that even for long term cleaning only the fouling on the surface of the membrane can be removed. The fouling that is caused by depth filtration remains in the membrane. This is the so-called “layered” fouling. The foulants are trapped in the nanofibrous structure by opening and closing of the pores by pressure changes during backwash of the membrane.

To make sure that the chemical reagents used for the cleaning of the membrane, were not harmful for the membrane, a novel tensile strength test was done in this study after the cleaning procedure. The original tensile strength was 13 MPa, after cleaning it was 11 MPa. The results of the strength test on the membrane showed that the strength of the membrane is only slightly effected by the used chemicals.

A

Figure 4.7 Influence of the cleaning procedure on irreversible fouling. a: before cleaning; b: after cleaning with 30 min 0.5% NaOCl and 30 min 0,2% HCl; c: after cleaning with 12 hours 0.5% NaOCl and 12 hours 0.2% HCl