Stabilised Colloids OPS

The filtration and rejection of stabilised colloids (OPS) was shown in section 5.8. The stable coUoids have a high negative surface charge and are not retained by the MF membranes, Adsorption on the membrane material is minimal - much less than in the absence of organics and at pH 3. 

All relationships are linear in at least one part of the graph, however, the cake filtration relationship is constant. This means that no cake is formed, which is indeed the observed situation during these experiments. While the overall linear relationships appear unrealistic, this shows that the blocking laws may indicate the correct mechanism - all filtration phenomena occur simultaneously or sequentially, except in the OPS case of small colloids where no cake formation occurs. 

As a function of primary colloid size the observation made is similar - for the small coUoids (75 nm), no cake formation is observed. For the 250 nm colloids cake formation is slightly higher but very close to zero, whereas for the large particles (500 nm) cake filtration is clearly evident. 

The blocking law analysis showed that particles do accumulate, block pores, and adsorb at the membrane surface, although cake formation is not always visible. Also, it is unknown how the model reacts to cake formation inside pore entrances. This is a possible scenario and may have a similar effect as the cake formed on the surface. A more quantitative analysis of the blocking laws of the systems used appears useful. 

As shown in section 0, MF only removes a very small amount of dissolved organics. To increase the organics rejection with MF, the organics need to be transformed into particulates. This can be done by coagulation, which, combined with conventional filtration, is currently the most abundant water treatment process. The interest in this investigation was to be able to compare the flux and rejection achieved with MF and pretreatment to other membrane processes (see following chapters). 

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Figure 5.4 Flux ratio (flux after collection of 800 mL permeate over pure water flux) as a function of primary colloid sit e for stable colloids (pH3), aggregates in the absence of organics (pH8), stabilised colloids (OPS), and aggregates (SPO).

Figure 5.9 Flux ratio as a function of calcium concentration (A) for stabilised colloids (OPS) and (B) for aggregates (SPO, Gl WP membrane).

Figure 4.19 Postulated structures (A) stable hematite colloids in absence of organics, (B) reaction limited aggregation (RLAJ, (C) diffusion limited aggregation (DLAJ, (D) SPO aggregates with organics, (E) OPS colloids stabilised with organics, (F) OPS colloids stabilised with NOM, (G) OPS colloids stabilised with organics and destabilised with calcium.

For the systems mixed in OPS order, no size was measured. The Malvern instrument signal was too low for these systems. Since the rejection of these stabilised systems was so low in MF, it is assumed that the size of these colloids is not larger than primar) colloids plus an adsorbed organic layer (thickness 1 to 2 nm). 

As shown in Table 5.5 (No 1, 2, 3), HA concentration was varied from 5 to 20 mgL" DOC, but the increase in concentration had only a marginal effect on colloid rejection. The aggregates were not redispersed due to the addition of organics, which stabilised the particles in the OPS case. The deposition of DOC (results not shown) on the membrane increases with concentration, but colloid deposition is constant at about 90%. Organic rejection decreases with increased concentration. 

In summary, there are two scenarios which cause the most severe flux decline. Firstly, poorly soluble organics at low pH (4,5) or in the presence of salt, which is often the case with surface waters, and secondly small colloids, that partially aggregate. Particles prepared with organics in the OPS order exhibit a low flux decline for the small colloids, due to a low rejection and an incomplete adsorption within the pores. The solution chemistr) is important, as pH influences the adsorption of organics and, thus, particle stabilisation. Additionally, calcium can destabilise the previously stable colloids. 

Figure 7.45 Ferric chloride addition in the presence of stabilised (OPS) and aggregated (SPO) colloids fPFC-SR membrane, 2.5 mM CaCl, 5 mgLi DOC HA, 10 m hematite (75 nm)).

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