each cycle and then analyzed. Table 5.4 shows the rejection of each organic model foulant in single solution for all four permeate cycles as well as the combinations.
Table 5.4: Rejection of each model foulant when filtered individually for each permeate cycle
Cycle Alginate B Humic Acid ABH ABHS
1 ≥ 86% ≥ 91% ≥ 88% 52% 55%
2 ≥ 86% ≥ 91% ≥ 88% 58% 55%
3 ≥ 86% ≥ 91% 83% 64% 59%
4 ≥ 86% 87% 85% 73% 62%
B: Bovine Serum Albumin
≥ is indicated for concentrations below the limit of quantification = 0.2 mg C/L
The single solutions were analyzed using the TOC instrument and the combinations were analyzed using LC- OCD
The rejection for all single model foulants was quite high and greater than 80% in all cases even though the size of the foulants is smaller than the membrane MWCO. However, despite the similar removals for all three model foulants with the tubular membranes, it is unlikely that the removal mechanism is the same since both humic acid and bovine serum albumin resulted in high levels of hydraulically irreversible fouling. Alginate rejection seems to remain relatively constant throughout all cycles. However, for humic acid and bovine serum albumin, by the fourth cycle rejection begins to decrease and this trend would be expected to continue considering the suspected fouling mechanism is adsorption, which can only occur temporarily at the beginning stages of filtration until adsorption sites are exhausted. Additional filtration cycles would be required to confirm this hypothesis. Konieczny et al. (2006) reported similarly high TOC removals of 100% of humic acid with microfiltration (MF). Considering that MF membranes are not expected to remove organics, adsorption is the most plausible removal mechanism.
The rejection results are somewhat higher compared to the flat-sheet study where the average TOC rejection was approximately 80% for bovine serum albumin, 75% for alginate and 50% for humic acid (Munla et al. 2011). If the mass filtered per membrane surface area is compared, the tubular membrane received approximately 600 mg/m2 whereas the flat-sheet membrane was exposed to more than double that amount with approximately 1,300 mg/m2. Therefore, it is possible that during the
flat-sheet experiments the membrane’s adsorption capacity was reached or exceeded, thus resulting in lower TOC rejection of humic acid and bovine serum albumin.
Silica is not included in Table 5.4 because inorganic concentrations cannot be measured by TOC analysis. However, a few preliminary supporting measurements of silica concentration were obtained through a commercial lab. The observed rejection ranged from 70-80% and silica concentration was increased in the backwash water. Considering that the size of the silica is averaged at 9 nm and a previous study by Calvo et al. (2008) measured a membrane pore size of 16.4 nm ±1 with scanning electron microscopy and 13.6 nm ± 0.7 with liquid-liquid displacement porosimetry for this membrane, a high silica rejection is unexpected. Possible explanations include aggregation of particles, thus increasing particle size, or if both the membrane and colloid are negatively charged there may also be an electrostatic repulsion effect at play. Some rejection may also occur as a result of the pore size distribution of the membrane as well as the colloidal particles, in which case some particles may be rejected through size exclusion. This particle size was specifically chosen to induce fouling because it is close to the pore size of the membrane, which has been associated with higher rates of fouling. However, in this study silica mainly contributed to hydraulically reversible fouling.
From Table 5.4 it can be seen that the overall rejection of DOC increased for both ABHS and ABH, more so for the latter. Rejection for both combinations was approximately 60%, which is within the range of the results observed for the flat-sheet study (ABH ~ 50% and ABHS ~60-80%). However, it was much lower than the rejections observed for the individual solutions. LC-OCD analysis was used for the combination solutions ABH and ABHS at 120 LMH to qualitatively assess which of the foulants were being rejected. This result is relatively similar to Munla et al. (2011), in which a trend of lower rejections was observed for combinations of model foulants. One hypothesis is that in the presence of silica, the foulant layer is looser thus allowing more foulants to pass through the membrane; this is supported by the results in Munla et al. (2011) where the rejection of solutions with silica were generally lower. For ABH however, the rejection may be lower due to less pore constriction and more of a cake layer formation, which may prevent humic adsorption within the pores. This cake layer may also explain the high hydraulic reversibility of this combination solution
The LC-OCD chromatograms for ABH and ABHS are shown in Figure 5.7. The different organic matter fractions are not baseline separated, however, they do have discernable peaks, particularly for the feed solutions. The LC-OCD chromatograms also confirm the use of BSA and alginate to represent biopolymers in natural water sources as they elute in the same time window biopolymers present in natural water elute.
For ABH, (Figure 5.7 part a) it can be seen that the biopolymer (in this case comprised of alginate and bovine serum albumin) concentrations in the permeate seem to decrease with each cycle but on average humic acid removal remained stable. Therefore, since humic acid rejection remained fairly consistent between cycles, while alginate and bovine serum albumin seem to show increased rejection with time, it is possible that theses two foulants are involved in the fouling layer build-up, which subsequently increased overall rejection from 52% to 73%.
For ABHS (Figure 5.7 part b) humic acid concentration was the lowest in the cycle 1 permeate. Therefore, rejection for humic acid was higher during the first cycle and then continued to decrease (peak in permeate increases), indicating likely initial adsorption. This trend is consistent with results of single humic acid filtration as shown in Table 5.4. Rejection of biopolymers (i.e. alginate and bovine serum albumin) remained quite constant throughout the cycles with only cycle 1 having a slightly higher concentration (i.e. lower rejection). Both simultaneously occurring opposite trends in concentration of these fractions resulted in a relatively constant DOC rejection over all four cycles (Table 5.4). The lower rejection for mixtures with silica was also observed for the flat-sheet study (Munla et al. 2011). One hypothesis is that silica interferes with humic acid adsorption either through interactions with the other model foulants or with the membrane surface itself.